Teaching and Learning About Reaction …...Teaching and Learning About Reaction Mechanisms In...

Teaching and Learning About Reaction

Mechanisms In Organic Chemistry

Meagan Ladhams Zieba

BSc (Hons)

This thesis is presented for the degree of Doctor of Philosophy in Chemistry, School of

Biomedical and Chemical Sciences at the University of Western Australia

June 2004

i

Abstract

This study was carried out to investigate the teaching and learning processes occurring

in the topic of reaction mechanisms in three tertiary level organic chemistry courses and

focussed on investigating perceptions about the importance of teaching and learning

about reaction mechanisms and about the difficult aspects of the topic.

Consistency was observed between students’ and lecturers’ perceptions of the

importance of teaching about reactions. Both groups discussed the usefulness of

mechanistic representations to generalise reaction processes and to predict reaction

outcomes.

There is some consistency between the perceptions of the difficulties inherent in

studying this topic, but the perceptions of lecturers and students are not necessarily the

same. Students’ demonstrated difficulties corresponded to the lecturers’ perceptions.

In the organic chemistry courses under investigation, students achieved many of the

explicitly stated aims that their lecturers identified. The students rarely achieved

implicit outcomes anticipated by the lecturer.

Lecturers demonstrate a tendency to use particular structural representations when

discussing certain types of reaction process. The study identified that students

commonly use these same types when working through particular reaction processes. In

addition, it was found that the use of a particular structure could cue students into

thinking about only one type of reaction process taking place in a given reaction.

The use of language that is consistent with a consideration of only single reaction

particles was also commonly observed in lectures. While this can be adequate in some

circumstances, other aspects of reaction processes are better considered in terms of

multiple reaction particles. Students tend to use single particle language when talking

about reaction processes.

The project proposes an integrated model, which takes into account the many levels

(macroscopic, single particle molecular, multiple particle molecular and intramolecular)

involved when describing reaction processes. It is felt that a consideration of the levels

discussed in this model is useful when teaching and learning about reaction

mechanisms.

iii

Acknowledgements

To Associate Professor Bob Bucat: Your enthusiasm for teaching chemistry has been an

inspiration to me, and I thank you for all your assistance, advice and understanding.

You are a wonderful supervisor and it has been a privilege to work with you.

To Dr Mauro Mocerino: I have you to thank for many things, my enthusiasm for the

teaching and learning of organic chemistry being just one. As a teacher, you inspired

me with your dedication to your students. As a supervisor, your guidance has always

been sincere and well appreciated. Thank you for everything.

To Professor David Treagust: Thank you for the time and energy that you have devoted

to supervising this project. You always ask me the right questions to keep me focussed

on the tasks at hand and gave constructive advice. Your assistance is appreciated.

While ethical considerations prevent me from naming the lecturers in this study, I thank

them for their participation. You allowed me to observe your classes and participated in

discussions with me. Thank you all for your time, efforts and interest.

My thanks to the students who gave their time to this study, allowing me the

opportunity to investigate their understandings. Their participation is most appreciated.

Thanks to Maree Baddock, whose unending patience and education background were

often called on; Janette Head, for her abilities to name compounds, follow curly arrows

and edit from across the room; and Sally Thompson, for always being able to articulate

answers to difficult questions.

Thanks also to the staff of Chemistry, School of Biomedical and Chemical Sciences for

providing the means to carry out this research. I also acknowledge the assistance of an

Australian Postgraduate Award.

I thank my family and friends for their help and friendship over my university career,

particularly my parents. I hope that my efforts have made you proud of me. Thank you

so much.

And finally, but most importantly, to my husband, Michael. You have always

understood me better than anyone and you know how much I have enjoyed the time I’ve

spent working on this research study. Thank you for always being there for me.

v

This thesis is dedicated to my parents, Dianne and Graeme Ladhams, who have given

me a beautiful life filled with love and laughter, and to my beloved in-laws, Paula and

Stan Zieba, who provided the same to the charming soul who is my husband.

vii

Table of Contents

1 Introduction ...............................................................................................................1

1.1 Introduction ...........................................................................................................1

1.2 Reaction Mechanisms ...........................................................................................3

1.3 Study Aims............................................................................................................4

1.4 Motivation and Justification of Aims....................................................................5

1.5 Research Questions .............................................................................................11

1.5.1 Lecturer Related Questions .....................................................................11

1.5.2 Student Related Questions ......................................................................12

1.6 Research Claims..................................................................................................12

1.7 Thesis Outline .....................................................................................................14

2 Theoretical Considerations .....................................................................................16

2.1 Introduction .........................................................................................................16

2.2 Naturalistic Research ..........................................................................................17

2.3 Theoretical Considerations .................................................................................20

2.3.1 Constructivism ........................................................................................20

2.3.2 Prototypes of Learning............................................................................21

2.3.3 Conceptual Change .................................................................................24

2.3.4 Information Processing Model................................................................25

2.4 Researcher’s Perspectives ...................................................................................28

2.5 Summary .............................................................................................................30

3 Context of the Study ...............................................................................................31

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3.1 Introduction......................................................................................................... 31

3.2 Universities ......................................................................................................... 32

3.2.1 University A............................................................................................ 32

3.2.2 University B ............................................................................................ 32

3.3 Courses of Interest .............................................................................................. 33

3.3.1 Chemistry 100 at University A ............................................................... 33

3.3.2 Chemistry 2XX at University A.............................................................. 35

3.3.3 Chemistry 3XX at University A.............................................................. 36

3.3.4 Chemistry 121/122 at University B ........................................................ 37



3.4 Lecturers..............................................................................................................40

3.4.1 Dr Anderson............................................................................................ 40

3.4.2 Dr Adams ................................................................................................41

3.4.3 Associate Professor Andrews..................................................................42

3.5 Students............................................................................................................... 42

3.5.1 Chemistry 100 Students .......................................................................... 43

3.5.1.1 Students Interviewed in 1999.............................................................. 43


3.5.2 Chemistry 2XX Students ........................................................................46



ix

3.6 Summary .............................................................................................................48

4 Subject Matter .........................................................................................................50

4.1 Introduction .........................................................................................................50

4.2 The Concept of Reaction Mechanism.................................................................50

4.3 Commonly Used Representations .......................................................................52

4.3.1 Appropriate Structural Representations ..................................................52

4.3.2 Curly Arrows...........................................................................................53

4.3.3 Formal Charges .......................................................................................54

4.3.4 Lone Pairs or Non-Bonding Electrons ....................................................55

4.4 Nucleophilic Substitution Reaction Mechanisms ...............................................55

4.4.1 SN1 Reaction Mechanism........................................................................56

4.4.2 SN2 Reaction Mechanism........................................................................58

4.4.3 SN1 vs SN2...............................................................................................58

4.5 Elimination Reactions .........................................................................................60

4.5.1 E1 Reaction Mechanism .........................................................................61

4.5.2 E2 Reaction Mechanism .........................................................................62

4.5.3 More Than One Elimination Product Formed ........................................62

4.6 Substitution Vs Elimination ................................................................................64

4.7 Intended Learning—Concepts and Skills ...........................................................64

4.8 Summary .............................................................................................................65

5 Pedagogical Content Knowledge ............................................................................66

5.1 Introduction .........................................................................................................66

x

5.2 Pedagogical Content Knowledge ........................................................................66

5.3 The Complexity of Teaching and Learning about Science................................. 69

5.4 The Complexity of Teaching and Learning About Chemistry ........................... 72

5.4.1 The Abstract Nature of Chemistry ..........................................................72

5.4.2 Macroscopic, Microscopic and Symbolic Representations ....................74

5.4.3 The Language Used in Chemistry........................................................... 78

5.5 The Complexity of Teaching and Learning About Reaction Mechanisms......... 81

5.5.1 Language and Representations Used in Reaction Mechanisms..............81

5.5.2 Literature Review of Previous Research.................................................84

5.5.2.1 Alternative Conceptions...................................................................... 84

5.5.2.2 Representational Difficulties............................................................... 85

5.6 Implications of Pedagogical Content Knowledge............................................... 85

5.7 An Integrated Model Guiding the Research .......................................................87

5.8 Summary ............................................................................................................. 90

6 Methodology ...........................................................................................................91

6.1 Introduction......................................................................................................... 91

6.2 Research Methodology .......................................................................................91

6.2.1 Qualitative Research ............................................................................... 91

6.2.2 Sampling ................................................................................................. 93

6.2.3 Ethical Considerations ............................................................................ 94

6.3 Data Collection Techniques ................................................................................ 96

6.3.1 Interviews................................................................................................96

xi

6.3.2 Other Techniques ....................................................................................97

6.3.2.1 Questionnaires.....................................................................................97

6.3.2.2 Examinations and Test Questions .......................................................97

6.3.2.3 Focus Groups ......................................................................................98

6.4 Trustworthiness ...................................................................................................99

6.4.1 Validity and Reliability ...........................................................................99

6.4.2 Validity and Reliability in Qualitative Studies .......................................99

6.4.3 Documentation and the Audit Trail.......................................................104

6.5 Analysis and Coding .........................................................................................105

6.6 Research Timetable...........................................................................................107

6.7 Research Design—Lecturers.............................................................................109

6.7.1 Lecturer Observations ...........................................................................109

6.7.2 Lecturer Interviews ...............................................................................109

6.8 Research Design—Students ..............................................................................110

6.8.1 Questionnaires.......................................................................................113

6.8.2 Interviews..............................................................................................113

6.8.3 Chemistry 2XX Focus Group 2001 ......................................................114

6.8.4 Laboratory Tests and Examinations......................................................114

6.9 Summary ...........................................................................................................114

7 Findings: Lecturers’ and Students’ Perceptions...................................................116

7.1 Introduction .......................................................................................................116

7.2 Students’ Prior Understandings.........................................................................117

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7.2.1 Introduction........................................................................................... 117

7.2.2 Research Tasks...................................................................................... 118

7.2.3 Findings................................................................................................. 118

7.3 Lecturers’ and Students’ Perceptions of Topic Importance.............................. 121

7.3.1 Introduction........................................................................................... 121

7.3.2 Lecturers’ Perceptions of Topic Importance......................................... 122

7.3.3 Students’ Perceptions of Topic Importance .......................................... 123

7.3.4 Comparison of Perceptions ................................................................... 128

7.4 Difficulties Associated With Writing and Interpreting Reaction Mechanisms 130

7.4.1 Introduction........................................................................................... 130

7.4.2 Lecturers’ Perceptions of Students’ Difficulties................................... 131

7.4.3 Students’ Perceptions of Difficulties .................................................... 136

7.4.4 Students’ Observed Difficulties ............................................................ 142

7.4.4.1 Introduction....................................................................................... 142

7.4.4.2 Mechanistic Representations............................................................. 142

7.4.4.3 Substitution Versus Elimination ....................................................... 153

7.4.5 Comparison of Difficulties Associated with Representing Mechanisms

.............................................................................................................. 159

7.5 Summary ........................................................................................................... 162

8 Findings From Chemistry 121/122 (Dr Anderson)............................................... 164

8.1 Introduction....................................................................................................... 164

8.2 Presentation of Coursework.............................................................................. 166

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8.3 Intentions Implemented in Class Presentation ..................................................169

8.3.1 Stated Intentions....................................................................................169

8.3.2 Implemented Intentions.........................................................................172

8.4 Students’ Achievements....................................................................................176

8.4.1 Introduction...........................................................................................176

8.4.2 Understandings of Curly Arrows ..........................................................176

8.4.3 Understanding Common Representations: Substitution vs Elimination

Processes...............................................................................................180

8.4.4 Understandings of Competing Reaction Processes...............................184

8.5 Summary ...........................................................................................................190

9 Findings From Chemistry 100 (Dr Adams) ..........................................................192

9.1 Introduction .......................................................................................................192

9.2 Presentation of Coursework ..............................................................................192

9.3 Intentions Implemented in Class Presentation ..................................................193

9.3.1 Stated Intentions....................................................................................193

9.3.2 Implemented Intentions.........................................................................196

9.4 Students’ Achievements....................................................................................199

9.4.1 Introduction...........................................................................................199

9.4.2 Understandings of Representations.......................................................200

9.4.2.1 Curly Arrows.....................................................................................200

9.4.3 Formal Charge.......................................................................................204

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9.4.4 Writing Mechanisms for Typical Functional Group Reactions Based on

Experimental Data ................................................................................ 206

9.5 Summary ........................................................................................................... 215

10 Findings From Chemistry 2XX (Associate Professor Andrews)...................... 218

10.1 Introduction....................................................................................................... 218

10.2 Presentation of Coursework.............................................................................. 218

10.3 Intentions Implemented in Class Presentation .................................................. 220

10.3.1 Stated Intentions.................................................................................... 220

10.3.2 Implemented Intentions......................................................................... 222

10.4 Students’ Achievements.................................................................................... 224

10.4.1 Introduction........................................................................................... 224

10.4.2 Understandings of Curly Arrows .......................................................... 224

10.4.3 Substitution and Elimination Processes ................................................ 227

10.4.4 Mechanistic Representations................................................................. 233

10.4.4.1 Effect of Unfamiliar Compounds on Students’ Abilities.............. 233

10.4.4.2 Relative Position of Particles in Representations ......................... 237

10.4.5 Structures as Cues for Reaction Type ................................................... 239

10.5 Summary ........................................................................................................... 244

11 Conclusions and Pedagogical Implications ...................................................... 247

11.1 Introduction....................................................................................................... 247

11.2 Research Claims and Related Implications....................................................... 247

11.2.1 Lecturers’ and Students’ Perceptions.................................................... 247

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11.2.1.1 Research Questions and Claims ....................................................247

11.2.1.2 Findings: Perceptions of Importance ............................................248

11.2.1.3 Findings: Perceptions of Difficulties Faced by Students ..............251

11.2.1.4 Pedagogical Implications of Findings...........................................253

11.2.2 Students’ Achievements in Their Studies .............................................257


11.2.2.2 Findings.........................................................................................258


11.2.3 Lecturers’ Teaching Strategies and Effect on Students ........................260


11.2.3.2 Findings.........................................................................................261


11.2.4 Single vs Multiple Particle Descriptions...............................................265

11.2.4.1 Research Claim Arising From Overall Study ...............................265


11.3 Summary ...........................................................................................................267

12 References .........................................................................................................269

13 Appendices........................................................................................................276

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Table of Figures

Figure 1.1: Representation of a mechanism involving carbocations and hydroxide ions.8

Figure 1.2: A representation of an elimination reaction mechanism using a curved arrow

system proposed by Eckert. ......................................................................................8

Figure 2.1: Prototypes of learning situations described by Pines and West. .................. 21

Figure 2.2: Information processing model showing the interaction between observed

events and their storage in short- and long-term memory. .....................................25

Figure 4.1: Different representations of 1,1-dibromoethane........................................... 53

Figure 4.2: Curly arrow notation symbolising the heterolytic breakage of an H-Cl bond

to form H+ ions and Cl- ions. ................................................................................... 54

Figure 4.3: A representation of a mechanism to explain the formation of ammonium

ions from ammonia and hydrogen ions. On the left, one H has a formal charge of

+1. On the right, the N in the ammonium ion is represented with a formal charge

of +1. .......................................................................................................................55

Figure 4.4: A generalised representation of a two-step SN1 reaction mechanism. ......... 57

Figure 4.5: A generalised representation of a one-step SN2 reaction mechanism. ......... 58

Figure 4.6: Space-fill representations of (i) primary, (ii) secondary and (iii) tertiary alkyl

bromides. There is increased crowding around the central carbon atom in (iii) as

compared to (i) and (ii). ..........................................................................................60

Figure 4.7: A generalised representation of an E1 elimination reaction mechanism. .... 61

Figure 4.8: Representation of an E2 elimination reaction mechanism. ..........................62

Figure 4.9: A structural representation of 2-bromobutane, which has five β-hydrogens,

indicated by *. ......................................................................................................... 63

Figure 4.10: 1-butene (i), cis-2-butene (ii) and trans-2-butene (iii), all of which are

possible products in an elimination reaction where 2-bromobutane is the substrate.

................................................................................................................................. 63

xvii

Figure 5.1: Johnstone’s triangle, representing the interaction between the macroscopic,

sub-microscopic and symbolic levels. ....................................................................76

Figure 5.2: Symbolic representations of (i) diatomic oxygen molecule, (ii) equations

representing a two-step reaction process whose first step is rate determining

(RDS), and (iii) a graphical representation of the process represented in (ii).........76

Figure 5.3: Some structural representations of ethanol: (i) ball and stick model, (ii)

space filling model, (iii) square planar structure, (iv) structural formula and (v)

molecular formula. ..................................................................................................79

Figure 5.4: Compound (i) has been prepared by Rybinskaya and co-workers and

contained a single cation. The charge associated with the cation is represented on

the top left-hand side of the diagram. Representations (ii) and (iii) are both

dications. These are structural isomers that are related to (i). The images are

scanned from Hoffmann and Torrence (1993, p. 62 – 6)........................................80

Figure 5.5: A scan of an description of reaction processes, including structural

representations, taken from McMurry (2000, p. 391).............................................82

Figure 5.6: Representation of a mechanism for the reaction between tertiary butyl

cations and cyanide ions..........................................................................................88

Figure 5.7: A framework based upon Jensen’s levels of chemistry model and

Johnstone’s multilevel thought construct................................................................89

Figure 7.1: Question 1 from the 2002 questionnaire administered to Chemistry 100,

121/122 and 123/124 students (see Appendix 6.12 for the complete questionnaire).

...............................................................................................................................119

Figure 7.2: Two mechanistic representations of the protonation of an alcohol.

Representation (i) is correct, as the curly arrow is representing electron movement.

The second representation, (ii), shows the curly arrow pointing in the incorrect

direction. ...............................................................................................................132

Figure 7.3: An equation used by Teo (2000, p. 24), modified from Kotz and Treichel

(1996, 427) and Brown, LeMay and Burnsten (1997, p. 271 - 3) for calculation of

formal charge. .......................................................................................................134

xviii

Figure 7.4: Structural representation demonstrating Dr Anderson’s perception of a

common error........................................................................................................ 135

Figure 7.5: Task given to students participating in focus groups. ................................ 139

Figure 7.6: Mechanistic representations drawn by Bill in his interview. In both

examples, Bill has represented nucleophilic attack on carbons not bearing leaving

groups.................................................................................................................... 143

Figure 7.7: Representations from Guy’s interview showing incorrect use of curly arrows

when H+ was involved in a reaction process......................................................... 145

Figure 7.8: Example representing Damon’s usage of curly arrows to (i) break and form

bonds and (ii) move electrons independent of atoms............................................ 147

Figure 7.9: Examples of students’ incorrect usage of curly arrows.............................. 148

Figure 7.10: Two mechanistic representations whose correctness students were asked to

comment upon. Although both contain errors, representation (i) is more correct, as

it shows H being removed from an adjacent C (with incorrect use of a curly arrow

between H and OH- at the top left). In (ii), H is being removed from the same C as

Br, which would lead to a product other than that represented being formed. ..... 149

Figure 7.11: Examination question given to Chemistry 100 students to probe their

understandings of formal charge........................................................................... 150

Figure 7.12: A student’s answer to an examination question in Chemistry 121/122. The

structural representation is correct, but its charge is not....................................... 151

Figure 7.13: Questionnaire task investigating Chemistry 2XX students’ understandings

of formal charge. ................................................................................................... 151

Figure 7.14: Representations of (i) protonated ethanol and (ii) protonated ethanamine.

The structures are shown without represented charges, as they were given to

students in an interview......................................................................................... 152

Figure 7.15: Example from an interview with Boris. This student demonstrated

difficulty when representing the formation of double bonds in elimination reaction

mechanisms........................................................................................................... 154

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Figure 7.16: Chemistry 2XX examination that probes students’ abilities to represent

substitution and elimination processes. Question a) was written by the lecturer and

is not a task designed by the researcher. ...............................................................155

Figure 7.17: Examples of examination question answers in which students’ mechanisms

are not consistent with the product represented. ...................................................156

Figure 7.18: Examples of reaction mechanisms represented by Chemistry 2XX students

in an examination in which OH- is shown attacking (i) Br and (ii) C—Br bond in 2-

bromobutane..........................................................................................................157

Figure 8.1: Dr Anderson’s use of δ+ and δ- symbolism in different structures............175

Figure 8.2: Representations of (i) a neutralisation process, (ii) an ionisation process and

(iii) the most common, but incorrect, use of curly arrows to represent process (ii)

pictorially. .............................................................................................................177

Figure 8.3: An examination task and the two most common (but incorrect) answers

given by students. Representation (i) correctly follows the curly arrows, but does

not account for formal charge. Representation (ii) is inconsistent with the two

curly arrows shown. ..............................................................................................178

Figure 8.4: Two questions in the 1999 Chemistry 122 examination and their answers.

...............................................................................................................................181

Figure 8.5: A modified question for the examination in 2000......................................183

Figure 8.6: The reaction between 2-pentanol and hydrobromic acid can conceivably lead

to the formation of four products; (i) 2-bromopentane, (ii) cis-2-pentene, (iii) trans-

2-pentene and (iv) 1-pentene. This reaction is represented as is common in organic

chemistry—as an unbalanced equation.................................................................187

Figure 8.7: Examination task from Chemistry 121/122 in 2000...................................188

Figure 9.1: Comparison of reaction coordinate diagram depicting energy for formation

of a primary carbocation (i) and a more stable secondary carbocation (ii) in an

addition reaction....................................................................................................197

Figure 9.2: Three questions in the 2000 examination for Chemistry 100.....................201

xx

Figure 9.3: Representations of (i) the correct answer for (a), (ii) a common, incorrect

answer (a carbocation) and (iii) a common answer in which students represented an

additional curly arrow and second structure. ........................................................ 203

Figure 9.4: Mechanism drawn by Carl to answer an examination question (question 2 in

Appendix 6.24). The mechanism is not consistent with his written explanation,

which talks about nucleophilic attack from ‘both sides’ of a carbocation............ 208

Figure 9.5: An example answer to the examination question shown in Appendix 6.22,

given by a first year student. ................................................................................. 214

Figure 10.1: Representation of a reaction mechanism for the reaction between

carboxylic acid derivatives and hydroxide ions. ................................................... 222

Figure 10.2: Example showing the use of mechanisms to explain other reaction

outcomes apart from nucleophilic substitution and elimination processes in his

course. ................................................................................................................... 223

Figure 10.3: Examples of incorrect examination responses to a substitution question.

The curly arrows can still be correctly interpreted as representing electron

movement to break and form bonds...................................................................... 225

Figure 10.4: Mechanisms representing (a) substitution and (b) elimination processes

written by two students ((i) and (ii)) in response to an examination question. The

substitution representations are correct, while the elimination processes are not

consistent with curly arrows representing electron movement. ............................ 226

Figure 10.5: Two tasks administered in an interview in 2000. ..................................... 228

Figure 10.6: Graeme’s represented mechanisms for the formation of (i) a substitution

product and (ii) an elimination product................................................................. 229

Figure 10.7: Guy’s mechanism to rationalise the formation of alkene compounds in a

reaction between 2-bromo-2-methylpropane and ethoxide ions........................... 230

Figure 10.8: Possible reaction mechanisms between 3-bromo-2-methylpentane and

hydroxide ion to produce (i) 2-methyl-3-pentanol, (ii) 2-methyl-2-pentene, (iii) cis-

4-methyl-2-pentene and (iv) trans-4-methyl-2-pentene……………………..232

xxi

Figure 10.9: (i) Dennis’ answer to example 1, (ii) Debra’s answer to example 2 and (iii)

the expected answer. .............................................................................................234

Figure 10.10: Two questionnaire questions. In (i), the hydroxide ion is shown on the

opposite side to the bromide in the alkyl halide. In (ii), it is shown on the same

side. The represented answers shown on the right hand side were the most

common in these tasks, which are consistent with the nucleophile attacking the

alkyl halide from the physical position in which it is represented........................237

Figure 10.11: Three tasks from an interview with second year students. .....................239

Figure 10.12: Representations of the two enantiomers of 2-butanol, (i) R-2-butanol (ii)

and S-2-butanol. ....................................................................................................240

Figure 10.13: Representations of (i) cis-2-butene and (ii) trans-2-butene. ...................241

Figure 10.14: Representations of drawings made by (i) Felix and Fabian, (ii) Fiona, (iv)

Fernando and (v) Frank and Felicity in answer to question 3. Felicity did not draw

an H on her structure. (iii) is a representation of 1-butene. .................................242

xxii

Table of Tables

Table 3.1: Topics and subtopics detailed in the course outline for Chemistry 100. ....... 34

Table 3.2 Organic chemistry syllabus for second semester Chemistry 121/122. ...........38

Table 5.1: Jensen’s classification structure model (1998). ............................................. 77

Table 5.2: A four-level classification table extension of Jensen’s model. Aspects of the

topic of reaction mechanisms have been incorporated into this classification

system. ....................................................................................................................89

Table 6.1: Research tasks and their associated research questions (RQs). The numbers

in the fourth column refer to the student-related questions outlined in section 1.5.2.

............................................................................................................................... 110

Table 8.1 Organic chemistry syllabus for Chemistry 122. ........................................... 168

Table 8.2: Task analysis of tasks (i) and (ii) from Figure 8.4....................................... 181

Table 8.3: Task analysis of the task from Figure 8.5.................................................... 183

Table 9.1: Topics and subtopics from Chemistry 100 course outline........................... 195

Table 9.2: Task analysis for three tasks in Figure 9.2................................................... 201

1

1 Introduction

1.1 Introduction

It is a moment firmly etched in my mind: my best friend and I staring open-mouthed at

the white board as our lecturer drew some pictures. The pictures were intended to be

representations of molecules in a reaction mixture. The curly arrows supposedly

showed us where electrons were moving to and from as bonds were broken and formed

in the process of changing starting material molecules into product molecules. And

then there was the phenomenon we dubbed the magic molecule; the molecule that

seemed to appear and disappear at the lecturer’s discretion. There was rarely any

explanation for the presence of this magic molecule. It appeared because the lecturer

needed it to represent a particular mechanistic step. Generally, it disappeared altogether

in the very next step. Sometimes, the same magic molecule appeared two or three times

over the course of a particular mechanistic representation. It always made very little

sense to us.

We both passed that course, studying together, memorising pictures, a couple of arrows

and our magic molecules. Obviously, we managed to reproduce them effectively,

because we passed! These pictures, these reaction mechanisms, which were meant to

help us understand how chemical reactions may happen, or predict what might be

produced from a given reaction, really meant nothing to us. And, from the comments

that our friends made while studying the same course, there were few students who felt

they understood how to write reaction mechanisms. It seemed a bit of a waste of all the

efforts of our many lecturers in telling us about this wonderful tool of the organic

chemist.

The motivation for this study was based, in part, on these personal experiences. As an

undergraduate student, I was often confused by my lecturers’ representations of reaction

mechanisms in lectures and laboratory classes. I generally felt that these pictures were

not particularly useful in helping me to construct appropriate understandings of reaction

processes. I didn’t know why these pictorial molecules did what the representation

suggested. I simply knew that I had to be able to draw these types of pictures the right

way in the examination.

2

My method for dealing with this difficult topic for examination purposes was simple. I

quickly learned to copy down all the lecturer’s notes and structural representations and

then tried to memorise the exact mechanism picture for tests and examinations. It never

seemed to me that there was any sense or reason to writing reaction mechanisms.

Represented mechanisms were just another chunk (or many chunks) of examinable

information to be memorised.

When I started my Honours project in fourth year, reaction mechanisms were still a

mystery to me. Over the course of that year, however, the usefulness of these

representations became apparent. My project involved a great deal of organic synthesis,

making compounds that had not been synthesised before. It was very common for a

group of students from our laboratory to crowd around a whiteboard and discuss ways

in which we might attempt to synthesise certain compounds. We thought about

reactions whose outcomes we knew and we suggested how we might modify them for

our own purposes. As we talked, we drew pictures. Our pictures usually included curly

arrows, formal charges and magic molecules. Representations of reaction mechanisms

were useful to us, helping us to consider possible reaction processes for synthetic

purposes. At other times, we drew our pictures to try to work out why certain reactions

hadn’t produced the compounds we expected. We suggested other possible ways that

we might overcome problems we’d encountered in our lab work. We drew more

pictures.

By the end of that year, some of the mysteries of reaction mechanisms had become

apparent to me. Structural representations and curly arrows no longer seemed quite so

confusing. I could now use mechanistic representations to help design a procedure to

use in the laboratory. I could talk about the chemistry of a particular reaction with my

supervisor or another research student, drawing reaction mechanisms to rationalise

different processes. The magic molecules were magic no longer; they were simply

reaction particles which were present in the reaction mixture, and which I chose to

represent only when they were required in the particular mechanistic step I was

drawing. There were still aspects of these representations that were a little confusing to

me, but I finally understood something very important about these pictures with their

arrows and charges. Representations of reaction mechanisms are a useful way to talk

about chemistry with other chemists.

3

Were my friends and I alone in our confusion about representations of reaction

mechanisms? There is little research evidence concerning this question, but Weeks

(1998, p. vii) and Eckert (1998) have suggested that some undergraduate students do

find reaction mechanisms difficult to understand. Discussions with organic chemistry

lecturers indicated that they are aware of some of the difficulties that their students face.

However, these lecturers consider reaction mechanisms to be important to students’

understandings of organic chemistry and therefore to be a necessary component of their

courses. Each of the lecturers participating in this research study commonly used

mechanistic representations in his coursework. All were interested to know more about

their students’ understandings of mechanisms. What did their students think these

representations meant?

1.2 Reaction Mechanisms

The study of organic chemistry at a tertiary level generally involves a great deal of

focus on the topic of reaction mechanisms. A reaction mechanism can be simply

defined as ‘the detailed, step-by-step description of the pathway by which reactants are

converted to product’ (Pine, 1999, p. 117).

The term reaction mechanism can be considered to have two levels of usage. The first

use is in terms of actual processes going on within a reaction mixture; which particles

collide with other particles, how many of these collisions are successful and result in

bond breakage and/or formation and how solvent molecules interact with the particles in

solution. Alternatively, the term is often used to describe a written representation, using

accepted conventions, that seeks to explain or predict reaction outcomes. This usage of

the term, generally accompanying a pictorial representation, is seen in many organic

chemistry textbooks (for example, Brown, 2000, p. 182; Bruice, 1995, p. 372;

McMurry, 2000, p. 385 – 8).

Mechanistic representations can be written for all types of reactions. This thesis

focusses on the teaching and learning related to the representation of reaction

mechanisms for two types of process; substitution and elimination reactions. The

specific types of mechanism that were investigated are referred to as SN1 and SN2

4

(substitution) and E1 and E2 (elimination) processes. The specifics of this subject

matter are discussed in more detail in Chapter 4.

Reaction mechanisms are commonly classified into a few different types, and these are

usually presented in textbooks and in lectures separately from each other in an attempt

to minimise confusion. Coppola (1996), however, has suggested that this classification

or ‘simplification’ of reaction mechanisms could lead to eventual confusion for students

because it denies development of the notion of competing reactions:

Sometimes what begins as a well-intentioned simplification ends up as an unnecessary

complication. The first lesson in choosing between competitive mechanistic pathways that

students encounter is the manifold of reactions represented by the uni- and bimolecular

nucleophilic substitution and elimination pathways . . . (SN2, E2, SN1, E1). Students tend to

perceive these as exclusionary predictive choices . . . The underlying usefulness of

substitution-elimination manifold is . . . that an understanding of competition between

pathways is key to understanding actual experimental results . . . the simplification that

implies exclusivity rather than competition is counterproductive to an understanding of the

fundamental relationships.

1.3 Study Aims

This study was undertaken to investigate the teaching and learning processes involved

during tertiary-level courses at two Western Australian universities that included the

study of reaction mechanisms in organic chemistry. The project focussed on both the

lecturers’ and the students’ understandings of the topic and on the strategies employed

by the lecturers in the teaching of their courses. The identification of pedagogical

concerns specific to the teaching and learning of reaction mechanisms was also of

interest.

More specifically, the study was designed to:

• investigate why reaction mechanisms are considered to be an important

topic in the study of organic chemistry and to compare the perceptions

that lecturers and students hold about the importance of reaction

mechanisms;

5

• identify the lecturers’ intended outcomes for students studying about

reaction mechanisms;

• identify any prior knowledge and conceptions relevant to the study of

reaction mechanisms that students hold before they commence studying

organic chemistry;

• investigate students’ abilities to perform mechanistic tasks and to apply

mechanistic concepts they have learned about;

• identify areas of pedagogical concern in the study of reaction

mechanisms in organic chemistry, including the teaching strategies that

lecturers use.

1.4 Motivation and Justification of Aims

Personal interest in the topic was the initial motivation for investigating students’ and

lecturers’ perceptions of why mechanistic representations were considered important in

the study of organic chemistry. I had possessed only a limited understanding of the

importance of studying these representations in my own undergraduate courses, which I

know influenced how I studied (memorised) these representations for examinations.

I felt that comparing the perceptions of participant lecturers and their students was a

good basis for the study. If students and lecturers had similar perceptions about the

importance of studying mechanistic representations, I would expect students to develop

more useful understandings of these representations than if their perceptions were vastly

different to their lecturers.

Lecturers’ perceptions of the importance of studying about reaction mechanisms will

influence the intended learning outcomes of their courses. These outcomes were

identified through discussion with the participant lecturers. The intended outcomes of

each course were then used to identify the types of tasks that were provided to each

group of students. Students were only asked to work through tasks that were

appropriate to the course they were studying.

6

According to constructivist models, people’s prior understandings can affect the way in

which we learn about new subject matter, which is an important consideration for this

research project. In general, first year chemistry students have not learned about

reaction mechanisms before starting their tertiary studies, but they would have seen

some of the representations, such as structural representations, previously. I aimed to

identify any previous understandings or conceptions that students might possess that

were relevant to the study of mechanistic processes and representations to determine

how these prior conceptions might impact on their abilities to construct useful and

appropriate understandings about reaction mechanisms.

The need to investigate students’ abilities to apply mechanistic understandings to their

study of organic chemistry and to consider areas of pedagogical concern related to the

teaching of this topic were motivated by the literature. While there exist few studies

investigating students’ understandings of mechanistic representations, many teachers,

lecturers and researchers have commented on the difficulties that students face in their

study of organic chemistry.

Schearer (1988) noted that there is a lot of information to be learned in an organic

chemistry course, the textbooks are generally large and lecture notes and supplementary

worksheets are copious. Many aspects of organic chemistry, such as nomenclature,

require students to memorise facts and information, which are later used in their study

of the topic. Jones (1997, p. 233) likened learning of organic structures to learning of

grammar and commented that writing reaction mechanisms requires students to ‘use

some of that grammar to write some sentences and paragraphs’.

Students’ difficulties with or lack of understandings about reaction mechanisms can

impact upon their performance in an organic chemistry course. A goal of introductory

organic chemistry courses is ‘to teach fundamental principles of organic reactivity

through the study of mechanisms’ which ‘eases the burden of memorization . . . by

demonstrating the interrelationships between reactions’ (McNelis, 1998).

There are several areas in which a student with limited understandings of mechanistic

representations might encounter difficulty. He or she might find it difficult to build any

useful understandings to explain observed reaction outcomes, such as the formation of

more than one product from the same starting reactants. Such a student would also find

7

laboratory work more challenging, as he or she would not be equipped to attempt such

tasks as understanding documented synthetic procedures which he or she may have to

undertake in the lab, or designing basic synthetic procedures of his or her own.

Additionally, troubleshooting or problem solving in the laboratory (determining how to

increase a reaction yield or explaining the formation of byproducts) would be nearly

impossible for a student who has limited understandings of reaction processes.

It is not simply the quantity of information that can lead to difficulties in the study of

organic chemistry. The study of reaction mechanisms also requires students to develop

understandings of the representational conventions used in communicating about these

particular models, as well as applying their understandings of chemical reactivity and

the likelihood of certain interactions occurring between compounds in given reactions.

Evidence suggests that learning to use mechanistic conventions correctly can be

challenging for students. Eckert (1998) and Weeks (1998, p. vii) have commented on

students’ difficulties in understanding the curly (or curved) arrow representation of

electron shifts that chemists use when drawing reaction mechanisms. Weeks (1998, p.

vii) stated that he had ‘observed that students avoid pushing electrons . . . Many

students never become comfortable with bond-making and bond-breaking steps in

organic mechanisms’.

Eckert (1998) suggested that students’ confusion with the curly arrow representation

might be due to the arrows not always ending at the same type of location. He pointed

out that while curly arrows generally indicate movement of electrons to atoms when

indicating bond formation, they are sometimes pointed at bonds.

In the following representation (Figure 1.1), curly arrow (a) represents formation of an

O—H bond and is pointed at an H atom. Arrow (b) represents formation of a C—C π

bond and is pointed at the line representing a bond between the two C atoms.

8

C C

H

H3C

CH3

CH3

H

HO H C C

CH3

H

H3C

H3C

OH

+

(a)

(b)

Figure 1.1: Representation of a mechanism involving carbocations and hydroxide ions.

Eckert commented that the inconsistency in the representation of curly arrows, such as

that shown above, ‘frustrates the thoughtful student seeking to find rhyme or reason in

electron arrows’. The author proposed a curved arrow system, which he believed is

helpful for improving students’ understandings of electron flow in mechanistic

representations. The mechanism represented in Figure 1.1 above is redrawn below

(Figure 1.2) using Eckert’s curved arrow system. Arrow (b) is drawn with a more

significant curve in it and ends at a carbon atom.

C C

H

H3C

CH3

CH3

H

HO H C C

CH3

H

H3C

H3C

OH

+

(a)

(b)

Figure 1.2: A representation of an elimination reaction mechanism using a curved

arrow system proposed by Eckert.

Eckert has used his new system in his teaching and commented that ‘students now

arrange electron arrows with more consistency and meaning, and write better

mechanisms’. He also suggested four rules to form a consistent curved arrow system:

(i) A curly arrow always begins at an electron pair (either bonding or

nonbonding). In a nucleophilic reaction, one curly arrow begins at an

electron pair on the nucleophile;

(ii) Electron pairs always remain attached to atoms;

9

(iii) A curly arrow always ends at an atom, not at a bond. If there is an

electrophilic atom in the representation, one arrow ends at that atom;

(iv) In a mechanistic step that shows a series of arrows, a curly arrow begins

beside the atom where the preceding arrow ends.

Wentland (1994) suggested a simple integration of understandings of electron flow to

assist students in learning about organic chemistry. He asserted that mechanisms could

be described by one or more of five simple steps, which he labelled ionisation,

neutralisation, 1,3-electron pair displacement, 1,3-electron pair abstraction and 1,5-

electron pair displacement. These steps can then be appropriately sequenced to allow

students to write reaction mechanisms for any type of chemical reaction.

Wentland also commented that these five mechanistic processes have been adapted to a

computerised format to assist students studying organic chemistry. The author added

that many of his students have attested to a greater understanding and appreciation of

organic chemistry following instruction in his courses.

Eckert (1998) and Wentland (1994) have both commented on observed difficulties with

one aspect of mechanistic representations. My own experience with studying

mechanistic representations would suggest that the curly arrow is not the only

representational difficulty that students face when learning about mechanistic

representations. It is likely that there are other representations, such as formal charges

and specific types of structural representations, that are poorly understood by students,

leading to them constructing weaker understandings about mechanistic representations

overall, and limiting their abilities to apply these representations to their study of

organic chemistry.

Several authors have made suggestions to facilitate students’ learning about reaction

mechanisms. It was these types of suggestions that prompted the researcher to consider

the pedagogical concerns related to the study of mechanistic representations.

McNelis (1998) detailed the use of mechanism templates in his classes to aid his

students in their understanding of what happens in each step of a mechanistic

representation. A survey of students provided with these mechanism templates found

10

that a majority of students utilised the templates, and found them easy to use. Most

students found that the templates helped with the understanding of the mechanism.

Harvey and Hodges (1999) offered four teaching strategies that they had used in their

classrooms to facilitate student learning. These four strategies were chosen as they

could be seen to address the needs of students with different learning styles. These

authors detailed their use of guided reading worksheets, dialogues, in-class worksheets

and role-playing exercises. Two of these strategies were described in terms of their

application to the study of reaction mechanisms.

Harvey and Hodges (1999) described the use of ‘dialogues’, which they felt helped to

‘gain greater insight about a student’s level of understanding’ and can reveal students’

misconceptions. These dialogues require students to explain a chemical phenomenon in

a written format for the benefit of someone who has little or no understanding of the

concept. The authors commented:

The challenge in creating these assignments is the presentation of a problem or observation

that is neither too specific nor too broad . . . A problem that is too broad impedes the

students’ ability to focus their understanding and makes it more difficult for the instructor

to identify a particular difficulty.

The example these authors have given in their discussion required their students to

explain, using a mechanism, why a given reaction resulted in a certain product.

Students’ answers to such a question were useful in identifying any misconceptions and

in giving the lecturer insight into students’ understandings about similar types of

reaction processes.

Harvey and Hodges also described a role-playing exercise in which groups of students

were required to present SN1 and SN2 mechanisms in dramatic style to the class. It was

felt that the ‘ideas involved in the two major types of nucleophilic substitution

reactions, SN1 and SN2, lend themselves well to this activity’. They commented that

‘the students who are actively engaged [in the role play] have a learning advantage over

those who are passive bystanders’. After this role-playing exercise had been carried

out, the authors compared various students’ responses to essay questions concerning

SN1 and SN2 reaction mechanisms. They found that students did better on essay

11

questions about the mechanism that they helped perform rather than the dramatisation

they simply watched.

1.5 Research Questions

The research questions under investigation were broken into two categories: lecturer-

related questions, and student-focussed questions. This is consistent with the project’s

focus on both the teaching and learning aspects of the topic. The research questions

were refined through initial discussions with lecturers and students, and by researcher

reflection of the aims of the project and the subject matter under investigation. These

research questions are detailed in the following sections.

1.5.1 Lecturer Related Questions

1. What perceptions do lecturers have of:

a. the purpose and importance of teaching about reaction mechanisms as a

tool for understanding in organic chemistry?

b. the difficulties students have at various levels of their tertiary education

when using representations of reaction mechanisms to rationalise or

predict reaction outcomes?

2. What do lecturers consider to be the essential knowledge of concepts required by

students at different educational levels to use reaction mechanisms in

explanations and predictions of reactions?

3. When teaching about reaction mechanisms:

a. what teaching strategies do lecturers employ, and are these strategies

different for students of different levels of education?

b. what reasons and motivations do lecturers have for choosing particular

teaching strategies?

12

1.5.2 Student Related Questions

1. What perceptions do students have of:

a. the purpose and importance of teaching and learning about reaction

mechanisms as a tool for understanding in organic chemistry?

b. the difficulties they encounter at various levels of their tertiary education


predict reaction outcomes, and how do these perceptions compare to

their demonstrated difficulties?

2. What understandings do students have of the important concepts and skills in the

topics of substitution and elimination reaction mechanisms in organic chemistry,

and which factors affect their abilities to demonstrate a facility with these

concepts and skills?

3. When using representations of reaction mechanisms, what strategies do students

employ when writing these representations?

1.6 Research Claims

As a result of this study, the researcher puts forward several research claims about the

teaching and learning of reaction mechanisms in tertiary organic chemistry courses at

the two universities involved in this research project. Evidence to support these claims

is presented in Chapters 7, 8, 9 and 10. These claims are discussed below.

Lecturers’ perceptions of the importance of teaching about reaction mechanisms in

organic chemistry are consistent with those of their students. However, although there

is some consistency between perceptions of the difficulties that students experience

when studying this topic, the perceptions of lecturers and students are not entirely in

agreement. Students’ demonstrated difficulties are consistent with lecturers’

perceptions about difficulties that they face in their studies.

13

Students generally achieve the explicitly stated aims that their lecturers have identified

for the various organic chemistry courses under investigation. In addition to these

explicit aims, however, there exists a second implicit group of outcomes anticipated by

lecturers. These outcomes are rarely articulated by the lecturers and are generally not

achieved by the students.


discussing certain types of reaction process. Three-dimensional representations are

generally used when writing substitution reaction mechanisms, whereas square planar

structures are used to represent elimination reaction processes. Although these

structural representations are useful for representing features of different reaction

processes, the use of three-dimensional structures appear to cue students into thinking

that a substitution reaction will occur, whereas square planar representations cues

students into thinking about an elimination process.

The language, including reaction mechanism representations, used in many instances in

lectures is consistent with a consideration of only individual particles in reactions, and

not with multiple particle interactions, such as competitive reaction processes.

Although this is appropriate in some circumstances, there are aspects of the topic that

require a consideration of the interaction between multiple reaction particles that require

students to visualise many particles in the reaction mixture, rather than to focus on just

one reaction event.

Students tend to display understandings that are consistent with them considering single

reaction events in a reaction process (single particle understandings), and not

understandings that are consistent with the consideration of many interaction between

many different reaction particles (multiple particle understandings). The understandings

displayed by students are generally consistent with the language and representational

style used by their lecturers in class and in their textbooks.

Students may use particular strategies when representing reaction mechanisms, but the

uses of these strategies does not necessarily indicate that students have deep

understandings of the multiple competitive processes going on within a reaction

mixture. These strategies are generally consistent with those demonstrated by the

lecturers in representing reaction mechanisms.

14

1.7 Thesis Outline

This chapter of the thesis gives a brief introduction to the study and the motivations for

undertaking this research. It includes a literature review of the previous research into

reaction mechanisms in organic chemistry. Much of this literature is based upon

lecturers’ observations in their own classes and not on specific research studies. The

research questions addressed are also introduced, as are the research claims that have

come out of the study. The structure of the remainder of the thesis is outlined below.

Chapter 2 involves discussion of the theoretical framework, including a brief literature

review of the educational considerations relevant to this study. This review includes a

consideration of the researcher’s perspectives on teaching and learning and how these

processes occur. These views impact upon the research design and implementation and

data analysis.

The context of the research study is introduced in Chapter 3 and includes a discussion of

the courses of interest, the relevant subject matter covered by these courses, and the

universities at which the courses are offered. The participating lecturers and student

volunteers are introduced in this chapter.

The subject matter is covered in Chapter 4. This chapter is concerned only with the

topics of substitution and elimination reaction mechanisms and is detailed to the level

expected of second year university students.

Pedagogical and content knowledge concerns related to the general fields of science and

chemistry are discussed in Chapter 5. This chapter includes a discussion of pedagogical

content knowledge considered specific to the teaching and learning about reaction

mechanisms in organic chemistry.

Chapter 6 includes a discussion of the methodology used in this research study and

involves a short literature review, as well as justifications of the tasks given to students

and lecturers and a timetable of the study.

15

The findings of the study are discussed in the next four chapters. These findings are

presented in response to the various research questions and research claims and are

discussed in the light of the theoretical and pedagogical content issues that have been

outlined in Chapters 2 and 5.

Chapter 7 discusses the finding in response to research questions L1 and S1, lecturers’

and students’ perceptions of representations of reaction mechanisms. Lecturers’

teaching strategies, students’ demonstrated understandings and their solving strategies

are discussed with respect to the three different courses that were investigated in this

study in Chapters 8, 9 and 10.

Pedagogical implications and conclusions discussed in chapter 11 relate directly to the

six research questions outlined in section 1.5 and support the research claims put

forward in section 1.6.

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2 Theoretical Considerations

2.1 Introduction

This chapter describes the theoretical models of teaching and learning that have been

used to frame the research. The discussion includes the appropriateness of the

theoretical framework in terms of the style of research that has been conducted. The

theoretical approach that the researcher chooses for a study influences the study’s

methodology and data collection techniques and analysis.

The design of a research study involves the consideration of an appropriate theoretical

framework. Lincoln and Guba (1985, p. 222) commented that ‘[a]n inquiry can almost

always be carried out from any of several theoretical perspectives’. They added:

From a conventional point of view, an important (but frequently overlooked) aspect of

design is to select the theory that provides the most power (leverage) in relation to the

problem [being investigated].

The framing educational theories shape the type of methodology that is suitable to

address the research questions. The data that the researcher gathers must be appropriate

for the type of study being conducted. This particular study was conducted within a

naturalistic, qualitative paradigm where the emphasis was on studying the teaching and

learning processes occurring in real classroom situations. The naturalistic paradigm is

discussed in section 2.2.

The theoretical models that were used in designing and describing the research process

must be consistent with this naturalistic approach. These models are discussed in more

detail in section 2.3.

The chapter also describes the researcher’s perspectives on teaching and learning, in

relation to the theoretical models discussed. The researcher’s perspectives are an

important consideration, as they can influence both the emergent design of the research

study and the data analysis that is carried out (Guba and Lincoln, 1981, p. 128):

One of the most difficult concepts involved in naturalistic inquiry is that of the inquirer as

instrument. He is at one and the same time instrument administrator, data collector, data

17

analyst, and data interpreter. He is, in the methodological terminology of educational

research, both an independent variable and an interaction effect.

Herron and Nurrenbern (1999) also considered the importance of a researcher’s own

theoretical perspectives on his or her research, commenting that ‘a chemical education

researcher’s working theory of learning shapes that researcher’s view and approach to

chemical education’. Indeed, how a researcher perceives teaching and learning to occur

can greatly influence the approach he or she uses when investigating students’ and

lecturers’ understandings in a course. Phelps (1994) commented that ‘[n]o one enters a

research setting objectively’:

A complete understanding of the researcher includes an explanation of the researcher’s

theoretical underpinnings and the role that theory will play in the collection and analysis of

data. The place of theory needs to be clear in the mind of the researcher . . . In this way, the

researcher makes everyone aware of theoretical assumptions and the perspectives being

used to view the research.

The researcher’s perspectives, which guided this study, are described in section 2.4.

2.2 Naturalistic Research

The research design adopted for this study is commonly described as a naturalistic

inquiry. This type of inquiry involves the study of natural settings: in the case of this

project, the study of the teaching and learning processes occurring in three tertiary level

organic chemistry courses about the same subject matter.

Naturalistic research is concerned with ‘several key assumptions’ (Guba, 1981) made by

researchers. These assumptions are related to perceptions concerning what constitutes

the reality under investigation, the interrelated nature of the relationships between the

researcher and the participant(s) in a study and the nature of truth statements or

generalisations that arise from the research that has been conducted.

Cohen, Manion and Morrison (2000, p. 21 – 22) described several ‘distinguishing

features’ of ‘naturalistic, qualitative, interpretive approaches’ to research:

• People act intentionally and create meaning from their experiences;

18

• Situations are affected by their context and are fluid and evolve over

time;

• Situations should be studied in their ‘natural’ state without the researcher

intervening in or manipulating the environment;

• People and situations are often not generalisable to any great degree;

• Remaining true to the situation being studied is paramount;

• People’s interpretations of events can influence their actions;

• Events can be interpreted/perceived on different levels by different

people;

• Reality itself is complex and multilayered;

• Some situations can require the use of ‘thick description’ to facilitate an

understanding of their context;

• It is important to look at situations from a participant’s point of view, not

the researcher’s point of view.

The naturalistic design was deemed to be most appropriate for several reasons. To

address the aims of the project (section 1.3), the researcher needed to observe classroom

situations and to discuss motivations and understandings with lecturers. The study also

aimed to investigate students’ understandings of aspects of the topic. The most useful

way to do this was to speak to students who had participated in the observed courses.

This study also addressed questions whose answers would necessarily be more

qualitative and descriptive by identifying prior understandings, difficulties,

misconceptions, perceptions, intended outcomes and areas of pedagogical concern.

There was also a strongly interpretive component to the data that was obtained

throughout this study. It was recognised from the outset of the project that data would

not be numerical or statistical—it would be narrative and descriptive. This type of data

(and the interpretation applied to elicit responses to the research questions posed) is

consistent with a qualitative, naturalistic style of research.

19

The current study was dependent upon interactions between the researcher and lecturers

and students. The most sensible way to obtain information about the teaching that

occurred in a particular class was to observe that class, but my presence in the

classroom made me a participant in the class, not simply an observer. Discussing

aspects of the courses with the lecturers might influence their teaching style or the

topics that they chose to cover. Interviewing students to probe their understandings of

particular aspects of the topic might affect their regular study habits (for example,

preparing for an interview by reading notes, which might not be a normal study habit).

Due to the nature of the study, these difficulties were unavoidable. To combat these

influences, however, detailed descriptions (thick descriptions) of the courses and

participants are given (see Chapter 3). These descriptions provide information on the

context of the study itself, which allows others to make judgements on the applicability

of its findings to their own situations. In naturalistic studies, generalisations and truth

statements are not possible (Guba, 1981):

[t]he naturalistic paradigm rests on the assumption that generalizations are not possible, that

at best what one can hope for are “working hypotheses” that relate to a particular context.

Thick descriptions also ensure a transparency to the methods that were used in finding

responses to the research questions posed in section 1.5.

Carrying out a research study also requires a consideration of the methodology to be

used. Lincoln and Guba (1985, p. 187) observed that naturalistic studies are always

carried out ‘in the natural setting, since context is so heavily implicated in meaning.

Such a contextual inquiry demands a human instrument’ for its implementation.

Qualitative research methodology was used in the course of the study (see section 6.2.1

for a discussion of qualitative research, and section 6.3 for details of the data collection

techniques used). This methodology was considered most appropriate to construct

responses to the research questions being investigated.

20

2.3 Theoretical Considerations

2.3.1 Constructivism

Constructivist models sees knowledge as being constructed by the student, based on his

or her experiences and observations, to make sense of the world around him or her.

What a student sees and experiences, not only in lectures and laboratory classes, but

also in the everyday world, will affect the knowledge he or she constructs to explain

these observed phenomena. This model has been summarised by Bodner (1986) in a

statement; ‘Knowledge is constructed in the mind of the learner’.

A constructivist approach to teaching and learning recognises the importance of the

prior conceptions that students hold when they come into a new learning situation.

These prior understandings influence how students construct new meanings. Keogh and

Naylor (1996) suggested similar implications of this model of education are that:

teachers need to find out the learners’ ideas in order to take them into account in their

teaching . . . to provide experiences which challenge the learners’ current understanding . . .

to help them restructure their ideas.

Herron and Nurrenbern (1999) also described a similar need for consideration of what

students already know:

[I]f students construct knowledge on the basis of what they already know rather than from

the perspective of being “empty vessels” to be filled with knowledge, then instruction could

be more effective when teachers know what students think about the world around them.

Pines and West (1986) described constructivist models as ‘making an important

distinction . . . between private understanding and public knowledge’. They commented

upon constructivists’ interest in the notion that a learner’s prior knowledge is important

to any new knowledge he or she constructs, describing this interaction as ‘the most

important ingredient in the process of meaningful learning’. They suggested that this

prior knowledge can be both ‘a bridge to new learning or a barrier’, commenting that

‘learning and the growth of understanding always involves a learner constructing . . . his

or her own private understanding of some part of the public knowledge’.

21

These considerations are important in the context of this research project. The study is

concerned with both teaching and learning, and the understandings that students

construct over the course of instruction. A constructivist perspective suggests the

importance of determining students’ prior understandings and/or misconceptions and

identifying how these might impact on the understandings that students construct when

learning about a particular topic, concept or idea.

2.3.2 Prototypes of Learning

The interaction between students’ prior knowledge and the new knowledge that they

might construct after instruction in a formal education setting (such as in a lecture) can

be described in several different ways.

Pines and West (1986) considered the interaction between what they termed formal

knowledge and spontaneous knowledge in terms of a vine analogy. This analogy put

forward four possible situations where formal and spontaneous knowledge could

interact and how the agreement between these two types of knowledge could affect

students’ abilities to integrate new information into their existing knowledge framework

(Figure 2.1).

Figure 2.1: Prototypes of learning situations described by Pines and West.

Sometimes, students’ prior conceptions and understandings are consistent with the new

information that they are taught in a formal education setting, and students are able to

use this new information to improve or benefit their formal knowledge. These students

can then merge the spontaneous and the formal understandings together to develop a

22

richer understanding of a concept or concepts. Pines and West (1986) described this as

a congruent situation. This situation is represented on the right-hand-side of Figure 2.1.

In contrast to the congruent situation is the conflict situation (left-hand-side of Figure

2.1), where the student’s already established knowledge clashes with that being

provided as formal knowledge. In this type of situation, a merging of the conflicting

understandings is not possible. Students can be left with rather confused understandings

of concepts. They may also compartmentalise their spontaneous and formal knowledge,

keeping them separate. Some students refuse ‘to play the rote learning game’. Mature

learning can be brought about in this conflict situation by finding a way to intertwine

the conflicting formal and spontaneous knowledge together. Bringing about this

intertwining can be difficult (Pines and West, 1986):

It demands the questioning of reality, abandoning ideas that have been established over a

long period and becoming committed to a new set of ideas that are completely incongruent

with the old ones.

These authors identify two other situations where spontaneous and formal knowledge

can interact. These are shown as the central vine representations in Figure 2.1. The

spontaneous/uninstructed situation can be described as one in which students have well-

developed spontaneous knowledge about a particular concept or understanding, but they

receive little or no knowledge from a formal source.

The formal-symbolic/zero-spontaneous situation can be considered the opposite of the

spontaneous/uninstructed situation. In this situation (Pines and West, 1986):

little spontaneous knowledge exists to interact with the formal symbolic knowledge

presented in school. The upward growing vine is nonexistent (or almost nonexistent),

whereas the downward growing vine is complex, elaborate, extensive, and well structured.

Many freshmen studying organic chemistry for the first time, for example, have no

spontaneous knowledge relevant to the learning of substitution reactions of benzene and its

derivatives. The student is attempting in this situation to acquire pure symbolic knowledge

bereft of intuitive, experiential underpinnings.

First year students’ spontaneous knowledge of reaction mechanisms and their

representations can be considered to be almost nonexistent. Students may hold some

intuitive understandings of particular representations that may be used when writing

23

reaction mechanisms, but their understandings of mechanistic processes and

representations are limited. This is not unexpected, as the representations are used

almost exclusively in organic chemistry classes and are rarely taught formally at the

secondary level of teaching. These students could be classified as likely to be in a

formal-symbolic/zero-spontaneous situation in terms of their learning about reaction

mechanisms and their representations.

Some second year students could also be considered as commencing their study of

organic chemistry with very little spontaneous knowledge about reaction mechanisms

and their representations. Some students have not been taught about these processes

before they enter second year chemistry. These students would fit into the same

category as first year students.

Other second year students, however, have had previous exposure to representing and

interpreting reaction mechanisms. These students may be classed as having some

‘spontaneous’ or prior knowledge of these ideas and concepts. Second year students

may have good understandings of reaction mechanisms from their previous study and

their conceptual development and the learning situation that they are in may be

consistent with the congruent situation described earlier in this section. Alternatively,

those with weaker prior understandings (or inappropriate understandings) of the topic

may encounter a conflict between the knowledge they already have and the formal

knowledge they are being taught.

These considerations for learning are important in the context of this research project.

Students’ educational level (first or second year) and background (have they studied

mechanisms before) will affect the level of what Pines and West called their

spontaneous knowledge. The vine analogy will be used to consider the possible

difficulties that might be faced by students when attempting to construct useful

understandings about mechanistic representations.

It also has implications for the types of task that may be used to probe students’

understandings of the topic. Different types of task will be more appropriate for

students with a greater level of spontaneous knowledge than for those who have a very

limited understanding of the topic or for those whose spontaneous knowledge consists

of inappropriate understandings.

24

2.3.3 Conceptual Change

Students are often confronted with new ideas and information that does not fit into their

current understandings or knowledge. Sometimes, this can be complementary to the

current understandings (described as a congruent situation by Pines and West, 1986).

Other times, it may be in conflict to students’ constructed understandings. In both

situations, students need to have reasons and motivations for attempting to assimilate

this new knowledge into their established understandings. Why would students attempt

to modify the understandings that they hold if they cannot see a reason for doing so?

Posner, et al. (1982) discussed four conditions necessary for the accommodation of new

knowledge or conceptual change. These conditions were described in the following

manner (Strike and Posner, 1985):

1) There must be dissatisfaction with existing conceptions. Scientists and students are

unlikely to make major conceptual changes unless they believe that less radical changes

will not work.

2) A new conception must be minimally understood. The individual must be able to grasp

how experience can be structured by a new conception sufficiently to explore the

possibilities inherent in it.

3) A new conception must appear initially plausible. Any new conception adopted must at

least appear to have the capacity to solve the problems generated by its predecessors, and to

fit with other knowledge, experience, and help. Otherwise it will not appear a plausible

choice.

4) A new conception should suggest the possibility of a fruitful research program. It should

have the potential to be extended, to open up new areas of inquiry and to have technological

and/or explanatory power.

It is expected that students studying a new topic, such as reaction mechanisms, will find

themselves confronted with understandings that are not part of their current

understandings, or that are inconsistent with their understandings. Students must then

choose how (and if) they will incorporate these new understandings into their existing

framework.

25

Events,

observations,

instructions

If these conditions for conceptual change are not met for students studying these

courses, then it is expected that students will not accommodate new understandings into

their existing frameworks, and would therefore not be able to demonstrate the same

level of understanding as students who have had the criteria for conceptual change met.

For these purposes, the conditions required for conceptual change are considered an

important aspect in this research study.

2.3.4 Information Processing Model

Johnstone has used an information processing model (Figure 2.2) in his discussions of

research findings in chemistry education, including some at the tertiary level (1983;

1984). This model describes the ‘filtration’ of sensory input, its processing in the short

term memory and the storage of knowledge in the long-term memory. The model also

describes a ‘working space’ where data processing can occur.

Figure 2.2: Information processing model showing the interaction between observed

events and their storage in short- and long-term memory.

In this model, the perception filter is affected by what a person already knows and

understands. While good understandings can help us filter out extra ‘noise’ from all the

information we receive each day, poor understandings or misconceptions can cause us

to ignore or screen out important or useful information, or to perceive irrelevant

information as important and attempt to make sense of it. This has important

implications for teaching and learning; if students are missing the ability to correctly

Perception

Interpreting

Rearranging

Comparing

Sometimes

branched,

sometimes as

separate

fragments

Working Space Storage

26

perceive and process the stimuli provided to them in the classroom, they may not be

able to construct meaningful understandings (Johnstone, 1997).

Johnstone (1997) described the effectiveness of storage of information in this working

memory model, and how this affects the type of learning that takes place. He also

discussed the ease of retrieval of information from long-term memory as being

dependent upon the way newly stored information is cross-linked to existing knowledge

in long-term memory. If information is well stored, with useful and appropriate cross-

linkages, it is easier to retrieve that information. Johnstone labelled this type of learning

meaningful learning. At the other end of the spectrum is the poorly filed, non-

connected information that is stored as separate pieces of information in a student’s

long-term memory. This kind of information is difficult to retrieve, and is almost

impossible to remember. This can often be the result of rote learning or cramming for

examinations or tests.

In addition, this model describes what Johnstone labelled the working space, where

processing of data occurs, forming links between incoming information and knowledge

already stored in the long-term memory. Johnstone’s work (1984) suggests that

working space is of a limited size, however, and this can restrict the amount of

processing that can be carried out. The size of the working space can have implications

for students’ abilities to process information about new or unfamiliar information.

No matter what their previous understandings of the topic, all students who are learning

about new subject matter will have some need to determine what is important to

understand and to make sense of their educational experience (if only for examination

purposes). Not only do students have to make sense of new information and

understandings, they then have to store it in their memories by linking it to existing

knowledge in an appropriate fashion. Students’ abilities to use their constructed

understandings to interpret new external sensory inputs are dependent upon the

strengths of the linkages within their existing knowledge frameworks.

Many things influence the manner in which we process new information and (perhaps)

link it to our prior understandings. What we learn is influenced by what we already

know, what perceptions we hold and how we are being taught. The importance of prior

27

understandings in the processing of information is consistent with constructivist

perspectives. Johnstone (1997) commented:

we have mechanisms by which we reduce the torrent of sensory stimuli to manageable

proportions, attending to what seems to be important, interesting, or sensational . . . we

have a filtration system that enables us to ignore a large part of sensory information.

. . .

We then have to ask how the filter works. It must be driven by what we already know and

understand. Our previous knowledge, biases, prejudices, preferences, likes and dislikes,

and beliefs must all play a part.

Johnstone (1984) has presented convincing evidence that most people have difficulty

processing more than five or six ‘chunks’ of information in their working space at one

time. If the information is in a foreign language, the number of chunks that can be

effectively processed in the working space appears to decrease (Johnstone and

Selepeng, 2001), suggesting that the need to translate a language takes up space in the

working memory.

When it comes to learning about something new or unfamiliar, such as new concepts in

a chemistry class, students may not have the same ability to ‘chunk’ information that

their lecturers do. The less a person is able to chunk information, the more pieces of

information he/she has to deal with. If a person can chunk relevant pieces of

information together, he/she can effectively process more information.

For example, to a lecturer, the formula ‘CH3COOH’ may be one chunk that means

‘acetic acid’. To students, the equation may very well be several chunks—a novice

might see it as six letters and one number, seven chunks. While the chunk a lecturer

perceives as ‘acetic acid’ could be linked to other understandings about the compound

(it is a clear, colourless liquid with a pungent odour, which contains a carboxylic acid

group that can undergo particular types of reaction), the seven chunks that a novice

learner sees tells him/her very little. Even if a student had a more developed

understanding of representing chemical formulae, his/her understandings may not be

sufficient for him/her to interpret things such as atomic connectivity or identify the

carboxylic acid functional group in this representation, chunks of information lecturers

might perceive that the students do understand.

28

Processing and making sense of the vast array of information that they can be presented

with in a lecture can be a very difficult task for students. Significant quantities of

information, stimuli and data are provided to students in the lectures that were observed

over the course of this study. Students may be taught about many different topics (or

aspects of a topic) during the course of the same lecture. Students need to make sense

of this input in some manner, and their prior understandings are going to be the filter

through which this is processed. It is therefore considered important in the context of

this research study to consider students’ prior understandings as an aspect of new

understandings that they construct over the course of their instruction.

2.4 Researcher’s Perspectives

The researcher believes that all students come into their respective courses with some

prior knowledge of the chemical principles to be taught. This knowledge may have

been acquired through formal education or it may be what Pines and West (1986)

referred to as spontaneous knowledge. Students’ prior knowledge may be appropriate

(and consistent with understandings accepted by the scientific community), but may

also consist of some misconceptions or alternative conceptions. For the purposes of this

thesis, the terms misconceptions or alternative conceptions are used to describe

understandings held by students that make sense to the students but that are in conflict

with formal knowledge accepted by the scientific community.

Students also have what can be called an existing knowledge framework: some way of

making sense of their understandings of a particular topic. As the course progresses, the

students may develop an enhanced or possibly even new understanding of the topic.

The students may even construct a new knowledge framework, or alter and add to their

existing framework, as a result of the instruction that they receive.

For students to modify, alter or add to their existing knowledge framework, the

researcher believes that they must have cause to be dissatisfied with the usefulness of

their current knowledge framework, and must have reasons for changing or replacing

their understandings. This notion is consistent with the proposed conditions necessary

for conceptual change (Posner, et al., 1982).

29

The researcher also believes that individuals construct their own understandings of the

world around them from both the formal and informal teaching that they are exposed to.

The understandings that they can construct of a particular topic or idea are dependent

upon both what they are taught about it (that is, what information they are provided

with) and their previous understandings and beliefs.

This leads the researcher to a belief that each person will have his or her own unique

understandings about particular topics. It is expected that no two people would

construct the exact same understanding of an idea or topic; nevertheless, a group of

people with similar understandings (for example, first year organic chemistry lecturers)

would have enough shared knowledge in their individually constructed understandings

that they could talk about and make sense of a topic or idea together. This model of

learning is consistent with a constructivist perspective (Bodner, 1986).

In this construction of understanding, the researcher believes that an important

consideration is how people process and store the information that they receive in their

formal and informal education. As a theoretical base, a model of learning described by

Johnstone (1997), based on information processing theory, guided the research. The

key features of this model are selective filtration of sensory input, the processing of

information in a ‘working memory’ of limited size and the storage of information in

long-term memory.

This research also assumes that the nature of the subject matter is as important as the

general pedagogical considerations to both the teaching and learning of reaction

mechanisms in organic chemistry. The subject matter has its own particular learning

demands and characteristics which make different aspects of the topic more difficult to

understand. The lecturer needs an ability to transform what he/she knows of the

particular subject matter into teachable (and learnable) content. Shulman (1987) named

this ‘special amalgam of content and pedagogy’ pedagogical content knowledge (PCK).

PCK refers to a teacher’s knowledge of how to teach a particular topic and involves an

understanding of what strategies help their students to construct meaningful

understandings about a course or topic, and of a variety of effective ways of

representing the ideas of that topic to students (Shulman, 1986). Pedagogical content

knowledge is discussed in more detail in Chapter 5.

30

2.5 Summary

The aims of this research project necessitate a qualitative approach. The answers that

the project seeks are narrative, not numerical or statistical. These answers are best

identified through observation of a teaching situation and interviewing and surveying

lecturers and students. The style of the research is therefore naturalistic in nature and is

based upon constructivist understandings of learning.

For the purposes of this study, constructivism is defined as the belief that learners

construct their own knowledge about different subjects and topics from their own

experiences, be they personal or formal. Because of the belief that knowledge is

constructed by each learner and influenced by the learner’s previous knowledge and

experiences, a person’s prior understandings can influence his or her abilities to learn

about and understand aspects of the course.

The interaction between students’ prior knowledge and the new knowledge they are

taught about is also an important consideration. Depending upon the appropriateness of

students’ spontaneous understandings, new conceptions may not necessarily agree with

the knowledge framework that the students have constructed previously. For students to

attempt to accommodate such information, conceptual change may be required.

Additionally, students may have limited spontaneous or prior understandings of the

conceptions, which means that they must endeavour to make sense of the new ideas and

construct an understanding in the absence of any intuitive knowledge.

One further consideration is that of the processing of information, which also impacts

on learning. The information-processing model proposed by Johnstone is an

appropriate model to use in this context. It takes into account students’ previous

knowledge, as well as the possibility of information overload and incorrect linkage of

information.

31

3 Context of the Study

3.1 Introduction

This chapter outlines the context of the study that is described in this thesis. The

context involves a consideration of the courses under investigation (including the

university at which they were offered), the lecturers who taught the courses and the

students who participated in face-to-face discussions in interviews and focus groups.

Detailed descriptions (referred to as ‘thick descriptions’ in the literature) are provided to

construct a complete picture of the setting in which the study was carried out.

In a naturalistic research study, thick descriptions of the setting under investigation are

an important and necessary requirement for informing readers about the nature of the

research undertaken. This descriptive information that is gathered from a research study

include a discussion of the nature of the project, the setting it is conducted in and the

participants in the study (Guba and Lincoln, 1981, p. 340).

Awareness of the conditions under which a study is conducted is important for readers

and researchers, not only so that they are aware of the context under which the study

was conducted, but also to help them to determine if the findings of such a study may be

applicable to their own situation. Lincoln and Guba (1985) commented:

[T]he naturalist cannot specify the external validity of an inquiry; he or she can provide

only the thick description necessary to enable someone interested in making a transfer to

reach a conclusion about whether transfer can be contemplated as a possibility.

The project was carried out at two universities, which are described in terms of the types

of chemistry courses they offer to their prospective students and the numbers of students

who normally take these courses. The study also sought to examine students studying

several different chemistry courses, so an explanation of the material covered in these

courses is also relevant.

Descriptions of the three lecturers who participated in the research study are also

included. The lecturers are described in terms of the approach they took when teaching

their course, as well as the length of time for which they had been teaching the course.

32

Finally, the student volunteers are described. Consistent with the argument presented in

section 2.3 concerning the importance of students’ prior understandings, their

educational level and background are described.

3.2 Universities

Students and lecturers at two tertiary institutions participated in this study. Although

the context of the study is described in detail, neither university is named and are

referred to as University A and University B. Both University A and University B are

located in Western Australia and offer several degrees in which students study

chemistry courses. While a large number of students study first year chemistry, the

number of second and third year chemistry students is much lower.

3.2.1 University A

Over a thousand students study first year chemistry at University A in any given year.

A large percentage of these first year students study chemistry as part of another degree

and do not go on to study chemistry in subsequent years.

In an average year, there are between 40 and 50 students enrolled in the second year

courses that cover organic chemistry. A smaller number study third year organic

chemistry courses.

Chemistry 100 and Chemistry 2XX were studied in-depth as part of this research

project. Some Chemistry 3XX students also participated in a single task in one year.

These three courses are discussed in more detail in section 3.3.

3.2.2 University B

The first year enrolment in chemistry courses at University B is quite large

(approximately 800 students). Similar to University A, the majority of these students

are studying chemistry as a requirement of a non-chemistry course. Many students do

not study chemistry beyond this requirement in first year.

33

First year students studying chemistry at University B can be separated into three

different groups. The first group are students who are studying a chemistry degree

(Chemistry 101/102). The second are those who are doing a non-chemistry degree with

a substantial level of chemistry after first year; for example, pharmacy or chemical

engineering; (Chemistry 121/122). The third group are students who require an

introductory level of chemistry only and will not study it after first year (Chemistry

123/124). These three courses are detailed in section 3.3.

Although the three units are similar, different lecturers taught different sections

(organic, inorganic and physical chemistry). Due to timetabling constraints, some

Chemistry 101/102 and 123/124 students attended the 121/122 lectures, and vice versa,

but most of the students in the 121/122 lectures were non-chemistry students.

3.3 Courses of Interest

3.3.1 Chemistry 100 at University A

The Chemistry 100 unit is a course for science students who intend to major in

chemistry or to study some chemistry units in second year. The course consists of three

lectures per week for the entire year (26 weeks of class), and covered inorganic, organic

and physical chemistry. The organic chemistry component of the course, which was

observed for this study, was covered in the last eight weeks of first semester.

The first semester of the Chemistry 100 course is described on University A’s website:

The first few weeks of the semester are spent revising and extending some of the basic

concepts in chemistry (acids and bases, equilibrium, atomic structure, bonding,

stereochemistry, nomenclature). The remainder of the semester is devoted to organic

chemistry, where the properties, preparation, reactions and the uses of the various classes of

organic compounds are discussed.

Several topics were covered in the organic chemistry section of this course. They were

detailed on a course outline that was provided to students at the end of the course. This

outline is summarised in table 3.1.

34

Table 3.1: Topics and subtopics detailed in the course outline for Chemistry 100.

Topic Subtopics

General Functional groups, nomenclature, physical properties, identification

Synthesis and interconversions of various classes of molecules

Structural isomers

Alkanes and

Cycloalkanes

Structure and nomenclature, reaction of alkanes

Conformations, relative stability, stereoisomerism in cycloalkanes

Alkenes and

Alkynes

Structure and nomenclature

Addition reactions (Markovnikov’s rule)

Stereochemistry Stereoisomers, stereocentres, enantiomers, diastereomers

Meso compounds, racemic mixtures, optical activity

Alkyl Halides 1o, 2o and 3o alkyl halides

Nucleophilic substitution reactions (SN1 and SN2)

Elimination reactions vs. nucleophilic substitution

Alcohols and

Ethers

1o, 2o and 3o alcohols, acidity/basicity, reaction with active metals

Nucleophilic substitution reactions—conversion to alkyl halides

Elimination (Zaitsev’s rule), oxidations, synthesis of ethers

Amines 1o, 2o and 3o amines, quaternary ammonium salts, SN reactions

Basicity of amines, reaction with acid, separation of amines

Aromatic

Compounds

Structure, resonance, molecular orbital view

Electrophilic aromatic substitution, reactions of nitrobenzenes,

aromatic rings, side chains, acidity of phenols, basicity of amines

Aldehydes and

Ketones

Reduction, oxidation of aldehydes, nucleophilic addition, reductive

amination

Carboxylic

Acids and

derivatives

Acidity, separation of carboxylic acids, non basicity of amides

Reduction of carboxylic acids and amides

Nucleophilic acyl substitution, esterification, ester hydrolysis

Spectroscopy Use of 1H and 13C nmr spectra, use of ir spectra

Reaction

Mechanisms

Reaction coordinate diagrams

Nucleophiles, electrophiles, leaving groups, use of curved arrows

1o, 2o and 3o carbocations and their relative stability

35

The second semester of Chemistry 100 was not observed for this study.

The students attended 11 laboratory classes each semester and performed organic

chemistry exercises (generally synthetic preparations of organic compounds) in seven of

these. These laboratory classes were held in the last weeks of first semester, which

were the same weeks that organic chemistry was taught in the lectures.

The lecturers provided the students with weekly problems sheets. The organic

chemistry lecturer assigned one or two of these questions as part of their laboratory pre-

work. Optional tutorial classes, held each week to assist students who may have

questions, generally had low attendance, with an average attendance of five students.

3.3.2 Chemistry 2XX at University A

Chemistry 2XX was observed for the second semester in 1999, the first semester in

2001 and for the entire year in 2000.

Chemistry 2XX is a group of second year chemistry units predominantly for chemistry

majors. It includes the units Chemistry 200, 246, 254, 256, 260, 270 and 280. These

units are prerequisites for many third year chemistry units and consist of lectures and

laboratories in at least two of the following topics: inorganic chemistry, organic

chemistry, physical chemistry and MAST (modern analytic spectroscopic techniques).

Chemistry 200, 260 and 270 were the only units in Chemistry 2XX that included the

organic chemistry component. Only students from these chemistry units took part in the

research study.

The organic chemistry component of the course is described on University A’s website:

The organic chemistry section (39 lectures) contains an extension of the first year treatment

of arenes, electrophilic substitution and aromatic-based drugs and dyestuffs. The

preparation and synthetic application of polyfunctional compounds are described together

with the application of the concepts of stereochemistry and mechanism. The chemistry of

the carbohydrates, amino acids, peptides and proteins, nucleosides, nucleotides and nucleic

acids, and the mutual relationship among carbohydrates, amino acids, nucleic acids and

proteins, are discussed.

36

The organic chemistry lecturer also provided his students with a more detailed course

outline at the start of his lectures (Appendix 3.1).

The organic chemistry component of Chemistry 2XX was studied over the entire

academic year and comprised approximately 20 lectures in each 13 week semester.

In 2000 and 2001, the topic of reaction mechanisms was introduced in first semester,

and applied throughout the course of the year. Two or three lectures in each year were

devoted to discussing aspects of these representations, with specific reference to

substitution and elimination reaction processes.

The lecturer gave out handouts, which he referred to in the course of the lecture, in most

classes. These handouts were a mixture of research papers, summary sheets and

practice problems.

During the year, the lecturer provided students with between six and eight problem

sheets, stating that the examination questions for both semesters were drawn directly

from these sheets. The lecturer used two or three lectures per semester as tutorial

sessions to work through the problems sheets with the students and advised students that

he was available outside of class time to assist them with problems that weren’t covered

in the tutorials.

Six laboratory sessions were held early in semester two. Seven different synthetic

procedures were carried out. Students were required to submit written reports for each

synthesis that they completed.

3.3.3 Chemistry 3XX at University A

Chemistry 3XX is a third year unit in which students are offered a variety of different

lecture topics. One topic is compulsory. These topics cover a wide range of subjects,

such as chemistry education, organometallic chemistry and heterocyclic chemistry.

One group of students from this course participated in this research study. They were

the students who sat the chemistry education topic in semester 2, 2000.

37

3.3.4 Chemistry 121/122 at University B

Second semester lectures in Chemistry 121/122 were observed in 1999 and 2000.

Approximately four lectures each year dealt specifically with substitution and

elimination reactions and the associated representations of reaction mechanisms.

Lectures were not observed in 2001 or 2002.

Chemistry 121/122 is a compulsory first year chemistry unit in a non-chemistry degree

at University B that covers organic, inorganic and physical chemistry. Students

attended three lectures and one three hour laboratory class each week. Students had

three different lecturers over the course of the year—one each for organic, inorganic and

physical chemistry. Their laboratory supervisors were the same in each semester for

organic, inorganic and physical chemistry experiments. Approximately one third of the

laboratory classes were devoted to organic chemistry experiments.

Students attended a compulsory tutorial class once a week. The students received a

mark for attendance and for attempting assigned tutorial problems, which they were

required to submit to their tutor at the end of the tutorial. This mark made up ten

percent of their grade each semester.

The organic chemistry content of these units was described in the university’s online

handbook. ‘Steroisomins’ is a typographical error and should read ‘stereoisomerism’.

Chemistry 121:

Organic - introduction. Bonding, nomenclature, structure, alkanes, cycloalkanes, alkenes

and alkynes. Benzene and derivatives, isomerism including steroisomins.

Chemistry 122:

Organic - alkyl halides, alcohols and phenols, ethers, aldehydes and ketones, carboxylic

acids, amines, oils, fats and waxes, carbonic acid derivatives, functional group

interconversion.

In 1999 and 2000, organic chemistry topics were discussed in one lecture per week in

both semesters (approximately 28 lectures each year). The course outline is shown as

Table 3.2.

38

Table 3.2 Organic chemistry syllabus for second semester Chemistry 121/122.

Section Topic

Functional groups Introduction to common functional groups.

Alkyl halides

Nomenclature, properties, preparation, reactions, nucleophilic

substitution reactions, SN1 and SN2 mechanisms, stereochemistry,

elimination, substitution verses elimination.

Alcohols, phenols

and ethers

Nomenclature and classification, properties, acidity, preparation,

reactions (dehydration, esterification, oxidation).

Polyhydric

alcohols

Geminal diols, glycols, glycerol and higher polyols, ethylene

oxide and nitroglycerin.

Aldehydes and

ketones

Nomenclature, nature of carbonyl group, preparation, reactions,

oxidation and reduction properties, tests.

Carboxylic acids

and derivatives

Nomenclature, properties, acidity, preparation, reactions.

Amines Nomenclature and classification, properties, basicity, preparation,

reaction, azo-dye formation.

Lipids: oils, fats

and waxes

Structure, properties, reactions (hydrolysis, reduction, oxidation),

soaps and detergents.

Interconversion of

functional groups

A review of the reactions of the functional groups considered.

In 2001, students studied slightly more organic chemistry (approximately 35 lectures),

with the relevant reaction mechanisms being covered at the end of first semester. This

change was made at the request of the Pharmacy department whose students were

studying this unit as part of their non-chemistry degree.

In 1999 and 2000, the content of the organic chemistry lecture courses in Chemistry

121/122 were essentially identical to Chemistry 101/102 and Chemistry 123/124. In

2001 and 2002, there were slight differences between the courses, due to the increase in

the number of organic chemistry lectures in Chemistry 121/122. As students only

participated in questionnaires early in both years, these changes did not impact the

study.

39


Chemistry 101/102 is a compulsory first year unit for chemistry students, covering the

topics of organic, inorganic and physical chemistry. Like Chemistry 121/122 students,

Chemistry 101/102 students attended three lectures and one three hour laboratory per

week. Over the year, approximately 28 lectures and seven laboratory classes were

devoted to the study of organic chemistry.

Different lecturers taught each of the organic, inorganic and physical chemistry

components. The organic chemistry lecturer for Chemistry 101/102 was not the same

lecturer who taught Chemistry 121/122.

The courses are detailed in the university’s online handbook, with the subject matter

relating to organic chemistry shown below.

Chemistry 101:

Reactions and properties of aliphatic and aromatic hydrocarbons and stereoisomerism.

Chemistry 102:

Reactions and properties of compounds containing common functional groups.

In 1999 and 2000, Chemistry 101/102 students did not have tutorials, but they did sit

regular tests. Students were required to complete a short test on a section of work by a

set date. Students who did not score 80 % or better in the test were required to re-take

the test. The students completed 20 tests (ten per semester) over the course of the year.

In each semester, three tests covered organic chemistry. In 2001, the students

completed on-line quizzes.


Chemistry 123/124 is a first year unit in non-Chemistry degree, covering physical and

organic chemistry. Students attended two lectures and one two hour laboratory a week.

Students attended a compulsory weekly tutorial, where they submitted their responses to

selected questions, and were marked on their attendance and for attempting to answer

the tutorial questions (10 % of final grade). They could also complete online quizzes.

40

Organic chemistry comprised in half the lectures and laboratories over the year. About

one third of the tutorial sessions covered organic chemistry problems. The description

of the organic chemistry content of these topics on the university’s website is essentially

identical to that of Chemistry 101/102.

3.4 Lecturers

Three lecturers at these two universities volunteered to take part in the research study.

To preserve their anonymity, all have been given pseudonyms. The personal pronouns

used in this thesis match the gender of the lecturer. A description of each of the

lecturers follows. A detailed discussion of their teaching is given in later chapters.

3.4.1 Dr Anderson

Dr Anderson was the organic chemistry lecturer for Chemistry 121/122. He had

lectured this course, or an identical first year course (Chemistry 101/102), for

approximately eight years.

Dr Anderson taught Chemistry 121/122 using a functional group approach. This

approach involved discussing the preparation and reactivity of compounds of different

functional groups (for example, alkanes, alcohols and ketones). Dr Anderson used this

approach to point out the similarities between reactions of particular functional groups.

Dr Anderson started his lecture with a short introduction that he referred to as the

Highlights of Last Lecture. In a few minutes, he briefly summarised the main points of

the previous week’s lecture and reminded students about what had been discussed in the

previous class. This summary was displayed on the overhead projector or written on the

whiteboard. Most students were observed to write notes on the highlights.

In addition to these highlights, Dr Anderson ended each of his lectures with a brief

reminder of what he called the take home message from that class, in which he told the

students what he considered to be the most important things from that day’s lecture.

The take home message mentioned at the end of each lecture always formed part of the

highlights discussed at the start of the next lecture.

41

When teaching, Dr Anderson asked questions of his students in an attempt to involve

them more in the class. In many situations where several choices were possible answers

to his questions, he asked students to vote on the correct answer by raising their hands.

The lecturer often used experimental evidence to introduce new concepts and theories to

the students. He employed this technique when teaching about substitution and

elimination mechanisms in second semester, citing data such as relative rates of reaction

or percentages of reaction products as a starting point to talk about new chemistry.

3.4.2 Dr Adams

Dr Adams was the organic chemistry lecturer for Chemistry 100 in 2000 and 2001. Dr

Adams had lectured this course for approximately ten years.

In 2000, he taught the topic using a functional group chemistry approach. Dr Adams

explained this to the researcher: ‘[w]hat I’ve been doing so far is, take a functional

group, learn how it behaves, and talk about the synthesis and the reactions of the

functional group type compounds’ (interview 23032000, line 31 – 33). The lecturer

added that he would like to take this approach further, depending upon the level of

interest of his students. He described this in the following manner (interview 23032000,

line (35 – 48):

What I’d like to do is say here’s a functional group, this is how it reacts, um, little bit of

synthesis, and then say here’s a, a system from industry, or a system in a semi-conductor, or

a biological molecule, what is it about the way it’s put together that makes it useful for

what it’s doing . . . [ ] . . . So, that’s just a little thing I would like to . . . put a bit more into

the course . . . [ ] . . . I would hope to cover the same sort of concepts, but make it more

practical, rather than more pure chemical (indistinct).

Dr Adams felt that such an approach might be more appropriate for particular students,

whereas ‘more analytical’ students would benefit from learning about mechanistic

principles in Chemistry 100.

42

3.4.3 Associate Professor Andrews

Associate Professor Andrews was the lecturer for the organic chemistry component of

Chemistry 2XX between 1999 and 2001. He had lectured this course for approximately

30 years.

Before starting each of his lectures, Associate Professor Andrews wrote brief notes on

the board to remind students of what they had covered in the previous lecture. He

generally discussed these points for a few minutes at the start of each class. He felt that

this was useful for two purposes: refocussing his students on the material that had been

covered previously and also allowing him to discuss points that he felt had not been

discussed adequately in the preceding lecture.

3.5 Students

In each of the units being studied, the students were given a questionnaire as a group at

least once during the year. Questionnaires were designed for each particular course.

Apart from researching students’ understandings of various aspects of reaction

mechanisms in organic chemistry, these questionnaires were used to call for volunteers

to participate in interviews and focus groups. In addition, the researcher attended some

laboratory classes to recruit volunteers.

In general, the student volunteers were very confident students, many of who received

high marks in their subjects. Several of the participants received the highest (or one of

the highest) marks for the chemistry subject that they were studying. Only one of the

student volunteers did not pass the chemistry topic he was studying. These observations

were consistent with comments made by Burns (1998, p. 18), who suggested that

volunteers are not likely to be a random sample; ‘They tend to be better educated, . . .

more intelligent . . . than non-volunteers’. For this reason, the student participants are

described in the following sections, to allow others to determine the applicability of

findings to their own teaching and learning environments.

43

Pseudonyms are used for all students who participated in interviews and focus groups.

The names and personal pronouns used when discussing students’ understandings are

consistent with the students’ gender.

The grades mentioned in the following sections are based on the following:

0 – 49 Fail

50 – 59 Pass

60 – 69 Credit

70 – 79 Distinction

80 – 100 High Distinction

3.5.1 Chemistry 100 Students

In 1999, 11 Chemistry 100 students participated in interviews. While several of these

students went on to study Chemistry 2XX in second year, only two of them (Guy and

Graeme) volunteered to participate in the study again in their second year. Both

students participated in at least two interviews (one in each year).

In 2000, four Chemistry 100 students were interviewed. None of these students went on

to study Chemistry 2XX in 2001 and were not interviewed in subsequent years.

Chemistry 100 was not observed in 2001, and no students were interviewed.

3.5.1.1 Students Interviewed in 1999

Barbara commented that she planned to study Chemistry 200 in second year because

she wanted to know ‘how the world works’, adding that, ‘chemistry’s fun, that’s why I

do it’ (interview 04101999, lines 3 – 6). She didn’t like organic chemistry all that

much, preferring the topics covered in the second semester of the course. Barbara was

awarded a distinction for Chemistry 100. In second year, she studied mathematics and

physics as a science-engineering student, studying no chemistry beyond first year.

44

Boris, a science student, commented that he enjoyed chemistry, was good at it and that

organic chemistry was a topic he particularly liked. Before he had started tertiary

studies, Boris had participated in the Chemistry Olympiad (an international extension

programme and competition for chemistry students) as a high school student. He

received a credit in Chemistry 100 and a pass in Chemistry 200 in second year.

Brian was interviewed together with Bill. Brian commented that he’d enjoyed studying

organic chemistry in first year, but would probably only continue studying it if it was

required in his course (materials chemistry). Brian received a credit in Chemistry 100.

He did not study organic chemistry in 2000.

Bill intended to study chemical engineering and commented that he wanted to continue

studying organic chemistry to complement his engineering degree—‘you do that

organic chem, like petroleum or something’ (interview 10091999, line 31). Bill

received a high distinction in Chemistry 100 and continued to study mathematics and

physics subjects. He did not study chemistry beyond a first year level.

Bob was studying Chemistry 100 as a component of a pharmacology course. He

admitted that he had enjoyed the organic chemistry section of the course, making

particular reference to the laboratory sessions. Bob received a pass mark, and continued

on to study biochemistry, molecular biology and pharmacology in his science degree.

Bruce, a science-engineering student, commented that he was considering studying

Chemistry 200 because this seemed like a natural progression, after he had studied

Chemistry 100 in first year. He received a distinction in the unit, and went on to study

second and third year chemistry units.

Brendon was a materials chemistry/mechanical engineering student. He intended to

study second year chemistry because he’d enjoyed it in first year but he hadn’t looked

into whether or not it was a requirement for his course. He was awarded a high

distinction in Chemistry 100 (he was the top student), and studied mathematics and

physics subjects in the second and third years of his degree, receiving high distinctions

in all the units he studied.

45

Belinda was a science student, who planned to continue studying chemistry. She

commented that she did not intend to study organic chemistry in second year, but when

asked if she was going to study Chemistry 200, which includes an organic chemistry

component she indicated that she hoped to do this. Belinda was awarded a credit grade

in Chemistry 100 and a distinction in Chemistry 200.

Benjamin was studying a science-engineering degree, with the intention of entering the

mining industry after completing his studies. He commented that, ‘chem might help

with metallurgy and most of it’ (interview 04101999, line 4 – 5). Benjamin was

awarded a distinction for Chemistry 100, and studied several chemistry courses in

second and third year.

Graeme, a science student, commented in first year that he intended to study Chemistry

200 in second year because he’d found chemistry to be so interesting in first year. At

the beginning of his second year, Graeme expressed an interest in continuing his

involvement with this research project. He was one of the two Chemistry 2XX students

in 2000 that was interviewed only once (Graeme’s interview was in second semester).

Graeme was awarded a high distinction in both Chemistry 100 and Chemistry 200. In

2001, he studied Chemistry 300, in which he was also awarded a high distinction.

Guy also volunteered to participate in the research project for a second year. Guy

enjoyed studying chemistry at high school, and this enjoyment had prompted him to

take some chemistry courses at university as part of his science degree. He said that he

had no particular favourite area of chemistry and added that he’d enjoyed the organic

chemistry section of the course, commenting that it was ‘[a] lot of rote learning, but, I

enjoyed the chemistry of it, it was good’ (interview 20091999, lines 12 – 13). He

received a distinction in Chemistry 100 and a credit in both Chemistry 200 and

Chemistry 300.


Christopher was a science-engineering student. He received a distinction in his

Chemistry 100 unit. He did not study any chemistry topics in the second year of his

degree.

46

Cathy commented that she found reaction mechanisms to be ‘tricky, so it kind of,

sometimes it confuses stuff’ (interview 16052000, lines 18 – 19). She was awarded a

high distinction in Chemistry 100. She studied molecular biology and genetics subjects

in the second year of her science degree.

Cameron was a science-engineering student in 2000. He changed to a science degree

later in his course. Cameron was awarded a high distinction in Chemistry 100. He

studied a chemistry special unit in second year.

Carl admitted to being a ‘bit behind in stuff’ (interview 160522000, line 98 – 99) and

was not very confident in his understandings of reaction mechanisms. He received a

credit mark in Chemistry 100. He studied environmental chemistry units in second year

as part of his environmental engineering degree.

3.5.2 Chemistry 2XX Students

Chemistry 2XX was not observed for all of 1999, and no students were interviewed. In

2000, six student volunteers were interviewed. Four of these students were interviewed

twice, once in each semester. One student (Dennis) was interviewed only in first

semester. Another Chemistry 2XX student (Graeme) was interviewed only in second

semester. In 2001, seven students participated in both interviews and a focus group. A

brief summary of each student follows.


Dennis studied Chemistry 100 in 1995 and receiving a pass mark. He was studying

organic chemistry to complement his biochemistry studies. He was interviewed only

once in 2000 (first semester). He received a credit for Chemistry 270 and studied a

chemistry special course in third year.

Debra was a Chemistry 100 student in first year, receiving a high distinction in the unit.

She studied Chemistry 270 in her second year, as this was the only course that fit into

her timetable. Given the opportunity, Debra said she would have studied Chemistry

200. Debra received a high distinction in Chemistry 270. She studied no chemistry

units in third year, focussing on microbiology in her science degree.

47

Damon studied Chemistry 100 in his first year, and was awarded a pass mark for the

course. He was studying organic chemistry as he considered it ‘useful to have organic

chem as part of a complete chemistry course’ (questionnaire, semester 1 2000). He was

a quiet student who had little to say in his interviews. He tended to simply agree with

comments made by the researcher. His Chemistry 200 mark was a pass. In third year,

he studied Chemistry 300, also receiving a pass mark. His third year mark was

adversely affected by a long-term medical condition that inhibited his ability to study

for extended periods of time and sit long examinations.

Dianne studied Chemistry 120 in first year, receiving a pass mark. She was intending to

do a double major (pharmacology/chemistry) to enable her to convert to a pharmacy

degree. She commented that she had only just been introduced to the concept of

reaction mechanisms when asked about the topic in her first interview, although on an

earlier questionnaire, she stated that she had studied reaction mechanisms in first year.

She was awarded a pass in Chemistry 270 and a pass in Chemistry 300 the next year.


Felicity was a science student. She studied Chemistry 100 in first year, receiving a high

distinction for the topic. In second year, she studied Chemistry 260. She was awarded

a high distinction, which was one of the highest marks for the unit.

Frank was a science student who studied Chemistry 100 in 1998, receiving a pass mark.

He returned to study Chemistry 270 in second year after a short break. Frank admitted

that he hadn’t attended many lectures in first semester, and often didn’t have copies of

notes or handouts prepared by the lecturer. Following a focus group, he borrowed

copies of the researcher’s lecture notes to assist him with his studies. Frank failed

Chemistry 270 in 2001.

Fred was a Chemistry 100 student in first year. He received a credit in the course. In

second year, he was enrolled in Chemistry 260. Although Fred attended most first

semester lectures, he was observed talking to friends during the majority of the classes

and did not take many lecture notes. Fred’s final mark for his second year course was a

credit. He was studying a science-economics degree.

48

Fernando studied Chemistry 100 in first year, and received a high distinction. In second

year, he was studying Chemistry 200. He received a high distinction in the course,

which was the highest mark of all the Chemistry 200 students. Fernando was a science-

law student.

Fiona studied Chemistry 120 in 2000. She was awarded a credit in the topic. She was

studying Chemistry 200 in second year, for which she received a credit. Fiona

commented that she’d had difficulties in her personal life around the time of the focus

group and interview that she’d participated in.

Fabian was a science-engineering student, who changed to a science degree in 2000. He

was a Chemistry 100 student in first year, receiving a distinction. He studied Chemistry

200 in second year and was awarded a credit.

Felix studied Chemistry 100 in first year as part of his science-engineering degree. He

achieved a high distinction, which was the top mark in the course. He studied

Chemistry 260 in second year, and was awarded a high distinction in the topic, again the

highest mark for the unit. In the first two years of his degree, Felix had received high

distinctions in all the units he studied.

3.6 Summary

The context of the research study involves a consideration of all the aspects that help to

define the study. This enables others to understand the findings described in the light of

the particular situation they came from and to establish if such findings are applicable to

their own teaching and learning situations.

In the case of this study, the context includes a description of the two universities at

which the investigated courses were taught, as well as a discussion of the particular

courses that were studied. In addition, both the lecturers who taught the courses and the

students who participated in interviews and focus groups were described as part of the

study context.

49

The discussion of the different courses studied for this research project given in this

chapter did not include any consideration of the subject matter that this study focussed

on. Mechanistic representations of substitution and elimination reaction processes were

discussed in detail in all of the courses studied. A description of this subject matter is

discussed in the following chapter.

50

4 Subject Matter

4.1 Introduction

This research project was involved with investigating the understandings of students

and lecturers in several courses about various aspects of teaching and learning about

reaction mechanisms in organic chemistry. The project focussed on two particular types

of reactions and the mechanistic representations used to rationalise experimental

observations in these reactions. These reaction types are substitution reactions and

elimination reactions. The following chapter discusses commonly used representations

and conventions relating to written reaction mechanisms. In addition, the subject matter

covered in first and second year chemistry courses relating to substitution and

elimination reactions is discussed.

4.2 The Concept of Reaction Mechanism

Hoffmann (1995, p. 143) defined a reaction mechanism as ‘a sequence of irreducibly

simple elementary chemical acts by which one molecule is transformed into another

one’. March (1992, p. 205) takes this explanation further:

A mechanism is the actual process by which a reaction takes place—which bonds are

broken, in what order, how many steps are involved, the relative rate of each step, etc. In

order to state a mechanism completely, we should have to specify the positions of all

atoms, including those in solvent molecules, and the energy of the system, at every point in

the process. A proposed mechanism must fit all the facts available. It is always subject to

change as new facts are discovered. The usual course is that the gross features of a

mechanism are the first to be known and then increasing attention is paid to finer details.

The tendency is always to probe more deeply, to get more detailed descriptions.

Although for most reactions gross mechanisms can be written today with a good degree of

assurance, no mechanism is known completely. There is much about the fine details which

is still puzzling, and for some reactions even the gross mechanism is not yet clear. The

problems involved are difficult because there are so many variables. Many examples are

known where reactions proceed by different mechanisms under different conditions. In

some cases there are several proposed mechanisms, each of which completely explains all

the data.

51

A reaction mechanism can be thought of as the actual processes that are going on within

a reaction vessel or in natural systems in transforming the starting material(s) into the

products. Chemists propose pictorial representations of these mechanisms to rationalise

reaction processes, based upon observable evidence about the reaction process; the rate

of reaction and the stereochemistry of starting materials and products. This evidence is

used to suggest a possible way (or ways) that this reaction might proceed. We model

these suggestions in the form of representations that are often called reaction

mechanisms. These representations are the pictures that we put onto the page (or, in

some cases, the computer simulation that we generate) as we attempt to explain how a

given reaction might proceed.

Representations of reaction mechanisms are not the real thing. As models, they have

certain strengths and weaknesses. Bent (1984) commented that:

Chemistry’s models are not like model airplanes: small versions of visible things.

Chemistry’s tangible models are large mechanical metaphors of small, invisible quantum

mechanical things.

. . . .

Indeed, to be useful, a model must be wrong, in some respects—else it would be the thing

itself. The trick is to see . . . where it’s right.

Mechanistic representations, by their very nature as models, are inadequate in some

aspects. For example, a mechanistic representation can show a reader very little in

terms of the multiple particle nature of a reaction process, nor is there much indication

of why this representation is appropriate to describe the real reaction process. A

structural representation used in a particular reaction mechanism may highlight one

portion of a molecule at the expense of another area in the same molecule.

Representations of reaction mechanisms generally only show one of the possibly many

reaction processes going on in a given reaction mixture and can therefore ‘ignore’ other

secondary reaction processes.

Laszlo (2002) commented that the writing of reaction mechanisms could encourage

‘confusion between a working hypothesis and a reliable conclusion, between the virtual

and the real’ in students’ minds. Students may equate the mechanistic representation

52

that their lecturer has represented with what is really going on within the reaction

mixture, ignoring the possibility of both unsuccessful reaction between particles and

other reactions that may also be occurring within the same reaction mixture. Students

might think that the representation is real, not that the reaction process is the reality.

The concept of reaction mechanisms involves the consideration of what is going on

within a reaction mixture at the molecular level, which is quite a challenging notion.

Laszlo (2002) argued that, in writing representations of reaction mechanisms, ‘what we

are truly doing is to replace a dynamic phenomenon, which we little understand, with a

stroboscopy of a succession of instantaneous successive static structures’.

This topic requires lecturers and students to consider things such as what particles are

present in a particular reaction mixture, the frequency of collisions between these

reactant particles, the energy and appropriate orientation required for a successful

collision to occur and the probability of a successful collision occurring. This involves

the breaking and formation of bonds, which can be represented using curly arrows to

indicate electron shift. Shapes of molecules, the stability of various species such as

transition states and intermediates and the competitive nature of some reactions are

essential factors in a topic where reaction mechanisms and their representations are

discussed and mechanistic processes are proposed for given reactions.

4.3 Commonly Used Representations

Representations of reaction mechanisms attempt to model a reality; what is going on a

reaction vessel. These models are pictorial in nature, utilising a variety of symbols and

structures to represent reaction mechanisms. As with most models, there are rules and

conventions associated with their use. The more common conventions are discussed in

the following sections.

4.3.1 Appropriate Structural Representations

When they are taking chemistry, organic chemists tend to use a variety of different

types of structural representations to explain and describe reactions through the writing

of chemical equations. Texts and lecturers use these different types of structural

53

representation to highlight specific aspects of chemical structure or bonding in a

particular compound. An example of this is shown in Figure 4.1, which represents 1,1-

dibromoethane using four different structures.

Br

BrCH3C

Br

HBrBr C

Br

H

C

H

H

H Br2CHCH3

(i) (ii) (iii) (iv)

Figure 4.1: Different representations of 1,1-dibromoethane.

Some structural representations, such as Lewis or square planar structures (i), show all

atoms and bonds. Other structural representations, such as the three-dimensional

representation (ii) show all atoms and most bonds, as well as attempting to portray some

suggestion of the shape of the molecule. Others ((iii) and (iv)) show more condensed

representations. In each of these structures, different features are emphasised.

It is common for particular structural types to be used for representing reaction

mechanisms of specific kinds of processes. For example, three-dimensional structures

such as (ii) in Figure 4.1 are often used in lectures and by textbooks to write reaction

mechanisms to explain substitution processes, as this representation focuses attention on

the carbon atom bearing the leaving group. The three-dimensional bond representations

are also useful when describing stereochemical inversion, which is discussed in detail in

section 4.4.2. Square planar structures (i) are commonly used in representations that

attempt to rationalise elimination processes, as they focus attention on the bond between

two carbon atoms, which is useful when describing the formation of double bonds, as

happens in elimination processes.

4.3.2 Curly Arrows

The presence of curly arrows is what distinguishes a mechanistic representation from a

chemical equation. Curly (curved) arrows are used to symbolise the movement of

electrons as bonds break and form in a reaction process (Figure 4.2). A curly arrow

represents the movement of a pair of electrons from the tail of the arrow to its head.

54

H Cl H + Cl

Figure 4.2: Curly arrow notation symbolising the heterolytic breakage of an H-Cl bond

to form H+ ions and Cl- ions.

The textbook recommended by Associate Professor Andrews, a participating lecturer, to

his second year students details four rules that students need to follow when using the

curved arrow symbolism (McMurry, 2000, p. 164 – 6). Examples supporting each of

the four rules are also shown. McMurry commented that ‘[i]t takes a lot of practice to

used curved arrows properly in reaction mechanisms’.

4.3.3 Formal Charges

The formal charge on an atom in a molecule is essentially a comparison between the

number of outer-shell electrons that an atom has as an isolated atom, and the number it

has formally assigned to it in a given structure, if we assign to the atom half the

electrons in each bond and all its non-bonding electrons.

Atoms can have formal charges of any value. In general, only non-zero formal charges

are represented. These are shown on atoms in structures as and . In organic

chemistry courses, students are most likely to see them represented on oxygen, nitrogen,

carbon, hydrogen and halogen atoms. Formal charges may be present on isolated atoms

or ions (such as Cl- or H+) or on or near particular atoms in molecular species (such as N

in Figure 4.3).

As curly arrows are used to represent the movement of electrons that result in (or

accompany) the breaking and forming of bonds in a particular reaction process, formal

charges are representations of the result of these electron shifts. This enables a chemist

to keep track of electrons in a particular representation. An example of this is shown in

Figure 4.3.

55

N

HHH

H

N

HHH

H

Figure 4.3: A representation of a mechanism to explain the formation of ammonium

ions from ammonia and hydrogen ions. On the left, one H has a formal charge of +1.

On the right, the N in the ammonium ion is represented with a formal charge of +1.

4.3.4 Lone Pairs or Non-Bonding Electrons

Organic chemistry reactions quite often include oxygen, nitrogen and halide atoms in

the reacting compounds. These atoms are of particular interest because, when in a

compound where they have what might be considered a normal number of bonds (two

for oxygen, three for nitrogen and one for halides), they have at least one lone pair of

electrons associated with them. Reaction mechanisms rationalise and explain reaction

outcomes using electron movement, so understanding which atoms do have lone pairs

of electrons (which therefore might be available for interactions with other species and

might form bonds) can be important in the study of this topic.

4.4 Nucleophilic Substitution Reaction Mechanisms

Organic substitution reactions involve replacement of one substituent in a carbon-based

compound by another. In a substitution process, one bond in each reactant molecule is

broken and a new bond is formed. This type of reaction can be represented by the

following general equation:

R X R Y+ Y - + X -

Substitution reactions are termed nucleophilic when the attacking group (Y-) is a

nucleophile; compounds that ‘act by donating or sharing their electrons’ (Sharp, 1990,

p. 283). They are typically negatively charged or contain at least one lone pair of

electrons. The starting material, R—X, is referred to as the substrate and X- as the

leaving group.

56

The rates of reaction of substitution processes can be categorised into three types

according to experimentally observed dependence of reaction rate on concentration of

reactants:

• The rate of reaction is proportional only to the concentration of the

substrate and is independent of the concentration of the nucleophile;

• The rate of reaction is proportional to the concentrations of both the

substrate and the nucleophile;

• The rate of reaction is not identifiable as either of the above types.

These three types of reaction rate dependencies are an indication that not all substitution

reactions proceed via the same reaction process.

Two commonly described reaction mechanisms, each of which is consistent with one of

the reaction types outlined above, are discussed in detail in many chemistry courses.

These are labelled substitution, nucleophilic, unimolecular (SN1) and substitution,

nucleophilic, bimolecular (SN2) reaction mechanisms.

4.4.1 SN1 Reaction Mechanism

Substitution reactions that have a reaction rate that is proportional only to the

concentration of the substrate can be modelled by an SN1 reaction mechanism (Figure

4.4). In this representation, the substrate R—X has been depicted as a central carbon

attached to three alkyl or aryl groups (R1, R2 and R3) and the leaving group, X.

It is believed that the product of the first rate-determining step is a planar carbocation.

This rate-determining step is consistent with the rate of reaction being dependent only

on the concentration of the substrate, because it is the only reactant involved.

Bond formation between the central carbon of the planar intermediate and a nucleophile

is thought to occur in the second, rapid step of the suggested mechanism. This is based

upon the following observation: in reactions where the substrate is a chiral compound,

the formation of racemic mixtures has been noted. Experimentally, the formation of a

racemic mixture is observed in the ability of a solution of the compound to rotate the

plane of polarised light.

57

Figure 4.4: A generalised representation of a two-step SN1 reaction mechanism.

Chiral compounds are those whose molecules have at least one stereocentre: a carbon

atom that has four different groups attached to it. A chiral molecule does not have a

plane of mirror symmetry. A test for this is that a molecule with a structure that is the

mirror image of it is not superimposable upon it. These mirror images are referred to as

R and S isomers. A common analogy in textbooks and lectures explains this by

comparing a left hand to a right hand.

If a bulk amount of chemical contains molecules of a chiral substance that are all of one

isomer (that is, they are all R isomers or all S isomers), a solution of the substance will

rotate the plane of polarised light. If the bulk contains an equal (or very close to equal)

amount of molecules of both the R and the S isomers, it is referred to as a racemic

mixture of the isomers and a solution of the substance will not rotate the plane of

polarised light. In cases where the racemic mixture is not an exactly equal mix of the

two isomers, a small amount of rotation of light may occur, but it will be close to zero.

The second rapid step of the SN1 reaction mechanism is consistent with the observation

that racemic mixtures are formed from chiral compounds. According to this

mechanism, there is an equal likelihood of nucleophile attack on either face of planar

carbocation intermediates—bond formation on one side giving rise to molecules of the

R isomer and on the other side to the S isomer. The probability of these collisions

C X

R1

R3R2

rdsC

R1

R3R2

+ X

C

R1

R3R2

C Y

R1

R3R2

CY

R1

R3R2

Y

and

racemic mixture

C

R1

R3R2

Y

and

58

between nucleophiles and carbocations being successful are approximately equal,

leading to the formation of equal amounts of the two isomers.

4.4.2 SN2 Reaction Mechanism

Substitution reactions that are observed to have a reaction rate that is proportional to the

concentrations of both the substrate and the nucleophile can be modelled by an SN2

reaction mechanism (Figure 4.5).

C X

R1

R3

R2C

R1

R3R2

XY Y C

R1

R3

R2

+ XY

transition state

Figure 4.5: A generalised representation of a one-step SN2 reaction mechanism.

An SN2 mechanism proposes the formation of a negatively charged transition state as

each effective collision between nucleophile and substrate molecules occurs. The

dotted lines in this transition state indicate the simultaneous breakage of the C—X bond

and formation of the C—Y bond. This bimolecular step, which involves both reactants,

is consistent with the observed reaction rate dependence.

In SN2 reactions with a chiral substrate, stereochemical inversion has been observed.

Stereochemical inversion refers to a modification in the spatial arrangement of atoms

around a stereocentre in a chiral compound. This inversion is consistent with the

rationalisation given in an SN2 type reaction that the attack of a nucleophile molecule on

the central carbon of the substrate is more likely to be effective if the nucleophile is

attacking the central carbon atom from the direction that is opposite to the position of

the leaving group. This type of attack is often referred to as backside attack.

4.4.3 SN1 vs SN2

Nucleophilic substitution reactions may proceed by either of the two described

mechanistic pathways. It has been observed that reaction conditions, such as the nature

of the substrate, the type of nucleophile and leaving group, and the solvent used

59

(Brown, 2000, p. 185), can mean that substitution is more likely to proceed via either

one of these reaction pathways.

There are reactions, however, about which experimental evidence suggests that

substitution occurs via both mechanisms occurring simultaneously, or where evidence

cannot conclusively suggest by which mechanistic pathway substitution takes place

(March, 1992, p. 305). These reactions fall into the third category that is mentioned in

section 4.4. In these reactions, competing SN1 and SN2 reaction processes may be

occurring, or a different type of reaction mechanism (or mechanisms) may be

proceeding.

The structure of the substrate has been experimentally observed to influence the type of

rate dependence (section 4.4) that a reaction process follows. Chemists use rate

dependence as evidence of a particular type of mechanistic process, SN1 or SN2.

Primary (1o) and secondary (2o) alkyl substrates are more likely to proceed via an SN2

process, whereas tertiary (3o) substrates are more likely to proceed via SN1. This is

thought to be due to two factors—the relatively high stability of carbocations formed

from more substituted tertiary alkyl halides favouring SN1 type reactions, and the less

sterically hindered arrangement of less substituted (primary and secondary) alkyl

halides favouring SN2 reactions.

The stability of tertiary carbocations appears to be a rather unusual term to use to

describe what is known to be a very reactive species. In the context of the reactivities

(or lack thereof) of primary, secondary and tertiary carbocations, stability is used as a

relative term. Carbocations formed from tertiary alkyl halides have been isolated

experimentally, while those from primary and secondary alkyl halides are much harder

to prepare and isolate (Sykes, 1986, p. 104). Tertiary carbocations have also been

observed to be formed under less vigorous conditions than their primary and secondary

equivalents, and are therefore classed as more stable than the primary and secondary

carbocations.

As described in section 4.4.1, the first rate-determining step of an SN1 reaction

mechanism involves the formation of a planar carbocation. The carbocation is a

reactive species, which can undergo rapid reaction with a nucleophile, as is described in

the second step of an SN1 mechanism. The more stable of the carbocations (those

60

formed from tertiary substrates) require less energy to form than primary and secondary

carbocations and are easier to form. For these reasons, tertiary substrates are most

likely to proceed via an SN1 reaction process.

Steric hindrance or crowding of the central carbon atom has also been observed to

influence the type of reaction pathway that a reaction may follow. An SN2 process

proposes a one-step process, in which five groups are crowded about the central carbon

in a negatively charged transition state. The less crowded a substrate is, the more likely

a substitution reaction is to proceed by this type of process. Figure 4.6 shows

representations of three alkyl bromides.

The increased substitution in (iii) means that the central carbon atom becomes less

accessible to possible nucleophilic attack (McMurry, 2000, p. 394). This is consistent

with the observation that primary and secondary alkyl halides are more likely to

undergo bimolecular substitution reactions than are tertiary alkyl halides.

(i) (ii) (iii)

Figure 4.6: Space-fill representations of (i) primary, (ii) secondary and (iii) tertiary

alkyl bromides. There is increased crowding around the central carbon atom in (iii) as

compared to (i) and (ii).

4.5 Elimination Reactions

An elimination reaction results in the formation of alkenes from saturated starting

materials. This type of reaction involves the removal of a hydrogen atom and another

group, often a halide, from adjacent carbons on molecules of the starting material, with

61

formation of a π bond. In its simplest case, an elimination reaction can be represented

by the following general equation, where Y¯ is a nucleophile (often termed a Lewis

base), X¯ is the leaving group and the R groups are alkyl or aryl substituents. As with

substitution reactions, the substituted alkane can be referred to as the substrate.

C CR

H

R'

R'''

R''

X

C C

R''

R'''

R

'R

+ Y - + Y-H + X -

The rates of many different elimination reactions have been determined experimentally,

and found to fit into three categories, similar to those described for nucleophilic

substitution reactions (section 4.4).

The different types of model that are used in explaining and rationalising elimination

reactions are rarely covered in as much detail as representations of substitution reaction

mechanisms in first and second year chemistry courses, and so their treatment in the

following sections is brief.

4.5.1 E1 Reaction Mechanism

The E1 mechanism is attributed to elimination reactions in which the rate of reaction is

dependent upon only the concentration of the substrate. A mechanism for this reaction

is represented in Figure 4.7.

C CR

H

R'

R'''

R''

X

C CR

H

R'

R''

R'''

+ X rds

C CR

H

R'

R''

R'''

Y

C C

R''

R'''

R

R'

+ Y-H

Figure 4.7: A generalised representation of an E1 elimination reaction mechanism.

62

The E1 reaction mechanism is represented as a two-step process, whose first, rate-

determining step is the formation of a carbocation. This process is consistent with the

observation that the rate of these elimination reactions is proportional only to the

concentration of the substrate.

4.5.2 E2 Reaction Mechanism

The E2 mechanism (Figure 4.8) is ascribed to elimination reactions in which the rate of

reaction is proportional to the concentrations of both the substrate and the nucleophile.

The breakage and formation of the bonds occurs simultaneously and both the substrate

and the nucleophile are involved in this one-step process.

This model is in agreement with the experimental observation that the rates of these

elimination reactions are proportional to the concentrations of both the substrate and the

nucleophile.

C CR

H

R'

R'''

R''

X

C C

R''

R'''

R

'R

+ Y-H + X

Y

C CR

H

R'

R'''

R''

X

Y

Figure 4.8: Representation of an E2 elimination reaction mechanism.

4.5.3 More Than One Elimination Product Formed

Elimination reactions often have more than one possible product, in those cases where

the carbon bearing the leaving group has more than one β-hydrogen. β-Hydrogen is a

term used to describe a hydrogen attached to the carbon next to the carbon bearing the

leaving group. 2-Bromobutane (Figure 4.9) has five β-hydrogen, each of which has the

potential to be removed by a nucleophile. The different possible products are shown in

Figure 4.10.

63

*

*

*

*

*

H C C C C

H

H

H

Br

H

H

H

H

H

Figure 4.9: A structural representation of 2-bromobutane, which has five β-hydrogens,

indicated by *.

H C C C C

Hi

H

H

Br

Hiii

Hii

H

H

H

C C

H

H

H

CH2CH3

C C

H3C

H

CH3

H

C C

H3C

H

H

CH3

(i)

(ii)

(iii)

Figure 4.10: 1-butene (i), cis-2-butene (ii) and trans-2-butene (iii), all of which are

possible products in an elimination reaction where 2-bromobutane is the substrate.

In many cases, even though several elimination products may be theoretically possible,

only one is observed. This may be produced by a much more probable mechanism than

the pathways that would give rise to the other possible products.

The abstraction of a different β-hydrogen atom may result in the production of a

different elimination product. Depending upon the structure of the compound, there are

two types of isomer that can be formed; geometric (cis/trans or E/Z) isomers and

structural isomers. Figure 4.10 shows an example of geometric isomers ((ii) and (iii)).

The atoms are joined together in the same order with the double bond between the same

carbons. The difference is in the relative position of the groups on either side of the

double bond. Compound (i) is a structural isomer of both (ii) and (iii); there is a

structural difference between these compounds. In this case, the double bond in (i) is

between different carbons.

64

4.6 Substitution Vs Elimination

Comparing the substitution and elimination mechanisms represented in the preceding

sections shows some similarities between the SN1 and E1 models, and between SN2 and

E2 mechanisms. Formation of a carbocation is represented as the first step in both SN1

and E1 mechanisms. Both SN2 and E2 reactions are consistent with attack on the

substrate by the nucleophile.

Generally, substitution is the predominant reaction if both reactions are possible. The

substitution process involves less bond reorganisation than elimination and is more

energetically favourable (Pine, 1999, p. 471).

Several factors can influence the competition between substitution and elimination in a

reaction process. These include the nature of the substrate, the nature of the

nucleophile/Lewis base, the nature of the solvent and the reaction temperature

(McMurry, 2000, p. 408). Manipulation of these factors experimentally can vary the

proportions of the substitution and elimination products produced.

4.7 Intended Learning—Concepts and Skills

One of the aims of this research project was to identify how well students could

demonstrate their understandings of these skills and concepts (research question 2,

section 1.5.2).

For the purposes of this study, the required skills and concepts have been identified

through formal and informal discussion with lecturers and consideration of the

coursework and the textbook. These skills and concepts have been previously discussed

in the relevant sections in this chapter and are summarised in Appendix 4.1. Students’

mastery of these skills and concepts were probed through various tasks, which were

prepared for each group of students. Tasks are discussed in greater detail in section 6.7.

65

4.8 Summary

The discussion given in this chapter has indicated some of the complexities associated

with the subject matter, as well as outlining the intended understandings that students

were expected to gain after studying such a course. The subject matter description has

been limited to the specifics of substitution and elimination reactions and their

mechanisms. This study investigated students’ and lecturers’ understandings of these

two processes only.

The particularities of the subject matter have implications for the teaching strategies that

a lecturer chooses to use when teaching about a topic or course. Lecturers’

understandings of the topic and its difficulties and nuances influence the manner in

which they approach their teaching about it. Chapter 5 discusses pedagogical content

knowledge that is specific to the study of science, with particular reference to the topic

of reaction mechanisms in organic chemistry.

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5 Pedagogical Content Knowledge

5.1 Introduction

This chapter discusses the notion of pedagogical content knowledge (PCK); the

particular subject-based knowledge or understandings that give rise to skills to enable a

teacher or lecturer to develop ways to teach about a specific topic. The chapter includes

a discussion of PCK, incorporating a consideration of the importance of an

understanding of the difficulty (or perceived difficulty) of learning about the subject

matter being discussed in a general sense.

A consideration of the pedagogical content knowledge issues related to the complexity

of learning about science follows, with particular reference to the challenges associated

with teaching and learning about chemistry. The specific context-based issues that are

related to the teaching and learning about the subject of reaction mechanisms and their

representations are then discussed. The discussion relates to specific theoretical

educational issues that were raised in Chapter 2.

A particular model for considering the specific subject matter related to the study of

reaction mechanisms is then introduced. Finally, this chapter addresses some of the

issues that arise from a consideration of the pedagogical content knowledge that are

related to learning and teaching about reaction mechanisms.

5.2 Pedagogical Content Knowledge

Shulman (1986) commented on the manner in which teachers’ skills were examined and

reviewed in the 1870s and compared them to those of the 1980s. Over that hundred-

year period, the focus in these reviews had moved from a requirement for teachers to

exhibit a high level of subject matter understanding in the 1870s to requiring a

demonstration of good classroom management skills in the 1980s. According to

Shulman (1986), in the 1870s little attention was given in these reviews and

examinations to the how to teach aspect of being a teacher; the emphasis was on good

subject matter understanding. In the 1980s, teacher demonstration of subject matter

67

understanding had all but disappeared, with teacher reviews focussed on a teacher’s

ability to manage a classroom.

Do either of these skills alone guarantee a good teacher? Neither subject matter

understanding nor classroom skills take into account to particularities of teaching about

specific subject matter in a classroom situation. Shulman (1986) argued that being able

to teach a class well was not simply related to either exceptional subject matter

knowledge or excellent classroom management skills:

What we miss are questions about the content of the lessons taught, the questions asked,

and the explanations offered . . . Where do teacher explanations come from? How do

teachers decide what to teach, how to represent it, how to question students about it and

how to deal with the problems of misunderstanding?

Geddis (1993) claimed that the exceptional teacher possessed far more than just good

subject matter understanding or classroom management skills. He argued that such a

teacher needed skills that were particular to teaching about his or her specific field:

[t]he outstanding teacher is not simply a ‘teacher’, but rather a ‘history teacher’, ‘a

chemistry teacher’, or an ‘english teacher’. While in some sense there are generic teaching

skills, many of the pedagogical skills of the outstanding teacher are content specific.

Shulman (1986) asserted that the interaction between teaching practice and subject

matter understanding formed a specific type of understanding or knowledge,

pedagogical content knowledge; knowing how to teach about the specific subject

matter. According to Shulman (1987):

[pedagogical content knowledge] represents the blending of content and pedagogy into an

understanding of how particular topics, problems, or issues are organized, represented, and

adapted to the diverse interests and abilities of learners, and presented for instruction.

Shulman made reference to three important aspects in the consideration of pedagogical

content knowledge about a particular topic. He argued that for a lecturer or teacher to

develop the ability teach a specific subject well, he or she needed to consider:

1) the aspects of the topic that make it easy or difficult to learn, including

alternative conceptions that students may bring to the topic or may form as a

result of the course;

68

2) effective teaching strategies to help students construct appropriate

understandings and to demonstrate why previously held misconceptions are

inappropriate and should be modified or replaced;

3) many ways of representing the topic’s contents and ideas for students.

The pedagogical content knowledge related to a topic is specific to the collection of

ideas, concepts, relationships and representations that constitute that topic. The

particularities of the content that is to be taught (and learned and understood) influence

both the lecturers’ methods of teaching the topic and the students’ abilities to learn

about it. Geddis (1993) commented:

In order to be able to transform subject-matter content into a form accessible to students,

teachers need to know a multitude of particular things about the content that are relevant to

its teachability. This pedagogical content knowledge is a result of the interaction of content

and pedagogy. It is knowledge about the content that is derived from consideration of how

best to teach it.

There are many questions concerning pedagogical content knowledge that can be

addressed when considering teaching about particular subjects. What issues or

difficulties might be influenced by particular symbolism and specific language used by

lecturers and textbooks? Are there particular visualisation demands related to

developing understandings of the subject matter? Do students need understandings of

other topics (for example, mathematics or physics) to enable them to develop

understandings about a particular subject matter? What misconceptions about the topic

do students bring to their classes? Are there misconceptions that might arise out of the

course itself? What effective strategies can the lecturer engage in to assist the students

in their learning and to help them create meaningful understandings? How does the

lecturer transform his/her knowledge and understandings into a form that is teachable

and learnable? These types of questions must all be considered when contemplating the

PCK associated with teaching and learning about reaction mechanisms.

69

5.3 The Complexity of Teaching and Learning about Science

Teachers and researchers attest to the fact that science is hard to teach and to learn

(Carter and Brickhouse, 1989; Johnstone, 1991; Millar, 1991). Johnstone (2000)

commented upon the decreasing numbers of students who choose to study chemistry,

adding that ‘[f]or normal daily living, most people believe that they need no knowledge

of chemistry’. Indeed, people may consider chemistry to be too difficult to learn or to

understand, or that it has limited impact on and interest for their lives.

Millar (1991) suggested several features that he considers intrinsic to science that

contribute to its difficulty. Firstly, students often feel that they get little in the way of

reward for the struggle to understand science. Secondly, learning about science

involves a reconstruction of meanings that they might already hold or feel that they

understand. A third contribution is that there exists a tension between the nature of

science as concerning both knowledge (science is some sort of shared understanding)

and enquiry (science is finding out new things), which can be confusing to many

learners. Finally, learning about science involves learning about abstract ideas and

concepts to explain observable phenomenon, which can be difficult for students to begin

to make sense of.

Johnstone (1991) commented that ‘the fact that many pupils claim that science is hard

to learn might suggest that ‘it’ is not being successfully transmitted’ to students. He

discussed the challenges posed in teaching and learning about science in terms of four

aspects; the appropriateness of laboratory work that students do, the abstract nature of

the concepts that students are expected to learn about, the specific and difficult language

issues associated with talking about science, and the need for multilevel thought when

teaching and learning about science.

What commonalities exist between the difficulties suggested by Millar and Johnstone?

Both authors suggest that the complexity of learning about science can be related to the

abstract nature of the concepts that are taught. Science is intrinsically abstract, and

students can be required to learn about things that they have never seen or can never

hope to see. For example, in early high school, students are taught about atoms, which

will probably only ever be seen as a picture in a text. Students often have nothing real

70

or observable to link these abstract concepts to and have no way of knowing how these

scientific notions are related to the everyday world in which they live.

Compounding this abstract nature is the manner in which science can be talked about.

Millar (1991) suggested that ‘the difficulty lies in the ‘distance’ between the language

of science and the vernacular language’. In describing science as involving ‘logical

chains of argument, couched in abstract language’, Millar commented that this

unfamiliar language makes understanding or interpreting the logic of a scientific

argument a more challenging task for students, thereby increasing the perceived

difficulty of the topic.

Irrespective of their fields, scientists can often talk to each other using words that mean

nothing to many non-scientists because, like most fields of study, science has its own

jargon. Two chemists might talk to each other about kinetic stability and reactive

intermediates and make sense of the conversation. Students may not have this same

understanding of the scientific language being used by their lecturers. The task of

understanding and comprehending the chemistry that their lecturer is discussing or

attempting to explain is made more difficult by the use of what is essentially a foreign

language to students. There is also the possibility that the use of this foreign language

(or a student’s inappropriate translation of such a language) might lead to students

constructing poor or inappropriate understandings about science topics.

In conducting a study to investigate the effect that such a translation factor had on

students’ abilities to perform particular tasks, Johnstone and Selepeng (2001)

investigated the effects that language translation had on students’ abilities to write down

sequences of numbers in forward and reverse order. It was found that when English-

speaking students were given the number strings in English, they could manipulate

number strings of, on average, six numbers. When the students were given the same

test in French, a language they had all been studying for three years, the average number

string dropped to just over four, an average decrease of 25 %.

Johnstone and Selepeng discussed this decrease in recall ability in terms of the memory

working space available to the students. This working space is part of an information

processing model (Johnstone, 1997) that has been discussed previously (section 2.5).

Students could remember longer English number strings because they weren’t taking up

71

space in their working memory translating the French numbers into English first. Their

available working space appeared to be decreased by the translation requirement. This

observed translation difficulty is equally true for students struggling to understand

unfamiliar science words, such as reaction kinetics, wavelength or enthalpy.

Learning about science requires students to process and filter a lot of new information

and to construct useful understandings from information that is often presented in a

different manner than more everyday concepts. People form understandings of

everyday concepts by observing examples and non-examples, by identifying common

factors and similarities, by experimenting and by trial and error (Johnstone, 2000). For

example, a child might learn that sugar is sweet and salt is sour by tasting the two, or

learn that plants require water to survive after watching unwatered plants die.

These types of observations and experimentation may not be possible when learning

about some facets of science, due, in part, to the abstract nature of the concepts to be

learned and understood (Johnstone, 2000) and the models that are commonly used in

science classes to explain them. A student cannot learn that a hydrogen atom contains

one proton and one electron by looking at an atom of hydrogen or by experimenting

with a sample of colourless, odourless hydrogen gas. Learning about science, making

sense of the concepts that are being taught in a classroom, can be a lot more difficult

than learning about more everyday ideas and notions because much of science

knowledge is not tangible.

Teaching and learning about science can be made more challenging by the many

different models that are used in scientific explanation and rationalisation. Models are

used as a way of thinking about and attempting to explain the observable world. For

example, the observed reaction between sodium metal and water can be modelled using

an equation. Solid sodium chloride dissolving in water can also be modelled using an

equation, or also as a many-particle representation, such as that simulated by Tasker, et

al (1996, 1997). Models are also an important aspect of communicating about science,

allowing the reduction of complex notions to enable students and lecturers to

concentrate on key aspects of the phenomenon (Gilbert, 1993, p. 5). However, it is

important for students and lecturers to recognise and appreciate the limitations of these

72

representations: ‘any formalism has a formal aspect, it is by necessity no more than an

approximation, a travesty of reality’ (Laszlo, 2002).

5.4 The Complexity of Teaching and Learning About Chemistry

All of the factors described in the previous section affect the complexity involved in

teaching and learning about chemistry. Other aspects that are particular to the study of

chemistry, such as the use of structural representations and equations and specific

chemistry language, can also complicate the teaching and learning about particular

topics within the study of chemistry. These aspects are discussed in the following

sections.

5.4.1 The Abstract Nature of Chemistry

Gabel (1999) stated that many of the concepts that students are expected to learn in

chemistry are inexplicable without their teacher using some type of model. This same

point extends to many experiments in which students make observations about a

reaction or process that is explained to them in terms of molecules or particles.

Johnstone (1982) described these types of explanations as micro (or microscopic)

explanations. Gabel added:

In order to understand the microscopic level, a person must be capable of associating

particles with models or analogies. Likewise, the model must be associated with symbols.

The abstract nature of chemistry can make it a challenging topic to learn about for a

variety of reasons. Often, students will be taught about particular reactions or

phenomena using only models such as molecular-level explanations of particle

interactions, reactions and reaction mechanisms. In many cases, students may not even

see a particular reaction occur. Rather, the process is described in a book or shown on a

blackboard with no clear linkage developed between an observable process and the

often highly symbolic explanation provided. There can sometimes be little indication

what the real reaction might look like (the reaction might be a solid dissolving in a

colourless liquid, or two coloured liquids mixing to form a solid) just the equation or

73

explanation that generally discusses the interaction between individual (invisible to the

eye) particles in a reaction.

Chemistry students can be required to look beyond the physical substance or reaction

that they see in a laboratory situation and to learn explanations and build understandings

based upon things that cannot be seen. To use an example relevant to the research

reported in this thesis, students might prepare 1-bromobutane in the laboratory as a part

of learning about nucleophilic substitution reactions (described in Section 4.4). This

compound can be prepared by adding 1-butanol (a colourless liquid) and hydrobromic

acid (a pale brown liquid) together in a flask and heating them with sulfuric acid. After

heating the reactants for thirty minutes, the coloured liquid is distilled, separated from

an aqueous layer and dried. The product, 1-bromobutane, is also a colourless liquid

with no apparent odour. In the classroom, discussing nucleophilic substitution

reactions, this preparation is likely to be represented as either (or both) an equation or a

reaction mechanism. Neither the equation nor the reaction mechanism shows the

student anything that represents the physical process of mixing and heating two liquids,

then extracting a colourless liquid from the mix. Students are expected to make this

link between the physical reaction and the models used to represent the process.

Forming understandings of the link between physical reactions and models used to

represent them can be made more difficult if the reaction itself shows no visible sign of

any process occurring. The process of mixing a very dilute solution of hydrochloric

acid and a dilute solution of sodium hydroxide involves combining one colourless liquid

with another colourless liquid. There is little indication of reaction—no colour change,

no evolution of gases and no real evolution of heat if the solutions are dilute enough.

Reaction does occur but can only be inferred from the observation (using a pH probe or

coloured indicator) that the pH has changed. In this case, where no physical evidence of

reaction might be present, students must nevertheless make sensible links between the

observable reaction and the model (equation, reaction mechanism, other representation)

that is used to rationalise the process.

Johnstone (1991) commented on the above point, arguing that ‘there is no immediate

sensory way to get at’ some scientific ideas, for example, the central notion of the

element. An element is described as ‘[a] substance which cannot be further divided by

74

chemical methods . . . An element is defined by its atomic number’ (Sharp, 1990, p.

154). Exactly what does this mean to a student? Yellow powdered sulfur may be

shown as an example of an element. So might a lump of lead, a vial of oxygen gas or

liquid mercury. What is the commonality between these different things that makes

them all elements? Johnstone (1991) maintained that the distinguishing features of an

element ‘exist only in the mind’ of those who created the explanations. We can’t see

atoms. Students can’t tell simply by looking at yellow sulfur or liquid mercury that

both consist only of atoms of one type and that neither can be divided into smaller

particles by chemical means.

5.4.2 Macroscopic, Microscopic and Symbolic Representations

Chemists often try to explain observable facts by talking about what particles within a

reaction mixture are doing. The structural representations that chemists draw in

equations and mechanisms are pictorial attempts to model the atomic composition and

connectivity of molecules in a compound or provide some type of information about

molecular shape, size or spatial orientation. Although the particles in a reaction mixture

are moving, vibrating or rotating in space and colliding with each other or the walls of a

reaction vessel, the structural representations that chemists draw are pictorial attempts to

model the atomic composition and connectivity of molecules in a compound or provide

some type of information about molecular shape, size or spatial orientation. Two

dimensional on paper representations generally cannot show a lot about particle

movement.

Chemists may describe the presence of different types of particles in their explanations,

from macromolecules to atoms or electrons. These particles may be shown in an

equation, as structural representations or as formulas. In some cases, such as in reaction

mechanisms, interaction between two or more particles might be represented (Figure

4.4, 4.5, 4.7 and 4.8 represent different reaction mechanisms). Often, there is no

obvious link between the model used to explain an observed phenomenon and the actual

reaction process itself. These are two different levels of explanation or understanding.

Johnstone (1982) refers to these levels as the macro and the micro level, later renaming

the second level sub-micro (1991).

75

The macroscopic level refers to the bulk properties of a given system; those

characteristics that we can perceive or measure. These are generally visible properties;

colour, shape, texture, odour, density, solubility (Johnstone, 2000). Examples of

macroscopic discussion include: freezing water to make ice; water and ethanol mixing

together; the rate of a given reaction is proportional to the concentration of only one of

its starting materials. In explaining or rationalising when teaching macroscopic

observations, scientists often use language that is applicable to the sub-microscopic

level.

Hoffmann and Laszlo (1991) stated that, as a ‘mature science’, chemistry has shed its

‘childhood habit’ of describing compounds only by their visual, macroscopic properties.

These authors made the point that while chemicals used to be given names and

descriptions that were consistent with their observable properties, this does not happen

now. They argue that:

[chemistry] has become a microscopic science. Explanations nowadays go routinely . . .

from the microscopic scale to the observable: from the way the electrons are distributed in a

dye molecule to its colour; . . . from bond energies to the superior tensile strength of

Kevlar.

An explanation at the sub-microscopic level rationalises the observed (or observable)

system in terms of particles: molecules, atoms and ions. These descriptions must be

consistent with known experimental data. This explanatory, particulate level is not

necessarily one that we can see or measure. Sub-microscopic explanations often

involve the use of models that scientists have developed that are useful and appropriate

for rationalising observable properties. For example, chemists rationalise the

macroscopic observations about water freezing, a water/ethanol mixture and reaction

rates using appropriate models. The presence of hydrogen bonding between molecules

of water and molecules of ethanol could be part of an explanation of the miscibility of

these two liquids. The rate dependence of a reaction whose rate is proportional only to

the concentration of one reactant might be explained by saying that one of two reaction

steps in a process is rate determining and that this rate determining step only involves

molecules of one reactant.

76

The macroscopic and sub-microscopic examples given here are descriptive and word-

based explanations. Chemists also commonly use a completely separate language, the

language of representations, to help describe and rationalise the macroscopic and sub-

microscopic processes. Both the word-based and the pictorial symbolic representations

can be considered as interacting with the macro and the sub-micro levels of explanation

(Figure 5.1).

Figure 5.1: Johnstone’s triangle, representing the interaction between the macroscopic,

sub-microscopic and symbolic levels.

These representations can take many different forms; for example, words, equations,

formulae, graphs, structural representations and diagrams. Some examples are shown in

Figure 5.2.

R X R X+RDS

YRYR +

(i) (ii) (iii)

Figure 5.2: Symbolic representations of (i) diatomic oxygen molecule, (ii) equations

representing a two-step reaction process whose first step is rate determining (RDS), and

(iii) a graphical representation of the process represented in (ii).

O O

Symbolics

Macro

Sub-micro

E

77

This highly symbolic nature of chemistry communication is one aspect of its language.

The representational language is discussed in greater detail in section 5.4.3. In addition

to this language of symbols and representations is a written and spoken language whose

words and terms are often specific to the teaching and learning of chemistry. It is this

written and spoken language that chemists use to talk about chemistry’s abstract

explanations, as well as how they describe the use of symbols, structures and

representations in chemistry to students.

While Johnstone’s levels of chemistry are a useful way to think about pedagogical

content issues related to the study of reaction mechanisms, are they enough? Students

studying about reaction mechanisms will see some experiments as macroscopic

examples of reaction processes, to which they will apply their sub-microscopic models

of mechanisms to explain reaction outcomes. In addition, many of the explanations and

rationalisations about which students are taught consider what is going on between

particles in the reaction mixture—this is also a sub-microscopic consideration.

However, reaction mechanisms can consider a third level—what is going on within

individual molecules—and we need to consider a different model of levels of chemistry.

In a paper directed at teachers of chemistry, Jensen (1998) extended Johnstone’s

multilevel notion when he asked, ‘Does Chemistry have a logical structure?’ His

interest was if logical relationships existed between the concepts and models that are

discussed in most chemistry courses. He suggested a classification scheme to categorise

the information that is presented in a general chemistry text (Table 5.1).

Table 5.1: Jensen’s classification structure model (1998).

Composition and

Structure Dimension

Energy Dimension Time Dimension

Molar

Level

Molecular

Level

Electronic

Level

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Jensen’s molar classification is analogous with Johnstone’s macroscopic level.

Similarly, Jensen’s molecular and Johnstone’s sub-micro levels can be considered to

correspond. Unlike Johnstone, however, Jensen suggests the possibility of a third level

of classification, the electrical level. This classification takes into account electronic

interaction within molecules, which is an important consideration in teaching and

learning about reaction mechanisms, for example, the movement of electrons when

bonds break and form.

Jensen’s classification scheme also suggests three dimensions, which take into account

three different aspects of chemical concepts; those related to structure, energy and time,

all of which are important considerations in mechanistic chemistry. This is discussed in

more detail in section 5.7.

5.4.3 The Language Used in Chemistry

Each field of science has its own particular language that is used to talk about and

describe processes, understandings and accepted explanations. Furthermore, science

lecturers use words with other common meanings to describe something very different

to the common meaning within the context of their particular course (Cassels and

Johnstone, 1983).

Chemistry is no different. Its language ranges from terms used in everyday life (naked

flame) to terms specific to the discipline (stereochemical inversion, a term used when

describing SN2 reaction mechanisms, as described in section 4.4.2). As with any

foreign language, there is a translation aspect involved for students learning about

chemistry. Students need to turn the chemistry words into a form that makes sense to

them so that they can begin to understand the content being discussed. As students

become more proficient at understanding the chemistry language, their need to translate

can decrease. Translation may eventually become an automatic process.

One form of language used a good deal in chemistry is the language of structural

representations. Gabel (1999) suggested that the ‘primary barrier’ to understanding

chemistry is the subject is often taught predominantly at a very abstract, symbolic level.

She added that ‘even the symbols that chemists use can be interpreted in several ways’.

Fe can be used when representing one atom of iron, or to indicate an entire sheet of the

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metal. Hoffmann and Laszlo (1991) observed that ‘[c]hemical structures are among the

trademarks of our profession’. Open any chemistry text and you will see a variety of

structural representations used in explanations and rationalisations of many different

observations.

Chemists use structural representations and formulae to communicate about

compounds. Ethanol, for example, is a colourless, pleasant-smelling liquid with a

boiling point of 78.3oC that is miscible with water. Each molecule of ethanol is

believed to contain two carbon atoms, six hydrogen atoms and an oxygen atom joined

together in a specific arrangement. Chemists represent molecules of ethanol in a variety

of symbolic ways (Figure 5.3), none of which bear any resemblance to the actual

appearance of the colourless liquid ethanol.

CH3CH2OH

(iv)

C2H6O

(i) (ii) (iii) (v)

Figure 5.3: Some structural representations of ethanol: (i) ball and stick model, (ii)

space filling model, (iii) square planar structure, (iv) structural formula and (v)

molecular formula.

Each of these representations tells a chemist something different about a molecule of

ethanol. The first four structures are unambiguous representations of ethanol. The

molecular formula (v) is not unique to ethanol; it also represents dimethyl ether, which

can be represented as CH3OCH3, a flammable gas with a boiling point of –25oC (Lide,

1991, p. 3-322). Chemists may recognise the possible ambiguity of representation (v),

whereas students may not understand enough of the language of chemistry to appreciate

H C

H

H

C

H

H

OH

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the problem with this type of representation. The possibility of ambiguities in structural

representations increases with the complexity of the structure being represented.

The use of structural representations can cause difficulty in teaching and learning about

chemistry, but it is a necessary part of communication between chemists. Hoffmann (in

Hoffmann and Torrence, 1993, p. 61 – 6) told of a letter he had written to a fellow

chemist, Margarita Rybinskaya, concerning the synthesis of an organometallic

compound consisting of an osmium atom ‘sandwiched’ between two cyclopentadienyl

rings. Rybinskaya and her co-workers (1989) had already synthesised a compound with

a single carbonium ion ((i) in Figure 5.4) and Hoffmann suggested synthesising a

dication, a compound with two carbonium ions. Rybinskaya and her co-workers

endeavoured to synthesise this dication, while Hoffmann and his colleagues worked

through theoretical calculations. When the synthesis was completed, the two chemists

shared their results; Hoffmann’s theoretical calculations and experimental results from

Rybinskaya’s synthesised compound. It was only then that these two chemists

discovered that they had not been discussing the same compound.

(i) (ii) (iii)

Figure 5.4: Compound (i) has been prepared by Rybinskaya and co-workers and

contained a single cation. The charge associated with the cation is represented on the

top left-hand side of the diagram. Representations (ii) and (iii) are both dications.

These are structural isomers that are related to (i). The images are scanned from

Hoffmann and Torrence (1993, p. 62 – 6).

Both Hoffmann and Rybinskaya had been talking about making a dication, but neither

had ever sketched their target structure for the other. There is more than one dication

that is related to compound (i), as Figure 5.4 shows. Hoffmann had carried out

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calculations for the molecule represented as (ii) in Figure 5.4, while Rybinskaya had

prepared a different compound, represented as (iii). Hoffmann concluded, ‘In my

earlier letter, I had violated a basic chemical principle: I failed to supplement my words

with a chemical structure’.

Chemists often use this structural language to talk to each other, just as they use spoken

and written language, which can also be specific to their topic. In some cases, such as

the example mentioned above, words may not be sufficient to communicate a message

even to a fellow chemist. Sometimes, structural representations are a central aspect of

an explanation or rationalisation. Sometimes, the nature of the topic to be discussed or

the question to be answered demands the use of structural representations to

communicate effectively. The discussion and explanation of reaction mechanisms is an

example of a topic where the structural language plays a very important part in its

representation and discussion.

5.5 The Complexity of Teaching and Learning About Reaction

Mechanisms

5.5.1 Language and Representations Used in Reaction Mechanisms

There are many words that are used in topics covering reaction mechanisms in organic

chemistry that are sometimes difficult for students to understand. Some examples of

such words are found in the following passage, describing the key ideas of the SN2

process (McMurry, 2000, p. 390):

At this point, we have two important pieces of information about the nature of nucleophilic

substitution reaction on primary and secondary alkyl halides and tosylates:

• The reactions occur with inversion of stereochemistry at the carbon atom.

• The reactions show second-order kinetics, with the rate law:

Rate = k x [RX] x [Nu:-]

This text is given as an introduction to SN2 reaction processes. These points are drawn

from experimental results that have been discussed previously in the chapter and are

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followed by a more specific description of the reaction process. Even in this relatively

short passage, however, there are words and phrases that may cause difficulty for

students. Some words, such as primary and secondary, are everyday words used in a

chemical context. Others, such as tosylates and nucleophilic, are part of a specific

science language.

In addition to the difficulty posed by understanding the words themselves, the

translation of the chemistry language into a more understandable form can also be a

challenging process. If students need to translate these words to give them some

meaning, the working space or memory available for processing any new information

can be restricted by these translation demands (Johnstone, 1997, Johnstone and

Selepeng, 2001).

Representations of reaction mechanisms are highly symbolic, utilising appropriate

structural representations, arrows, charges and graphs. The diagram in Figure 5.5, from

the Chemistry 2XX text (McMurry, 2000, p. 391), shows an example of a pictorial

representation of an SN2 type of reaction process. Similar representations can be found

in many other organic chemistry texts.

Figure 5.5: A scan of an description of reaction processes, including structural

representations, taken from McMurry (2000, p. 391).

83

The diagram in Figure 5.5 shows three structures, representing the starting material

((S)-2-bromobutane), the transition state and the product ((R)-2-butanol). Each

structure is represented with bold and dashed bonds in an attempt to indicate the three-

dimensional shape of the structure. Arrows are shown on the first representation, which

students need to interpret as electron movement to break and form bonds. To

understand these illustrations, students need to be able to follow both the written words

and the symbolic representations, each of which are describing reaction processes in

terms of movement of atoms and electrons.

The language used in the accompanying explanations in Figure 5.5 is quite specific and

focuses only on the successful interaction between one molecule of (S)-2-bromobutane

and one hydroxide ion. These descriptions may imply to students that the represented

structures shown are real, exact pictures of what is going on in a reaction mixture. In

addition, such phrases as ‘the nucleophile –OH uses its lone pair of electrons to attack’

might indicate some sense that nucleophile molecules know what they’re supposed to do

in a given reaction process. Finally, focussing on the interaction between single

molecules of the two reactants may lead students to consider only individual particles in

a reaction process, ignoring the fact that many particles are involved in any one reaction

process and that not all particles undergo successful collision resulting in reaction.

Explanations such as that shown in Figure 5.5 often give little indication that a

representation of a reaction mechanism is an explanation based upon experimental data.

It is important for students and lecturers to be aware that the proposed reaction

mechanism is the result of a hypothesis resulting from a great deal of laboratory

experimentation. These mechanistic representations can be useful to help chemists

think about the processes that may be going on in a reaction vessel, but they are simply

a model of a process that experimental evidence suggests might be occurring, and are

not the real reaction. Laszlo (2002) made the point that:

[S]tudy of any reaction mechanism is experimental in work. It is hard work, using a large

number of diverse tools . . . [A]fter we have done all this work, which may take months,

sometimes years, we allow ourselves to couch the results in the language of reaction

mechanisms . . . [we] will make the findings understandable at a glance. But, before

making use of the formalism, we conducted a study of reality. The experimental work was

antecedent upon its description, it had to be.

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The type of structural abbreviation shown in Figure 5.5 does have its benefits.

Representations of these types are used to write reaction mechanisms for a reason. The

structure is relatively easy to draw (and for students to copy quickly) and it directs a

reader’s attention onto the C at the centre of the representation. This indicates the

important or interesting part of the represented molecule; the C—Br, focussing on the

portion of the particle that will undergo reaction.

These symbolic representations are models of reality, not real indications of shape, size

or movement. In a text, a reaction mechanism might show the interactions between two

equally sized molecules, drawn with an appropriate orientation to each other, an array of

curly arrows represented between certain areas of the molecules. This does not mean

that in reality the molecules are static, similarly sized particles that know they have to

interact in a certain way. Specific structural representations are chosen when

representing different types of reaction mechanisms; parts of a molecule may be

highlighted, others may be ignored.

5.5.2 Literature Review of Previous Research

5.5.2.1 Alternative Conceptions

There exists a large body of research concerned with students’ alternative conceptions

and misconceptions in science (for example, Cros, 1988; Fensham, 1992; Garnett,

Garnett and Hackling, 1995). According to Shulman (1986), an important part of

pedagagogical content knowledge is that a teacher needs to know about the ‘conceptions

and preconceptions’ that students hold concerning specific topics. He added:

If those preconceptions are misconceptions, which they so often are, teachers need

knowledge of the strategies most likely to be fruitful in reorganising the understandings of

learners, because those learners are unlikely to appear before them as blank slates.

What common conceptions do students bring to the study of mechanisms? For the

majority of first year students (as well as some second year students), reaction

mechanisms is a new topic that they have never formally covered, so their

preconceptions may be linked to alternative understandings of representations used in

reaction mechanisms. These representations, for example, curly arrows, structural

representations and chemical equations, might be similar to something they have seen in

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another context, leading to understandings that are not appropriate in the context of

reaction mechanisms.

While much research concerning students’ alternative conceptions and misconceptions

has been carried out, there is little, if any, research that addresses students’

misconceptions or alternative conceptions concerning mechanisms and their

representations. Authors (Eckert, 1998; Weeks, 1998, p. vii) have commented on the

difficulty that students perceive when using mechanistic symbolism such as curly

arrows. However, there is little research to suggest that students do hold alternative

conceptions about the meaning implied by these representations. Research has also

implied that students may hold poorer understandings of some structural representations

(Bodner and Domin, 2000; Chiu and Fu, 1993).

5.5.2.2 Representational Difficulties

Representational difficulties may cause students to avoid using curly arrows in their

mechanistic representations, or they may use them incorrectly, for example, pointing

them in a direction that is not consistent with electron movement. Eckert (1998)

suggested that students’ difficulties with this representation might arise from the

inconsistencies in where curly arrows are commonly pointed. Curly arrows can be

pointed at either atoms or bonds, depending upon the type of reaction process that is

being represented.

McNelis (1998) commented that the most common question students ask concerning

reaction mechanisms is ‘Why did that happen?’ An appreciation of the chemical

reactivity of particular processes is often something that students are lacking. This lack

of appreciation can lead to students representing mechanisms that correctly utilise curly

arrows and structures, but that may not be chemically sensible.

5.6 Implications of Pedagogical Content Knowledge

The study of reaction mechanisms involves its own language, conventions and

representations. The particularities of the subject matter and an understanding of how

this can impinge upon its learnability can influence the challenges that students might

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face in their attempts to understand the subject matter. This understanding should

therefore influence how lecturers decide to teach about it. When considering the

teaching of reaction mechanisms in organic chemistry, there are several implications of

the pedagogical content knowledge discussed in this chapter.

An important consideration in teaching about mechanisms and their representations are

the reasons why the subject is being covered. How are reaction mechanisms useful for

students studying organic chemistry? What sorts of understandings does the lecturer

intend the students to gain from learning the material? Do the students understand and

appreciate why reaction mechanisms are useful in their study of organic chemistry?

Structural representations are a language that is specific to chemistry and students may

not know enough about their use to appreciate why a lecturer uses one particular type in

his representation of a particular reaction process. Students’ understandings of the

usefulness and appropriateness of specific structural representations are likely to be very

different to the understandings that their lecturers hold. Apart from this difference,

might the use of specific structural representations cue students into thinking about

specific types of reaction processes? If students are interpreting a particular structural

representation used in a specific question to suggest a reaction process, is this

appropriate for the level of understanding that their lecturer wants them to achieve?

There is the added complexity related to the fact that chemistry can be talked about on

more than one level. In a lecture, when the teacher moves between a discussion of a

reaction’s molar (macroscopic) properties to a molecular (sub-microscopic) explanation

of these observed properties, he or she understands that he or she is talking on two

different levels, one of which is real and one of which is imaginary or explanatory.

Students may never have seen the macroscopic process that the lecturer is describing

and may not have a good understanding of the relationship between this observable

process and the explanation that their lecturer is describing. They may not even

appreciate that the explanation is not real, but simply an appropriate model that fits all

the available data about the real process.

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5.7 An Integrated Model Guiding the Research

The dimensions of composition and structure, energy and time in Jensen’s classification

structure (Table 5.1) are particularly applicable to the topic of reaction mechanisms.

Experimental evidence based upon reaction rates (time) has been used to suggest

different types of reaction mechanisms that occur (sections 4.4 and 4.5). Energy

considerations can often be discussed in terms of stability of intermediates (section

4.4.3) or the likelihood of one product being produced in greater quantities in a given

reaction than another possible product (section 4.6). These are all aspects that are

discussed to some level in first and second year organic chemistry courses, such as

those described in this thesis.

The classification table is not entirely suited to consider aspects within the topic of

reaction mechanisms. In a representation of an SN1 mechanism (Figure 4.4, p 57), the

curly arrows shown are representing the movement of electrons to break and form

bonds, an electronic type of representation. However, a molecular representation is also

shown, as the interactions represented are between two different types of reactant

particles. The symbolic representational level is also not incorporated into this model.

In mechanistic chemistry, the distinction between the molecular and electrical levels is

not always clear. Representations of single molecules often describe electrical

characteristics such as positions of lone pairs on atoms or bonds between atoms. Many

electrical aspects of the topic can also be considered as molecular level representations.

For example, an interaction between carbocations and nucleophiles (represented in

Figure 5.6) could be classed as both a molecular and an electrical aspect of the topic

and discussed on both these levels. This interaction can be described as molecular

because it involves the representation of individual molecules of particular chemicals.

It can be considered an electrical aspect of the topic because the representation includes

the use of a curly arrow, which is a symbolic representation of electron movement

between molecules as reaction occurs.

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CH3

C

CH3H3C

C N

Figure 5.6: Representation of a mechanism for the reaction between tertiary butyl

cations and cyanide ions.

To remove any possible confusion, it is suggested that the term intramolecular be used

in the place of electrical in attempting to classify aspects of the topic of reaction

mechanisms. The intramolecular level allows for the classification of aspects of the

topic that are concerned with electron and nucleus positions within molecules.

As this research project was developed, it became apparent that a consideration of a

molecular level was too limited for the topic of reaction mechanisms. It was felt that

the molecular level would be more appropriate if it were divided into two sub-levels:

the single particle level and the multiple particle level. Reaction processes are the

result of interactions between multiple reaction particles. Often, these are simplified

into single particle representations, leading students to construct understandings built

upon (erroneous) perceptions that only successful single particle interactions occur in

particular reactions.

The inclusion of these sub-levels into the adapted Jensen framework leads to a four-

level classification scheme (Table 5.2) that is suitable to describe the levels at which

reaction mechanisms can be taught about and learned about. The three dimensions

(structure and composition, energy and time) remain unchanged, as it is felt that these

already cover the possible levels that reaction mechanisms can be taught and learned

about.

Representations of reaction mechanisms are symbolic interpretations at the molecular

and intramolecular levels of phenomena observed on a molar level. Each of these levels

and dimensions can be modelled using a variety of symbolic representations. In this

type of classification system, the symbolic level overarches the four other levels, as

represented in Figure 5.7.

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Table 5.2: A four-level classification table extension of Jensen’s model. Aspects of the

topic of reaction mechanisms have been incorporated into this classification system.

Composition and

Structure Dimension

Energy

Dimension

Time Dimension

Molar Level Descriptions of reactions;

observed optical activity

Experimentally

observed stability of

carbocations

Rates of reaction

Molecular

Level: Multi-

Particle

More than one reaction

product, racemic mixtures

Competing reactions The frequency and

probability of effective

collisions

Molecular

Level: Single

Particle

Structure of reactants,

stereochemical inversion,

chirality

Energy profile

diagrams

Representations of rate

determining steps

Intramolecular

Level

Bond positions,

stereocentres

Carbocation

stability in terms of

structure

Transition states and

intermediates

Figure 5.7: A framework based upon Jensen’s levels of chemistry model and

Johnstone’s multilevel thought construct.

The overarching symbolic level provides chemists with a manner in which to talk about

mechanisms and their representations. These symbolic representations can, however, be

describing one of several different levels of mechanistic chemistry; molar, multiple-

particle molecular, single particle molecular and intramolecular; and it is important that

Molar

Single particle

Intramolecular

Symbolic

Multi-particle

90

lecturers are aware that their students do not necessarily have the same facility in

switching between the various levels and dimensions as do their lecturers. The nature

of the model shown in Figure 5.7 indicates that understanding of particular aspects of

the topic of reaction mechanisms will be enhanced by students’ abilities to operate in

the most appropriate level of this framework.

5.8 Summary

Pedagogical content knowledge involves knowing not only subject matter, or generic

teaching skills, but also understanding specific aspects about the teaching of a particular

topic. This incorporates an understanding of what makes the topic easy or difficult to

learn, the types of alternative conceptions that students may hold when they commence

their study of the topic and ways to assist the students to construct useful and

appropriate understandings of the topic.

Chemistry is a complex subject that presents challenges to both the teacher and the

learner. The complexity of the discipline can be linked to the abstract nature of the

concepts and ideas that are involved and to the multiples levels at which the material

can be observed, explained and represented. Johnstone (1982, 1991) suggested a

triangular model, which incorporated the macroscopic and microscopic aspects of

science, together with the symbolic methods of representing such information. Jensen

(1998) presented a three-level table for the classification of chemistry concepts, based

upon considerations of the molar, molecular and electrical aspects and their relationship

to the dimensions of structure and composition, energy and time.

The topic of reaction mechanisms can be classified according to a model that is a

combination of Johnstone’s triangle and Jensen’s table. The researcher designed this

integrated model as a tool for guiding the research. This model incorporates four levels;

molar, molecular multi-particle, molecular single particle and intramolecular; all of

which can be considered according to Jensen’s three dimensions of composition and

structure, energy and time. A symbolic level of representation that overarches all four

of these levels is the language that many chemists use when talking about their subject

matter.

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6 Methodology

6.1 Introduction

This chapter, which describes the research methodology that was used to carry out the

research, includes a review of relevant literature, as well as descriptions of the tasks

prepared. The research methods that were employed in this study were consistent with

a qualitative study and included interviews, focus groups, questionnaires and

examination and test questions. Oral and written data were collected, coded and

documented. As the student participants were at different educational levels and

studying different courses, tasks were designed to be appropriate for students in these

different classes.

6.2 Research Methodology

6.2.1 Qualitative Research

Qualitative research involves the detailed documentation of events in a particular

situation. This detailed documentation, particularly of the research setting and the

participants, is often described as providing thick descriptions of the research context.

One of the aims of this type of research is to identify the meanings that particular events

hold for both those who participate in them and those who observe them (Erickson,

1998). In an educational sense, this type of approach is appropriate to help researchers:

• [gather] detailed information about implementation;

• to identify the nuances of subjective understanding that motivates various participants in a

setting;

• to understand and identify change over time.

This research is ‘a field of inquiry in its own right’ (Denzin and Lincoln, 1994, p. 2):

Qualitative research is multimethod in focus, involving an interpretive, naturalistic

approach to its subject matter. This means that qualitative researchers study things in their

natural settings, attempting to make sense of, or interpret, phenomena in terms of the

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meanings people bring to them. Qualitative research involves the studied use and collection

of a variety of empirical materials—case study, personal experience, introspective, life

story, interview, observational, historical, interactional, and visual texts—that describe

routine and problematic moments and meanings in individuals’ lives. Accordingly,

qualitative researchers deploy a wide range of interconnected methods, hoping always to

get a better fix on the subject matter at hand.

Phelps (1994) wrote about qualitative research as an ‘established methodology

appropriate for studying the dynamic classroom situation’. She commented that a clear

understanding of what is currently being done in the classroom is necessary when trying

to study and improve the teaching and learning of chemistry. Qualitative research is,

according to Phelps, inductive, beginning ‘with a set of observed phenomena from

which patterns are identified and a theory developed’. Qualitative researchers tend to

see reality as subjective and truth as being constructed in the mind of each individual.

Analysing qualitative data is a continuous and on-going process. Due to the nature of

data analysis, which is being carried out at the same time as data are being collected, the

researcher’s understandings and interpretations of data are subject to change as the

study progresses (Phelps, 1994). Qualitative data are often reported in narrative form,

with interpretation and analysis of the data.

The qualitative research approach is a useful method for studying teaching and learning

situations, but it does have its limitations. One of the major criticisms levelled at this

type of research is that it is difficult to achieve results that are valid and reliable (Burns,

1998, p. 13). Being often narrative, not numerical, ascertaining the trustworthiness and

generalisability of qualitative data is not a simple task.

Another difficulty with these types of investigation is the time-intensive aspects of such

studies, which can run for several years (Burns, 1998, p. 13). Repeating or attempting

to reproduce a qualitative study would require a researcher to devote large quantities of

time. However, not only is the research itself time consuming, data analysis and

interpretation is also very time-intensive, which can increase the difficulty of attempting

to reproduce a qualitative study.

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6.2.2 Sampling

Erlandson, et al. (1993, p. 82-3) commented that there are two decisions that a

researcher must make in his or her sampling technique; who and what to study, and who

and what not to study. Lincoln and Guba (1985, p. 201) believed that in qualitative,

naturalistic research, the ‘purpose of sampling will most often be to include as much

information as possible’.

The courses that were studied were self-selected by the lecturers’ willingness to have

classes observed and to participate in interviews. The degree to which each course (and

its students) was investigated was determined as the study progressed. For example,

discussion with a former secondary school chemistry teacher identified some possible

alternative perceptions that students might hold, leading to a wide-scale questionnaire of

first year students in all courses.

The sizes of the chemistry classes observed for this project were such that it was

impossible to use some research methods on all students. For example, it was not

feasible to interview all of the approximately 120 students in the Chemistry 100 course.

Apart from the logistical difficulty associated with interviewing and analysing

transcripts for over one hundred students, it was anticipated that not all of them would

opt to participate in the study.

Where possible, the understandings of all students in the classes were investigated,

using such tools as questionnaires or examination questions. Students who did not wish

to participate in the questionnaires were informed that there was no compulsion to do

so. Although the examination question was a compulsory question, it was designed in

conjunction with the lecturer to serve his aims as an indication of the students’

understandings of a particular concept. The fact that the researcher could use this

question to gather data was secondary to that.

There was no limit set on the number of participants who could take part in the study.

However, there were never more volunteers in a particular year group than could be

successfully managed by the researcher, so all volunteers were asked to participate in

the study. Some volunteers did not participate in the study for personal reasons. The

majority of students who volunteered to participate were interviewed at least once.

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6.2.3 Ethical Considerations

Due to the participation of both students and lecturers in this study, there are many

ethical considerations that must be taken into account. Participants (or potential

participants) must be well informed about the research project, so that they can make an

informed decision about whether or not to participate. Consequently, it is necessary for

the researcher to anticipate how data collection, analysis and interpretation will be

carried out, so that he or she can reliably inform potential volunteers of the research

conditions (Erickson, 1998).

Students were asked to volunteer to participate in interviews and focus groups for this

study. This was consistent with the University’s Human Ethics policy, which states that

‘[t]he free consent of participants must be obtained before research is undertaken’

(UWA Human Research Ethics Committee, 2002).

Although this approach was deemed to be the most appropriate method of selecting

study participants, there are some drawbacks with calling for volunteers that must be

considered (Burns, 1998, p. 18):

[T]hey are not likely to be a random sample of the population. They tend to be better

educated, of a higher social class, more intelligent, more social, less conforming and

possess a higher need for approval than non-volunteers.

Another difficulty with calling for volunteers is that, while they may be free to choose

whether or not to participate in the study, they do not feel they have a choice (Burns,

1998, p. 18). To combat this, Burns outlined an ethical code of conduct for researchers,

detailing the importance of the following considerations (p. 22 – 23):

• that risks to participants are minimised by procedures which do not expose subjects to risk;

• that risks to participants are outweighed by the anticipated benefits of the research;

• that the rights and welfare of participants are protected. The research should avoid

unnecessary psychological harm or discomfort to the subject;

• participation should be voluntary;

• the subject has a right to know the nature, purpose and duration of the study, i.e. informed

consent. Participants should sign an informed consent form which outlines the study, who

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is conducting it and for what purpose, and how it is to be carried out, also providing

assurances of confidentiality and voluntary participation. The participant should sign

acknowledging that they freely consent to participate. Should the subject be below the age

of consent or incapacitated due to age, illness or disability, a parent, guardian or responsible

agent must sign.

• the subject should be free to withdraw at any time without penalty;

• information obtained is confidential;

• participants are debriefed after the study.

The Office of Research Ethics at the University of Western Australia approved this

project. All students who were invited to participate in aspects of the study were

informed that their participation (or non-participation) was completely voluntary, and

would have no bearing on their marks in the chemistry subject that they were studying.

Students were also reminded that if they chose to volunteer to participate in interviews

or focus groups, they could withdraw from the project at any time without penalty.

Volunteers who participated in interviews and focus groups were provided with an

information sheet detailing the aims and intended outcomes of the project (see

Appendix 6.1, which was provided to one group of students). Students were asked to

sign a consent form, indicating that they understood the aims and intentions of the

project and agreed to participate in the various aspects of the study (Appendix 6.1).

The anonymity of the study participants was preserved by the use of pseudonyms.

While some students did know the identities of other student volunteers (some students

participated in focus groups together), lecturers were not aware of which of their

students had volunteered to participate in the research project. Outcomes of this

research were discussed with the participating lecturers, but the identities of student

volunteers were not revealed to them.

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6.3 Data Collection Techniques

6.3.1 Interviews

Semi-structured interviews were deemed to be most appropriate for the study because it

meant that the researcher was not constrained to asking only particular questions, but

could follow students’ understandings and thought processes on related topics.

Erlandson et al. (1993, p. 86) described a semi-structured interview as one that is

‘guided by a set of basic questions and issues to be explored, but neither the exact

wording nor the order of questions is predetermined’. These researchers added that,

although a semi-structured interview is something like an informal conversation,

researchers must be sure to use language that is clearly and easily understood by the

interviewee. Questions must be worded in a manner that will elicit useful and pertinent

information from students.

Burns (1998, p. 331 – 2) believed that a semi-structured interview ‘permits greater

flexibility than the close-ended type’ of interview. He commented on the advantages of

the semi-structured interview:

1) Rapport between researcher and participant increases, particularly when the

participant is interviewed more than once;

2) The focus is on the participant’s perspective and understanding;

3) The participant can use their own language to answer questions and describe

understandings;

4) The participant is on equal status to the researcher.

Although this style of interview is useful to help participants feel comfortable and to

provide the researcher with information, there are some difficulties associated with the

use of interviews. Burns claimed that, ‘the comparability of the information between

informants is difficult to assess and response coding difficulties will arise’. Each

interview respondent is treated as a different person and because respondents are not

necessarily asked the same questions in a semi-structured interview, this can lead to the

gathering of data that are more difficult to code and analyse.

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The researcher designed tasks and questions specifically for students in different classes

and appropriate language was used when talking with students about their

understandings of various aspects of the topic. For any given interview, all students in a

particular group were asked to work through the same tasks. The order of the questions,

as well as the specific wording of the questions, were not necessarily the same in each

interview but depended upon the responses given by each student.

6.3.2 Other Techniques

6.3.2.1 Questionnaires

Cohen, Manion and Morrison (2000, p. 245) described the advantages of using

questionnaires as a means of collecting information. Questionnaires generally lead to

structured data that are straightforward to analyse. Questionnaires are, however, limited

in their scope, often due to constraints on the length of the questionnaire that can be

administered, and can be time consuming to develop and refine. Depending on the

structure of the questionnaire, data analysis can sometimes be time intensive as well.

Pribyl (1994) recommended that student participants be told the goal of the research,

that their participation is voluntary and will not influence their grades in the subject and

that their responses are confidential. He commented that by employing these techniques

when conducting his own questionnaires, he has had over ninety percent of the students

in a class complete them.

Questionnaires administered during lectures in this study were useful for gaining a

broad, general feeling for students’ understandings of particular aspects of the subject

matter. These types of data collection tools were economical, in terms of both money

and time, and enable large numbers of students to be involved in the study. They were

also useful for identifying areas of difficulty that students may have, or for suggesting

aspects of the topic that should be investigated to a greater depth in interviews.

6.3.2.2 Examinations and Test Questions

Examinations and test questions can be very useful in following up information from

earlier in the course (such as students’ abilities to perform certain types of tasks), or

identifying students’ understandings about particular aspects of the course. The biggest

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difference between questionnaires and examinations is that in many cases, the student is

compelled to answer the examination question. He or she cannot decline to participate,

as can be done in a questionnaire. The ethical considerations of this are addressed by

writing of the examination question in conjunction with the course lecturer.

Examination questions must be designed by the researcher to be appropriate for both

assessment purposes and research purposes. They must first address the assessment

requirements of the lecturer for his course. In some cases, the lecturers reworded or

restructured questions proposed by the researcher. As the primary purpose of these

types of questions must be as a suitable assessment tool rather than a research probe,

sometimes the researcher modified particular questions to suit the needs of the lecturer,

the assessment and the students.

Another aspect to be considered is the level of stress that a student is placed under in an

examination situation. Students can sit six or seven examinations in a two week period,

which means that the answer they give in a particular question may not be his or her

best understanding of the concept. The student may give an answer that he or she thinks

the lecturer wants to hear or give an answer whose accuracy is compromised by the

stressful examination situation. These issues must be considered when analysing

students’ responses to examination questions. For this reason, findings that were

identified through students’ examination responses were investigated in other situations,

such as interviews and questionnaires.

6.3.2.3 Focus Groups

Focus groups, described as a type of group interview in which the discussion of interest

is that between the participants and not between the interviewer and the participants

(Cohen, Manion and Morrison, 2000, p. 288), allow the participants’ views to emerge

from the group discussion. As a result of the interactions between participants, focus

groups can often produce a large amount of data in a relatively short amount of time.

Morgan (1988, p. 11) commented that focus groups could be a useful tool for several

different purposes. These include:

• Orientation to a particular field;

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• Generating hypotheses based upon respondents’ comments;

• Evaluating possible research sites or participant populations;

• Developing themes or topics for later investigation in a study;

• Gathering feedback or confirming previous studies (triangulation).

6.4 Trustworthiness

6.4.1 Validity and Reliability

Validity and reliability of results are both extremely important considerations in all

research and are not necessarily conditions that occur together. Results can be reliable

without being valid. Reliability refers to the dependability and consistency of results.

Are the results obtained repeatable? Validity is a determination of whether or not the

research has measured what the researcher was intending to (Burns, 1998, p. 259, 271).

Phelps (1994) remarked that ‘[a]ll researchers should be concerned about the credibility

of their work’. In the physical sciences, the validity and reliability of an experiment or a

process can be proven by repetitive measurements. In the case of chemical education

research, particularly a qualitative study, the validity and reliability is an issue

(Maxwell, 1992; Mishler, 1990).

6.4.2 Validity and Reliability in Qualitative Studies

Cohen, Manion and Morrison (2000, p. 105) have commented upon the importance of

validity in both qualitative and quantitative research, adding that the way in which these

concepts are addressed varies for the two types of research. They stated that in

qualitative data:

validity might be addressed through the honesty, depth, richness and scope of the data

achieved, the participants approached, the extent of triangulation and the disinterestedness

or objectivity of the researcher.

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Metz (1994) mentioned six different factors that can limit the perceived reliability and

validity of qualitative studies. These factors include:

1) The unique and complex nature of the participants in the study;

2) The difficulties encountered in observing teaching and learning situations

and the responses of participants, and the influence of the researcher’s

personal values and attitudes on these observations;

3) The difficulties in the replication of data from one study to another;

4) The interaction between researcher and participants, which can actually alter

the participants’ normal behaviours;

5) The number of variables present in an educational study and the difficulty

encountered in trying to control these variables;

6) The difficulties faced in measuring responses and outcomes in a qualitative

study.

Cohen, Manion and Morrison (2000, p. 106) also stated the need for the validity of the

research to be faithful to the type of research being conducted. The principles of

naturalistic, qualitative methods are focussed on the natural setting being the primary

source of data, the use of thick descriptions and descriptive data to document the project

and the involvement of the researcher in the area being studied. The researcher must

consider these principles when determining how the reliability and validity of the

findings can be established.

Mishler (1990) commented that inquiry-guided studies are often evaluated by more

traditional or experimental researchers as lacking in scientific rigour, due to ‘the mis-

application of experiment-based criteria and methods for validation’. He discussed the

need for a ‘new approach to validation . . . that takes into account the distinctive

features’ of qualitative and inquiry-guided studies. This author suggested that an

important issue in determining the ‘validity’ of the findings of a study was ‘whether the

relevant community of scientists evaluates reported findings as sufficiently trustworthy

to rely on them for their own work’.

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Guba and Lincoln (1989, p. 233) mentioned the need to identify appropriate standards

for the evaluation of data from qualitative research studies, suggesting three approaches:

consideration of (i) the trustworthiness criteria, (ii) the authenticity criteria and (iii) the

contribution to quality by the nature of the research process itself.

The evaluation used in a research study must be consistent with the style of the study

itself. Guba and Lincoln reminded researchers that (p. 251):

there are many ways to assess the goodness of a fourth generation evaluation. It is not and

need not be the case that such evaluations are sloppy, corner-cutting, or unmindful of

standards. Quite the opposite. It ought to be evident that the most basic question is this:

“What standards ought apply?” We have described serval ways to respond to this question

. . . Each set has utility for certain purposes. The trick is not to confuse the purposes. It is

also important to keep in mind that goodness criteria, like paradigms, are rooted in certain

assumptions. Thus it is not appropriate to judge constructivist evaluations by positivistic

criteria or standards, or vice versa. To each its proper and appropriate set.

To this end, it was determined that evaluation of data would be considered in terms of

this trustworthiness. Many of the authenticity criteria appear to be more appropriate for

studies in which the participants were involved for extended periods of time; for

example, ontological authenticity considers how participants’ understandings and

conceptions matured through their participation in the study. In the study described in

this thesis, many student participants were only interviewed once, making a

consideration of this type of authenticity quite difficult. The collected data are,

therefore, considered in terms of the four criteria of trustworthiness referred to by

Earlandson, et al. (1993, p. 28 – 35); credibility, transferability, dependability and

confirmability; which are now discussed in more detail with reference to this particular

research study.

Guba and Lincoln (1989, p. 233) referred to the four criteria of trustworthiness (namely,

credibility, dependability, confirmability and transferability) as parallel or foundational

criteria, as they ‘are intended to parallel the rigor criteria that have been used within the

conventional paradigm for many years’. These criteria are related to (or at least

intended to parallel) the standards of validity, reliability and objectivity that are applied

to more conventional data.

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Credibility describes the ‘compatibility of the constructed realities that exist in the

minds of the inquiry’s respondents with those that are attributed to them’ (Earlandson,

et al., p. 30). A credible study can be accomplished through the use of a variety of

techniques (Lincoln and Guba, 1985, p. 301 - 316) that include:

1) Prolonged engagement in the study environment, persistent observation and

triangulation of data;

2) Peer debriefing;

3) Analysis of negative cases;

4) Archiving of raw data to enable its use in later validation of hypotheses and

checking of referential adequacy;

5) Checks of data and analyses by participants.

The credibility of the study reported here was assessed in several ways. Where

possible, more than one source was used to build responses to the research questions,

allowing for triangulation of data. As can be seen in section 6.8, several different tasks

were often designed to address the same research question. This gave the researcher

confidence that findings from one task were a reflection of students’ understandings and

not the result of a flawed task. In addition, the study was carried out over several years.

This allowed the researcher more time to develop an understanding of the three courses

being investigated and to interact with the lecturers teaching the courses.

Research findings were also discussed with other members of the research group at

regular group meetings. At some of these meetings, raw data from tasks was viewed by

the group and then discussed in light of the particular research question that the task was

designed to address. If there existed disagreement between researchers as to the

findings from a particular task, other data or new tasks were used to support and

confirm findings.

Finally, raw data were stored in its original form (tape, handwritten notes) for future

reference. This enabled the researcher to return to raw data at a later date, should any

disparities or inconsistencies arise in the future.

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The extent to which findings can be applied in other contexts is a measure of the study’s

transferability (Lincoln and Guba, 1985, p. 316). A researcher must provide enough

information about the context of his or her study to enable readers to determine if the

findings are transferable to their own situations. This requires the use of thick

description to adequately illustrate the context of the study and a sampling method,

which will result in richly detailed data (Earlandson, et al., 1993, p. 33).

To enable others to determine if the findings of this study are applicable in their own

teaching situation, thick descriptions of the setting and its participants were provided in

Chapter 3. The context includes a consideration of the two universities at which the

three courses were taught, as well as the subject matter covered in those courses. An

understanding of the background of both the lecturers and the students who participated

in the study is also required.

Dependability is concerned with the stability of the data over time (Guba and Lincoln,

1989, p. 242). These authors commented that the dependability criterion generally

excludes methodological changes made by the researcher, adding that ‘[i]n conventional

inquiry, . . . alterations in methodology . . . would render reliability greatly suspect’ (p.

242). In the case of qualitative studies, however:

methodological changes and shifts in constructions are expected products of an emergent

design dedicated to increasingly sophisticated constructions. Far from being a threat to

dependability, such changes and shifts are hallmarks of a maturing—and successful—

inquiry.

An important aspect of a naturalistic study is that the changes and modifications (to

researcher questions, tasks) need to be well documented and explained. This allows for

a dependability audit, which an external reader can use to check upon the processes

used in carrying out the research (Earlandson, et al., 1993, 34).

A fourth criterion that can be used in judging a study’s trustworthiness is its

confirmability. This criterion is a measure of the degree to which the findings produced

are a result of the focus of the inquiry and not of the particular thoughts or biases of the

researchers involved (Lincoln and Guba, 1985, p. 318). Although the naturalistic

researcher accepts that he or she plays a significant role in the study, there is also an

ability to show the confirmability of the data, as claims, assertions and findings can all

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be tracked back to their original sources (Lincoln and Guba, 1989, p. 243). Sufficient

documentation is required to enable external readers to identify the sources from which

raw data have been drawn to support claims and assertions made by the researcher

(Earlandson, et al., 1993, p. 35).

Both dependability and confirmability of a study are related to the documentation

associated with the study and its findings. This is discussed in section 6.4.3.

6.4.3 Documentation and the Audit Trail

An important facet of qualitative, naturalistic research is the need to document the

course of the research and the analysis of data from the study. In addition to

documenting both the raw and the analysed data, this procedure involves keeping good

records of research plans, reasons for modifying research tasks and emerging theories or

hypotheses. This audit trail is essential in allowing external readers to justify the

dependability and confirmability of the study, and to evaluate its trustworthiness.

For an independent assessment to be made of the dependability and confirmability of

the data presented and analysed, the study itself must be clearly and carefully

documented (Guba and Lincoln, 1981, p. 122). Data that are collated for auditing

purposes can be classified according to type. Lincoln and Guba (1985, p. 319) cited the

six audit trail categories identified by Halpern (1983):

1) Raw data;

2) Data analysis and reduction;

3) Products resulting from the reconstruction of data, such as reports;

4) Notes on research process;

5) Notes relating to intentions, expectations and aims;

6) Information relating to development of instruments and tasks.

A large quantity of raw data (lecture notes, interview tapes, students’ worked tasks) was

collected over the course of this study. Before analysis was made of any hard copies of

raw data (for example, students’ answers to a task), copies were made and the original

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documents stored to preserve their integrity. Tapes of interviews and lectures were

stored after transcription.

Data analysis was carried out on copies of original data or on transcripts before being

tabulated in electronic documents for ease of reference. These coded hard copies were

also stored. Some coding was transferred to electronic documents through the use of

colour coding on transcripts or documents.

Before any research task was administered to students, the researcher constructed a

detailed justification to enable others to understand her motivations for preparing that

particular task. The justification also included suggestions of possible responses that

students may give to particular tasks and an analysis of what such responses might

indicate about a student’s understanding. An example of a justification for one task is

shown in Appendix 6.2.

The researcher kept a journal to detail the planning and implementation of the project.

Mills (2000, p. 63) suggested that keeping a journal is a useful way for a researcher to

keep track of both observations and feelings about his/her project. The researcher’s

journal included such things as proposed tasks, justification for task or research question

modification, notes on research discussions with supervisors and summaries of findings.

Parts of the journal were hand-written notes. Others were electronic documents and

files.

All electronic documents were transferred to compact disc for ease of storage. These

included transcripts, coded data, presentations and papers related to research findings

and all tasks administered to students.

These documentation procedures were carried out to ensure that the trustworthiness of

the data and findings from this research study could be assured.

6.5 Analysis and Coding

The analysis of qualitative data is a time-consuming and involved process. Lincoln and

Guba (1985, p. 333) commented that:

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The process of data analysis, then, is essentially a synthetic one, in which the constructions

that have emerged (been shaped by) inquirer-source interactions are reconstructed into

meaningful wholes. Data analysis is thus not a matter of data reduction, as is frequently

claimed, but of induction.

Indeed, analysis of qualitative data is an ongoing process, beginning as soon as data

collection commences (Erlandson, et al., 1993, p. 111). As the data are analysed and

the data collection continues, the researcher’s perspectives and understandings can be

coloured and changed by previously collected data (Phelps, 1994). Data analysis does

not occur in a vacuum (Earlandson, et al., 1993, p. 113):

It must be in the forefront of the researcher’s mind that data analysis occurs during data

collection . . . The concept of looking at phenomena, stepping back and analyzing them,

and drawing conclusions based on the resulting analysis is one of the major trappings of

traditional research.

The analysis of qualitative data involves the researcher interacting with the collected

data, which leads to the formation of new ideas or hypotheses. The collection of

subsequent data can be affected by these new ideas, which can be changed or modified

after the analysis of new data. Earlandson, et al. described some to the methodological

tools that the researcher may employ while carrying out data analysis (1993, p. 115):

1) Triangulation, where several different sources are used to provide

information on the same events or hypotheses;

2) Developing working hypotheses based upon the data collected so far;

3) Testing hypotheses, by designing tasks for research participants or carrying

out member-checks.

More than one researcher can carry out analysis to check the trustworthiness of data

analysis. Earlandson, et al. (1993, p. 127) commented that this approach can strengthen

the analysis of the collected data so long as the analysts are competent, that is, they

must have some level of understanding of the research project and its subject matter.

In this study, the researcher principally carried out the analyses. Other researchers

assisted in the analysis and coding of data from some tasks to verify the researcher’s

interpretations. Students’ understandings of different aspects of the topic were

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investigated using a variety of methods (triangulation) to confirm the researcher’s

interpretations of their understandings. In addition, tasks were designed throughout the

course of the study to investigate the usefulness of working hypotheses put forward by

the researcher.

6.6 Research Timetable

Three different chemistry courses; Chemistry 121/122, Chemistry 100 and Chemistry

2XX; were observed in detail over the course of the project. Students from a Chemistry

3XX course and Chemistry 101/102 and 123/124 also participated in questionnaires.

In 1999, lectures in both Chemistry 121/122 (14 weeks) and Chemistry 2XX (13 weeks)

were observed in second semester.

Preliminary interviews were conducted with a group of Chemistry 100 students in

second semester. Chemistry 121/122 and 101/102 second semester examinations and

laboratory tests were also analysed.

In 2000, lectures were observed in Chemistry 100 (8 weeks in first semester), Chemistry

121/122 (14 weeks in second semester) and Chemistry 2XX (26 weeks over the year).

Four Chemistry 100 students participated in semi-structured interviews towards the end

of their study of organic chemistry in semester one. Several questions were also written

for the first semester Chemistry 100 examination.

In second semester, Chemistry 121/122 and 101/102 students answered a questionnaire,

questions in a laboratory test and examination questions.

The Chemistry 2XX students were given questionnaires twice, once at the beginning of

each semester. Six student volunteers participated in two series of interviews, one in

each semester. Four of these students were interviewed twice. One student was

interviewed only in first semester, another only in second semester. An examination

question was also written for the Chemistry 2XX students.

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A questionnaire was administered to students studying the chemistry education topic

offered as a part of the Chemistry 3XX course in second semester.

In 2001, lectures were observed only in Chemistry 2XX (13 weeks) in first semester.

Chemistry 2XX students were also asked to complete a questionnaire at the beginning

of their course. Seven student volunteers participated in a focus group discussion and a

series of interviews in semester one.

Questionnaires were administered to Chemistry 100 and 121/122 students early in first

semester, even though their lecture courses were not observed.

In 2002, large-group questionnaires were administered to three different first year

groups; Chemistry 100, Chemistry 121/122 and Chemistry 123/124. These

questionnaires were all administered early in the semester. The Chemistry 100 students

had not studied any tertiary organic chemistry when they completed a questionnaire.

The Chemistry 121/122 students had had several lectures in organic chemistry, but had

not begun to study reaction mechanisms. The Chemistry 123/124 students had had

several organic chemistry lectures, including one introductory lecture about reaction

mechanisms and their representations.

Dr Adams, the Chemistry 100 lecturer, was interviewed in mid-1999 (when on study

leave and not teaching the course) and in early 2000, before he commenced lecturing to

the Chemistry 100 students.

Dr Anderson, who lectured to the Chemistry 121/122 students, was interviewed five

times; once in mid 1999, just after commencing his second semester lectures, once in

early 2000, before semester started, and again in early 2001, also before semester had

started. He was also interviewed twice in mid-2002.

The organic chemistry lectures in Chemistry 2XX were given by Associate Professor

Andrews, who was interviewed three times, twice in the mid-year break in 1999 and

once in early 2000, before lectures had started for the year.

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6.7 Research Design—Lecturers

6.7.1 Lecturer Observations

The researcher attended and observed organic chemistry lectures in all of the courses

that were studied. All lectures were tape recorded and class notes taken. Some lectures

in Chemistry 100 were also videotaped. The researcher collected all handouts and

practice questions provided to the students. The researcher’s notes were colour-coded

to indicate what had been written on the board and what the lecturer had said.

The researcher transcribed audio recordings of lectures (or portions of lectures) that

specifically related to representations of reaction mechanisms. The transcripts were

coded to identify teaching objectives that were explicitly discussed with the students.

Representational strategies used by the lecturers when describing reaction processes

were identified.

The researcher used lecture notes and class transcripts to compile a description of the

lecturers’ presentation of class work. Lecturers were subsequently interviewed about

their motivations for using particular representations or language in their classes. The

descriptions were then supplemented with the lecturers’ explanations and motivations

for choosing to teach the topic in a particular manner.

6.7.2 Lecturer Interviews

Interviews with the three lecturers who participated in the research study were designed

to investigate their reasons and motivations for teaching about representations of

reaction mechanisms, and to identify any difficulties or problems that they believed

were common amongst their students. Lecturers were asked to describe the course they

taught and asked about notes and handouts and texts they recommended.

The researcher carried out the interviews, either alone or together with her supervisor.

Each lecturer was interviewed several times during the research project. Each interview

was tape-recorded, transcribed by the researcher and coded. Interviews were analysed

to address research questions L1 and L2 (section 1.5.1).

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Initial interviews with each lecturer were coded to identify categories of responses for

each research question. Subsequent interviews were categorised in a similar manner.

Where necessary, additional categories were created from data from later interviews.

6.8 Research Design—Students

Many different types of research task were designed for use in this study and they are

described in the following sections. Each of the tasks administered to students is

summarised in table 6.1. A more complete description of how each task was designed

to address particular research question(s) is included as Appendix 6.3.

Table 6.1: Research tasks and their associated research questions (RQs). The numbers

in the fourth column refer to the student-related questions outlined in section 1.5.2.

Appendix Source Course R.Q. Specific aspect of Research Question

6.4 Questionnaire 121/122 2 Understanding and representation of curly

arrows.

6.5 Questionnaire 2XX 2 Understanding of term reaction

mechanism.

6.6 Questionnaire 2XX 2 Understandings of:

1. Formal charge;

2. Curly arrows;

3. Relative positions in representation.

6.7 Questionnaire 2XX 2 Understandings of:

1. formal charge;

2. curly arrows;

3. relative positions in representation.

6.8 Questionnaire 3XX 2 Understanding and representation of curly

arrows.

6.9 Questionnaire 121/122 2 Understanding and representation of curly

arrows.

6.10 Questionnaire 100 2 Understanding and representation of curly

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arrows.

6.11 Questionnaire 2XX 2 Understandings of mechanistic

representations.

6.12 Questionnaire 100,

121/122

2 Understandings of mechanistic

representations.

6.13 Questionnaire 100,

121/122

2 Understandings of mechanistic

representations.

6.14 Interview 100 1

2

3

Understandings of:

1. term reaction mechanism;

2. why mechanisms are important.

Rationalising reaction mechanisms;

Understanding of multi-particle nature of

reactions;

Understandings of curly arrows;

Predictive abilities in competitive

processes;

Translation between structures.

Solving strategies used.

6.15 Interview 100 1

2

3

Understandings of:

1. term reaction mechanism;

2. why mechanisms are important.

Formal charge calculation;

Understanding of curly arrows;

Understanding of link between

experimental and mechanisms.


6.16 Interview 2XX 1

2

Difficulties encountered by students in

representing mechanisms.

Formal charge calculation;

Understandings of types of bond breakage

and outcomes;

Interpretation of curly arrows.

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3 Solving strategies used.


3

Understandings of competitive reaction

processes.



3

Appropriateness of mechanistic

representations;

Formal charges;

Perception of link between structural

representation and reaction outcome.


6.19 Focus

group

2XX 1

3

Understandings of usefulness of teaching

about reaction mechanisms;

Perceptions of difficulties associated with

mechanisms.


6.20 Lab test 121/122 2 Understandings of competitive reaction

processes.

6.21 Lab test 121/122 2 Understandings of competitive reaction

processes.

6.22 Examination 121/122 2 Understandings of curly arrows and

formal charge.

6.23 Examination 121/122 2 Understandings of:

1. curly arrows and formal charge;

2. link between experimental and

mechanisms;

3. competitive reaction processes.

6.24 Examination 100 2 Understandings of:

1. curly arrows and formal charge;

2. link between experimental and

mechanisms.

6.25 Examination 2XX 2 Understandings of competitive reaction

processes.

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6.8.1 Questionnaires

Questionnaires were administered to students in each of the courses at various times

throughout the year, generally at the beginning of semester. The questionnaires were

administered either in the first five minutes of a lecture or the last five minutes, in

accordance with each lecturer’s requirements. The researcher was introduced to the

students by the lecturer and gave a brief explanation of the questionnaire and what the

students were expected to do.

Students were asked to complete the questionnaires by themselves, and not to share

answers with or help their neighbours. Most students responded to the questionnaire,

although some chose not to participate.

Each of the questionnaires comprised a maximum of four tasks and was no larger than

one double-sided A4 page. They were kept relatively short to enable students to

complete them in five to ten minutes. In some questionnaires, students were asked to

explain their answers in short, written statements. The tasks that were asked in

questionnaires were ones that the researcher wanted general data or larger numbers of

responses. The outcomes of many of these tasks were followed up in interviews.

Questionnaires that were administered early in the year were also used to recruit student

volunteers for later interviews. Only students who volunteered for the research project

put their names on their questionnaires. All other questionnaires were anonymous.

When each questionnaire was designed, possible answers to each question were

suggested, along with explanations of what type of understanding each response

represented. Students’ responses were categorised according to these answers. In some

cases, a small number of new categories were created during the analysis of data.

6.8.2 Interviews

Semi-structured interviews were conducted with student volunteers from Chemistry 100

and Chemistry 2XX. Students were asked to complete a series of tasks, writing down

answers and talking through their responses aloud. The researcher collected the

students’ written responses. All interview sessions were tape recorded and transcribed

by the researcher and subsequently students’ responses were coded.

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At the completion of each interview, each student could ask the researcher to go through

any questions that he or she did not understand. This section of the discussion was not

tape recorded. If a student had displayed a particularly inappropriate understanding of

mechanistic representations in his or her interview, the researcher offered the student

assistance and often worked through a problem to help explain his or her difficulties.

6.8.3 Chemistry 2XX Focus Group 2001

Chemistry 2XX students participated in a focus group session, which was designed to

address a research question that had proved difficult to discuss in an interview situation;

students’ perceptions of the difficulties in studying mechanisms (research question

S1b). The focus groups consisted of three or four students, and lasted 40 minutes.

6.8.4 Laboratory Tests and Examinations

The researcher designed a number of test and examination questions, and a selection of

these was given to the various course lecturers. The participating lecturers evaluated

these questions, selected appropriate ones, and included them in tests and examinations

in their courses.

These tasks were designed to address specific aspects of research question S2 that the

researcher wanted a large sample size for. The students’ responses were tabulated and

coded according to categories determined when the questions were designed.

Additional categories were created during analysis as required.

6.9 Summary

A qualitative, naturalistic research methodology was employed in this study. Different

techniques (questionnaires, interviews, focus groups) were used to elicit students’

understandings of aspects of the topic of reaction mechanisms. Classes were observed

and the lecturers interviewed to build an understanding of their perceptions of teaching

about reaction mechanisms and to gain insight into the teaching strategies used in their

classes.

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Trustworthiness of the data was ensured through the use of triangulation and through

thorough documentation of the research process. This documentation allows for the

credibility and dependability of data to be confirmed by an external source, if required.

Transferability of the findings is assisted through the use of thick descriptions (Chapter

3) to allow other researchers to determine the study’s applicability to their own teaching

situations.

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7 Findings: Lecturers’ and Students’ Perceptions

7.1 Introduction

This chapter discusses findings that relate to research questions L1 and S1:

L1: What perceptions do lecturers have of:

1. the purpose and importance of teaching about reaction mechanisms as a tool

for understanding in organic chemistry?

2. the difficulties students have at various levels of their tertiary education when

using representations of reaction mechanisms to rationalise or predict reaction

outcomes?

S1: What perceptions do students have of:







Quotations from transcriptions of interviews with students and lecturers are included

where appropriate. In the quotes, . . . indicates a pause of less than five seconds. Where

transcripts have been edited, . . . [ ] . . . is used to indicate that some original data has

been omitted. Words added for clarity are inserted [in square brackets].

Due to similarities in the findings related to these research questions across the three

courses investigated for this study, all findings are reported together in this chapter.

The source of each set of data (course, university and year) is clearly identified when

the relevant findings are discussed.

The discussion considers students’ prior understandings of aspects of mechanistic

representations. A consideration of any prior understandings that students possess is

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important, as alternative conceptions or misconceptions can impinge upon students’

learning. Students can ignore or reject new explanations and ideas if they conflict with

beliefs they already hold (Stepans, Beiswenger and Dyche, 1986).

A consideration of the lecturers’ motivations for teaching about this topic is discussed.

This includes a description of both the lecturers’ and the students’ perceptions of why

reaction mechanisms are important in the study of organic chemistry. These

perceptions are compared and contrasted, and any differences examined in terms of

their implications for teaching and learning about mechanistic representations.

Lecturers’ perceptions of the difficulties students face while studying the topic are also

described. The lecturers’ perceptions of the difficulties students face is one aspect that

can influence the manner in which they teach the course and the strategies they use in

their lecturing (Shulman, 1986). These views are compared with the difficulties

students believe that they encounter in their study of reaction mechanism and with the

difficulties that the students who participated in the study exhibited when working

through tasks and problems.

From the findings, the researcher claims that lecturers’ perceptions of the importance of

teaching about reaction mechanisms in organic chemistry are consistent with those of

their students. However, although there is some consistency between perceptions of the

difficulties that students experience when studying this topic, the perceptions of

lecturers and students are not entirely in agreement. Lecturers’ perceptions of students’

difficulties are more consistent with students’ demonstrated difficulties.

Pedagogical implications arising from these findings are discussed in Chapter 11.

7.2 Students’ Prior Understandings

7.2.1 Introduction

By identifying students’ prior understandings, it is possible to determine which were

inconsistent and to identify if students underwent any conceptual change in regards to

these understandings. In addition, participant lecturers could be made aware of these

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prior understandings, which would allow them to direct their teaching towards assisting

their students to construct more useful understandings about representing reaction

mechanisms. The tasks completed by the students and the findings from the research

tasks directed at identifying these prior understandings are discussed in the following

sections.

7.2.2 Research Tasks

Students’ prior understandings about representations of reaction mechanisms were

investigated using a series of questionnaires (see Appendices 6.9 – 6.13). The aim of

these questionnaires was to investigate students’ interpretations of common

representations in reaction mechanisms before they commenced studying chemistry at a

tertiary level. In all but one case (Appendix 6.11), the questionnaire was administered

to large numbers of first year students before they had commenced their study of

reaction mechanisms. These first year students were from both University A and

University B.

The questionnaire attached as Appendix 6.11 was completed by second year students at

the beginning of their course of study. Approximately half of these students had not

studied reaction mechanisms previously, as they had completed a different first year

course that did not include this topic. The other half of students had studied Chemistry

100 in first year, and had therefore studied reaction mechanisms approximately eight

months earlier in first year.

7.2.3 Findings

The questionnaire shown as Appendix 6.9 was designed to investigate if first year

students spontaneously used any kind of arrow representation to explain reaction

outcomes before learning about them in organic chemistry. It was administered to a

group of Chemistry 121/122 students very early in semester one. The task outcomes

suggested that students generally did not use arrows of any sort to rationalise reaction

outcomes. The majority of students wrote word explanations of the represented

processes. Many students also expressed confusion as to what the questionnaire was

intended to ask them.

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The students’ confusion led to a reanalysis of the appropriateness of the task and the

research question it was intended to probe. After discussion with other researchers, it

was decided that students’ spontaneous use of curly arrows was not the important issue.

What the researcher was interested in investigating was how students interpreted curly

arrows before they had been taught about these representations in their chemistry

course. Consequently, it was decided that the validity of the findings from this original

questionnaire was suspect, due to a poorly designed task. No results from this task were

included in the findings of the research project.

The outcome of students’ responses to this task was a restructuring of the research

question and a rewritten task. Tasks addressing the restructured question were

administered as a questionnaire to Chemistry 100 (Appendix 6.10) and Chemistry 2XX

(Appendix 6.11) in 2001 and to Chemistry 100, 121 and 123 in 2002 (Appendices 6.12

and 6.13). The questionnaire findings indicated that many students considered curly

arrow representations to indicate movement of either atoms or bonds. For example, in

question 1 in the 2002 questionnaire (Figure 7.1), students commonly selected answer

(a) as the more correct one. This response is not consistent with the convention adopted

in chemistry.

The reaction between water molecules and hydrogen ions is represented in the equation below:

HO

H HO

H

H

+ H

In your opinion, which of the following is the most correct representation of the above reaction?

HO

H

HH

(a) (b)

HO

H

Figure 7.1: Question 1 from the 2002 questionnaire administered to Chemistry 100,

121/122 and 123/124 students (see Appendix 6.12 for the complete questionnaire).

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Students’ explanations of why they had selected this particular representation as the best

mechanistic representation was often in terms of the relative movement of the H+ and

H2O molecules:

H+ is only a proton, so it has a very small size and is charged, so it will be attracted to the

lone electron pairs. The electron pairs will also be attracted to the H+, but the smaller H+

will have greater acceleration and velocity towards the H2O.

Because H+ is being added to the H2O molecule, not removed from the H3O+ molecule.

The hydrogen ion forms a bond with the oxygen in water molecule to form hydronium ion.

Hydrogen is pulled to bond with the lone pairs.

As H+ is smaller the bigger H2O is more likely to attract it.

Similarly, in other questions on these questionnaires, students were more likely to give

responses that were consistent with arrows representing atom movement, rather than

electron movement. In some cases, even with this incorrect interpretation of the curly

arrow representation, the correct reaction outcome was predicted. This was more likely

to occur when the question represented substitution processes. However, when

ionisation reactions (Appendix 6.10, question 2a) or elimination processes (Appendix

6.11, question 2) were represented in a task, students’ responses exposed inappropriate

interpretations. Students who begin their study of the topic with such misconceptions

need to develop more appropriate understandings and interpretative abilities to assist

them in learning to represent reaction mechanisms.

Pedagogically, this has implications for the teaching of the course. If lecturers are

aware of the prior understandings and possible misconceptions that their students may

possess, this can affect the way that they choose to teach their course (Shulman, 1986).

Lecturers can develop techniques to help their students learn to replace their incorrect

understandings with more appropriate ones. The usefulness of such strategies is

dependent upon the appropriateness of lecturers’ perceptions of their students’

alternative conceptions and prior understandings.

One particular strategy was described by Dr Anderson to attempt to improve one

group’s understandings of curly arrows (interview 13082002). He turned a large-group

lecture into a tutorial, putting problems on the board and asking students to work

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through the problems together. While students worked through the examples, Dr

Anderson walked around the lecture theatre, answering any questions and assisting

students with the tasks. Dr Anderson felt that the task was beneficial to both him and

the students. He’d intended it to be a short task at the beginning of a lecture, but it

proved so useful that he devoted an entire lecture to the large tutorial.

7.3 Lecturers’ and Students’ Perceptions of Topic Importance

7.3.1 Introduction

The researcher was interested in identifying the perceptions held by both students and

lecturers as to the importance of studying about reaction mechanisms in organic

chemistry. It was felt that if lecturers and students held different perceptions about the

topic’s importance then this could lead to differences in the way lecturers attempted to

teach about the subject and how the students tried to understand what was taught. In

contrast, if the students’ and lecturers’ perceptions were similar, a good fit was more

likely to exist between the teaching strategies used by the lecturers and students’

abilities to accommodate new ideas.

Lecturers’ understandings of the importance of teaching about reaction mechanisms will

influence the manner in which they choose to teach about it, the strategies they employ

in their teaching and the examples that they use in their classes. Students’

understandings of the importance of the topic may influence the depth to which they

seek to understand the representations and the time that they devote to developing

useful and usable strategies for representing mechanisms to assist their understandings.

The lecturers’ perceptions of the importance of studying about reaction mechanisms are

discussed in the next section. This is followed by a consideration of the students’

perceptions of the topic’s importance in section 7.3.3 and a comparison between the

perceptions of the lecturers and those of their students in section 7.3.4.

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7.3.2 Lecturers’ Perceptions of Topic Importance

Each of the lecturers was asked in an interview situation about their understandings of

the purpose of teaching about reaction mechanisms. The three participant lecturers

made similar comments when asked what they felt were the purposes and importance of

teaching about reaction mechanisms in their classes.

Each of the lecturers considered representations of reaction mechanisms to be an

important conceptual tool for their students to understand and to use in their studies.

All three lecturers commented that they had made a conscious decision to teach about

mechanisms in their classes. Their reasons and motivations for choosing to discuss

reaction mechanisms in their courses were linked to their understandings of the purpose

and importance of the topic and its place in the study of organic chemistry.

Dr Anderson acknowledged that organic chemistry is an information-rich subject. He

added that he believed students could use an understanding of the chemical processes

occurring in a reaction vessel as a predictive tool. In his opinion, this reduced the need

for the students to attempt to memorise many different reactions, thereby reducing the

amount of information they were required to process. He commented (interview

09081999, lines 17 – 21):

I use the analogy to my students that they all know how to add. They may not know the

answer the sum of a couple of big numbers, but they all know how to work it out. I try to

keep, put mechanisms in the same bracket. They might not know the answer to the

reaction, but if they know the process, they can get to the answer.

This suggests an agreement with the need for the creation of useful and appropriate

structures and constructs for understanding in the information processing model that

was described by Johnstone (1983, 1984) that was discussed in section 2.5. Students

need to relate relevant pieces of information together appropriately to enable them to

correctly file new information and understandings, which allows them to easily retrieve

such information.

Associate Professor Andrews made similar comments to this one by Dr Anderson

concerning his own perceptions of the usefulness of teaching about reaction

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mechanisms. He considered the purpose of reaction mechanism representations to be

(interview 28061999, lines 8 – 10):

to put a common theme so that you can predict, given a reagent and a starting material,

using mechanism you can predict the reaction what will happen.

Associate Professor Andrews’ comments also suggested a manner in which students

could develop solving strategies using knowledge of reaction mechanisms. He hoped to

teach his students to recognise the patterns in reaction mechanisms so that they could

use similar types of representations when solving different problems.

Similarly to both Dr Anderson and Associate Professor Andrews, Dr Adams recognised

the predictive nature of reaction mechanisms. He thought that this aspect of the topic

might help students who were struggling to understand the vast quantities of

information that was presented to them in their organic chemistry classes, by presenting

them with recognisable patterns that were relatively simple to follow and to apply to

other similar reactions, thereby simplifying the volume of information they felt they

were required to process.

In conclusion, the participant lecturers described two main motivations for teaching

students about mechanisms. These were:

1) to help students to generalise reactions and to enable them to apply their

understandings to different or unfamiliar reactions;

2) to allow students to predict products that may be formed by reaction of

particular starting materials.

Both of these were identified by the lecturers as being related to reducing the amount of

information that students needed to memorise in each course.

7.3.3 Students’ Perceptions of Topic Importance

Lecturers’ motivations are an important consideration in the manner in which they

chose to teach about a particular topic of a course. If a lecturer is teaching a topic in a

particular manner, to emphasise the aspects he/she feels are crucial for a student to

understand, and students do not have the same perception of the topic’s importance,

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students may not have the motivation to learn about the topic and may therefore not

grasp its intricacies. For this reason, students’ perceptions of the importance of reaction

mechanisms were also investigated.

The students who were asked about their perceptions of the importance of teaching and

learning about reaction mechanisms in organic chemistry courses were at various stages

of their course. One group of first year students was interviewed several months after

completing the organic chemistry component of Chemistry 100 (Appendix 6.14). A

second group was interviewed towards the end of their study of organic chemistry in

Chemistry 100 (Appendix 6.15). Second year students were also asked about their

perceptions in a questionnaire at the beginning of their second year of study (Appendix

6.3), as was of a different group of second years mid-way through their first semester of

Chemistry 2XX in a focus group (Appendix 6.19).

Students’ responses to questions ranged from general explanations to very specific

reasons that they considered mechanisms to be important in the study of organic

chemistry. Some students made general comments about representations of reaction

mechanisms being useful in helping them to understand how a reaction process might

occur; ‘It gives you a better understanding of how it, how it works’ (Guy, Chemistry

100, 1999), ‘it’s just understandings of what’s happening, what’s going on’ (Bruce,

Chemistry 100, 1999); ‘if you didn’t have it . . . [ ] . . . if you didn’t show what’s

happening, you’d just have to accept it’ (Cathy, Chemistry 100, 2000).

Other students (Barbara and Graeme, both Chemistry 100, 1999 and Cameron,

Chemistry 100, 2000) made specific comments about why they perceived reaction

mechanisms to be an important topic to cover in their study of organic chemistry.

Barbara, for example, commented upon the usefulness of reaction mechanisms

(interview 04101999, lines 69 – 74):

[I]f you don’t have a pattern, if you don’t have a system to follow, you’re just floating

(laughs) and it, you’re learning equation after equation, rather than looking for trends and

themes which can help you extrapolate to things you don’t know. . . . [ ] . . . Because you

can’t, you can’t learn everything. So, yeah, it’s important to learn the pattern behind it.

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Cameron’s understandings of the importance of mechanisms were, like Barbara’s,

related to the application of reaction mechanisms for generalising reaction processes

(interview 18052000, line 36 – 39):

I think, if we get a better understanding of, like, looking at the mechanisms and figuring

them out for ourselves, like now, when we get to sort of, more complex reactions, things

like that later, they won’t have to, make, re-teach the mechanisms, or like have to teach

them, so we can, they can just run through the more elaborate reactions a lot faster.

Felix, Fernando and Fiona (Chemistry 2XX) had similar perceptions about the

usefulness of reaction mechanisms for generalising information (focus group 07052001,

line 233 – 254):

Felix: Yeah. Whereas, like, if you actually just go through the mechanism, like

mechanisms are a good way to remember stuff.

Interviewer: Ok. Why do you find them a good way to remember stuff? Why do they

help you?

Felix: Um . . . because you can sort of generalise, like a lot of reactions . . . you

don’t have to remember like every single reaction.

Interviewer: Ok.

Felix: You can just look at something, if you can remember the mechanism,

you’ve got a good chance of being able to do it.

Interviewer: Ok.

Fernando: I think that’s what it is, just remembering the whole list of reactions . . .

but if you can just generalise it down to one simple, or not-so-simple

mechanism, then it’s easier to remember for. For example (indistinct).

Interviewer: Ok.

Fiona: Yeah, I’d agree. Especially if you were coming up against a compound

which, um, you may not have seen before, etcetera, you know . . . I mean,

knowing, knowing the classifications with the mechanism, you can look

at it and say, ‘yeah, I can see a carboxylic acid there, I know that if I put

base there this will happen’ . . .

Interviewer: Mmm hmm.

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Fiona: . . . and, um, just so that you know, anything that you might do to it . . .

you can sort of roughly guess what might happen then and therefore you

can use it in your chemistry further on. I mean, it’s definitely, definitely

the advantage as a classification . . .

Barbara, Cameron, Felix, Fiona and Fernando’s views were consistent with those

described by the lecturers. They believed that they could use the patterns they could

perceive in mechanistic representations to extend their understandings to unfamiliar

reactions or situations, which helped to simplify and chunk the information presented.

These were not the only examples of students’ whose perceptions of the importance of

studying reaction mechanisms were consistent with those of their lecturers. Both first

and second year students mentioned the usefulness of reaction mechanisms as a

predictive tool. In his interview as a Chemistry 100 student, Graeme commented that

he felt that ‘if you draw your molecule, and this molecule, you can sort of say, righto,

mechanisms, and you do little arrows and stuff, and you can predict what would

happen’, (interview 13091999, line 31 – 32). Felicity (Chemistry 2XX) made a similar

comment to Graeme’s (focus group 09052001, line 311 – 319):

going through functional groups studying the whole course, then that does make sense . . .

[ ] . . . saying alcohols for example, then say, extrapolate it out to some big alcohol that

we’ve never heard of before . . [ ] . . . And try and predict what happens.

Fiona (Chemistry 2XX) also made mention of the predictive capabilities of reaction

mechanisms (focus group 07052001, line 246 – 253):

if you were coming up against a compound which, um, you may not have seen before,

etcetera . . . [ ] . . . you can look at it and say, ‘yeah, I can see a carboxylic acid there, I

know that if I put base there this will happen’ . . . [ ] . . . and, um, just so that you know,

anything that you might do to it . . . you can sort of roughly guess what might happen then

and therefore you can use it in your chemistry further on.

In addition to their predictive ability through generalisations, covering reaction

mechanisms in organic chemistry was also considered by students to be useful for

revision purposes. When second year students were asked why they felt that their

lecturer had introduced the topic early in semester, they commented that it was helpful

to refresh their memories and assist them with their revision. These introductory

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lectures were revision for Felicity, Fabian and Fred (Chemistry 2XX) who had all

studied reaction mechanisms in Chemistry 100 (focus group 09052001, line 494 – 515):

Fabian: Just all the revision from last year.

Interviewer: Yeah. Ok. So, do you think that that was included for any other purpose

apart from revision from last year?

Fabian: I saw it just as revision, to get your mind back into (indistinct) brush up.

Felicity: (indistinct)

Interviewer: Just revision?

Felicity: Yeah.

Interviewer: Ok.

Fred: Yeah, revision, but also to focus what, the part of the revision that was

relevant to that part of the course (indistinct).

Interviewer: Ok, so, giving you just a little bit you might need, well, that he wanted

you to have fresh in your mind?

Fred: Well it was also (indistinct).

Felicity: Yeah, I suppose all those concepts are so fundamental, and . . .

Fred: I think I missed that lecture then.

Fabian: They’re the sort of things that they’re just planted into your mind back in

first year, especially like curly arrows, indicate, or the double head,

whatever it may be, it’s just sort of like (indistinct) an electron . . .

(indistinct) . . . just a refresher of that . . .

Interviewer: Yep.

Fabian: It wasn’t learning any new, or anything like that.

Students’ responses to questions about the reasons for studying about reaction

mechanisms and their representations were observed to be consistent across all the

groups of students who participated in interviews and questionnaires. All student

groups described reaction mechanisms as being useful for explaining reaction processes

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and to enable them to predict reaction outcomes. Students’ commented that this would

help them to apply general reaction mechanisms to unfamiliar reactions to assist them in

their understandings.

One group of first year students and the second year students also identified the

usefulness of reaction mechanisms for generalising reaction processes. This is useful to

assist students in chunking information as they learn about organic chemistry.

Chunking helps students in their processing of information, as one chunk of information

(for example, the propositional knowledge ‘alkyl halides can undergo nucleophilic

substitution reactions’) requires less processing than many pieces of information, which

the preceding statement may be to a student who has not learned and understood the

general reaction process that is classed as a nucleophilic substitution reaction

(Johnstone, 1984).

Second year students perceived the study of reaction mechanisms to be helpful for

revision purposes, refreshing their memories of what was taught in first year at the start

of their second year course. These students were the only ones who had studied

reaction mechanisms in detail before being interviewed about their perceptions of the

topic’s usefulness. They were the only students who could have been expected to

perceive reviewing or revising the use of mechanistic representations as a reason why

reaction mechanisms were taught in their class.

7.3.4 Comparison of Perceptions

Students’ and lecturers’ perceptions of the importance of teaching and learning about

reaction mechanisms in organic chemistry correspond to a large extent. Both lecturers

and students identified the usefulness of these types of representations in generalising

information about reaction processes and predicting reaction outcomes as two of the

reasons that the topic is taught. Students and lecturers both made reference to how this

could be applied to unfamiliar starting materials to predict reaction products.

Many textbooks identify these same reasons for teaching about reaction processes.

Sykes (1986, p. 1) commented on the use of a mechanistic approach to both explain

‘existing facts’ about reaction processes and to predict the outcomes of changed or new

reaction conditions. Weeks (1998, p. vii) added that mechanistic representations could

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be helpful in changing organic chemistry from a ‘bewildering array of facts to a unified

science’, suggesting that representations of reaction mechanisms can be useful in the

construction of appropriate understandings by students.

Two other student perceptions were identified. Some students commented that they

considered reaction mechanisms to be a way of explaining how reactions might proceed.

Some second year students also commented on the usefulness of covering the topic

early in their course for revision purposes, as it helped to remind them about

mechanistic representations and their uses and applications in organic chemistry.

The lecturers did not mention that the representations were useful because they detailed

how a reaction might proceed. However, this description is quite similar to the

lecturers’ own definitions of what a reaction mechanism is. Dr Anderson (interview

09081999, lines 38 – 9) defined a reaction mechanism ‘as a step-by-step description of

how we think the reaction may occur’. Dr Adams described the process to his students

as ‘the details of what is happening during a reaction when you’re thinking at the

molecular level’ (lecture 11042000, line 195 – 6). Associate Professor Andrews

(lecture 02032000, lines 63 – 4) describes reaction mechanisms to his students as ‘the

sequence by which this set of events [in a reaction] occurs’.

The second student perception about the importance of reaction mechanisms in organic

chemistry is that some students consider the topic to be useful for revision purposes.

The students who held these perceptions were all students studying Chemistry 2XX

who had been Chemistry 100 students in their first year at university. For these

students, some of the mechanistic principles covered in their Chemistry 2XX course had

already been discussed in some detail in Chemistry 100, and therefore could be

considered revision work by the students.

Some Chemistry 2XX students had not previously studied Chemistry 100. In each year,

approximately half of the students studying the organic chemistry component of

Chemistry 2XX had studied Chemistry 100, and therefore mechanistic principles, in

first year. The other half of the students had studied other chemistry courses, which

either contained no examinable mechanistic content or whose mechanistic content was

unknown.

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Associate Professor Andrews recognised that his students have different backgrounds in

organic chemistry and for this reason chose to teach basic mechanistic principles early

on in his course (interview 22022000, line 62 – 66):

I assume that they’ve just done the basics . . . [ ] . . . I’ll go over it again, that’s largely the

function of the introductory and review lectures.

The lecturer realised that these beginning lectures will be revision for approximately

half of his students, but discussed the topic at the start of his course for the benefit of the

students who had not completed the Chemistry 100 course, as well as to refresh the

memories of the former Chemistry 100 students.

7.4 Difficulties Associated With Writing and Interpreting Reaction

Mechanisms

7.4.1 Introduction

The way that a lecturer chooses to approach the teaching of a particular subject can be

dependent upon many different factors. Topics are discussed to a level that is

considered appropriate for the students’ understandings, their prior knowledge of the

topic and the importance of the topic in the course being taught. In addition, lecturers

often use their understandings of the difficulties faced by previous students to assist

them in pointing out possible areas of difficulty to their students and to identify useful

teaching strategies.

The three lecturers participating in this study outlined several difficulties that they felt

students experienced in their study of reaction mechanisms. This is discussed in section

7.4.2. In addition to identifying these difficulties, the lecturers discussed some of the

ways in which they taught about the topic with these perceived difficulties in mind.

Specific examples the lecturers’ applications of their pedagogical content knowledge in

teaching about reaction mechanisms are described in later chapters (Chapters 8, 9 and

10).

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Students commented on the difficulties that they feel are associated with learning about

reaction mechanisms. It was felt that students’ perceived difficulties could influence

the way in which they approached their study of the topic. This might also influence the

degree to which they consider the advice and help given by the lecturer in their classes.

If a lecturer perceives a certain aspect to be difficult but a student does not, the student

may not pay particular attention to the lecturer when he is discussing that difficulty and

how a student might overcome it. Students’ perceived difficulties are described in

section 7.4.3.

In addition to students’ perceived difficulties, their demonstrated difficulties were also

examined. These are discussed in section 7.4.4. Many of their lecturers’ perceptions of

what students found difficult in the topic of reaction mechanisms were based upon

lecturers’ observations of mistakes or misunderstandings that students demonstrated in

laboratory work, tests, assignments and examinations. Students do not necessarily

perceive that these demonstrated mistakes are misunderstandings.

In conclusion, the perception of lecturers and students and students’ observed

difficulties are compared in section 7.4.5. Many of these demonstrated difficulties were

exhibited by students at both universities, in all courses and in both year groups.

7.4.2 Lecturers’ Perceptions of Students’ Difficulties

When asked about his perceptions of students’ difficulties when studying about reaction

mechanisms, Dr Anderson stated (interview 09081999, line 228 – 232):

I mention the word mechanisms and a wall goes up, they go ahh, I can’t do it, it’s too hard.

I don’t think they’ve got any idea where to start. Um, when it’s written down in front of

them, a lot of them can sort of see, ok, I can see what’s going on here, it makes sense.

When it’s not there, they don’t where to start, and I think that’s a problem, I mean, how do

you know what goes where and what happens?

By far the most common student mistake lecturers mentioned was the incorrect use of

curly arrows in representations, particularly when H+ was involved in the mechanism.

An example of this is shown in Figure 7.2.

Dr Anderson commented upon the fact that ‘the most common [error] is having the

arrow showing not where the electrons are going but what’s moving’ (interview

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090819999, line 72). Dr Adams and Associate Professor Andrews made very similar

comments regarding commonly observed errors. In attempts to help students to learn

the correct use of this representation, all three lecturers commonly reminded students

about the curly arrow representation in their lectures, describing bond breakage and

formation in terms of the movement of electrons (see pages 354, 361 and 384 for

examples).

C O

H

HH H

H

C O

H

HH H

H

(i) (ii)

Figure 7.2: Two mechanistic representations of the protonation of an alcohol.

Representation (i) is correct, as the curly arrow is representing electron movement.

The second representation, (ii), shows the curly arrow pointing in the incorrect

direction.

Dr Anderson went so far as to modify his teaching approach based upon his

observations of the difficulties his students had demonstrated in using curly arrows. He

mentioned that he had stopped using curly arrows in some of his representations

(interview 13082002, line 224 – 250):

Interviewer: What actually made you think that the arrows weren’t helping in this case,

in these sort of cases here? Why did you stop using them for a little

while?

Dr Anderson: Well. It’s something that . . . we gave them the . . . in first semester, we

talk about the radical substitution photochemical halogenation. Because I

don’t give, I don’t use arrows for that section, they do initiation,

propagation, termination . . .[ ] . . . Um, and I just draw a series of

equations. You’ve got chlorine, breaks to give two chlorine atoms, um,

chlorine atom reacts with methane to give . . . et cetera. And when we

asked the question in the exam, they were quite happy to, they could be

just regurgitating, but that question didn’t stump people, it was done quite

well . . . throughout. Ah, whereas any questions, like the electrophilic

addition reaction . . . where you’ve got an alkene and you’re adding H,

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HBr. Drawing arrows, it was all over the place. Um . . . some of them

had the right intermediates, but the arrows were going back to front. Um .

. . others just were moving arrows all over the place. They seemed to

recognise that you had to add the H plus first. Um . . . but the arrows

weren’t reflecting that so . . .

Interviewer: Yeah.

Dr Anderson: I got . . . I was beginning to think that that was confusing the issue. Um .

. . I’ve decided to go back to arrows, because . . . it might confuse the

issue a little bit at the beginning but if I take a little bit more time now to

go on about these two electrons are now being shared by these atoms

instead of these atoms . . . [ ] . . . Um . . . this nucleophile’s got two

electrons and it’s going to share it with this carbocation . . . to form a

bond. And I . . . you’ll see me on the board with my fingers going . . . [ ] .

. . Moving them around.

Dr Anderson added that he has since decided to use curly arrows more in his course and

build up on their use from earlier on in the year (interview 13082002, line 277 – 289):

Dr Anderson: . . . when you, if you teach curly arrows starting with alkene additions, the

first arrow they see is coming from a double bond to form a bond with

something else . . . [ ] . . . So, it’s not an atom going to something else.

It’s a bond going to something else . . . [ ] . . . Um . . . and I think that’s an

important issue. When they’re faced with . . . that, it’s much harder to

interpret that (draws picture of addition to C=C bond) as atom movement.

Interviewer: Because the arrow’s going from the double bond between the carbons to

the H plus.

Dr Anderson: Yeah. So, in this case here, it’s the problem with this sort of thing. They

don’t know whether they’re going to share here or share there . . . [ ] . . .

Um . . . but I think that . . . I build from that now, and I think that that’s . .

. that helps.

In addition to these representational difficulties, the type of students taking a course

could also affect how difficult the topic of reaction mechanisms may be perceived to be.

Dr Adams discussed his perceptions of the difficulty of the topic in terms of two groups

of students, commenting that what he referred to as the ‘analytically minded’ students

generally found the topic to be easy. He added that the students who found learning

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about reaction mechanisms difficult were generally those who had to memorise

everything in their study (the rote learners, as described by Johnstone, 1997) and

therefore found it difficult to remember things such as the correct direction to point

curly arrows.

A second representational difficulty mentioned was students’ understandings of formal

charges. Dr Anderson spoke about this understanding, making reference to a previous

study done by a research student (Teo, 2000). Her study of first year students

discovered that they were able to calculate formal charges under certain circumstances.

Students were given a formal charge equation (Figure 7.3) and shown some commonly

represented structures.

Atom Formal

Charge

= Group

Number

- Number non-

bonding electrons

- ½ (number bonding

electrons)

Figure 7.3: An equation used by Teo (2000, p. 24), modified from Kotz and Treichel

(1996, 427) and Brown, LeMay and Burnsten (1997, p. 271 - 3) for calculation of

formal charge.

The students were asked to write down the group number, number of lone pairs and

number of bonding electrons around a specific atom (in one case, oxygen in hydroxide,

water and hydronium, in the other, nitrogen in ammonia, amide and ammonium) before

calculating the formal charge on that atom. Over two thirds of the students could

calculate correct formal charges in this type of example. However, that number

dropped to less than half of the students interviewed when they were asked to calculate

formal charges on atoms in protonated ethanol molecules and nitronium ions. In this

particular question, students were not first asked to write down the group number or the

numbers of non-bonding and bonding electrons. Teo also commented that a small

number of students appeared to confuse formal charge with polarity (Teo, 2000, p. 24 –

26).

Another comment by Dr Anderson related to the extra difficulty added to mechanistic

representations by the type of structural representation (for example, square planar or

three dimensional structural representations) that is used. He believed that the extra

135

translation aspect of a mechanistic representation that used a different or unfamiliar

structure could increase its difficulty for students (interview 15022000). This view is

consistent with the findings of Johnstone and Selepeng (2001), who found that the use

of a different language increased students’ difficulties in processing simple information.

Structural representations can be considered part of the specialised language of the

chemist (Hoffmann and Laszlo, 1991), which may increase the difficulty students face

in learning about many aspects of organic chemistry, including reaction mechanisms.

Dr Anderson made specific reference to a structure-related difficulty associated with

reaction mechanisms in an introductory lecture to his class (lecture 17081999, line 144

– 154). The associated diagram he used while discussing this error is shown in Figure

7.4.

Something that I want to point out . . . a common mistake that students make when drawing

an SN2 mechanism, is this reaction . . . they have the nucleophile and R groups . . .

something like that, no good. Ok? . . . If you’ve copied it down, put a cross through it so

you can see that it’s wrong. The OH is not attacking the methyl group of ethyl iodide, it

always attacks the carbon bearing the leaving group. If you formed an OH bond here, this

carbon here (indistinct) and the iodine is . . . the mistake doesn’t happen so often when you

actually draw out the hydrogens. The problem is for those people who use the short cut

method, like so, because they forget about the number of hydrogens here, or . . . the other

method that I often use, with the, just the lines. There are dangers with both of these if you

don’t remember that this has got three hydrogens. The attack’s not happening here, it all

happens on the carbon with the leaving group.

CH3CH2 IHO CH3CH2 IHO CH3CH2OH + I

Figure 7.4: Structural representation demonstrating Dr Anderson’s perception of a

common error.

Associate Professor Andrews commented that he had observed students using correct

mechanistic representations (for example, curly arrows to represent the movement of

electrons to break and form bonds) to explain a reaction process in a way that is not

chemically viable. In the words of the lecturer, some students ‘don’t take notice of

simple acid-base theory’ (interview 28061999, line 119) and will often write

mechanistic representations that are consistent with explaining the observed starting

136

materials and products in a given process, without considering whether or not a specific

step in their represented mechanism is chemically sensible and likely to happen.

This lecturer also discussed the implications of simplifying reaction mechanisms for

students’ understandings. Associate Professor Andrews made reference to the students

perceiving reaction mechanisms to be a way to work out exactly what would happen in

a given reaction process (interview 28061999, line 159 – 168):

[W]hen you show um, a fairly sophisticated pathway, just as an example, um, a lot of

students might think that this happened and whoever devised the pathway had a brilliant

idea, and got somebody to sit down and do it, and again, I try and remind them that this is

the final thing, but there’s been a lot of effort that’s gone in to developing each step in the

sequence. . . . [ ] . . . you just try to say that you can use theory, and it’s useful in allowing

you to predict, but . . .[ ] . . . because you’re always operating on different molecules, whilst

the mechanistic theory is useful in saying this is likely to happen, there are other factors

that may not allow that reaction to happen, and you have to explore the alternatives.

Associate Professor Andrews felt that students did not necessarily understand that

mechanisms were representations of reality, based upon actual experimental work. His

suggestion was that students might think that mechanistic theory will tell them what will

happen when two particular starting materials were mixed, not that the theory suggests

possible pathways, which can be affected by other reaction conditions (for example,

temperature or concentration of reagents). This is consistent with comments made by

Laszlo (2002), who reminded readers that ‘[t]he experimental work was antecedent

upon its description, it had to be.’

7.4.3 Students’ Perceptions of Difficulties

Students’ perceptions of the difficulties that they faced in studying about reaction

mechanisms were also investigated. Two groups of students were asked about the

difficulties they felt they’d encountered while studying about reaction mechanisms.

First year students were asked in an individual interview format, while the second year

students were questioned in focus groups.

Interviews with first year students did not identify many specific difficulties that

students believed that they had encountered in their study of mechanisms. Most of the

students made very general comments about the difficulty of learning about the topic;

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for example; ‘I think it was difficult, at first, to get a grasp of it’ (Graeme, Chemistry

100, 1999) and ‘I had no idea what was going on, but, after you see enough of them

done it was, ok’ (Brendon, Chemistry 100, 1999).

Brian (Chemistry 100, 1999) compared the perceived difficulty of the topic to other

subjects he had already learned about in chemistry (interview 10091999, line 39 – 48):

Brian: I found it a lot more difficult than the stuff we did on, um, acids and

bases, we did, we covered that briefly. Um, yeah, I don’t think I spent

enough time just going over practicing reactions and remembering them

and stuff, because it’s, you know, I could understand the basics of it, but

when it got messy, and complex, I found it pretty difficult, yeah.

Interviewer: So was there anything in particular that you found difficult, or was it just

the whole concept of it was just different?

Brian: Yeah, yeah, it was different, it wasn’t like anything we really did in high

school, whereas a lot of the stuff that we’ve done this year has just been

extensions of that. So, it’s, you know, you’ve got the basics there, so, um,

yeah, I guess, just that it was different. It was just more difficult to get

the concepts.

Two students (Barbara and Benjamin, Chemistry 100, 1999) made reference to specific

difficulties that they had encountered in their studies of reaction mechanisms. Barbara

commented on the amount of information involved in mechanistic representations,

calling it an ‘overload’ (interview 04101999, line 35), which reflect comments made by

Johnstone (1997) relating to the previously described information processing model

(section 2.5). Barbara also mentioned her perception that there was not necessarily just

one reaction mechanism for a process; ‘there’s different ways of doing the same thing . .

. [ ] . . . And that’s scary, because, you can’t be sure if you’re right’ (interview

04101999, line 44 – 51).

Benjamin made mention of the difficulty involved in visualising what is actually going

on in the reaction process. He also commented on the specificity of a reaction

mechanism; that he ‘didn’t know why that thing specifically happened, and not

something else’ (interview 04101999, line 24). The representative nature of reaction

mechanisms was not clear to Benjamin. He did not demonstrate clear understandings

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that mechanisms were used to represent or explain observable, experimental evidence.

Benjamin appeared to consider mechanistic representations as reality, not as one way of

representing reality.

Three of the nine Chemistry 100 students who were interviewed in 1999 did not

perceive mechanistic representations to be difficult to study. These students

commented that mechanistic representations were ‘relatively easy’ (Bill), ‘pretty

straightforward’ (Guy) and that the processes made sense (Boris). All three of these

students demonstrated reasonable understandings of mechanistic representations,

although they each also demonstrated some misconceptions about these representations.

These will be discussed further in section 7.4.4.

Chemistry 2XX students interviewed in a focus group situation about their perceptions

gave much more specific answers than those asked in individual interviews. This was

influenced by both the interview situation and by the type of question asked, as well as

by the students’ level of study. Their increased exposure to the topic may have

influenced their abilities to discuss difficulties associated with mechanistic

representations in more detail than other student participants.

The interviewed students were simply asked if they found mechanistic representations

to be difficult and what was difficult about them. The focus group students were given

a series of equations (Figure 7.5) for which they were asked to draw appropriate

reaction mechanisms. They were then asked to rank these mechanistic representations

in order of difficulty and then asked what about a specific reaction had made the

representation more difficult. The focus group situation allowed for reasonable

discussion between the student participants, which was helpful in identifying students’

perceived difficulties.

Two of the students (Fernando and Frank) commented that the use of the term racemic

mixture in equation (iv) made the reaction mechanism more difficult to write. These

students commented that they didn’t know or couldn’t recall what racemic mixture, a

commonly used term in organic chemistry, actually meant. In this case, it is the specific

chemistry language that has caused difficulty. As researchers such as Cassels and

Johnstone (1983) and Johnstone and Selepeng (2001) have shown, this language issue

can cause difficulty for students of many different topics.

139

(i)

ClC

H3C

H3CH3C

CH3

C

CH3H3C

+ Cl

(ii)

CC

H H

CH3

CH3

C C

CH3

CH3

H

H3CH3C

+ OH + H2O

(iii)

C C

CH3

CH3

Br

H

CH3

H3C C C

CH3

CH3

H3C

H3C

+ OH + H2O + Br

(iv)

ClC

H

H3C

H3CH2C CNC

H

H3CH3CH2C

NC C

H

CH3CH2CH3

+ CN + Cland

racemic mixture

(v)

C Br

H

H3CH3C

H3CO C

H

CH3

CH3

+ OCH3 + Br

Figure 7.5: Task given to students participating in focus groups.

140

Felix and Fiona described another factor that influenced the students’ perceptions of the

difficulty of particular tasks. These students commented that reaction processes in

which more events happened (in terms of bond breakage and formation) were harder to

represent. More complex processes, such as elimination reactions, are generally

explained by reaction mechanisms that contain more curly arrows and show more bond

breakages and formations than simpler processes, such as substitution reactions. These

more complex representations require more information to be processed and

understood. Information overload is more likely as the amount of information in a

particular representation increases (Johnstone, 1997).

Fabian’s comment about the ease of representing a reaction mechanism for equation (i)

is also linked to the number of events going on in the process. In this task, ‘there’s only

one thing really happening’ (focus group 09052001, line 86), which makes representing

a mechanism a much simpler task. As the number of pieces of information to be

processed is small, information overload is much less likely. Several students (Felicity,

Fred, Frank and Felix) commented about the changes in stereochemistry between

reactants and products that are represented in equations (iv) and (v). While Felix and

Felicity found this helped them visualise the reaction process, Fred and Frank

commented that it made the tasks more difficult. This suggests that it is the students’

abilities to interpret and understand particular representations that influence their

abilities to make sense of the representations.

Different types of structural representations can also affect students’ abilities to

understand and interpret mechanistic representations. This is shown in the following

discussion (focus group 07052001, line 146 – 174):

Frank: [T]he only other thing I would have said was a problem was with number

five, the thing which immediately threw me was, ah . . . I know, every

time I looked at it, since it was, ah, reversed on the other side . . .


Frank: . . . going . . . I mean . . . obviously you know that the, the ah . . . the

methyl groups that are on it . . . since they’re, since the way they’re

represented is reverse, is H3C . . .


141

Frank: . . . not CH3, originally, I was looking at it and I just couldn’t figure out

exactly what the hell was wrong with it . . . It was just . . . yeah, it

completely threw me.

Interviewer: Ok, so, it was representation here that was the issue.

Frank: Mmm.

Interviewer: Does anybody else have a problem with that, I mean . . .

Felix: Actually, I found it sort of helped me, because I remembered that yeah . .

. the stereochemistry was like reversed, and that was the reaction where it

comes in from the back and it flips it around.

Interviewer: Ok. Ok.

Felix: So, like . . . yeah, and it helped me here as well . . . in number four.

Interviewer: In the racemic mixture one as well.

Fiona: I think it depends on what you’re brought up with. Like I was always

brought up, um . . . from previous chemistry, that you always put the

carbon next to the bond and the hydrogen, whatever, on the other side of

the bond.


Fiona: So I didn’t have a problem with the um, the swapping of the . . . um . . . of

the CH3 as the reverse, mainly because it, I think it shows, more

accurately . . . how the um . . . the bonds (indistinct) the um . . .side chains

. . .

Interviewer: The connectivity is shown better like that (indicates H3C)?

Fiona: Yeah. Yeah. But I personally think it was probably just due to the fact

that that’s how I was always taught (indistinct) . . .

Fabian also remarked that the nature of the type of question asked in this task affected

its level of difficulty. His comments were related to something that wasn’t asked in the

focus group task; the difference in difficulty between representing explanatory

mechanisms, where students knew the reaction outcomes, and predictive mechanisms,

142

where they were provided only with the starting materials (focus group 09052001, line

207 – 218).

It’s probably made a lot easier by having the product written there . . .[ ] . . . Like, if it was

just giving us (indistinct), you know, say, with the third reaction, just had with the OH, and

tell us what it forms, and give the reaction mechanism . . . [ ] . . . you’d probably have to

think about it a bit harder, than you do, you can’t, it’s not as obvious straight in front of

you, what’s going to happen . . . [ ] . . . you know, you sort of, your mind is trailing through

all the different pathways that it could take, and then, so that’s almost trial and error, unless,

you know, you have done enough of the same problems to know what is going to happen.

7.4.4 Students’ Observed Difficulties

7.4.4.1 Introduction

Apart from those aspects that students in these two groups commented directly upon as

being more complex, other difficulties were also observed as student volunteers worked

through different tasks. These difficulties are discussed in detail in the following

sections.

7.4.4.2 Mechanistic Representations

Students who perceived reaction mechanisms to be a relatively straightforward topic

demonstrated some difficulties when it came to actually representing reaction processes.

Bill, Guy and Boris all commented that they had experienced limited difficulties in

understanding reaction mechanism representations. Although these students generally

demonstrated good understandings of the representations, all three exhibited some

difficulties in representing reaction mechanisms.

Bill’s interview suggested that he had reasonable understandings of representations of

reaction mechanisms. He was generally able to use mechanistic representations to work

through tasks and to suggest reasonable reaction mechanisms. In his interview, he

represented the formation of carbanions while working through reaction mechanisms.

These are feasible structures, but ones that are not discussed in any detail in the

Chemistry 100 course. This indicates that Bill might have begun to construct useful

understandings about how reaction mechanisms are used in predicting possible reaction

outcomes.

143

However, he did use inappropriate representations in some tasks. One

misrepresentation that Bill was observed to make on two occasions was to represent

nucleophile ions attacking substrate molecules on a carbon other than that bearing the

leaving group (Figure 7.6). This error was similar to one described by Dr Anderson as

‘common’ (section 7.4.2).

Figure 7.6: Mechanistic representations drawn by Bill in his interview. In both

examples, Bill has represented nucleophilic attack on carbons not bearing leaving

groups.

In these examples, the mechanisms drawn by Bill’s mechanisms are not feasible

reaction pathways. Bill appears to demonstrate a misunderstanding of the

representation of atomic connectivity in these types of structures. In both cases, Bill has

shown the nucleophile attacking the carbon atom at one end of the hydrocarbon chain,

while the leaving group is lost from the carbon at the other end of the chain. Bill has

exhibited this difficulty with both line-angle (1) and square planar (2) structural

representations, which might suggest that this difficulty was not necessarily particular to

the type of structural representation used.

In the simple examples shown above, Bill’s misunderstandings may not be obvious to

him, as he believes he is showing a mechanism that is consistent with the represented

product. In both cases, the starting material contains only a single functional group (OH

in example 1, Cl in example 2). Reaction with the nucleophile (Br- and OH-

respectively) results in a product that also contains only one functional group. Bill

perceived that his representations indicate the formation of these products, although the

144

representations do not show an appropriate movement of electrons in rationalising these

processes.

Bill also appears not to have noticed that his mechanistic representations would leave

one C in each structure with five bonds and the other with three. In more complex

representations, containing more that one functional group, where this inappropriate

understanding cannot be perceived to lead to the expected answer, Bill will require more

useful understandings of both structural and mechanistic representations to draw

sensible reaction mechanisms.

Like Bill, Guy demonstrated good general understandings of mechanistic

representations. One observed difficulty in his first year interview was that he tended to

represent curly arrows pointing in an incorrect direction when H+ ions were involved in

the reaction process (Figure 7.7). Although his use of this mechanistic convention was

not appropriate, the description he gave when explaining his answers were generally

consistent with describing bond breakage and formation in an appropriate manner

(interview 20091999, line 150 – 173):

Interviewer: So, what’s attacking onto the carbon?

Guy: I was thinking maybe the H would attack first, and draw off the hydroxide

group . . .

Interviewer: Oh, ok.

Guy: . . . and the Br would be left behind. Or, would come in to take the place,

because this is um . . .

Interviewer: Ok, you said it was a substitution to start, a substitution reaction to start

off with . . .

Guy: Yep. Mmmm.

Interviewer: Ok. You’ve got . . . the H plus present in solution. That’s there for a

reason. If this reaction isn’t carried out under acidic conditions, it doesn’t

work.

Guy: Ok.

Interviewer: So why would your H plus attack the carbon? (20 seconds silence).

145

Guy: It wouldn’t, really.


Guy: That would be attracted to the hydroxide group, because it’s very

electronegative.

Interviewer: Ok.

Guy: And then that would make that (indicates OH area) quite, um, unstable, so

this (indicates H2O) would come off . . .


Guy: . . . just add an electron pair. And, so this (indicates terminal carbon)

would be highly positive, which would attract your bromine . . .

Interviewer: Ok.

Guy: . . . bromine in to take its place.

Interviewer: Ok. Can you draw that on there?

Guy: Mmm (approximately 90 seconds silence). Something like that.

Figure 7.7: Representations from Guy’s interview showing incorrect use of curly

arrows when H+ was involved in a reaction process.

Guy was not the only student to demonstrate this misuse of curly arrows in first year.

Graeme, Bob, Belinda, Bill, Bruce and Benjamin all drew representations with curly

arrows pointed in a direction inconsistent with that used in reaction mechanisms

146

(indicating direction of atom movement, not electron movement) in at least one

instance, generally when acid (represented as H+) was involved in the process being

represented. However, by the time Guy was interviewed in second year, he consistently

represented curly arrows in the correct manner to indicate electron movement. Many of

the other students interviewed had developed good understandings of the curly arrow

representation by second year, and this incorrect usage of curly arrows does not appear

to occur to the same degree as was observed in first year courses.

Even though most students demonstrated good understandings of the curly arrow

symbolism by second year, there were still a small number of students who did not. An

example of this is Damon, a Chemistry 2XX student in 2000. In his first interview,

Damon used a single curly arrow to represent both the breakage and the formation of

bonds in a substitution process ((i) in Figure 7.8). He also used similar representations

in his second interview. When questioned, Damon explained this representation

(interview 28032000, line 11 – 18):

Interviewer: Um, so, the negative charge on the nucleophile . . .

Damon: Mmm hmm.

Interviewer: . . . is forming a bond between what things? The . . . you’re indicating

movement there (indicating arrow).

Damon: Ah, between the X, which is the . . . um, yeah, so the X is becoming

negatively charged . . .

Interviewer: Ok.

Damon: . . . and separating from this group (indicates carbon).

Interviewer: Ok. And what’s happening to the nucleophile then?

Damon: Ah, it’s, it’s joining this group (indicates carbon).

In separate tasks later in the same interview, Damon expressed an understanding that the

curly arrows represented pairs of electrons moving independently of atoms ((ii) in

Figure 7.8). Damon used curly arrows in this manner in two separate examples. In both

cases, he had been asked to suggest the product formed in one step of a represented

reaction mechanism. He commented (interview 28032000, line 185 – 188):

147

So, the HO’s, um, well, two electrons’ been transferred to the H on the main group

(indistinct), and that still leaves, appears to still have a negative charge on that HO, and

then the, um, the bond from the H-C bond gets transferred down to the C-C bond, and

becomes a double bond there . . .

Damon demonstrated these difficulties with the curly arrow symbolism throughout his

study of Chemistry 2XX. He demonstrated different understandings of curly arrows for

different situations. This indicated that Damon had constructed a variety of

understandings of the curly arrow representation that he used in what he considered to

be the appropriate situation.

Later discussion (in interviews and in after-interview discussion) with Damon indicated

that he did not feel his understandings of curly arrows were inadequate. Even when

faced with examples where his understandings of mechanistic representations did not

seem to make sense, Damon used his understandings in a manner that he felt was

satisfactory, which generally did not answer the researcher’s questions. As Damon

could not see any fault with his explanations, it was often difficult to draw him further

on any discussion. He considered there to be no reason to modify or discard his

understandings, and so did not attempt any changes to his understandings. The

conditions for conceptual change (Strike and Posner, 1985) were not met in this

instance, and no conceptual change was seen to occur in Damon’s understandings.

Figure 7.8: Example representing Damon’s usage of curly arrows to (i) break and form

bonds and (ii) move electrons independent of atoms.

(i)

(ii)

148

Other examples of misuse of curly arrows were seen in a Chemistry 2XX examination

question in 2000 (Appendix 6.25). Students used arrows to represent what appeared to

be atom movement ((i) in Figure 7.9) or to show something else (see (ii) and (iii) in

Figure 7.9). In many cases, without having interviewed the particular student, it can be

difficult to interpret what the student was thinking.

C

BrCH3

H

HHCH3

HO-

OHCH3

H

H CH3

HCHCH3

HCH3Br

(i) (ii) (iii)

Figure 7.9: Examples of students’ incorrect usage of curly arrows.

The top arrow in (i) appears to be indicating atom (H) movement, although the second

arrow in the same representation appears to be moving the C—H bond onto the C. The

arrows in (ii) appear to be indicating electron movement, but this movement is broken

up into stages, with the arrows moving from atom to bond and then from bond to bond.

It is also difficult to determine how many electrons are being moved by each arrow, as

there are two curly arrows representing the formation of the C—C double bond. In (iii),

the curly arrow is probably representing the formation of a double bond, although it is

unclear where the electrons to make this bond are coming from.

As the curly arrow representation is central to mechanistic representations,

misunderstandings about the correct manner in which to use it can be detrimental to a

student’s ability to successfully interpret and represent reaction mechanisms. While a

particular misunderstanding may still lead to appropriate answers in certain types of

situations (for example, students can successfully interpret and represent nucleophilic

substitution mechanisms with an atom-movement understanding of the curly arrow

representation), the misunderstandings will not allow students to interpret or represent

the correct answer for all mechanistic representations. If students find that their method

of representing and interpreting curly arrows does not work when attempting to

understand a new type of mechanism or reaction process, it can cause confusion and

149

difficulty and they will predict wrong product(s). However, such a conflict does occur,

it may promote conceptual change in students’ understandings.

An interview with Chemistry 2XX students in 2000 identified another representational

difficulty, relating to the position of the hydrogen being removed in an elimination

process (Figure 7.10). Some students’ responses indicated that they perceived the

position from which a hydrogen was abstracted (ie, from the carbon bearing leaving

group Br or from the adjacent carbon) did not affect the reaction outcome or make a

particular mechanism inappropriate. Students did not appear to realise that the product

shown in (ii) would not be formed by the processes indicated by the curly arrows.

C C

H

Br

H

CH3

CH3

H

C C

H

Br

CH3

CH3

H

H

HO

C C

H

CH3

CH3

H

+ Br + H2O

OH

C C

H

CH3

CH3

H

+ Br + H2O

(i)

(ii)

Figure 7.10: Two mechanistic representations whose correctness students were asked to

comment upon. Although both contain errors, representation (i) is more correct, as it

shows H being removed from an adjacent C (with incorrect use of a curly arrow

between H and OH- at the top left). In (ii), H is being removed from the same C as Br,

which would lead to a product other than that represented being formed.

Some of the second year students appeared not to understand that removing the H from

the same carbon as the Br ((ii) in Figure 7.10) would not produced the alkene shown.

Two carbon atoms in the resulting structure would have an incorrect number of bonds;

the carbon on the left would have five bonds, while the carbon on the right would have

three. Some students appeared not to notice this inconsistency.

150

This inability to notice inappropriate reaction mechanisms could have implications for

students’ abilities to correctly represent reaction mechanisms for particular processes, or

to make informed judgements about the feasibility and chemical correctness of reaction

mechanisms that they might find in their textbooks, laboratory manuals or chemistry

papers during their studies.

Various tasks also identified that determining charges on atoms and molecules (either

formal or overall) could be challenging for students. First year students in Chemistry

100 could generally assign formal charge to the simple example given to them in their

examination, in which they were specifically asked to determine formal charge on a

structure (Figure 7.11).

Consider the structure A. There are no non-bonding electrons (lone pairs) on

this structure.

(A)

a) On the structure, indicate any formal charges that exist.

b) What is the overall charge of the structure A?

Figure 7.11: Examination question given to Chemistry 100 students to probe their

understandings of formal charge.

However, when the example was included in a reaction mechanism, as it was in the

Chemistry 122 examinations (Appendices 6.20 and 6.21), students tended not to assign

formal charge as a part of their answer (see example in Figure 7.12). In many cases,

while the structural representation drawn by the student was correct, he or she did not

indicate a positive charge on the protonated product. The lecturer (Dr Anderson)

commented that this surprised him, as he felt that his students should be able to

correctly determine charges on different species in a reaction mechanism.

CH2CH3

C

CH3H

151

Figure 7.12: A student’s answer to an examination question in Chemistry 121/122. The

structural representation is correct, but its charge is not.

This difficulty with understanding formal charges was not limited to first year students.

In a questionnaire completed by 35 second year students (Figure 7.13), second year

students were asked to indicate the formal charges on each atom in a positively charged

molecule. The overall molecular charge was shown on the structure given to the

students. Only one third of the students correctly answered the question. Another third

did not even attempt the question. The remaining students answered the question

incorrectly. Whether the students were unfamiliar with the term formal charge or

simply did not know how to assign charge to atoms was not clarified by this particular

task, leading to students’ understandings being investigated in interviews.

Consider the positively charged protonated alcohol represented in the structure below.

In the spaces provided above, indicate the formal charges on each atom.

Hii C O

Hiii

Hi

Hiv

Hv

+

Figure 7.13: Questionnaire task investigating Chemistry 2XX students’ understandings

of formal charge.

C = ……………... Hiii = …………….

O = …………….. Hiv = …………….

152

In an interview (tasks 2 and 3 in Appendix 6.16), Fiona was asked to determine the

overall charges present on represented molecules. She decided that molecules such as

(i) and (ii) in Figure 7.14 had no overall charges.

C O

H

H3C

H

H

H

N C

H

HCH3

HH

H

(i) (ii)

Figure 7.14: Representations of (i) protonated ethanol and (ii) protonated ethanamine.

The structures are shown without represented charges, as they were given to students in

an interview.

When asked why there was no charge associated with these structures, Fiona responded

by explaining in terms of the number of electrons around each atom, appearing to

incorrectly link the concepts of formal charge and stable octets of electrons (interview

14052001, line 177 – 187):

Fiona: Um . . . You’re looking at the um . . . ah . . . just for sort of grad . . . ah,

looking at the individual atoms themselves, mainly the actual oxygen and

the carbons, and just seeing if they, um . . . following the octet rule about

sharing bonds, bonds or separate, just electrons in shells . . .


Fiona: . . . would count up to eight.

Interviewer: Ok.

Fiona: And then, just looking, if there is one that doesn’t fit that rule, and the

hydrogens are two of course, because they don’t follow the octet rule . . .

yeah, and if there is a specific thing that isn’t cancelled out or made up to

eight . . . well then, you know you’ve got a charged molecule. Yep.

This apparent difficulty with representing charge on molecules in a reaction mixture has

implications for students’ abilities to use mechanisms to explain and predict reaction

outcomes. If students have difficulty determining charge on simple structures, it is

likely that attempting to represent formal charge as a result of electron movement in a

153

reaction mechanism may also be challenging. In simple reaction mechanisms, such as

those students may encounter in first year, this may not be a particular problem, as

formal charges may not be in simple representations. However, for a student in second

or third year, who may need to write a complex mechanism for a reaction process or

interpret a represented mechanism for a reaction, this difficulty with representing formal

charges could impact on their ability to understand the representations of reaction

mechanisms.

Formal charge can also be an indication of possible areas of reactivity in a molecule, so

not representing formal charge (or not being able to determine its value) may lead to

difficulties when rationalising reaction products or predicting reaction outcomes using


7.4.4.3 Substitution Versus Elimination

Boris commented that he had few difficulties understanding reaction mechanisms.

Unlike the previously mentioned students, Boris had demonstrated good understandings

of the use of curly arrows. The appropriateness of Boris’ understanding of substitution

processes was well demonstrated in his interview. He commented that these types of

processes were ‘easy’, and was able to represent mechanisms for these reactions.

However, he exhibited some difficulty in representing reaction mechanisms for

elimination processes. This difficulty was associated with his attempts to work out the

bond breakages and formations that might happen to produce the given product in the

reaction. Boris could not rationalise how a double bond might be formed in an

elimination reaction. An example of an elimination task Boris attempted is represented

in Figure 7.15.

He explained his thought processes as he worked through the example (interview

15091999, line 272 – 294):

[I]n the first one, you’ve got a chlorine sitting there all by itself, which is very

electronegative . . . [ ] . . . Ok, I guess occasionally what is going to happen is the chlorine,

which is very electronegative, well, compared to the carbon anyway, is going to . . . remove

the two, the electron pair from its bond and break away . . . [ ] . . . Um, so you’ve got this

chloride ion suddenly, and then you’ve got a tertiary carbocation, which, um, with a

positive, let’s see, with a positive charge, you’ve got a positive charge on it, which the

154

hydroxide can sit on . . . Let’s see, I’m going to have to redraw this . . . um . . . Um . . . Ok,

I’ll draw it like that so that it’s a simple carbon with three methyl groups and a chloride, ah,

chlorine on it . . . Alright, so let’s see, if you’ve got a carbocation . . . in the centre of three

CH3 groups . . . and then you’ve got a hydroxide ion . . . that can attach itself easily enough

. . . um . . . hmm . . . I’m not really sure. I’m not really sure for this one . . . [ ] . . . Alright,

but I’ll just say, I guess that once you’ve got a hydroxide on there, um . . . no, I’m not sure

where you’re going to get the other, where you’re going to draw the other hydrogen from.

Well, it’s presumably got to come from our carbon skeleton itself, but I’m not sure how the

oxygen’s going to pick it up, so, I’ll leave that.

Figure 7.15: Example from an interview with Boris. This student demonstrated

difficulty when representing the formation of double bonds in elimination reaction

mechanisms.

Boris was not the only student to find representing elimination reaction mechanisms to

be more challenging than substitution mechanisms. A second year examination

question that asked students to represent reaction mechanisms for both substitution and

elimination processes (Figure 7.16) was answered by 37 students, and exposed some

difficulties with the representation of elimination processes.

Half of the students correctly represented a substitution reaction mechanism. Only eight

of the students could represent an appropriate mechanism for the elimination process

referred to in c. All eight of these students had correctly answered the substitution

question as well as the elimination question. There were also some common errors

noticed in students’ responses.

155

Consider the SN2 reaction represented below:

CH2CH3

C

BrH3C

H

CH2CH3

CHO

CH3

HOH Br

a) Assign the configurations at the stereogenic carbons of (A) and (B)

b) Suggest a reaction mechanism for the conversion of (A) to (B)

c) The reaction of (A) with hydroxide ions can also lead to some amount of

2E-butene. Draw a reaction mechanism that rationalises this observation.

Figure 7.16: Chemistry 2XX examination that probes students’ abilities to represent

substitution and elimination processes. Question a) was written by the lecturer and is

not a task designed by the researcher.

Approximately one third of the students sitting the examination made the task of

representing a reaction mechanism difficult by drawing incorrect starting materials or

products in their answer booklet. Although the starting material was represented on the

examination paper and the elimination product was clearly named, there were some

students who did not draw (or, in the case of the starting material, copy) the appropriate

structures. It is difficult to tell from the examination question whether students’ ability

to represent an appropriate reaction mechanism is influenced by their inability to draw

correct structures from a chemical name, or if it is their difficulties with representing

reaction mechanisms that has led them to represent an incorrect product structure.

Another error made by about one third of the students was drawing mechanistic

representations that were not consistent with the product they had drawn. In these types

of answers (see Figure 7.17 for some examples), it can be difficult to interpret students’

understandings of mechanistic representations and to identify particular deficiencies in

their understandings.

156

CH2CH3

BrCH3

H

-OHC

C

HCH3

CH3H

+ Br + H2O

C

C

CH3

H

H

H C

H

H

HBrHO -

C CCH3

H

H3C

H

2-E-butene

Figure 7.17: Examples of examination question answers in which students’ mechanisms

are not consistent with the product represented.

These types of representation suggest that students are aware that curly arrows should

be part of a mechanistic representation but that they do not understand how curly arrows

should be used. For example, mechanism 1 uses ‘fishhook’ arrows, which are used to

represent the movement of a single electron. Only two of these arrows are shown on the

representation, but the product shows the formation of a double bond, loss of Br (and

presumably its bond) from the substrate and the formation of H2O. The bond

rearrangement indicated by the product is inconsistent with the electron movement

indicated in the mechanisms. Example 2 includes three curly arrows. These arrows are

not consistent with the product of the reaction, as they indicate the abstraction of an H

atom from the same carbon as the Br atom and do not indicate the loss of the Br- ion or

the formation of the C-C double bond. Again, it appears that the student knew that

curly arrows should appear in reaction mechanisms but did not understand what the

representation meant and how it should be applied to the particular reaction.

A small number of students (five of the thirty seven who sat the examination)

demonstrated an unexpected misunderstanding in this question. They gave answers in

which they represented hydroxide ions attacking either Br or the C—Br bond on the

alkyl bromide structure (Figure 7.18). This type of attack was never shown to students

in their lectures, nor was it in any textbook and was an unexpected response to the

question.

1

2

157

CH2CH3

BrCH3

H OH

OH CH3 H

CH3 H

H2O + BrOH +

CH2CH3

BrCH3

H -OH

H

CH3

H3C

H

+ H2O + Br-

(i)

(ii)

Figure 7.18: Examples of reaction mechanisms represented by Chemistry 2XX students

in an examination in which OH- is shown attacking (i) Br and (ii) C—Br bond in 2-

bromobutane.

Both the researcher and the lecturer felt that most students would consider that

nucleophilic OH- would not attack Br on an alkyl bromide, or attack a bond. The

electron richness of such areas should lead students to an explanation that limited

attraction was likely to occur between a negatively charged nucleophile and an area

where electrons were concentrated, such as in bonds or around halides. The fact that

several students did represent this type of interaction in answer to this particular

question might indicate that at least some students held inappropriate understandings

about areas where more electrons might be concentrated in a molecule, or about

interactions between electron rich and electron poor areas in molecules. These types of

misunderstandings have implications for other chemistry topics as well as for the study

of reaction mechanisms.

Due to the difficulties that were exposed by this question, this examination question was

investigated further with five-second year volunteers in an interview situation (Task 3 in

Appendix 6.17). Four of the five interview students had seen the question previously

when they’d sat the examination. Guy and Graeme had both answered the substitution

and elimination questions correctly in the examination. Damon and Debra answered the

substitution question correctly and the elimination question incorrectly. Dianne had not

sat the examination.

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The elimination question caused difficulties for most of the students in the interview

situation. While Guy and Graeme both represented correct reaction mechanisms for the

process, Guy originally suggested an incorrect reaction mechanism. After some

consideration, however, Guy realised that his representation was not appropriate to

explain an elimination process and was able to suggest ways to represent an appropriate

reaction mechanism. In doing so, Guy indicated that his understandings of mechanisms

were appropriate for interpreting both substitution and elimination processes. He was

able to use his constructed understandings to make sense of reaction mechanisms for

two different types of process, as was Graeme.

Debra, Dianne and Damon all demonstrated difficulty in representing an elimination

reaction mechanism. Damon did not even attempt to draw a reaction mechanism for the

process, commenting that he did not know how to do it. This type of process, with its

increased amount of implicit information, was much more difficult for these three

students than the substitution mechanism that they had previously drawn.

Both Guy and Graeme were successful in their attempts to represent a substitution

mechanism. However, of the three students who had difficulty making sense of the

elimination process in the task, only Debra could correctly represent a substitution

mechanism. Dianne and Damon both demonstrated misunderstandings of curly arrows.

Damon continued to use one arrow to represent both bond breakage and formation as he

had in his first interview ((i) in Figure 7.8), commenting that ‘I think it represents both

the breaking of the bond, and the making of the new bond’ (interview 30082000, line

151 – 152). Dianne did not demonstrate a consistent use of curly arrows, but she

appeared to consider each curly arrow as moving a single electron.

Debra commented that her difficulty with deciding how an elimination reaction

proceeds was linked to the way the structure of the starting material was drawn, adding

that she wasn’t aware that the starting material could undergo an elimination process,

indicating a lack of understanding of the competitive processes that may go on in a

reaction mixture. This lack of understanding could cause problems for Debra in future

studies, where competing reaction processes become more of a central issue. This may

also indicate that Debra has formed a mental link between the type of structural

159

representation and reaction outcome. Evidence to support such cueing is presented in

greater detail in section 10.4.5.

7.4.5 Comparison of Difficulties Associated with Representing Mechanisms

The study of reaction mechanisms was perceived to be challenging by lecturers and

some students. Although both groups perceived the topic to be complex, many of the

students’ perceived difficulties did not correspond to their lecturers’ perceptions.

Lecturers felt that the representational aspects of the topic increased its difficulty.

Students felt that amount of information, the degree of visualisation required and the

terminology associated with the topic made it more difficult to study.

However, similarities were observed between the difficulties perceived by the lecturers

and those exhibited by the students when working through mechanistic problems. In

general, these related to representational aspect of reaction mechanisms, such as curly

arrows, formal charges and structural representations. This similarity between lecturers’

perceptions and students’ observed difficulties makes sense, as lecturers have based

their perceptions on the errors and misconceptions they have observed in their students

over the years, and generally not by asking students what they perceived to be difficult.

Some students used curly arrows in a manner that was not consistent with convention.

In first year, students often used curly arrows to represent atom movement, not electron

movement. This type of misrepresentation was most common when H+ was involved in

the mechanism. Apart from using curly arrows to represent atom movement, other

misuses of these representations included using arrows to represent the movement of a

single electron (Dianne) and single arrows to (inappropriately) represent both breaking

and forming bonds (Damon). Some students demonstrated poor understandings of the

curly arrow representation; by their use of multiple arrows or not representing all bond

breakages and formations with curly arrows.

Students’ difficulties with curly arrow representations might be linked to their limited

prior knowledge of this type of representation and with mechanistic representations in

general. Because they appear to have limited spontaneous knowledge about these

representations, students need to construct new understandings from the information

and examples they are provided with in their lectures. Pines and West (1986) describe

160

this type of learning situation as a formal-symbolic/zero-spontaneous learning situation.

Students are being required to construct meaning about a highly symbolic conceptual

tool; representations of reaction mechanisms; which they would not have encountered

before tertiary level chemistry study. As Pines and West commented, ‘[t]he student is

attempting in this situation to acquire pure symbolic knowledge bereft of intuitive,

experiential underpinnings’, which can increase the difficulty involved in constructing

appropriate understandings about the topic.

Perhaps as a reflection of their previous lack of understanding of these types of

representations, some students appear to have misconceptions about the use of these

representations that may be consistent with representing some reaction processes (for

example, substitution reactions) but not others (elimination). This can impact upon

their abilities to represent appropriate reaction mechanisms for different types of

reaction processes.

One lecturer felt that some students demonstrated poor understandings of the commonly

used formal charge representation. Students had difficulty working out charges on

molecules in some tasks. This difficulty was seen in students from both first and second

year courses. Often, students were able to correctly represent product structures from a

mechanism, but did not show an appropriate charge (or charges) on the product(s).

Students’ difficulties with formal charges might be linked to inappropriate

understandings about curly arrows. If a student perceives an arrow to represent atomic

movement only, then it might make sense to him or her to simply move the appropriate

atoms on a structure. To be able to determine that a mechanistic representation should

have resulted in a charge transfer of some description, students need to know that a

curly arrow represents the movement of a pair of electrons.

In the case of a student like Fiona, the difficulty with representing appropriate formal

charge appears to be caused by poor linkage of concepts in her mind. When she has

processed new information about formal charge, she has, in the words of Johnstone

(1997), misfiled this information, relating it to a concept that is actually appropriate for

useful understandings.

161

Both lecturers and students mechanisms perceived the use of different types of

structural representations as affecting the difficulty of reaction. Dr Anderson’s belief

that this extra translation could increase the difficulty of representing reaction

mechanisms was supported by the actions of several students who redrew structural

representations while working through tasks. If students made minor errors in

redrawing (translating) their structures, it impacted on their abilities to represent

appropriate mechanisms. Research has suggested that this translation aspect impacts

upon many different aspects of learning; it is not a difficulty that is solely related to the

representation of reaction mechanisms (Johnstone and Selepeng, 2001).

Students perceived the representation of reaction mechanisms to be a difficult topic for

reasons that were very different to their lecturers. They commented upon general

aspects of the topic, such as the perceived correctness of represented mechanisms and

the information overload they experienced (Barbara). Information overload can be quite

common when studying new or unfamiliar topics, as students have not yet developed

the ability to effectively chunk information while they are working with it.

This limited information processing may also be related to students’ observed

difficulties with representations of reaction mechanisms that include more events

occurring and therefore require more curly arrows. If a student is unfamiliar with these

representations, each arrow and structure (or even part of a structure) can conceivably

be one piece of information that they have to process and work with in the limited space

of their working memory.

Some students commented that they felt that the difficulty of reaction mechanism

problems could be increased by the use of unfamiliar terminology in the question. This

is also related to the difficulty in processing new information in their minds. On top of

this, students may have prior understandings or misconceptions that relate to the use of

terminology, which can affect their abilities to understand the information included in

descriptions like racemic mixture.

Students also commented that reaction mechanisms could be difficult to understand, as

it can be hard to visualise how reactants in a reaction mixture interact with each other to

form products. Visualisation is a skill that students may not have had (or needed) to

develop before they started studying chemistry. Developing an ability to visualise

162

reaction processes can be a difficult and time-consuming exercise, but it can also

increase the richness of a student’s understanding of concepts such as mechanisms.

Difficulties associated with the use of language and the need to visualise reaction

processes was not commented upon by the three participant lecturers. This is not to say

that the lecturers did not consider these to be difficult issues. There is much to suggest

that the language and symbolism of chemistry contributes to its difficulty (Hoffmann

and Laszlo, 1991; Johnstone, 1991; Millar, 1991). As described in sections 5.4 and 5.5,

the language of chemistry, be it verbal, written or pictorial, is one aspects of what makes

chemistry (and other sciences) hard to learn. It may be that the three lecturers

interviewed in this study did not mention these difficulties in their consideration of the

difficulties associated with studying reaction mechanisms, as they did not consider these

difficulties to be specific to the study of reaction mechanisms. Scientific language,

much of which has other, everyday meanings, is used in a far broader range than just

when teaching and learning about reaction mechanisms in organic chemistry. Similarly,

visualisation difficulties can be encountered in a much wider range of topics than just


7.5 Summary

There is good agreement between students’ and lecturers’ perceptions of the importance

of teaching about reaction mechanisms in organic chemistry. Both agreed that reaction

mechanisms were useful for both generalising reaction processes and predicting reaction

outcomes given the starting materials. Students suggested two other important reasons

for teaching about reaction mechanisms. Their first consideration was that mechanisms

were useful for showing what was going on in a reaction process. Second year students

commented upon the importance of learning about reaction mechanisms as a revision

tool.

Studying reaction mechanisms and their representations was considered to be a

relatively difficult by lecturers and most students. Although both commented on the

complexity of the topic, there was a difference between the perceptions of the lecturers

and of the students, as many of the students’ perceived difficulties did not match up to

163

the perceptions of the lecturers. There were some similarities noted between the

perceived difficulty that related to representational aspect of reaction mechanisms, such

as curly arrows, formal charges and structural representations.

Students exhibit a common misconception that is related to the use of the curly arrow

representation in reaction mechanisms. Many students who have not studied reaction

mechanisms perceive the representation to mean the movement of atoms or molecules,

not electrons. In general, students perceive the curly arrow to mean the movement of

the largest atom or molecule in a particular representation. This alternative conception

does not alter students’ abilities to represent or interpret substitution reaction

mechanisms, but it does affect their abilities to both calculate formal charge (which

results from the movement of electrons in breaking and forming bonds) and to make

sense of elimination reaction mechanisms, which are slightly more complicated than

substitution processes.

The three lecturers who participated in this study were all aware of this difficulty and

felt that they addressed this difficulty in their teaching. This perception was supported

by the abilities students developed later in the course. The lecturers’ intentions, course

presentation and teaching strategies will be discussed individually in the following

chapters. The abilities of students of each of these lecturers will also be discussed in

detail in the relevant chapters.

164

8 Findings From Chemistry 121/122 (Dr Anderson)

8.1 Introduction

In addition to investigating students’ and lecturers’ perceptions of the importance of

learning about reaction mechanisms in organic chemistry and the difficulties that

students may face when studying about these representations, four other research

questions were investigated.

L2: What do lecturers consider to be the essential knowledge of concepts required

by students at different educational levels to use reaction mechanisms in


L3: When teaching about reaction mechanisms:



b. what reasons and motivations do lecturers have for choosing particular

teaching strategies?

S2: What understandings do students have of the important concepts and skills in

the topics of substitution and elimination reaction mechanisms in organic

chemistry, and which factors affect their abilities to demonstrate a facility with

these concepts and skills?

S3: When using representations of reaction mechanisms what strategies do students


The findings from these questions are most appropriately discussed in terms of the

context of each of the three different courses. Although similar subject matter was

covered in all of the courses, each lecturer emphasised different aspects of reaction

mechanisms. Different teaching strategies were used in each course, which might have

an impact on the type of solving strategies the students adopted. In addition, the

Chemistry 2XX students studied a more advanced level of mechanistic representation

than the Chemistry 100 and Chemistry 121/122 students.

165

The teaching of Dr Anderson, Dr Adams and Associate Professor Andrews are

described and discussed in chapters 8, 9 and 10 respectively, along with a discussion of

their students’ achievements in tasks. Each chapter commences with a description of

any typical behaviour that the lecturer engaged in during his classes, such as

commencing with a recap of the last lecture or questioning students during his lectures.

This is followed by a description of what the lecturer intended to teach in his course,

with respect to reaction mechanisms in organic chemistry. The means by which these

intentions were identified is also described. The lecturer’s intentions are compared to

his coursework presentation and related to the issues raised in Chapters 2 and 5.

A detailed description of each lecturer’s presentation of the coursework related to the

substitution and elimination reaction mechanisms including teaching strategies is

attached in the appendices. These descriptions are based upon lecture notes, field notes

and lecture transcripts recorded by the researcher as she observed each of the lecturer’s

classes.

Scans of diagrams from lectures are copied from the researcher’s field notes, which

were reproductions of what the lecturer wrote on the board in the class. Diagrams of

students’ responses are either direct scans of their representations or ChemDraw

reproductions of the structures drawn. In the case of both Dr Anderson and Associate

Professor Andrews, where more than one year of teaching was observed, the

coursework presentation in both years will be described and any differences or

similarities between these presentations discussed.

Following this is a discussion of the students’ demonstrated abilities in interpreting and

representing reaction mechanisms at various stages during the course. As each group of

students (both from different courses and different years in the study) were asked to

work through a variety of different tasks, each of the tasks is described briefly.

These descriptions of lecturers’ presentations and students’ demonstrated abilities

support the following claims made by the researcher:



166


lecturers. These outcomes are rarely articulated by the lecturers and are generally not

achieved by the students.



generally used when writing substitution reaction mechanisms, while square planar

structures are used to represent elimination reaction processes. Although these structural

representations are useful for representing features of different reaction processes, the

use of three-dimensional representations can cue students into thinking about only a

substitution process, while square planar representations can cue them to think about

only an elimination process.

The language used in many instances in lectures is consistent with a consideration of

only individual particles in reactions, and not with multiple particle interactions.


require a consideration of multiple reaction particles. Students tend to display single

particle understandings of reaction processes.



understandings of the multiple processes going on within a reaction mixture. These

strategies are generally consistent with those demonstrated by the lecturers in

representing reaction mechanisms.

Pedagogical implications arising from the findings described in the following chapters

are discussed in Chapter 11.

8.2 Presentation of Coursework

Dr Anderson’s classes were observed in second semester for two consecutive years

(1999 and 2000). In 2000, his final two lectures, in which there was little discussion of

substitution and elimination reaction mechanisms, were not observed. Detailed

descriptions of Dr Anderson’s teaching are included in Appendix 8.1.

167

In both 1999 and 2000, the topic of reaction mechanisms and their representations was

introduced in semester 1 when students were learning about the chemistry of alkenes.

Because of this, by semester 2, when they studied substitution and elimination reaction

mechanisms, students had already been exposed to the curly arrow representation and

shown how to calculate formal charge, although Dr Anderson did not generally use the

term formal charge in his lecturers; he used the term charge.

There were 13 organic chemistry lectures in second semester at University B. The

lecturer covered substitution and elimination reaction mechanisms as a part of his

discussion of the chemistry of alkyl halides between lectures two and six. The

recommended textbook was Organic Chemistry: A Brief Survey of Concepts and

Applications by Bailey and Bailey (1995 and 2000). Typed lecture notes prepared by

another lecturer at University B several years earlier were also recommended reading.

Dr Anderson rarely made reference to the recommended text or the prepared notes

while teaching.

At the beginning of semester, Dr Anderson provided his students with a course outline

that detailed the topics they were going to study. Although the presentation of the

outline was slightly different in 1999 and 2000, the same aspects of the course were

mentioned on the outline, which is summarised in Table 8.1.

Each week, Dr Anderson started his lecture with a short introduction that he referred to

as the Highlights of Last Lecture. In a few minutes, he summarised the main points of

the previous week’s lecture. This summary was commonly displayed on the overhead

projector or written on the whiteboard. Most students were observed to write notes on

Dr Anderson’s highlights. In addition to these highlights, Dr Anderson ended each of

his lectures with a brief reminder of what he called the take home message from that

class, in which he told the students what he considered to be the most important things

from that day’s lecture. The take home message mentioned at the end of each lecture

always formed part of the highlights discussed at the start of the next lecture.

When teaching, Dr Anderson asked questions of his students to attempt to involve them

in the discussion. In many situations where there were several possible answers to his

questions, he asked students to vote on the correct answer by raising their hands.

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The lecturer often used documented experimental evidence to introduce new concepts

and theories to the students. He employed this technique when teaching about

substitution and elimination mechanisms in second semester, citing data such as relative

rates of reaction or percentages of reaction products as a starting point to talk about new

chemistry (Table A8.1 on page 371 in Appendix 8.1).

Table 8.1 Organic chemistry syllabus for Chemistry 122.

Section Topic

Functional groups Introduction to common functional groups.

Alkyl halides

Nomenclature, properties, preparation, reactions, nucleophilic

substitution reactions, SN1 and SN2 mechanisms, stereochemistry,

elimination, substitution versus elimination.

Alcohols, phenols

and ethers

Nomenclature and classification, properties, acidity, preparation,

reactions (dehydration, esterification, oxidation).

Polyhydric

alcohols

Geminal diols, glycols, glycerol and higher polyols, ethylene

oxide and nitroglycerin.

Aldehydes and

ketones

Nomenclature, nature of carbonyl group, preparation, reactions,

oxidation and reduction properties, tests.

Carboxylic acids

and derivatives

Nomenclature, properties, acidity, preparation, reactions.

Amines Nomenclature and classification, properties, basicity, preparation,

reaction, azo-dye formation.

Lipids: oils, fats

and waxes

Structure, properties, reactions (hydrolysis, reduction, oxidation),

soaps and detergents.

Interconversion of

functional groups

A review of the reactions of the functional groups considered.

Dr Anderson generally used language that was consistent with a single particle

visualisation of reaction processes. He commonly represented unbalanced chemical

equations and drew representations of reaction mechanisms showing interactions

between single molecules of substrate and nucleophile. These types of representations

were consistent with those shown in the recommended textbook.

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In 2000, the lecturer was observed to use the curly arrow representation less in his

introductory lectures, particularly when discussing substitution reaction processes. For

an example of this, compare Figure A8.1 (from 1999) and Figure A8.13 (from 2000) in

Appendix 8.1. While the lecturer is describing similar processes (nucleophilic

substitution of alkyl halides) he has chosen not to use the curly arrow representation in

the latter diagram. As has previously been described in section 7.4.2, Dr Anderson had

consciously decided to use fewer curly arrows when teaching about reaction

mechanisms. He felt that the curly arrows may be confusing to the students and was

attempting to improve the students’ understandings of mechanistic representations

without using curly arrows.

However, in an interview after observation of his course had ceased, the lecturer

commented that after attempting this technique, he decided that it was more appropriate

to teach using curly arrows and spend more time on helping his students understand

what the representation was attempting to show. The researcher did not observe the

course in the years where Dr Anderson returned to teaching about curly arrows in more

detail.

8.3 Intentions Implemented in Class Presentation

8.3.1 Stated Intentions

Dr Anderson’s teaching intentions were identified through interviews with him and an

analysis of the course outline provided at the beginning of the course. The lecturer

discussed this course outline with his students when he presented them with the outline.

The lecturer was first asked to define the knowledge that he considered essential for the

students to have at the completion of Chemistry 121/122. He began his discussion by

commenting that, ‘to have a fair grasp of arrow pushing and mechanisms, you really

have to have a fair grasp of molecular shape, or structure’ (interview 23012001, line 902

– 903). He also commented that he considered students’ understandings of atomic

connectivity in a molecule to be very important.

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Dr Anderson also considered it important that students could recognise areas of polarity,

which he referred to as ‘sites of electron deficiency and electron richness’, in a

represented molecule. He linked this to findings from a recent research study (Teo,

2000), which was carried out at the same time as some data collection for this PhD

thesis, that identified that students sometimes experienced difficulty in identifying the

attacking part of a nucleophile, the electron rich portion.

The lecturer added that he felt it was important for students to be able to speak the

language of reaction mechanisms. He made particular reference to the linguistic aspects

of the curly arrow representation as being an important part of the understanding of

mechanistic representations.

As was discussed in section 5.5.1, there are many linguistic aspects that are important in

the study of this topic. Students need to develop an understanding of the written and

spoken words used in the description of the topic (for example; racemic mixture,

stereochemical implications) and of the representational aspects of the topic that include

curly arrows and formal charges.

Dr Anderson was also asked to comment upon a proposed list of essential knowledge

for the understanding of mechanistic representations that was prepared by the researcher

(Appendix 8.2). The lecturer commented that he considered most of the knowledge

suggested by the researcher to be essential. He again suggested that an understanding of

polarity (or areas rich in charge) was important to students’ understandings, pointing out

that it hadn’t been considered by the researcher. However, he disagreed with the

researcher’s suggestion that an understanding of common words (such as substitution

and elimination) was necessary for students to construct good understandings of the

processes described. Dr Anderson argued that a student did not need to know that a

reaction was described as a nucleophilic substitution to be able to correctly represent a

mechanism for the process. He did not comment on how students’ understandings (or

lack thereof) of common mechanistic terms (such as racemic mixture or stereochemical

inversion) might affect their abilities to interpret what was being taught in his lectures,

even though he commonly used such terms in his teaching.

The section referring to the study of substitution and elimination reaction mechanisms

was presented as part of the alkyl halides section of the course; ‘reactions of alkyl

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halides, nucleophilic substitution reactions, stereochemistry and mechanism of SN1 and

SN2 reactions, elimination reactions, substitution versus elimination’. The lecturer

expected that students would be able to represent mechanisms to explain and predict

outcomes of unimolecular and bimolecular substitution reactions, and discuss the

stereochemical implications.

The course outline also indicated that students would be required to develop an

understanding of both SN1 and SN2 substitution processes, elimination reactions and of

the competition between substitution and elimination reactions in particular reaction

mixtures. This competition suggests an implicit need for students to develop

understandings of the multiple particle nature of reaction processes.

When asked by the researcher if he felt that the ability to consider the implications of a

multiple particle representation is important in the study of reaction mechanisms, Dr

Anderson agreed that this ability would be useful (interview 23012001, line 1061 –

1073):

The teaching of . . . SN1, it just dawned on me, with a multiple particle representation would

make a world of difference, because I think, the way we teach it, it cannot, you have a

carbocation that can either attack above or attack below, students may find that if it attacks

above, you get this isomer, attacks below you get this isomer . . . [ ] . . . and depending on

what day you do the reaction, you might get the above attack, or the below attack. And . . .

whereas if I had a situation where it says, ok, now we’ve got . . . a hundred carbocations in

here, and on average, fifty percent of the times they’re going to come from here, and fifty

percent of the times they’re going to come from there, so, you get fifty and fifty of each . . .

[ ] . . . If I taught it like that, maybe it would make a little more sense. With SN2, they’re

always going to come from the back.

Although Dr Anderson did comment upon the usefulness of a multiple particle

understanding of reaction processes to help students learn about some aspects of

reaction mechanisms, these comments were made after observation of his courses had

ceased. As Dr Anderson’s courses were not observed after this, it is not known if he

implemented any particular strategies to develop his students’ abilities to visualise

multiple particle reaction processes. For this reason, an appreciation of the multiple

particle nature of mechanistic processes will not be considered to be an explicit and

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intended learning outcome of Dr Anderson’s courses in the two years that the class was

studied.

8.3.2 Implemented Intentions

Dr Anderson did take time to explain typical representations related to reaction

mechanisms to his students. He believed that understanding these representations

would assist students in their learning about reaction mechanisms. This is consistent

with his views that students need to learn the language of these representations to be

able to make sense of reaction mechanisms. This is also consistent with the views of

Millar (1991), who believed that science topics were made more difficult due to the

‘distance’ between scientific and everyday language, and with Laszlo (2002), who

commented upon the need for students (and lecturers) to be aware of the limitations of

representations and language used in teaching and learning about chemistry.

Examination of lecture transcripts has identified several instances where the lecturer

described linguistic and representational aspects of reaction mechanisms. In several

lectures, Dr Anderson described the use of curly arrows and the representation of formal

charges in reaction mechanisms, the importance of molecular shape and connectivity

and the need to consider polarity of molecules (or bonds within molecules).

In one of his introductory lectures on reaction mechanisms, Dr Anderson reminded the

students what curly arrows were used to represent. His students had been taught about

curly arrow representations in the previous semester (lecture 03081999, line 118 – 122):

I talked about arrows last semester, just to refresh your memory, the arrow indicates the

movement of two electrons . . . the movement of two electrons, and all they’re saying is

these ones there, instead of being shared, or belonging to the oxygen only, it’s now going to

be shared between the oxygen and the hydrogen. So, we’re saying that these two electrons

are going now be shared between oxygen and hydrogen, to give us our . . . our oxonium

species . . .

In a later lecture, Dr Anderson gave an example (Figure A8.5 in Appendix 8.1) of how

to represent reaction mechanisms using curly arrows (lecture 17081999, line 114 –

118):

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I want to refresh your memory on how we draw these mechanism processes out with

arrows. So . . . SN1, if we consider as an example . . . tertiary butyl chloride, or 2-chloro-2-

methylpropane . . . what we’re saying is, the electrons that were in this carbon-chlorine

bond are going to no longer be shared between the carbon and the chlorine, they’re going to

reside on the chlorine atom, so we end up with . . . a carbocation, and a chloride ion.

The lecturer also explained his method of determining formal charge on atoms to his

students, without using the term formal charge, instead referring to it as simply charge.

He used an example structure in an equation to show students how he calculated this

charge (Figure A8.1 in Appendix 8.1). He gave students a similar example in 2000,

using the third equation represented in Figure A8.13 in Appendix 8.1 to work through a

charge calculation (lecture 08082000, line 13 – 20):

Why do we have a positive charge? Well, if you look at this . . . on this side of the reaction,

the overall charge is zero, neutral, neutral. So, therefore, on this side, the overall charge

must also be zero. So, if this is a negative (indicates halide), this beast (indicates RNH3)

must be positive . . . The reason it is positive is that the nitrogen of the ammonia has used

its two electrons (indicates lone pair on :NH3) to form the carbon-nitrogen bond, and it’s

now sharing a pair of electrons, instead of having them all to itself, it’s effectively lost one

electron, and so if it’s charged, it goes from neutral to plus.

Dr Anderson used this style of explanation on many other occasions when he worked

through examples where charged species (such as halide ions or carbocations) were

formed in a reaction process. The lecturer was consistent in his manner of describing

the determination of charge on a species by comparing the number of electrons an atom

had access to in its starting material structure with the number it had access to in the

product structure.

The lecturer commonly discussed atomic connectivity as he described particular

reaction processes. He called students’ attention to which atoms were connected with

each other, often describing reaction processes in terms of the change in connectivity;

that is, which bonds were broken and which bonds were formed to change starting

materials into products. This approach formed part of Dr Anderson’s introductory

lectures on the chemistry of alkyl halides. There were many instances identified in Dr

Anderson’s lectures where he described functional groups or their reactions in terms of

atomic connectivity:

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Phenols are similar to alcohols. They have an OH group. The only difference is that the

OH group here is attached directly to a benzene ring, whereas here, the OH is attached to a

alkyl chain.

(lecture 27071999, line 64 – 66)

Aldehydes, ketones, carboxylic acids, esters, anhydrides and amides are all related. Can

you see they’re related in they’ve all got carbon double bond oxygen? Each one. That

carbon double bond O is referred to as a carbonyl group, and in some sense all have very

similar tendencies because of this carbonyl group.

(lecture 27071999, line 78 – 81)

R-X has got to become an R-Y. For that to happen, the R-X bond, obviously is broken,

somewhere along the way, because there is no R-X in the product. And, somewhere along

the way, the R-Y bond must be formed.

(lecture 10081999, line 237 – 239)

[T]he carbon-iodine bond . . . is being broken as the carbon oxygen bond, the alcohol, is

being formed . . . So . . . what would that tell you about the approach of the hydroxide ion .

. . ? How would you expect the hydroxide ion to, um, react with the carbon . . . ?

(lecture 15082000, line 196 – 199)

Dr Anderson mentioned the shape of molecules as a defining characteristic of whether

or not substitution processes would proceed by either SN1 or SN2 reaction mechanism.

He used a simple analogy (that a pen and its lid must be correctly aligned to each other

and pushed together with appropriate force for the lid to stay on the pen) to help explain

to his students the importance of correct orientation and appropriate force of collision

between molecules for successful reaction to occur. He also used an overhead slide

(Figure A8.8 in Appendix 8.1) to demonstrate how molecular shape, in this case, the

crowding of a particular atom, could influence the type of reaction process that is more

likely to happen.

The lecturer often indicated the polarity of reactants when working through reaction

mechanisms. This was not limited only to the classes in which Dr Anderson discussed

substitution and elimination reaction processes. In many different lectures, Dr

Anderson used the representations δ+ and δ- when discussing the chemistry of

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particular compounds to indicate areas of possible reaction in a molecule. Examples are

shown in Figures A8.16 and A8.18 in Appendix 8.1 and in Figure 8.1.

Figure 8.1: Dr Anderson’s use of δ+ and δ- symbolism in different structures.

In terms of the outcomes detailed on the course outline, Dr Anderson devoted a lot of

lecture time to discussing the mechanisms and stereochemical implications of

substitution reactions. He used both static (written on the whiteboard or on overhead

slides) and animated (ChARMs, Capon, 1996) representations to detail these processes

to the students. He felt that using these different types of representations did help his

students to gain an appreciation of mechanistic representations. However, all of the

representations that he used in his lectures were consistent with a single particle

description of the processes he was describing.

Dr Anderson’s discussion of elimination processes was much briefer than that of

substitution processes, which appears to be consistent with the outline intentions (Table

8.1). Substitution processes were described over the course of several lectures.

Elimination reaction processes were discussed in only one or two lectures each year.

Additionally, the lecturer covered competition between substitution and elimination

processes by using one example reaction and indicating changes in product composition

under different conditions (Figure A8.12 in Appendix 8.1). There was no discussion of

the inherent multiple particle nature of the reaction mixture that would allow for the

formation of different reaction products from the same reaction mixture. No reaction

mechanisms were represented for a competitive reaction process. Students were only

shown an equation and told about the particular amounts of each product produced at

different reaction conditions.

As has been described in section 5.7, reaction mechanisms can be used to explain both

the formation of a single molecule of product from interaction between single molecules

of reactants, or to describe multiple particle processes, such as the interaction between

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many particles in a reaction process. A single particle description may be suitable for

describing the formation of a single product from a given reaction process, but will not

be sufficient for describing a reaction in which more than one product is formed.

8.4 Students’ Achievements

8.4.1 Introduction

The understandings of Dr Anderson’s students were investigated from 1999 to 2002.

The instruments used to probe their understandings were questionnaires (at the start of

either semester between 2000 – 2002, Appendices 6.4, 6.9 and 6.12), laboratory test

questions (mid-way through semester 2 in 1999 and 2000; Appendices 6.20 and 6.21)

and examination questions (end of semester 2, 1999 and 2000, Appendix 6.22 and

6.23). The laboratory tests were conducted in laboratory classes that Dr Anderson

supervised. Due to timetabling restrictions, the students in these laboratories did not

necessarily attend Dr Anderson’s lectures even though they attended his laboratory

class.

The questionnaires conducted in 2001 and 2002 when the course was not being

observed were carried out to investigate students’ prior understandings of arrow

representations. The results of these questionnaires have been discussed previously in

section 7.2.

8.4.2 Understandings of Curly Arrows

As students’ understandings of the curly arrow representation was something that the

lecturer felt increased the difficulty of the study of the topic (section 7.4.2), these

understandings were investigated in a questionnaire early in semester 2, 2000 (questions

shown as (i) and (ii) in Figure 8.2). Dr Anderson had used curly arrows in his classes in

semester 1, so the expectation of both the lecturer and the researcher was that students

would be familiar with their representation and would be able to use curly arrows in an

appropriate manner.

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C Br

CH3

CH3H

CH3

C

HCH3

+ Br

CH3

C

HCH3

+ OH CHO

CH3

CH3H

C Br

CH3

CH3H

CH3

C

HCH3

+ Br

(i)

(ii)

(iii)

Figure 8.2: Representations of (i) a neutralisation process, (ii) an ionisation process

and (iii) the most common, but incorrect, use of curly arrows to represent process (ii)

pictorially.

Analysis of the questionnaire indicated that many Chemistry 121/122 students did not

use curly arrows when they represented reaction processes. Only a third of the students

surveyed actually used any kind of arrow in response to being asked to ‘represent this

description pictorially on the represented equation’. The majority of students who did

use curly arrow representations used them in an appropriate manner for question (ii) ((i)

in Figure 8.2), which was an example of what Wentland (1994) referred to as a

neutralisation process. The students demonstrated less facility with curly arrows in the

ionisation (Wentland, 1994) process (question (i), shown as (ii) in Figure 8.2). The

most common arrow representation drawn by the students involved pointing an arrow

from the C to the Br ((iii) in Figure 8.2), which is an incorrect use of the curly arrow

representation. Correct curly arrow usage is from the C—Br bond to the Br atom.

The results from this questionnaire suggest that most students find neutralisation

processes ((i) in Figure 8.2) easier to represent than ionisation processes (ii). Students

do not need to have an appropriate understanding of curly arrows to draw a correct

representation of a mechanism for a neutralisation reaction. The correct mechanism

(with a curly arrow from the OH- to the central C of the carbocation) could also be

interpreted as representing atom movement, which is a simpler concept for students to

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grasp. An ionisation process is more difficult to represent using arrows to show atom

movement.

The most common representation (iii), with an arrow from the C atom to the Br atom, is

not consistent with either atom or electron movement. Exactly what students were

attempting to represent with this type of arrow is unclear. As interviews were not

conducted with Dr Anderson’s students (due to very limited numbers of student

volunteers), no further information was gathered concerning the understandings that

lead to this type of misrepresentation.

Dr Anderson also felt that students’ understandings of formal charge symbolism were

sometimes a stumbling block to them constructing good understanding of mechanistic

representations. As a result of discussions concerning this perceived problem, a

substitution question in the end of semester examination was designed to investigate

students’ understandings of charge transfer in mechanistic representations. The

examination question is shown in Figure 8.3, together with the most common (and

incorrect) responses given by students.

C Cl

CH3

HH

H3N

CH3N

CH3

HH

CH2N

CH3

HH

or

(i)

(ii)

Figure 8.3: An examination task and the two most common (but incorrect) answers

given by students. Representation (i) correctly follows the curly arrows, but does not

account for formal charge. Representation (ii) is inconsistent with the two curly arrows

shown.

This examination question had two aims. It was used to identify if students could

correctly interpret curly arrows in a representation to indicate what product would be

formed from the represented bond breakages and formations. The question was also

179

designed to investigate students’ understandings of the implications of moving electrons

between atoms in a represented mechanism, in terms of charge transfer that must occur

if electron positions are changing. A similar task asked in the 1999 examination (see

question (iii) in Appendix 6.22) suggested that while some students (approximately one

third of those sitting the examination) could follow simple curly arrow representations

to correctly represent the structures that would result from particular bond breakages

and formations, the majority of students did not perceive that there was a charge transfer

associated with these events. Only three correctly represented a charged product in

response to this question in 1999.

Similar results to the 1999 findings were seen in the responses to this question in the

Chemistry 121/122 examination in 2000. Two thirds of the students gave answer (i)

(Figure 8.3) in the examination. This finding suggests that most of the students have a

good understanding of the curly arrow as representing bond formation and breakage in

this particular example. However, these students have demonstrated a limited

understanding of the representation of charges on structures as an outcome of the

electron movement that causes the breaking and formation of bonds. The students’

abilities to represent a correct structure but to not represent the charge suggests that

students’ constructed understandings of the curly arrow representation appear to be

incomplete, missing the important link between representing electron movement and

charge transfer.

This may be the result of students forming inappropriate links between information they

have learned in their classes, what Johnstone (1997) referred to as a misfit (or misfiling)

of new information. It may be that students have not perceived a link between electron

movement and formal charges and have simply filed them as separate ideas of facts in

their minds. Alternatively, while students may have encountered tasks like this before,

they may not have perceived that there were deficiencies or inconsistencies in their

understandings. If the students had not felt that there was a conflict between their

understandings and what their lecturer was teaching them about, they had no need to

attempt to construct new, more workable explanations and would have continued to use

their prior understandings (Strike and Posner, 1985).

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A much smaller number of students (15 %) gave the answer represented as (ii) in

response to this question. This structure is not consistent with what the curly arrows are

intending to represent (there are only two Hs represented on the N in the product, not

three as indicated in the starting structure) and may indicate that these students have

constructed an incorrect or inappropriate understanding of the curly arrow

representation. This may also indicate that some students consider a protonated amine

compound (RNH3+) to be unstable, unfamiliar or unlikely to form in a reaction process.

Of the 78 students who sat the examination, only six gave an answer that showed the

correct structure and the correct charge. This would suggest that only a small number of

Chemistry 121/122 students had formed appropriate understandings of the link between

the curly arrow representation to break and form bonds and associated charge transfer,

as the curly arrows are representing the movement of electrons in the breaking and

formation of bonds. This difficulty has implications for students’ future studies in

organic chemistry. It impacts their abilities to use reaction mechanisms to generalise

reaction processes and to predict reaction outcomes. For example, if a student does not

understand how to represent formal charges, he/she might not be able to identify

reactive areas in a molecule, which might lead to difficulties in representing

mechanisms. Representations of reaction mechanisms are useful tools for designing

laboratory processes and students whose abilities to write reasonable mechanisms are

compromised by them not understanding the link between formal charge and curly

arrows might experience difficulties in their future studies when they are required to

design their own laboratory procedures to make particular compounds.

8.4.3 Understanding Common Representations: Substitution vs Elimination

Processes

It was observed that students found it easier to follow curly arrow representations when

the process represented was a substitution reaction rather than an elimination reaction.

For example, in one of the questions in the 1999 examination, students were asked to

‘Give the products of the following transformations’ (Figure 8.4). Example (i) is an

elimination process and (ii) is a substitution process. Expected answers are also shown.

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CH3

C

H

H3C CH2

Br

HO

C C

H

H

H3C

H3C

+ H2O + Br

CH3CH2CH2 OH2

Br

CH3CH2CH2Br + H2O

(i)

(ii)

Figure 8.4: Two questions in the 1999 Chemistry 122 examination and their answers.

An analysis of these two tasks shows that task (i) requires students to carry out more

steps or sub-tasks than task (ii). This is shown in Table 8.2.

Table 8.2: Task analysis of tasks (i) and (ii) from Figure 8.4.

Task (i) Task (ii)

Interpret the curly arrow from –OH to H as the

formation of a bond between O and H.

Interpret the curly arrow from Br-

to C as the formation of a bond.

Represent an O—H bond. Represent a C—Br bond.

Interpret the curly arrow from C—H to C—C as

the breaking of the C—H bond and the

formation of a C—C double bond.

Interpret the curly arrow from C—

O to O as the breaking of a bond.

Remove the C—H bond. Remove the C—O bond.

Represent a C—C double bond. Calculate any charge present on

atoms in the products.

Interpret the curly arrow from C—Br to Br as the

breaking of a bond between C and Br and the

formation of Br-.

Represent the resulting structures.

Remove the C—Br bond.

Calculate charge present on atoms in products.


182

While the majority of the students (greater than 90 %) could correctly answer the

substitution question (ii), over half of the students could not predict the correct answer

for (i), which represents an elimination process. This is consistent with the findings

described in sections 7.4.3 and 7.4.4, which suggested that students found examples

with more information more difficult to interpret and represent. As there is more

information to be processed in examples similar to (i) than to those like (ii), this type of

question supports the notion of the possibility of information overload when dealing

with new information (Johnstone, 1997).

There was one incorrect answer given to question (i) by a majority of the students

sitting the examination. This most common incorrect answer was a compound with an

alcohol structure (ie; the students had indicated the OH group attaching onto a carbon in

the represented structure). These types of structure were consistent with the students

interpreting the curly arrows to represent atom movement (OH moves towards C to

form C—OH bond) rather than electron movement. These results suggest that the

students’ understandings of curly arrows are not appropriate for correctly understanding

reaction mechanisms. It seems that the students’ limited understanding of this

mechanistic representation is hindering their abilities to correctly interpret simple

elimination mechanisms.

As a result of these findings, which were discussed with Dr Anderson prior to him

commencing his teaching in 2000, a similar examination question was asked of the

Chemistry 122 students in 2000.

The question was simplified somewhat as a result of the findings from the 1999

examination. In 2000, the question contained two curly arrows, not three, and the

structure was drawn out a little differently, with all the bonds to two of the carbons

represented (Figure 8.5). The intention was to identify if simplifying the elimination

question would make the task easier for the students to understand. If so, this would

support the notion that the example used in the 1999 examination might have been more

difficult due to the extra translation steps (or interpretation) required by the sub-tasks.

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H

C

H

H3C C

CH3

CH3

HO

C

H

H3CC

CH3

CH3

+ H2O

Figure 8.5: A modified question for the examination in 2000.

A task analysis for this mechanism is shown in Table 8.3. This task has fewer steps or

sub-tasks than that shown in Figure 8.4.

Table 8.3: Task analysis of the task from Figure 8.5.

Task

Interpret the curly arrow from –OH to H

as the formation of a bond between O and

H.

Represent an O—H bond.

Interpret the curly arrow from C—H to

C—C as the breaking of the C—H bond

and the formation of a C—C double bond.

Remove the C—H bond.

Represent a C—C double bond.

Calculate any charge present on atoms in

the products.


In 2000, the examination results suggested that in this form, students found the question

easier to follow than the similar question asked in 1999 ((i) in Figure 8.4). Three

quarters of the students represented an appropriate structure in response to this question

in the 2000 examination. This was a marked improvement over the students’

demonstrated abilities in 1999. As in 1999, the percentage of students in the 2000

examination who correctly predicted the elimination product was, however,

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nevertheless lower than those who could represent correct structures for substitution

processes (85 %).

The large increase in the number of students who were able to represent correct

elimination products in the 2000 examination in comparison to 1999 supports the

hypothesis that the manner in which the question was represented in 2000 made the task

easier for students to interpret. The task involved less interpretation steps, which

decreased the number of pieces of information that the student had to consider in

determining an answer to the question, which, according to Johnstone (1997), will make

it easier for students to process the information.

8.4.4 Understandings of Competing Reaction Processes

The researcher believes that the phenomenon of competing reaction processes required

a multiple particle explanation at the molecular level (Table 5.2 on p. 89). As has been

described, Dr Anderson did not use these types of explanation when teaching about

competitive reaction processes, although he wanted students to be able to describe

competing reaction processes. Several tasks designed to probe students’ understandings

of this aspect were included in two laboratory tests in 1999 and 2000 (Appendices 6.20

and 6.21) and in an examination question in 2000 (question 1 in Appendix 6.23).

In both laboratory tests, students were asked to identify an organic byproduct that could

be produced in a particular reaction whose major product was formed by substitution.

The correct answer to the test question was 1-butene, the product of an elimination

reaction. Dr Anderson had discussed the formation of this product in his pre-laboratory

discussion before students had attempted the laboratory work, the week before they sat

the laboratory test. Almost 50 % of the 1999 students could correctly identify this

possible byproduct, compared to only 25 % in 2000.

There appeared to be several difficulties that the students faced in answering this

question correctly. The language used in the question may have made the task more

difficult for students to answer. Many of the students did not appear to understand the

term organic byproduct, as they suggested water as a possible organic byproduct.

Water is a byproduct of one of the possible reactions that may occur, but it is not

considered an organic compound. While these students did appear to understand the

185

term byproduct, they did not demonstrate an understanding of the scientific meaning of

organic in their answer.

Sharp (1990, p. 289) describes the term organic as relating to ‘compounds of carbon’,

compounds which often contain hydrogen in addition to carbon, and can also contain

oxygen, nitrogen, halides, sulfur and phosphorous in their structure. Dr Anderson felt

that the students should have been able to understand this term, but the task results

suggest that many of them did not. This finding provided evidence that the students had

difficulty understanding written language in the context of chemistry (Cassels and

Johnstone, 1983).

Another difficulty that the students faced appeared to be related to misunderstandings

about how alkenes could be formed from alcohols. Even when students did consider the

possibility of elimination processes happening, many did not appear to have a clear

understanding of the process that could form an elimination product from 1-butanol.

This was indicated by the fact that many students commented on the possibility of the

formation of 2-butene. A quarter of the students in 1999 suggested this as a possible

byproduct, while 15 % of the 2000 students included this in their answer.

2-Butene is not a possible product of the reaction between 1-butanol and phosphoric

acid, the starting materials in this laboratory exercise. Students who gave this response

may also be confusing the formation of an elimination product in a given reaction with

discussion of Zaitsev’s rule from both first and second semester classes. Zaitsev’s rule

is based upon observation of the more common elimination products in reactions. The

students had been taught that when several elimination products are possible, the more

substituted alkenes are most likely to be produced. 2-Butene is a more substituted

alkene than 1-butene, but it is not a possible reaction product.

The response to this question may indicate that some students have formed

inappropriate linkages between Zaitsev’s rule and possible elimination products in

particular reactions, which will have implications for their abilities to correctly use

mechanisms to rationalise and predict reaction outcomes, as well as to design laboratory

syntheses.

186

In addition to the students who suggested the formation of 2-butene, a similar

percentage of the students who sat the test in 2000 (15 %) named the byproduct simply

‘butene’ without drawing a structure, not indicating the position of the double bond that

was formed. This may be because students did not realise that there is more than one

compound whose name is ‘butene’, or that they do not understand the rules associated

with nomenclature in naming organic compounds. As many students did not draw

structures, it is not possible to determine whether students were considering appropriate

compounds or not.

This linguistic difficulty has implications beyond the study of reaction mechanisms.

Organic chemistry students need to develop a strong understanding of and ability to use

the conventions associated with nomenclature. In the classroom and the laboratory,

certain assumptions are often made about students’ abilities to understand and interpret

names of chemicals. If students do not fully understand how to name compounds, this

can affect their ability to carry out laboratory procedures and to communicate

effectively about chemistry. Additionally, there may be a safety aspect to not

understanding nomenclature—some compounds have remarkably similar names but

vastly different chemical properties and inherent dangers.

In 2000, students were asked one additional question in the laboratory test:

Based on the results of this laboratory experiment, what species would you expect to be

produced by the reaction of 2-pentanol and hydrobromic acid in the presence of

concentrated sulfuric acid? Represent this as a reaction.

This task was designed to enable students to use what they had learned about the

reaction between 1-butanol and hydrobromic acid and apply it to a different situation.

The example used was the reaction between 2-pentanol and hydrobromic acid. There

are four possible products of this reaction; 2-bromopentane, a substitution product, and

three elimination products, 1-pentene and cis- and trans-2-pentene (Figure 8.6).

187

H3CCH

CH2

H2C

CH3

OHH3C

CHCH2

H2C

CH3

Br

conc HBr (i)

HC

C

H2C

CH3

CH3

H

H3CC

C

H2C

CH3

H

H

(ii)

(iii)

H2C

HC

CH2

H2C

CH3

(iv)

Figure 8.6: The reaction between 2-pentanol and hydrobromic acid can conceivably

lead to the formation of four products; (i) 2-bromopentane, (ii) cis-2-pentene, (iii)

trans-2-pentene and (iv) 1-pentene. This reaction is represented as is common in

organic chemistry—as an unbalanced equation.

It was expected that most students would recognise the possibility of a substitution

process in this example as the laboratory had focussed on a similar process. As 2-

bromopentane contains a stereocentre, there is the possibility of forming two

enantiomers, R- and S-2-bromopentane. However, students were not expected to

recognise this in the test, as chirality had not been discussed in detail in the pre-

laboratory discussion, so ‘2-bromopentane’ was accepted as an appropriate answer. The

majority of the students did recognise this as a possible reaction product. A small

number (six of 77) suggested that one or more elimination products might be formed in

this reaction. No student suggested all four possible products. The few students who

did mention the possibility of the formation of 2-pentene did not comment on its

possible geometric isomerism.

Many of the students who sat this test suggested the formation of only the substitution

product. This suggests that students do not have well-developed understandings of the

possibility of competing reaction processes and that they seem to consider that a

reaction will result in only one product. Additionally, these findings indicate that

students are more likely to suggest the formation of a substitution product than an

elimination product, which is the type of reaction that they cover in more detail in the

Chemistry 121/122 course.

188

Students’ understandings of competition between substitution and elimination reactions

were also investigated in a question in the end of semester examination in 2000 (Figure

8.7). Students were asked to select which of four structures represented molecules of

possible products from a given reaction. The structure (1) would be the result of a

substitution process while the three other structures are elimination products. A

majority of students (85 %) recognised the substitution product as a possible product.

Although all four structures were possible reaction products, only three students

recognised this.

Consider the reaction between (+)-3-bromo-2-methylpentane and hydroxide ions.

C Br

(CH3)2CH

HCH3CH2

+ OH

Which of the following represented organic compounds are possible products of this

reaction? There may be more than one possible product of this reaction.

CHO

CH(CH3)2

HCH2CH3

C C

H

CH3

(CH3)2CH

H

C C

CH3

H

(CH3)2CH

H

C C

CH2CH3

H

H3C

H3C

Figure 8.7: Examination task from Chemistry 121/122 in 2000.

The results from these test and examination tasks indicated students’ understandings of

the possibilities of competition between substitution and elimination processes are

189

limited. As the results from this task shows, students can often predict possible

substitution products, but can struggle when it comes to suggesting possible elimination

processes. There may be several reasons for this.

The laboratory class was held at the time where Dr Anderson had taught the students

about substitution process and was just commencing his discussion on elimination

processes. While the chemistry and mechanistic representations of substitution

reactions are covered in a great deal of detail in the course, elimination processes and

the possibility of competition between elimination and substitution are not covered to

the same depth. It is possible that this perceived emphasis on substitution processes has

influenced students’ understandings of what might be occurring in particular reactions.

The test was investigating students’ understandings of a laboratory exercise in which

they were preparing and collecting 1-bromobutane. As the laboratory work (and all the

chemical tests that the students performed) was involved with the formation of a

substitution product, this may have focussed students’ attention on only the possibility

of substitution processes.

Additionally, the frequent use of single particle language and representations in their

lectures may have impacted upon students’ abilities to construct appropriate

understandings of the possibility of competing substitution and elimination processes in

particular reaction mixtures. It would be difficult for students to construct useful

understandings of more than one reaction process occurring between particular starting

materials if they only consider single molecules of the starting materials, as was done in

their lectures and in the recommended textbook. While students might nevertheless

realise that both substitution and elimination processes might happen in a particular

reaction mixture when considering only single molecules, it is difficult to use this type

of representation to rationalise the formation of products of both a substitution reaction

and an elimination reaction (in differing yields) from a single particle explanation.

A workable understanding of the competitive processes that may occur in a reaction

mixture requires students to think about what is happening between many particles in a

reaction, not just what is happening between one molecule of an alkyl halide and one

nucleophile, as is commonly represented by lecturers and in textbooks. Students’

inabilities to recognise the possibility of competing reaction processes (and the

190

probabilistic nature of reaction processes) may be an indication that they have not

formed understandings of these multiple particle interactions in reaction processes.

8.5 Summary

Dr Anderson addressed his stated intentions throughout the teaching of his course. His

explicit intentions relating to reaction mechanisms were discussed in detail. His

implicit intention (the ability to describe and consider competing reaction processes,

which implies an understanding of the multiple particle, probabilistic nature of reaction

processes) was not articulated in his teaching.

The language Dr Anderson used in his teaching was consistent with single particle

descriptions of reaction processes, even when discussing competing reaction processes.

He commonly used particular types of structural representations to teach about

particular types of reaction processes: three-dimensional representations for substitution

processes and square planar for elimination processes.

Although Dr Anderson’s students had encountered curly arrow representations in first

semester, early in second semester the majority of students did not appear to use the

representation to rationalise reaction processes. It was observed that students were

more likely to use these representations correctly in neutralisation processes ((i) in

Figure 8.2) than in ionisation processes ((ii) in Figure 8.2).

By the end of the course, Dr Anderson’s students demonstrated good understandings of

some mechanistic conventions in particular examples. Their abilities to interpret

mechanistic tasks appeared to be affected by the number of curly arrows that were

represented on a particular structure and the presence of a neutral nucleophile, such as

NH3, which could lead to the formation of a charged species. This suggested that many

of his students held incomplete understandings of the curly arrow representations and

determining associated charge transfer (formal charge).

Students also demonstrated more of a facility with recognising the possibility of

substitution processes occurring than elimination processes. When students’

understandings of the possible competition between these processes were investigated,

191

few students could identify all possible substitution and elimination reaction products

from given starting materials. Similarly, a large percentage of students gave only a

single product as possible from a particular reaction. Commonly, this single reaction

product was a substitution product, not an elimination product. This may be influenced

by the fact that substitution processes are discussed in more detail than elimination

processes in the lecture course. The students performed a substitution reaction in the

laboratory, but did not do any elimination reactions in second semester. They had

performed an elimination reaction in first semester when studying the chemistry of

alkenes.

The notion of more than one reaction product was also discussed only briefly in the

lecture course. The amount of time allocated to talking about the possibility (and

probability) of competing reaction processes was comparable to the time devoted to

discussing elimination reactions. This was significantly less time than was used in

covering substitution reactions and their mechanisms.

Finally, the nature of the language and the structural representations used in the lecture

course may have influenced students’ understandings of competing reaction processes.

Although the lecturer did discuss the possibility of competing reactions, both competing

substitution (SN1/SN2) and substitution/elimination competition, the structural

representations and language that he used when describing individual processes refer to

images of single particles. Students who are thinking along similar single particle lines

may experience difficulty when they are asked to consider a situation (such as the

formation of more than one product) that is difficult to rationalise when thinking about

only one particle of nucleophile and one particle of substrate. It does not make sense to

consider the formation of both 1-butene and 1-bromobutane from a single 1-butanol

molecule. Helping students to develop multiple particle understandings of reaction

mixtures would be beneficial to their understandings of the presence of more than one

product in particular reaction mixtures, and considering the competitive nature of

reaction processes.

192

9 Findings From Chemistry 100 (Dr Adams)

9.1 Introduction

This chapter discusses the teaching and learning that occurred in Dr Adams’ Chemistry

100 course. It will follow a similar structure to chapter 8, describing the lecturer’s

teaching intentions, the strategies he used when teaching about reaction mechanisms

and the intentions that he implemented in the teaching of his course. In addition to this,

the students’ abilities to work through mechanistic tasks are described and discussed.

These tasks were designed by the researcher to address certain aspects of the research

questions detailed in section 1.5. The lecturer’s intentions and achieved aims are related

to the theoretical and pedagogical issues raised in previous chapters, as are the students’

demonstrated abilities to represent reaction mechanisms.

The evidence is presented to support the research claims expressed on pages 165 - 166

of Chapter 8.

Pedagogical implications are discussed in Chapter 11.


Dr Adams commenced teaching the organic chemistry component of the Chemistry 100

course approximately five weeks into first semester. A different lecturer had introduced

general chemistry in the first few weeks of semester.

Students had 22 organic chemistry lectures in the last eight weeks of semester. The

recommended textbook was Brown’s Introduction to Organic Chemistry (2000).

Detailed descriptions of Dr Adam’s teaching are included in Appendix 9.1.

Dr Adams commonly used single particle representations when writing reaction

mechanisms. His language was generally consistent with a consideration of single

particles. The lecturer used these types of representation and language even when he

was discussing processes that implied a multiple particle explanation, such as the

formation of a racemic mixture from a chiral compound by an SN1 reaction mechanism.

193

Dr Adams described his teaching in the Chemistry 100 lectures as what the researcher

classified as a functional group chemistry approach (interview 23032000, line 31 – 33):

[w]hat I’ve been doing so far is, take a functional group, learn how it behaves, and talk

about the synthesis and the reactions of the functional group type compounds.

The lecturer added that he would like to take this approach further, depending upon the

level of interest of his students. He described this in the following manner (interview

23032000, line (35 – 48):

What I’d like to do is say here’s a functional group, this is how it reacts, um, little bit of

synthesis, and then say here’s a, a system from industry, or a system in a semi-conductor, or

a biological molecule, what is it about the way it’s put together that makes it useful for

what it’s doing . . . [ ] . . . So, that’s just a little thing I would like to . . . put a bit more into

the course . . . [ ] . . . I would hope to cover the same sort of concepts, but make it more

practical, rather than more pure chemical (indistinct).

Dr Adams felt that such an approach might be more appropriate for particular students,

whereas more analytical students would benefit from learning about mechanistic

principles in the course.



Dr Adams was asked in interviews about the outcomes he wanted his first year students

to achieve from his course. He commented that he would like his students to finish the

course with an understanding of (interview 23032000, line 159 – 166):

some basic organic chemistry. It would be nice if they could come out of the course at the

end knowing a little bit about mechanisms, learning how to draw simple ones, knowing the

difference between nucleophilic and electrophilic, and being able to know that if they’ve

got a carbonyl compound and something that’s nucleophilic, the chances are that one will

react with the carbonyl comp . . . (indistinct) of the other. That sort of thing. Um, get a

feeling for polar and non-polar things, what might dissolve in water and what won’t . . . and

a bit about how to synthesise molecules, different functional groups, and how to, what

reactions different functional groups will undergo.

194

At the end of the lecture course (lecture 20), Dr Adams presented the students with a

course summary (Table 9.1), detailing the topics that had been covered in the organic

chemistry section of Chemistry 100. He had commented previously to the researcher

(interview 23032000) that he only gave out course outlines towards the end of his

course, when he knew what material he had covered.

The lecturer reminded the students that the information pertaining to each section of the

outline had been covered in the textbook, in the lecturers and in the laboratory classes.

Students were also advised to check on the answers for the laboratories and to work

through their weekly worksheets and past examination papers as part of their study prior

to sitting the Chemistry 100 examination.

Dr Adams’ course outline detailed twelve specific topic areas that had been covered in

the eight-week organic chemistry component of Chemistry 100. Each of these topics

was described in terms of several sub-topics. The topics that specifically relate to the

types of mechanistic representation that are being investigated in this research study are

the topics of stereochemistry, alkyl halides, alcohols and ethers and reaction

mechanisms.

In addition to this topic-by-topic description, the course outline also made specific

reference to Dr Adams’ reasons for teaching about reaction mechanisms. The students

were provided with an indication of the detail they were expected to know about the

various mechanistic representations and the manner in which Dr Adams anticipated that

they would be able to demonstrate their understandings of the representations:

Reaction mechanisms have been discussed in detail during lectures to show why certain

reactions proceed the way they do. Many of the mechanisms are complex, and you are not

expected to know all of them. You should, however, be able to use “curved arrow”

notation to outline the mechanisms of SN1 and SN2 reactions, electrophilic addition

reactions of C=C bonds, and electrophilic aromatic substitution. You should also be able to

use resonance theory to explain:

(i) why phenols are more acidic than alcohols

(ii) why amines are more basic than anilines

(iii) substituent effects in electrophilic aromatic substitution reactions

195

Table 9.1: Topics and subtopics from Chemistry 100 course outline.

Topic Subtopics

General Functional groups, nomenclature, physical properties, identification

Synthesis and interconversions of various classes of molecules

Structural isomers

Alkanes and

Cycloalkanes

Structure and nomenclature, reaction of alkanes

Conformations, relative stability, stereoisomerism in cycloalkanes

Alkenes and

Alkynes

Structure and nomenclature

Addition reactions (Markovnikov’s rule)

Stereochemistry Stereoisomers, stereocentres, enantiomers, diastereomers

Meso compounds, racemic mixtures, optical activity

Alkyl Halides 1o, 2o and 3o alkyl halides

Nucleophilic substitution reactions (SN1 and SN2)

Elimination reactions vs. nucleophilic substitution

Alcohols and

Ethers

1o, 2o and 3o alcohols, acidity/basicity, reaction with active metals

Nucleophilic substitution reactions—conversion to alkyl halides

Elimination (Zaitsev’s rule), oxidations, synthesis of ethers

Amines 1o, 2o and 3o amines, quaternary ammonium salts, SN reactions

Basicity of amines, reaction with acid, separation of amines

Aromatic

Compounds

Structure, resonance, molecular orbital view

Electrophilic aromatic substitution, reactions of nitrobenzenes,

aromatic rings, side chains, acidity of phenols, basicity of amines

Aldehydes and

Ketones

Reduction, oxidation of aldehydes, nucleophilic addition, reductive

amination

Carboxylic

Acids and

derivatives

Acidity, separation of carboxylic acids, non basicity of amides

Reduction of carboxylic acids and amides

Nucleophilic acyl substitution, esterification, ester hydrolysis

Spectroscopy Use of 1H and 13C nmr spectra, use of ir spectra

Reaction

Mechanisms

Reaction coordinate diagrams

Nucleophiles, electrophiles, leaving groups, use of curved arrows

1o, 2o and 3o carbocations and their relative stability

196


Dr Adams used strategies in his lectures that were aimed at helping his students develop

appropriate and workable understandings of mechanistic representations. He defined

commonly used mechanistic representations, such as curly arrows, to his students. He

called curly arrows a useful ‘conceptual tool’ as he described their use to students in an

introductory lecture (see page 382 in Appendix 9.1). Nucleophiles were defined as

electron donors that could attack areas that were electron poor. Dr Adams discussed

these types of compounds several times over the course of his lectures.

Dr Adams addressed the common types of reactions that compounds undergo based

upon their functional group. He discussed substitution and elimination as types of

reaction that alkyl halides and alcohols undergo:

[T]he dominant reaction of alkyl halides, and to a lesser extent, alcohols, is nucleophilic

substitution . . . So, I’ll give you a general example, then a real example, then I’ll talk about

details of the mechanisms.

(lecture 10052000, line 112 –114)

So, there, there’s five different functional groups that you can make from an alkyl halide

using this nucleophilic substitution reaction . . . and there are many other ones . . . that you

can make.

(lecture 10052000, line 146 –148)

The other very important reaction for alcohols and alkyl halides is elimination . . . now,

remember, earlier in the course, we talked about addition reactions. If you have an alkene .

. . you can some molecule X—Y, you can add X and Y across the double bond . . . you can

do (indistinct). Well, elimination is the reverse of that process.

(lecture 16052000, line 167 – 171)

The lecturer gave a very comprehensive discussion of substitution and elimination

reaction mechanisms. Substitution reactions were described to the students as having

two ‘extreme’ types of process, SN1 and SN2. Both of these reaction pathways were

defined for the students in terms of the types of molecules of starting materials

197

(nucleophile and substrate or only substrate) that were involved in the rate-determining

step.

Dr Adams described both substitution and elimination reactions in terms of bond

breakage and formation, and demonstrated many examples using curly arrows and

different types of nucleophiles and leaving groups.

Dr Adams’ discussion of substitution reactions also included a consideration of

experimental evidence, such as reaction rate dependence on reactant concentrations and

stereochemistry of reaction products. His demonstration of stereochemical inversion

using a large movable model (see description on pages 385 - 386 in Appendix 9.1) was

used to show students how the shape of a reactant molecule might change as reaction

processes.

The relative stability of carbocations was also discussed as an aspect of substitution

reactions. According to Dr Adams, the stability of carbocations increasing as they

became more substituted. Primary carbocations were less stable than secondary, which

were less stable than tertiary. He explained this in terms of the possibility for what he

referred to as ‘charge dilution’; in a tertiary carbocation, there were more atoms (and

therefore more electrons) to ‘dilute’ the positive charge. He indicated this relative

difference in stability in a reaction coordinate diagram in a lecture he gave early in

semester that dealt with addition reactions (Figure 9.1).

Figure 9.1: Comparison of reaction coordinate diagram depicting energy for formation

of a primary carbocation (i) and a more stable secondary carbocation (ii) in an

addition reaction.

(i)

(ii)

198

In this diagram, the energy level of the transition state in the formation of a primary

carbocation is higher than that of the transition state in the formation of a secondary

carbocation. Dr Adams also supported this difference in stability with experimental

evidence of the existence of carbocations (lecture 10052000, line 237 – 238):

with carbocations, you can actually make it to study spectroscopically, you can make in an

nmr tube, for example, and study it, and it has standard sorts of carbon-carbon bonds,

standard bond lengths, and so on.

Dr Adams also used reaction coordinate diagrams (Figures A9.2 and A9.5 in Appendix

9.1) in his explanations of the energy requirements of the two different substitution

processes. The diagrams were used to describe the formation of transition states and

intermediates, as well as compare the energy of activation in each of the steps in an SN1

reaction process.

When describing elimination reactions, the lecturer did not discuss two ‘extreme’

mechanisms as he had done with substitution processes. This was consistent with the

outcomes described in the course outline, which did not specify that students would

learn about two types of elimination process. The lecturer also drew students’ attention

to the possibility of more than one elimination product from any given reaction,

describing how structural isomers may be formed. The formation of geometric

(cis/trans) isomers was also mentioned briefly, but students were given no specific

examples. Dr Adams discussed Zaitsev’s rule as a way of predicting which elimination

products were more probable.

Towards the end of his lectures, Dr Adams discussed competition between substitution

and elimination processes on the chemistry of alkyl halides and alcohols. Although the

lecturer talked about what type of reaction process (substitution or elimination) might

proceed or be favoured under certain reaction conditions, these processes were not

described as competing in the sense that they may occur simultaneously in the same

reaction mixture. In lecture 15, Dr Adams used two separate examples using different

substrates and nucleophiles to discuss possible substitution and elimination processes

(Figure A9.9 in Appendix 9.1). He did not give an example of a reaction process in

which both substitution and elimination products were formed from reaction between

the same starting materials.

199

The one discussion of reaction competition that Dr Adams mentioned was when he

described the possibility of both SN1 and SN2 processes happening in the same reaction

mixture (lecture 10052000), but this discussion was brief, and mentioned while

describing stereochemical inversion in SN2 reaction processes.


9.4.1 Introduction

A small number of Chemistry 100 students participated in exploratory interviews with

the researcher in 1999. These interviews were carried out to investigate the

appropriateness of particular tasks for probing students’ understandings. Dr Adams did

not teach the course in that year, so these students’ achievements and abilities are not

discussed in relation to the lecturer’s intentions and his course presentation.

Dr Adams’ students were involved in the research study through their participation in

interviews (2000), examination questions (2000) and questionnaires (2001 and 2002).

As with Dr Anderson’s first year students, the questionnaires were conducted to

investigate students’ prior understandings of commonly used representations before

they commenced learning about reaction mechanisms and their associated

representations. These findings were discussed in section 7.2.

Due to the fact that Dr Adams did not cover elimination reaction mechanisms in great

detail, it was determined to be inappropriate to probe students’ understandings of these

processes in any great detail. Similarly, as the lecturer did not focus on competition

between substitution and elimination processes in his classes, this aspect was not

investigated in the research project. Students’ understandings of the relative stabilities

of carbocations and reaction coordinate diagrams were also not investigated, due to time

constraints and limited numbers of students volunteers who participated in the study.

Students’ abilities to represent simple reaction mechanisms were investigated in an

interview situation with four student volunteers and also in the end of semester

examination. Students’ understandings of curly arrows and formal charge were

investigated in both this interview (Appendix 6.15) and the examination (Appendix

200

6.24). Students’ perceptions of the link between experimental evidence and appropriate

mechanistic representations were also investigated in both the interview and the

examination question.

9.4.2 Understandings of Representations

9.4.2.1 Curly Arrows

In interviews, four student volunteers were asked to describe what they thought a curly

arrow was used to represent when writing reaction mechanisms. Cathy commented that

she understood that curly arrows represented (interview 16052000, line 109 – 113):

tak[ing] the electrons here and put them on the bromine . . . [ ] . . . the ones where the arrow

starts. Take those and put them on the bromine.

Cameron and Christopher gave responses that were similar to Cathy’s. Carl described

the curly arrow representation in more detail, discussing not only what the

representation meant in terms of the movement of electrons, but also the implication of

the movement on the compound’s structure (interview 16052000, line 182 – 192):

Um, it’s the, the move of the electrons from this bond to the individual Br atom, the

bromide atom . . . So, it involves the electrons that are used in bonding, the covalent bond

between these two is broken, and the electrons, because that’s the, I thought the tail

signifies, where the tail is is where you’re getting the electrons from . . . [ ] . . . and where

the tip is is where they’re going to, so, I’m presuming that the, the bromide’s going to be

leaving this structure . . . [ ] . . . and it will have those, it will have a negative charge on it.

Each of the four students interviewed also demonstrated an ability to correctly follow

curly arrow representations in the three simple tasks that there were asked to complete.

The interviews were held just after Dr Adams had presented lectures on curly arrows, so

it was assumed that the representations were fresh in their minds.

These observations were consistent with the abilities that the majority of the Chemistry

100 students demonstrated in the examination, in which 74 students answered three

questions (Figure 9.2). These questions were representations of (a) a protonation

process, (b) a substitution reaction and (c) an elimination reaction. A task analysis of

each of these three processes is shown in Table 9.2.

201

Draw diagrams that represent the compound (or compounds) that are produced by the

transformations shown:

(a) (b) (c)

Figure 9.2: Three questions in the 2000 examination for Chemistry 100.

Surprisingly, the examination question that gave the students the most difficulty was

(a). It was anticipated that this question would be simple to answer. However, less than

half of the students could give the correct answer ((i) in Figure 9.3). A small number (5

of 74) showed the correct structure, but did not include the charge on the species.

About a third of the students gave an answer that was inconsistent with the curly arrow

representation.

The most common of these incorrect answers is shown as (ii) in Figure 9.3. A small

number of students represented a correct answer, then added an extra curly arrow to the

structure and drew a second structural answer ((iii) in Figure 9.3).

The results of this question appear to indicate that some students are unwilling to

suggest a structure such as (i) in answer to this question. Almost a quarter of the

students represented (ii) as the answer to this question. This product is not consistent

with the single curly arrow in the question. It may be that students were more familiar

with a carbocation representation (ii), having seen them quite often in their lectures,

than with the protonated alcohol species (i). It is suggested that the difficulties students

encountered with the protonation question might be to do with the unfamiliar nature of

the product structure they were expected to draw, which increased the level of difficulty

of the question beyond what the task analysis suggested. While students had

encountered examples in which protonated alcohols were formed (for example, see

202

Figure A9.6 in Appendix 9.1), the protonated compounds were not the final product in

any examples they had previously seen

Table 9.2: Task analysis for three tasks in Figure 9.2.

Protonation Substitution Elimination

Interpret curly arrow

as representing bond

formation between O

and H.

Interpret the curly arrow

from OH- to C as the

formation of a bond.

Interpret the curly arrow from –

OH to H as the formation of a

bond between O and H.

Represent O—H bond. Represent a C—OH

bond.

Represent an O—H bond.

Calculate overall

charge on molecule as

a result of shift.

Interpret the curly arrow

from C—Br to Br as the

breaking of a bond.


H to C—C as the breaking of the

C—H bond and the formation of a

C—C double bond.

Represent the resulting

structures.

Remove the C—Br bond. Remove the C—H bond.

Calculate any charge

present on atoms in the

products.

Represent a C—C double bond.

Represent the resulting

structures.


Br to Br as the breaking of a bond

between C and Br and the

formation of Br-.

Remove the C—Br bond.

Calculate any charge present on

atoms in the products.


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CH3

CO

H3C

H H

HCH3

C

CH3H

+ H2O

CH3

CO

H3C

H H

HCH3

C

CH3H

+ H2O

(i) (ii) (iii)

Figure 9.3: Representations of (i) the correct answer for (a), (ii) a common, incorrect

answer (a carbocation) and (iii) a common answer in which students represented an

additional curly arrow and second structure.

This difficulty with representing unfamiliar compounds might be expected of the novice

(first year) student. The unfamiliarity of the protonated alcohol that was the expected

answer for (a) might have made students unsure that they were correct to draw the

protonated structure, and they might have represented the more familiar structure of the

carbocation. The implication is that a good understanding of the curly arrow

representation might be overridden by students not recognising the appropriateness of a

particular (unfamiliar) structure, which could result in them giving an incorrect answer

to a mechanistic question. This is an example where there is conflict between the

understandings a student possesses (for example, that the unfamiliar protonated alcohol

is not a real species) and the new ideas or information being presented to him/her

(correctly following the curly arrow will result in the formation of the protonated

alcohol species). This conflict situation can lead to confusion of concepts within

students’ minds (Pines and West, 1986). Students need to have reasons to attempt to

understand new ideas and to assimilate them into the understandings that they currently

possess. In the case of these students, they appear not to have been provided with

sufficient motivation to address this conflicting situation.

Additionally, this unwillingness to represent unfamiliar structures when they are the

product of a represented mechanism may be a difficulty that is specific to the teaching

and learning of reaction mechanisms, part of the pedagogical content knowledge related

to the topic. Lecturers may need to consider that students might be unwilling to

represent unfamiliar compounds or structures and structure their course presentation

accordingly.

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The majority of students demonstrated an ability to interpret the curly arrow

representations correctly for both the substitution (b) and elimination (c) questions.

Both of these questions were similar to examples that students had seen over the course

of their lectures (see Figures A9.3 and A9.7 in Appendix 9.1), and were anticipated to

present limited difficulty to the students. Over 90 % of the students gave an appropriate

answer for task (b) (Figure 9.2). The number who could correctly respond to (c) (just

over 80 %) was also fairly high.

While the students’ facility with the substitution problem (b) was expected, it was

anticipated that a larger number of them might have trouble with question (c). The task

was more complex, involving three curly arrows and a different type of structural

representation. It was also dealing with a type of reaction (elimination) that had been

discussed in less detail in the lecture course. However, students had seen examples of

both substitution and elimination questions such as those asked in the examination in

their lectures, which may be linked to why students performed better on these questions

than on task (a). While charged species were formed as products in all three processes,

those species formed in (b) and (c) were Br -, a species that was probably more

recognisable to the students than protonated alcohol species formed in (a).

9.4.3 Formal Charge

In addition to being asked to demonstrate their facility with the representation of curly

arrows, the interviewed students were also probed about their understandings of and

abilities to calculate formal charge. The student volunteers utilised several different

methods of determining formal charge on atoms in a given species. Four different

methods (all of which are appropriate, if used correctly) were identified from the

students’ approach to solving tasks:

(i) Comparison, similar to the description favoured by Dr Adams, where the

number of electrons (from bonds and lone pairs) around an atom in a

compound is compared to the number of electrons in the outer shell of

the individual atom;

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(ii) Bond counting, where the number of bonds an atom normally has is

compared to how many it has in a particular compound and any

discrepancy means that the atom must have a charge of some type;

(iii) Reaction method, which involved considering what compounds or

molecular fragments may have been reacted together to form the

represented structure;

(iv) Balance of charge, where the overall charge on a product is determined

through the necessity of the charge of the starting materials being equal

to the charge of the products.

Two of the students utilised a mixture of these methods in their determinations of

formal charges on atoms in the interview. When determining the formal charge on an

oxygen atom in question 1 (Appendix 6.15), Christopher used both a bond method and a

balance of charge method to discuss the formal charge on a species (interview

18052000, line 38 – 42):

[W]hen it’s in the bonding situation, it will form two bonds, two bonding pairs, um, if

you’ve got an extra, a bond formed in here, so, this hydrogen, assuming this had only got

plus one when it comes in, if that is introduced with a plus one, then the overall system has

to have plus one total charges, the sum of all the constituent charges.

Cameron discussed formal charge for the same question in terms of both expected

number of bonds an atom should have, and a comparison of the number of electrons

around an atom (interview 18052000, line 53 – 64):

Cameron: Um . . . formal charge . . . well this . . . that shouldn’t have one . . . [ ] . . .

(indistinct) . . .

Interviewer: That carbon there? And why should that not have one?

Cameron: Um, because it’s sp3 hybridised . . .

Interviewer: Ok.

Cameron: . . . so it’s . . . um . . . got four possible, um, covalent bonds on it . . .

Interviewer: Yep.

206

Cameron: . . . H two O, is um . . . should only have two on it . . . so there . . . um . . .

I gather I can’t draw on this . . .

Interviewer: No, you can draw on that. That’s fine, that one’s for you . . . [ ] . . .

Cameron: Oh right. Um . . . I’ll just use . . . so this is a water molecule, basically,

that’s attached . . .

Interviewer: Yep. Ok.

Cameron: . . . to that . . . um . . . normally has, this should normally have, for a

neutral, it should have six, in its valence shell.

Interviewer: Ok, six . . . ?

Cameron: Six electrons in its valence shell . . .

Interviewer: Ok.

Cameron: . . . um . . . here it would only have five . . . so that, the formal charge

would be on the oxygen.

Students who could use more than one method to determine formal charge might have

an advantage over other students who use only a single method to determine formal

charge. The ability to use more than one calculation method demonstrates a richer

understanding of the formal charge concept. It also allows students the ability to double

check any formal charges they assign to a particular structure, which can be helpful in

situations where the assigned formal charge appears to be counter intuitive (such as an

oxygen atom being designated +1 formal charge in a protonated alcohol species, as

represented as (i) in Figure 9.3).

9.4.4 Writing Mechanisms for Typical Functional Group Reactions Based on

Experimental Data

The interviews and the examination also investigated students’ abilities to write reaction

mechanisms that were consistent with experimentally observed data, which allowed

students’ understandings of both SN1 and SN2 reaction processes to be investigated.

Additionally, as experimental data is the basis for proposing reasonable reaction

mechanisms, students’ understandings of this link between the real and the model was

an important consideration. In both the interview and the examination, students were

207

asked to represent and explain reasonable reaction mechanisms for reactions in which a

racemic mixture of two enantiomers was formed. The structures represented in each of

the questions (interview and examination) were different, but the tasks were relatively

similar.

Carl demonstrated very poor understandings of the links between experimental evidence

and proposed reaction mechanisms. His understandings were the weakest of the four

students. He appeared to have snippets of information in his mind, but they were poorly

linked and he often commented that he’d just read about a particular topic, but couldn’t

really answer the question being asked. Carl demonstrated understandings that were

consistent with what Johnstone (1997) described as finding ‘no connection on which to

attach the new knowledge’ in the long-term storage of the mind. Pines and West (1986)

suggested that students like Carl, who were studying a new and unfamiliar topic of

which they had no spontaneous understandings, were attempting to learn very symbolic

and abstract understandings in the absence of any prior understandings to help in his

knowledge construction.

Carl recognised that his understandings of reaction mechanisms were not particularly

strong. An example was when he was asked to explain the formation of a racemic

mixture (question six in Appendix 6.15). Carl commented (interview 16052000, line

305 – 306):

I remember that, yeah, there’s a special . . . time, when they, you get equal amounts

produced of the, the mirror images, but I’ve . . . I’m blank when . . .

Carl depended on his memory of the ideas presented in Chemistry 100, rather than

trying to construct understandings and linkages between the concepts being taught in

the class. This shallow approach to learning was an ineffective way to attempt to make

sense of the vast quantities of information presented about reaction mechanisms.

Even after extended discussion with the researcher, during which time the researcher

prompted the student with questions, Carl couldn’t explain the reaction outcomes in

terms of a reaction mechanism.

208

When Carl’s response to a similar question in the end of semester examination was

analysed (question 2 in Appendix 6.24), Carl still showed weaker understandings than

the other interviewed students. His written explanation:

This is an SN1 reaction that involves a carbocation that is relatively stable. Bromine is the

nucleophile that attacks the carbocation to produce the enantiomers. Bromine attacks

equally from both sides of a carbocation to produce equal amounts of the enantiomers.

was consistent with a much richer understanding than he had demonstrated in his

interview, but the represented mechanism did not reinforce this. The mechanism that he

drew (Figure 9.4) to accompany his description showed a bromide ion attacking a

carbocation from only one side. There was no indication in his representation of

bromine attacking ‘equally from both sides of a carbocation’. Carl’s representation may

also be linked to an inability to represent the other structural orientations, or to his

perception that other representations were not necessary to support his written

description. As this representation was drawn in the examination, it is not possible to

comment upon Carl’s reasons for representing only one bromine ion and one

carbocation.

Figure 9.4: Mechanism drawn by Carl to answer an examination question (question 2

in Appendix 6.24). The mechanism is not consistent with his written explanation, which

talks about nucleophilic attack from ‘both sides’ of a carbocation.

The other three students (Cathy, Christopher and Cameron) were all able to use their

understandings of reaction mechanisms and experimental evidence to explain the

experimental results that were detailed in the interview question. All three students

demonstrated good understandings of the two types of substitution processes they had

been taught about, including the stereochemical implications of each process.

Cathy originally suggested that both SN1 and SN2 processes were occurring in the

reaction mixture, resulting in two enantiomers (interview 16052000, line 184 – 209):

209

Cathy: some of them have (indistinct), some of the methoxide . . .

Interviewer: Yep.

Cathy: . . . things have just knocked out the chlorine, instead of, like, come in this

way, and gone that way (student indicating attack from behind central

carbon and Cl going from other side).

Interviewer: Ok.

Cathy: Is that right?

Interviewer: With the . . . your little hand movements there, you’re meaning, coming

(indistinct) . . .

Cathy: Like, behind . . .

Interviewer: Yep.

Cathy: . . . and, like, made it into the S.

Interviewer: Ok. But to make the R, what has happened? Can you explain that again

for me?

Cathy: It just, went, just changed places with the chlorine.

Interviewer: Ok. If it just changed places with the chlorine . . . what’s happened to the

chlorine-carbon bond? What happened to the electrons there?

Cathy: Mmm, well, I’d guess that the chlorine would take them both . . .

Interviewer: Ok.

Cathy: . . . and become chloride ion . . .

Interviewer: Yep.

Cathy: . . . and because this (indicates methoxide) is negative, this just joins on.

Interviewer: Joins onto the carbon there?

Cathy: Yep.

Interviewer: Ok.

210

Cathy: Because this would, this, like go away, and then this has a plus, and then

this goes, this negative one comes in here when it’s like this.

After a brief discussion of reaction processes, the researcher commented that Cathy had

been discussing two different types of reaction mechanism that Dr Adams had taught

about in the lecture course. Cathy agreed with this comment (interview 16052000, line

270 – 281):

Cathy: Oh, little SN1 and SN2?

Interviewer: SN2, yeah.

Cathy: Yeah.

Interviewer: Is one of those more likely to rationalise the production of equal amounts

of these two products, do you think?

Cathy: Oh . . . (ten seconds silence) . . . well, if you got, if it was definitely equal

amounts, then it wouldn’t have been SN2 . . .

Interviewer: Ok.

Cathy: . . . because then it would be all, S-, because it would all go in here, and

push it over (indicating backside attack at central C).

Interviewer: Ok.

Cathy: So, it would have to be the other one.

Christopher also explained the reaction outcome by considering that both SN1 and SN2

reaction processes were occurring in the same reaction vessel (interview 18052000, line

200 – 235):

Christopher: Well, I know how, there’s two ways in which this can react, um . . . I’ve

been told that, we’ve been told . . .

Interviewer: Ok.

Christopher: . . . um, and, if, either reaction mechanism can be affected by . . . one is

affected by the polarity of the mixture, and one is not, various things like

that. Um, but, SN2, the actual reaction mechanism is that you have,

partial, formed, a bond formed between that methyl group . . .

211

Interviewer: The methoxide, yeah.

Christopher: . . . the methoxide, and, um, this carbon, central carbon . . .


Christopher: . . . um, just like an attraction between the electrons in there, and, well,

protons in there and, again, protons there, electrons in there, so you’ve got

a temporary weak bond formed between there (indicating methoxide and

central carbon of carbocation) and . . .

Interviewer: Ok.

Christopher: . . . we have low electron density in here, so that the stronger attraction

for, um . . . the electrons with this proton, with this positive of the protons

in here, than there is in here, so, um . . . a more stable state, or lower

energy of the bonds in formed if this carbon, I mean chloride (indicating

Cl on representation of starting material), then leaves . . .

Interviewer: Ok.

Christopher: . . . is the leaving group . . . um . . . forming that (indicates S-enantiomer)

and you have your chloride ion. Um, with this, you have a . . . this

chlorine leaving, um . . . it’s like, I suppose, an equilibrium type thing . . .

um . . . and then we have this (indicates methoxide) taking its place . . .

Interviewer: Ok.

Christopher: . . . That’s an SN1 reaction. Um . . . the fact that they’re fifty percent, fifty

percent . . . (ten seconds silence) . . . that would indicate something about

the conditions in which they’re . . . um . . . reacting in. If the conditions

are such that one is more likely than the other, then that would, you would

be able to distinguish between the actual (indistinct).

Interviewer: When you’re saying one is more likely than the other, are you talking

about one product is more likely than the other . . .

Christopher: Yeah.

Interviewer: . . . or one reaction mechanism?

Christopher: Well, one product is formed by one reaction mechanism . . .

Interviewer: Ok.

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Christopher: . . . the other product is formed by the other . . .

As Christopher continued to discuss the possible reaction processes, he realised that the

experimental evidence provided about the racemic mixture produced was consistent

with only one type of reaction mechanism, the SN1 mechanism (interview 18052000,

line 264 – 281):

Christopher: actually . . . sorry, this could actually . . . when this SN1 can form . . .

forms a planar . . .

Interviewer: Yep.

Christopher: . . . triagonal . . . forms a planar structure, and . . . something’s wrong . . .

Anyway, um . . . this could be SN1 as well (indicates S-enantiomer).

Interviewer: Ok . . . So you’re saying that both of those products could be the result of

an SN1 type reaction?

Christopher: Mmm (nods).

Interviewer: Ok. What is it about the, you’ve drawn a planar compound here (indicates

carbocation student has drawn) . . . what is it about that planar compound

that makes you think that both are, both could be the result of an SN1 type

reaction?

Christopher: Well, it’s the transition structure that’s formed . . .


Christopher: . . . um, and, when this leaving group, chlorine leaving group leaves . . .


Christopher: . . . and then, um, I suppose there’s equal likelihood that the methoxide,

methoxide could come from either side and attach, and therefore create

either one tetrahedral structure like that, or one like that.

Both Cathy and Christopher demonstrated that they held good understandings of both

SN1 and SN2 processes and of the implications of experimental data on the

appropriateness of reaction mechanisms to describe a reaction process. Both were able

to work through their understandings of reaction mechanisms to arrive at

representations that were consistent with described experimental data. Although neither

213

of these students could discuss the most appropriate mechanism immediately, both

demonstrated that they had constructed well-formed and well-linked understandings of

the two types of substitution mechanism that had been discussed in their course. This

linking and appropriate storage of information enabled them both to discuss different

mechanistic representations with the researcher and to determine which was more

appropriate to describe the experimentally observed outcome that had been detailed.

Unlike Cathy and Christopher, Cameron’s response when asked this task was very

definite (interview 18052000, line 214 – 226):

Cameron: It has to be SN1.

Interviewer: Ok. Why do you say it has to be SN1?

Cameron: Because there’s, um . . . because there’s two types of, um, product formed

. . .

Interviewer: Ok.

Cameron: . . . or the racemic mixture, there, it indicates that there was a um, there

was a carbocation, cation formed, which, um, because it’s planar, the, um

. . . the . . . attacking nucleophile can attack from either side, which sort

of, um . . . results in two different formations of, oh, two . . . yeah, the two

enantiomers in the product . . .

Interviewer: Ok.

Cameron: . . . so, um . . . the first step must have been . . . it’s SN1 because the first

step, um, includes only the, only one of the reactants, which is the R-

enantiomer over here at the start . . .

Interviewer: Ok.

Cameron: . . . so, that’s sort of reacting with itself, and the Cl’s a leaving group,

without the, um . . . the ion coming in to start with, and the ion only

attacks after it’s formed the carbocation.

Cameron’s understandings of the link between experimental data and mechanisms

appeared to be better developed than Cathy’s and Christopher’s. While the other two

students had needed to discuss and work through the two types of mechanistic processes

they’d been taught about before they could explain why one mechanism was more

214

appropriate, Cameron did not need this discussion. He demonstrated a good

understanding of the link between experimental evidence and the implications this has

for appropriate mechanistic representations in this instance.

A similar question to this was asked in the end of semester examination (question 2 in

Appendix 6.24). Students were asked to explain the formation of a racemic mixture in a

given substitution reaction. When answering this, Cathy, Christopher and Cameron

were all able to describe a mechanistic process that was consistent with the

experimental evidence provided. Each of these students also wrote a mechanistic

representation that indicated the possibility of nucleophilic attack onto both faces of a

carbocation, resulting in the formation of a racemic mixture. Carl’s response to this

question has been previously discussed and is shown in Figure 9.4.

Analysis of the responses given by all the students in the course to this examination

question indicated that many could use an appropriate reaction mechanism to rationalise

provided evidence. Of the 74 students who sat the examination, over three quarters

responded to the question by representing and/or describing an appropriate reaction

mechanism. An example response is shown in Figure 9.5.

Figure 9.5: An example answer to the examination question shown in Appendix 6.22,

given by a first year student.

The most common response given by Chemistry 100 students was one that showed

nucleophilic attack on both sides of a carbocation to produce a mixture of two

enantiomers. A small number of students also commented upon the planar shape of

carbocations and the probability of equal attack onto either face of the carbocations,

215

resulting in an equal mixture of the two enantiomers. These types of responses were

consistent with what students had been taught in their course by Dr Adams.

A small number of students also gave answers that were similar to Carl’s (Figure 9.4).

These students gave both a written explanation and a pictorial mechanism in their

response to the examination question. Although they referred to the possibility of

nucleophilic attack onto either side or face of a carbocation, leading to two products, the

pictorial representations the students used to support their descriptions were not

consistent with what they had written, showing nucleophilic attack from only one

direction. This might be simply for ease or speed of drawing representations in the

examination, or because students perceived that their written descriptions were

complete explanations.

The inaccuracy (or inappropriateness) of representing the formation of a racemic

mixture with a mechanism that shows nucleophilic attack on only one face of a

carbocation is an issue that is specific to teaching and learning about substitution

reaction mechanisms. If it is the case that students do not perceive why they should

have to show nucleophilic attack on both faces of a carbocation to explain racemic

mixture formation, then lecturers need to be aware of these perceptions and to address

them in class to assist students in forming suitable understandings.

As there was no way to follow up on students’ responses in the examination, the reasons

above are speculative.

9.5 Summary

Dr Adams covered the intended topics in his course presentation about reaction

mechanisms. His intention to discuss competing reaction processes implied the need for

a multiple particle understanding of reaction mixtures, but this was not articulated.

He tended to use single particle language when teaching about reaction mechanisms.

He also commonly used three dimensional representations when talking about

substitution reactions and square planar representations when teaching about

elimination processes.

216

The majority of the Chemistry 100 students demonstrated a good understanding of the

curly arrow representation. While students demonstrated slightly stronger abilities in

representing the outcomes of a substitution process than an elimination process, a

majority of students were still able to represent appropriate structures in the elimination

process. Substitution processes had been covered in detail in their course, so it was

expected that students would demonstrate good understandings of these types of

reaction mechanisms. Elimination processes had not been covered in the same detail,

but students still demonstrated good understandings of the mechanistic representations

of this type of process.

Surprisingly, students demonstrated most difficulty in a mechanistic process that

involved only one curly arrow. The difficulty with this problem may have been linked

to the fact that it represented a protonation process, which was something that the

students had only seen as part of a reaction process, not as the only step in a mechanism.

Many students correctly drew an extra curly arrow on this representation, indicating the

production of carbocations and water. This was similar to examples that they had

encountered in their lectures (see Figure A9.6 in Appendix 9.1).

Students demonstrated a good ability to determine formal charge on a representation of

a simple carbocation species. These types of compound were frequently represented in

the lectures and in the textbook, and so students had seen these structures many times.

It was expected that, by the end of the Chemistry 100 course, the majority of the

students would be able to represent formal charges on such species.

Interviews with student volunteers identified the different methods that students used to

calculate formal charge. Two of the students used more than one of these methods to

determine formal charges.

Many of the students were able to demonstrate good understandings of the links

between experimental evidence and appropriate mechanistic representations. In the

examination, a majority of the students demonstrated a good understanding of the

mechanistic implications of the formation of a racemic mixture; they could suggest the

possibility of an SN1 reaction mechanism to explain the observed racemic mixture. A

small number of students provided an appropriate written explanation, coupled with an

inconsistent pictorial representation. This may have been a factor of the examination

217

situation (students did not have time to represent two reaction mechanisms), or the

students may simply have felt that their representation was appropriate to indicate their

understanding of the process.

Overall, the Chemistry 100 students demonstrated good abilities to represent simple

mechanistic processes. Their abilities appeared to be affected slightly by the type of

reaction process that was being represented (students were slightly better at interpreting

a substitution mechanism than an elimination question in an examination). Students

also appeared to be more comfortable representing structures that they had seen often in

lectures (such as alcohols and alkenes as products from substitution and elimination

processes) than those that were less common, or not what they considered to be the final

product in a reaction (such as a protonated alcohol).

218

10 Findings From Chemistry 2XX (Associate Professor Andrews)

10.1 Introduction

This chapter discusses Associate Professor Andrews’ Chemistry 2XX course and the

teaching and learning that occurred within the context of this class. The chapter follows

a similar structure to both chapters 8 and 9. Pedagogical implications of these findings

are discussed in Chapter 11.


Associate Professor Andrews lectured the organic chemistry component of Chemistry

2XX in 2000 and 2001. Detailed descriptions of Associate Professor Andrews’

teaching are included in Appendix 10.1.

He provided handouts to his students in most lectures. Some of these handouts were

problems sheets or (on one occasion) answers to problems on these sheets. The lecturer

commented to the researcher that he generally avoided handing out answers to problem

sheet questions so that students didn’t feel that there was only one right answer to a

particular problem. He also reminded students often over the course of the lectures that

the questions on their examination would be drawn from these problems sheets. He also

discussed the answers to at least some of these questions in lectures.

The lecturer believed that providing his students with these problem sheets and telling

them that the examination questions would be drawn from these would help the students

focus on developing understandings of the subject matter discussed. He felt that an

‘outside of the exam’ situation would help them develop the ability to apply their

knowledge to different circumstances. When asked why he chose to draw examination

questions from a pool of tasks students had already seen and, in some instances, worked

through with the lecturer, he commented (interview 16102002, line 8 – 15):

Because I believe that the exam situation is not the best way of examining it . . . no, let me

go back . . . I believe that the . . . the exam only can measure knowledge . . . [ ] . . . or recall

. . . of knowledge. Anything beyond that, where you want them to apply and develop um . .

219

. ideas, I think you need to have a situation outside the exam situation. So, all I . . . um . . .

I was after essentially is presenting them with a range of topics, makes the syllabus, and for

them to acquire some knowledge and to be able to recall that knowledge . . . in an exam

situation.

Before starting each of his lectures, Associate Professor Andrews wrote brief notes on

the board to remind students of what they had covered in the previous lecture. He

generally discussed these points for a few minutes at the start of each class. He felt that

this was useful for two purposes: refocussing his students on the material that had been

covered previously and also allowing him to discuss points that he felt had not been

discussed adequately in the preceding lecture. He commented upon this when asked by

the researcher (interview 16102002, line 49 – 63):

A/P Andrews: I want them to essentially rem . . . focus . . . when they come in, they’re

coming in at different times, um . . . they can just sit down there and

perhaps that will, the summary that I put up from the previous lecture

with the most important section of the previous lecture are up there, so we

can connect, I can connect back to what I, the topic that we talked about

and perhaps they can as well. So it’s just a continuity thing.

Researcher: So, connecting what you’re going to talk about this lecture with what you

talked about last, last lecture and important points. Ok.

A/P Andrews: Yeah. Yeah. It does serve, in my case, it does serve another purpose, in

that frequently in the previous lecture you, one, I feel that I haven’t

emphasised a particular point . . .

Researcher: Mmm hmm.

A/P Andrews: . . . so . . . the, the summary at the beginning of the next lecture allows me

to give certain things their right emphasis if I haven’t done that already.

Researcher: Ok.

A/P Andrews: so there’s a patching up . . .

Researcher: Yeah, yep.

A/P Andrews: It can be a patching up thing as well.

220

The textbook recommended by this lecturer was McMurry’s “Organic Chemistry”. As

a new edition of the textbook was released in the first year this course was investigated,

students may have either used the 4th edition (1996) or the 5th (2000). Associate

Professor Andrews generally provided students with relevant chapters and pages when

introducing new topics in Chemistry 2XX. He was aware of the fact that students may

be using different editions of McMurry’s text. He also commented that any reasonable

organic chemistry textbook would be suitable for the course, should students already

have one, mentioning that students should check on the suitability of any different text

with him, to ensure that it was appropriate for the organic chemistry to be covered in

Chemistry 2XX.

It was common for Associate Professor Andrews to represent chemical equations and

reaction mechanisms as single particle processes. The language that he used in his

lectures was also consistent with this view. However, in some cases, such as discussing

competing reactions, Associate Professor Andrews occasionally used language that

demonstrated a more multi-particulate view.



In interviews with the researcher, Associate Professor Andrews described several

intended outcomes for his Chemistry 2XX course. He also discussed what he intended

to present to the students in the course. He commented that he intended to compare the

efficiency of laboratory reactions with reactions in nature, to show his students how

nature generally performs much ‘cleaner’ reactions (only one product is formed with

great efficiency) than the chemist can (interview 22022000, line 288 – 296):

I come back to the main aim that I have. I would like them to understand how, have some

experience how difficult or easy it is to do certain conversions in the laboratory, as well, of

course, in the process learning typical reactions of different groups, but I always have, I

always like to juxtapose what we can do in the lab and the limitations with what can be

done in nature, so, in the back of my mind is always this point. That we can do certain

things, we never get a hundred percent yield, we have to be brutal about it, very often

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generating, losing most of the compound, but look at what nature does, and how neatly,

now that we’ve got the basics, we can go on to appreciate what a proper chemist can do.

Associate Professor Andrews also commented that he intended to comment on the

stereochemical implications of enzyme catalysed and natural reactions. One of the very

important considerations when teaching about organic chemistry for Associate

Professor Andrews is that students need to learn to think in three dimensions, to

visualise the shapes of molecules and how they might interact with each other in a

reaction. This included a consideration of the movement of bonds and atoms within

molecules, not just the movement of the entire molecule.

The lecturer also commented that he wanted to help his students to develop

understandings of how reactions might proceed and the possible outcomes of given

reactions. He also wanted to make his students aware that reaction mechanisms are

often used to represent only one of the possible outcomes in a reaction. Associate

Professor Andrews wanted his students to appreciate that most reaction processes

generally do not result in only one product being formed.

Associate Professor Andrews was then asked to discuss what he thought it was

important for students completing his course to know before they went on to study third

year organic chemistry courses. He explained (interview 22022000, line 357 – 359):

If they can look at a reagent or an intermediate and be able to say whether it’s an

electrophile, a nucleophile, an acid or a base, then they can, that’s enough ammunition, or

information to lay, to tackle quite complex mechanistic steps.

In addition to his comments in interviews, Associate Professor Andrews’ teaching

intentions were identified through an analysis of the course outline provided to his

students. In both 2000 and 2001, the lecturer gave his students essentially the same

course outline at the start of semester one. This outline described how the lecturer saw

organic chemistry as systemised in a three part classification scheme; grouped by (i)

type of functional group, (ii) influence of inductive, resonance and steric effects and (iii)

category of reaction mechanism; to help simplify the amount of information students

have to process in the course. The section of the course outline relevant to reaction

mechanisms stated:

222

A third classification can be made on the basis of reaction mechanism, a description of the

course of a reaction. For example, there are many derivatives of carboxylic acids (-

COOH), oxygen esters, amides, acyl halides, anhydrides, all appearing slightly different.

However, on treatment of each with aqueous base they all undergo the same type of

reaction which follows a similar mechanism.

This description was followed by pictorial representations of reaction between

hydroxide ions and carboxylic acid derivatives (Figure 10.1).

Figure 10.1: Representation of a reaction mechanism for the reaction between

carboxylic acid derivatives and hydroxide ions.


Associate Professor Andrews was seen to implement his stated intentions in his

teaching of Chemistry 2XX. Many of the example reactions that the lecturer used in his

course were reactions that happened in the body or in nature—‘natural reactions’. He

described the function of enzymes as catalysts, detailing the rate enhancement that such

catalysts can produce. The majority of the second semester of his course covered the

chemistry of nucleic acids and biological macromolecules. As these lectures did not

cover substitution and elimination processes, this is not discussed in any further detail.

Associate Professor Andrews gave students a detailed account of the stereochemical

implications of the two different types of substitution processes he discussed. The

223

diagrams he used when talking about SN1 and SN2 mechanisms and their stereochemical

outcomes are shown in Figures A10.6 and A10.8 in Appendix 10.1. Questions that

related to the stereochemistry of products from particular reactants were covered in both

worked problem sheets and in the examination. Indeed, in the 2000 examination, the

question relating to stereochemistry (Appendix 6.23) was the only compulsory question

in the paper.

As reaction mechanisms were described as an important classification system, the

lecturer covered these processes in detail. Associate Professor Andrews detailed the

importance of reaction mechanisms in his first lecture in both 2000 and 2001, calling

students’ attention to its inclusion in his course outline. The specific representations

common to reaction mechanisms, such as curly arrows, formal charges and types of

bond breakages were discussed early in Associate Professor Andrews’ course.

Examples of diagrams used in these introductory lectures can be seen in Figures A10.2

(heterolytic and homolytic bond breakage), A10.3 (calculation of formal charge) and

A10.11 (curly arrow representation) in Appendix 10.1. The lecturer also discussed

competition between substitution and elimination processes (Figures A10.9 and A10.15

in Appendix 10.1).

There were many instances apart from those described in Appendix 10.1 in which the

lecturer used mechanistic representations to explain reaction processes or to comment

upon reaction outcomes. An example is shown in Figure 10.2.

Figure 10.2: Example showing the use of mechanisms to explain other reaction

outcomes apart from nucleophilic substitution and elimination processes in his course.

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10.4.1 Introduction

Associate Professor Andrews’ students participated in the research study in 2000 and in

2001. They answered questionnaires (2000, 2001) and examination questions (2000), as

well as participating in interviews (2000, 2001) and focus groups (2001). Students’

understandings of general mechanistic representations were investigated using several

different tasks. The specific tasks are discussed in more detail later in this chapter.

10.4.2 Understandings of Curly Arrows

Students’ abilities to interpret curly representations in simple tasks were seen to be

somewhat better in second semester than at the start of the course. This was an

expected observation, as many of the students had not studied reaction mechanisms

before commencing second year. These abilities were investigated through the use of

questionnaires with two different groups of students.

In a questionnaire administered in first semester (Appendix 6.11), a group of students

were asked to represent the outcome of a given mechanistic process (question 1) and to

select the most appropriate mechanistic representation to rationalise a particular reaction

(question 2). In this questionnaire, most of the students who had studied reaction

mechanisms before (former Chemistry 100 students) were able to answer both questions

correctly. Only half of the students who had studied Chemistry 120, a first year course

that does not cover reaction mechanisms, were able to answer both questions. In

comparison, in a second semester questionnaire with a different group of students (tasks

2 and 3 in Appendix 6.6), the majority of respondents were able to answer similar tasks

correctly.

Although students’ abilities to answer these particular types of tasks appeared to have

improved over the course of their study, they did not necessarily demonstrate good

understandings of all aspects of mechanistic representations. For example, many of the

students appeared to find it easier to represent reaction mechanisms for substitution

processes than for elimination reactions. Evidence of this was seen in students’ answers

to an examination question (Figure A10.7 in Appendix 10.1 and Appendix 6.25). While

225

half of the students could correctly represent a substitution mechanism in this question,

only a fifth could write a mechanism for the elimination process described.

It was observed that students were more likely to use curly arrows in an incorrect

manner when representing elimination reaction processes than when representing

substitution reaction mechanisms. Many of the students who did not represent the

expected SN2-type mechanism as an answer to the substitution question (for example,

students who showed an incomplete mechanism or an SN1 process) still demonstrated

an ability to use curly arrows in an appropriate fashion; these representations could be

interpreted as representing electron movement to break and form bonds. An example of

this is shown in Figure 10.3. Example (i) shows only one curly arrow to represent the

formation of a C—O bond, with no arrow to represent the breakage of the C—Br bond.

Example (ii) correctly uses curly arrows to represent bond formation and breakage, but

it is not consistent with the stereochemical outcome of the reaction.

CH2CH3

BrH3CH

CH2CH3

HH3C

CH2CH3

HO HCH3

OH

Twists to form stabletetrahedral shape (mirrored)

CH2CH3

BrCH3

HHO

CH2CH3

C

HOCH3

H+ Br-

(i)

(ii)

Figure 10.3: Examples of incorrect examination responses to a substitution question.

The curly arrows can still be correctly interpreted as representing electron movement to

break and form bonds.

Other difficulties faced by students in representing substitution and elimination reaction

mechanisms are described in more detail in section 10.4.3.

The types of responses shown in Figure 10.3 may indicate that although the curly

arrows appear to be correctly representing electron movement, students may still hold

226

inappropriate understandings of the curly arrow representation, which happens to be

applicable in the case of a substitution process but cannot be applied to representing an

elimination process.

Interpreting curly arrows to mean movement of atoms, not electrons is one way of

misunderstanding the representation that would lead to students showing correct

substitution processes and incorrect elimination mechanisms. Analysis of the students’

examinations suggests that very few students demonstrate this type of understanding.

Students who wrote a correct substitution process but an incorrect elimination

mechanism used curly arrows in a manner that was inconsistent with both the

movement of electrons (the correct use) or the movement of atoms (an incorrect but

observed use, which can be applied to give correct answers in a substitution process by

not in an elimination process). Examples are shown in Figure 10.4. In both cases, the

substitution representation (a) is correct, but the elimination process (b) is not.

Br

CH2CH3

H3CH

HO - C

C

CH3

H

H

H C

H

H

HBrHO -

CH2CH3

BrCH3

HHO-

CH2CH3

BrCH3

HHO- H3C C C CH3

OH

H

H

H

(i)

(ii)

(a) (b)

(a) (b)

Figure 10.4: Mechanisms representing (a) substitution and (b) elimination processes

written by two students ((i) and (ii)) in response to an examination question. The

substitution representations are correct, while the elimination processes are not

consistent with curly arrows representing electron movement.

The use of arrows in the elimination processes shown is incorrect, but not consistent

with an atom movement understanding of curly arrows.

227

In example (i), the student appears to move electrons onto the second C in the structure

to form a double bond between two carbons, with the loss of H on an adjacent C

(presumably as H-), leaving Br on the structure. This indicates that the student may

hold inappropriate understandings as to how to represent the formation of double bonds

between atoms by pointing the curly arrow at the existing single bond. The fact that H

is lost and Br remains indicates the student holds inappropriate understandings about

elimination processes.

In example (ii), the student’s elimination process (b) starts in the same manner as the

substitution process in (a), with the subsequent loss of OH and H, with curly arrows

indicating electrons moving onto the second and third Cs. This suggests that the student

either has inappropriate understandings about the use of curly arrows, or that he/she is

unsure of how an elimination process occurs and is simply using the representations that

he/she knows are associated with reaction mechanisms to attempt to represent the

process.

These inappropriate understandings have implications for writing and interpreting

reaction mechanisms for all types of reaction processes. Representing reaction

mechanisms requires an understanding of the experimental evidence and the chemical

viability of particular interactions occurring. If a student has limited understandings of

how to represent simple processes, such as the formation of double bonds in elimination

reactions, their abilities to apply their understandings to more complex reaction

processes is compromised.

10.4.3 Substitution and Elimination Processes

An increased difficulty with representing elimination reactions over substitution

reactions was noted in students’ responses in interviews in semester 2, 2000. Two

questions in this interview (Figure 10.5) were asked to investigate whether students can

determine which of four products (whose structures were represented) were possible

products of a reaction between given compounds. Both questions looked at students’

abilities to suggest possible substitution and elimination products from particular

starting materials. Students were also asked to demonstrate their understandings of

mechanistic representations by writing mechanisms to rationalise particular reactions.

228

1. Consider the following reaction between (S)-3-bromo-2-methylpentane and hydroxide ions:

C Br

(CH3)2HC

CH3CH2

H+ OH

Which of the organic products represented below are possible products of this reaction? Can you illustrate a reaction mechanism for each possible product.

CHO

CH(CH3)2

CH2CH3

H C C

H

CH3

(CH3)2HC

H

C C

CH3

H

(CH3)2HC

H

C C

CH2CH3

H

CH3

CH3

(1) (2)

(3) (4)

2. If 2-bromo-2-methylpropane (A) reacts with ethoxide ions (B) which of the following are

possible products of the reaction (ref: McMurry, 5th ed, pg 424):

C Br

CH3

CH3

CH3

+ CH3CH2O

C OCH2CH3

CH3

CH3

CH3

CCH3CH2O

CH3

CH3

CH3C C

H

H

CH3

CH3

C C

H

H

CH2CH3

CH3

(1) (2)

(3) (4)

For each possible product, illustrate a reaction mechanism that rationalises its production.

Figure 10.5: Two tasks administered in an interview in 2000.

229

In each of the two questions, at least one of the provided structures was that of a

substitution product, and at least two were that of an elimination product. In question 1,

all four structures are possible products of the reaction between (S)-3-bromo-2-

methylpentane and hydroxide ions. In question 2, structures (1) and (4) were different

ways of representing the same structure, which is a possible substitution product of the

reaction between 2-bromo-2-methylpropane and ethoxide ions. Compound (3) is a

possible elimination product. Structure (2) is not a possible product of the given

reaction.

(i) (ii)

Figure 10.6: Graeme’s represented mechanisms for the formation of (i) a substitution

product and (ii) an elimination product.

Graeme was able to write an elimination mechanism where OH- was the nucleophile but

had difficulty representing an elimination process showing a different (unfamiliar)

nucleophile (ethoxide, CH3CH2O-). If the unfamiliarity of the ethoxide representation is

the reason that Graeme could not correctly represent a mechanism, then this might

indicate poor linkage of knowledge in Graeme’s mind, which does not allow him to use

mechanisms as predictive and generalise tools.

Guy was the only student who attempted to rationalise the formation of alkenes in

question 2. Although he had talked about the possibility of forming alkenes in question

1, he had not attempted to rationalise this using a mechanism. Guy redrew the structure

of the starting material as a more square planar type of representation (as Graeme did),

before attempting to explain the formation of a double bond (Figure 10.7).

230

Figure 10.7: Guy’s mechanism to rationalise the formation of alkene compounds in a

reaction between 2-bromo-2-methylpropane and ethoxide ions.

Guy explained his representation (interview 14092000, line 155 – 163):

Guy: I see this coming in there . . .

Interviewer: Ok, the ethoxide. Ok.

Guy: . . . the ethoxide coming in, and then this hydrogen, because there’s a

double bond, this bond will move down to there (indicates C—H bond) . .

. between the carbons . . .

Interviewer: Ok.

Guy: . . . because you have too many electrons there, this, these electrons here

would move out to where there’s less density (indicates C—H bond, and

arrow moving towards C—C bond, and then to Br).

Guy was attempting to use the representations he had learned about to rationalise the

formation of a double bond. His response suggested a basic understanding of reaction

mechanisms. He correctly used curly arrows to represent the movement of electrons to

break and form bonds, recognising that some atoms (in his case, H and Br) are removed

as electrons are moved around. However, Guy did not notice that in attaching the

ethoxide group to one C, he drew a compound in which one C had five (not four) bonds.

This may indicate a weakness in Guy’s understandings of how elimination reactions

might proceed, specifically what types of interactions might occur between nucleophile

and substrate molecules to give elimination products. This has implications for both

examination and laboratory work. For example, Guy’s weaker understandings of

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elimination processes might make it difficult for him to design a laboratory procedure to

synthesise an alkene (2-pentene) or to explain how more than one product might be

produced from a particular synthesis.

The other three students who answered this interview question (Debra, Damon and

Dianne) did not attempt to rationalise the formation of alkene products. Dianne

commented that she thought alkene products may be possible, but she wasn’t sure how

they could be formed. Neither Debra nor Damon felt that these types of products would

result from these reactions.

The inability to answer the task may not be simply a reflection of the students

constructing poor understandings of elimination processes. This may also be linked to

the fact that the representations shown to the students, like those they had seen in their

lectures and textbooks, only showed single reaction particles. Competing reaction

processes, such as elimination and substitution, are difficult to explain and rationalise

using just a single particle description.

A single particle description leads students to consider only one (successful) interaction

between such particles. In the case of question 1 (Figure 10.4), students are considering

the interaction between one molecule of 3-bromo-2-methylpentane and one hydroxide

ion. Although there is more than one possible way that these two particles might

interact, each resulting in a particular product (as well as the possibility of the

interaction or collision of particles being unsuccessful and not resulting in reaction),

these two particles can interact in only one way, producing one product.

There are four possible products from the successful interaction of pairs of 3-bromo-2-

methylpentane and hydroxide particles. If hydroxide ions collide with the third carbon

(attached to the Br) in the carbon backbone of the substrate particles, substitution will

occur, resulting in the formation of molecules of 2-methyl-3-pentanol (1 in question 1 in

Figure 10.4) and bromide ions. If hydroxide ions collide with hydrogens on the second

carbon (CH(CH3)2) in substrate molecules, molecules of 2-methyl-2-pentene (4 in

question 1 in Figure 10.4) are produced. If hydroxide ions collides with one of the

hydrogens on the fourth carbon (CH2CH3) in substrate molecules, molecules of the

geometric (cis/trans) isomers of 4-methyl-2-pentene (2 and 3 in question 1 in Figure

232

10.4) will be produced. These four possible reactions (and proposed mechanistic

representations) are shown in Figure 10.8.

When considering interaction of a single molecule of 3-bromo-2-methylpentane and a

single hydroxide ion, only one of these products will result. It does not make sense to

suggest that more than one product molecules can be formed from a single alkyl halide

and a single nucleophile. To consider the possibility of more than one reaction product,

a chemist must be considering multiple particles—many alkyl halide molecules and

many hydroxide ions, which may interact with each other in a multitude of ways.

Lecturers, as experts, may understand single particle representations such as Figure 10.4

to be a condensation or a simplification of the multiple particle interaction that is

occurring in a reaction process and be able to predict several possible products from

Figure 10.8: Possible reaction mechanisms between 3-bromo-2-

methylpentane and hydroxide ion to produce (i) 2-methyl-3-pentanol, (ii) 2-

methyl-2-pentene, (iii) cis-4-methyl-2-pentene and (iv) trans-4-methyl-2-

pentene.

(i) (ii)

(iii) (iv)

233

such interactions. Second year chemistry students do not necessarily possess that skill

and are not taught it in this course.

A question relating to the examination question discussed on section 10.4.2 was also

asked in this interview. As it has been previously discussed in section 7.4.3, no further

comment will be made about students’ abilities. It is, however, interesting to note that

Guy, who demonstrated difficulty in rationalising elimination processes in the first two

questions in the interview (Figure 10.4), was able to correctly work through the

elimination example in question 3 (Appendix 6.17), suggesting that he began to link his

understandings about elimination mechanisms and representations more appropriately

as he worked through these tasks.

After Guy’s interview, he commented his understandings of elimination processes had

been improved through the interview discussion, and he felt that he would now be able

to give better answers to the first tasks. This discussion was not taped, but was noted in

the researcher’s field notes. This indicated that Guy held appropriate understandings of

elimination reaction processes in that he required to spend some time considering how

these elimination processes might have occurred. This could be the result of Guy

having constructed weaker links between relevant information when he was processing

it after learning about substitution and elimination reactions in class. Additionally, as

elimination processes were not covered to the same degree as substitution processes,

Guy may not have sufficient information available to him to construct such links

between his understandings of substitution and elimination reaction processes.

10.4.4 Mechanistic Representations

10.4.4.1 Effect of Unfamiliar Compounds on Students’ Abilities

Some students demonstrated an unwillingness to draw structures that they were

unfamiliar with or that they felt were not stable. This was seen in first semester

interviews with both Debra and Dennis. Neither of these students wanted to represent

structures that they were unfamiliar with, such as protonated alcohols (see Figure 10.6).

This was also seen in other tasks performed by different students (section 9.4.2.1).

234

In examples 1 and 2 of Task 4, the students were asked to represent the structure that

would result from a particular electron shift, which was represented by curly arrows

(questions shown as (i) and (ii) in Figure 10.9). The students added the circled arrows.

H3C C

CH3

O

CH3

H

H

Figure 10.9: (i) Dennis’ answer to example 1, (ii) Debra’s answer to example 2 and (iii)

the expected answer.

In both cases, the expected result was the same protonated alcohol ((iii) in Figure 10.9).

This type of structure had not been commonly represented in the Chemistry 2XX

lectures at the time of these interviews. Both Debra and Dennis drew additional

(circled) arrows on their representations, leading to different products; in example 1, a

carbocation and water ((i) in Figure 10.9, drawn by Dennis), in example 2, an alcohol

structure and H+ ((ii) in Figure 10.9, as drawn by Debra). In both cases, the final

products whose structures were represented were the result of electron shifts that the

students’ extra arrows had indicated, indicating an understanding of the curly arrow

symbolism. Similar products had be represented and discussed in their Chemistry 2XX

(iii)

(i)

(ii)

235

lectures at the time of the interviews more frequently than protonated alcohols such as

(iii).

Dennis was asked to explain the representation he had drawn in response to example 1

((i) in Figure 10.9). He commented (interview 23032000, line 323 – 363):

Dennis: Um . . . the hydrogen ion, which is a proton, would bond to the lone pair,

on the oxygen . . .

Interviewer: Ok.

Dennis: . . . and then that water molecule would be more stable for the oxygen, is

more stable with two oxygens hanging off it that it is with this carbon

structure hanging off one, off it as well, so . . .

Interviewer: Yep.

Dennis: . . . off it, so, it goes as water, and, um, that will leave behind the carbon

structure with, it’s still got its one, two, three, four, five, six, seven, eight,

that’s still got its um, (indistinct) that wouldn’t be there (indicating lone

pair), scribble that out, um . . . so it would be the oxygen with its eight,

and . . . would it have a lone pair there? No, it wouldn’t have a lone pair

there, it would be positive, at that point there. Yep.

Interviewer: So, you’ve indicated the formation of an oxygen hydrogen bond . . .

Dennis: Yep.

Interviewer: . . . to form the water. What two electrons in the first picture are

representing the formation of that bond? Where do they come from?

Dennis: Um, the lone pair of the oxygen.

Interviewer: Ok. Can you just point to it for me. Those ones? Ok. And, where have

the bonds, sorry, the electrons that form the bond between the carbon and

the oxygen gone?

Dennis: Um, with the oxygen to form another lone pair.

Interviewer: Ok, so they’ve actually moved as well?

236

Dennis: Yeah. So, I supposed we’d put another arrow into there . . . as that goes to

there, that would that would (indistinct) electrons from that bond would

go to there.

Interviewer: Ok, so you’re considering what may have happened, following that

event?

Dennis: Yeah.

Interviewer: Ok. Without considering what might have happened following that event

(indicating electron shift), this arrow here, the curved arrow towards the

hydrogen, without considering what else might have happened, could you

draw what would be the result of that electron shift? . . . (brief pause) . . .

Or, do you consider it as that happens, and the other thing (indicates

student’s arrow) follows along?

Dennis: Well, I’d consider they’d both happen simultaneously . . .

Interviewer: Ok.

Dennis: . . . as the hy, proton, proton bumps into the lone pair on that oxygen, it

would instantaneously take the, um, electrons from that bond with it.

Interviewer: With it? Ok. And why do you think it would do that? Just because the

water is a more stable compound?

Dennis: Yeah.

Interviewer: Ok.

Dennis: Because, um, the major premise of most of the ah, reactions, I sort of

believe is they go to a most, more stable . . .

This unwillingness to represent unfamiliar structures was similar to that seen in

students’ responses to tasks in Chemistry 100. Some students drew additional (correct)

curly arrows leading to a different product as part of their response to the question

asked. It appears that students were generally capable of manipulating the curly arrow

representations, but that their unfamiliarity with particular types of representations

meant that they would not represent a protonated alcohol like that shown in Figure 10.9.

It is suggested that the reason students added extra curly arrows to the tasks they were

237

given is due to their unfamiliarity with the expected answer ((iii) in Figure 10.9) and

with their familiarity with the products that they did represent.

10.4.4.2 Relative Position of Particles in Representations

The way tasks were presented to students can also influence the types of answers that

students may give. Associate Professor Andrews had expressed the hope that students

develop an understanding of the stereochemical implications of reactions, but the

researcher observed that some students’ perceptions of the direction of nucleophilic

attack in a substitution reaction appeared to be influenced by how the reaction is

represented. In a questionnaire (question 4 in Appendix 6.6), students were asked to

represent the products of a particular equation. Half of the students were given a

question with OH- represented on the left-hand-side of an alkyl bromide structure ((i) in

Figure 10.10). The other students were asked to complete the same question where OH-

was represented on the right-hand-side ((ii) in Figure 10.10). Answers that are

consistent with these relative positions of nucleophile and substrate structures are also

shown, which are the result of backside attack in (i) and frontside attack in (ii).

Draw the species that is most likely to be produced from the reaction represented below.

Include curly arrows on the representation to explain the production of these species.

BrC

CH3CH2

HCH3

OH +

BrC

CH3CH2

HCH3

+ OH

(i)

(ii)

OHC

CH3CH2

HCH3

+ Br

HO C

CH2CH3

HCH3

+ Br

Figure 10.10: Two questionnaire questions. In (i), the hydroxide ion is shown on the

opposite side to the bromide in the alkyl halide. In (ii), it is shown on the same side.

The represented answers shown on the right hand side were the most common in these

tasks, which are consistent with the nucleophile attacking the alkyl halide from the

physical position in which it is represented.

238

Thirty-five students answered this questionnaire question. Seventeen students answered

version (i) of question 4, the other eighteen answered version (ii). Over half of the

students (nine) who answered version (i) represented the structure whose

stereochemistry was consistent with the relative positioning of the nucleophile and the

substrate in the task, as represented in Figure 10.10. Only three of these students

showed a structure that was consistent with frontside attack. The remaining five

students gave different answers, none of which were consistent with either frontside or

backside attack. Over half of the students who answered version (ii) of the question

(10) represented a product that was consistent with frontside attack (and the relative

position of the nucleophile in the representation). Seven students represented a

structure that was consistent with backside attack. The remaining student gave a

response that was not consistent with either frontside or backside attack.

From these results, it is suggested that the relative position of alkyl halide structures and

nucleophiles in a representation may affect some students’ abilities to consider the

stereochemical outcomes of a particular reaction. Some students appear to consider the

positions of the representations as an indication of the direction of nucleophilic attack,

indicating a misunderstanding of the reality that the (single particle) equation or

mechanism is attempting to represent. Students do not appear to perceive that there are

many reaction particles in a given reaction, and that successful reaction is dependent

upon collision between particles with appropriate energy and orientation to each other.

This observed link between relative position in a representation and reaction outcome

has implications for the design of test and examination questions. If students answer a

question differently, based upon the positioning of the nucleophile and the substrate, the

relative position of structures may is important consideration. For example, the

examination question shown in Appendix 6.25 is described as an SN2 process, but the

nucleophile is represented on the right-hand-side of the structural representation, the

same side as the leaving group. Students may not perceive this as consistent with the

represented product, in which OH is shown attached to the left-hand-side of the

structure.

239

10.4.5 Structures as Cues for Reaction Type

A link between the type of structural representation used in a question and the response

a student gave to a particular question was also discovered. As has been described in

Appendix 10.1, Associate Professor Andrews used different types of structural

representations when teaching about substitution reactions (Figure A10.5 in Appendix

10.1) and elimination reactions (Figure A10.9 in Appendix 10.1). Substitution

processes were discussed briefly as competing processes using the same structural

representation shown in Figure A10.9, but, in general, substitution reactions were

represented using the structures shown in Figure A10.5.

To investigate if students used types of structural representations as cues to determine

reaction outcomes, student volunteers were asked to complete a task in an interview

(Appendix 6.17). Students were given three separate representations of two starting

materials (Figure 10.11) and asked to represent all the possible products of each

reaction.

C Br

H

H3CH3CH2C

H

C C

CH3

H

BrH

H3C

OH +

OH +

OH + CH3CH2CHBrCH3

1.

2.

3.

Figure 10.11: Three tasks from an interview with second year students.

With the exception of Fred, who did not write reasonable reaction products in any of the

questions, all of the students interviewed drew similar products as the result of reaction

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in question 1. All of the students but Fred also gave similar answers for question 2.

Each student’s answer to question 1 was different to their answer to question 2.

Students gave a substitution product, 2-butanol, as their answers for 1. Some students

(Felicity, Fabian and Felix) showed both enantiomers of 2-butanol in their answer.

Fiona and Fernando showed only the S-enantiomer of the alcohol; the enantiomer

consistent with backside attack of the OH- on the alkyl halide. Frank showed only the

R-enantiomer, which could be considered consistent with front side nucleophilic attack

(Figure 10.12).

CHO

H

CH3

CH2CH3C OH

H

H3CH3CH2C

(i) (ii)

Figure 10.12: Representations of the two enantiomers of 2-butanol, (i) R-2-butanol (ii)

and S-2-butanol.

These students all showed an alkene, an elimination product, as their answer for

question 2. With the exception of Felix, the students represented cis-2-butene as the

product of this reaction. Felix suggested trans-2-butene as the product of this reaction,

attempting to rationalise his answer in terms of product stability that had been discussed

by Associate Professor Andrews in the lectures (interview 14052001, line 227 – 239):

Felix: Um . . . I’m just sort of guessing (laughs) . . . but, I know you have, like

isomers here . . . and stuff, but . . .

Interviewer: Ok.

Felix: . . . I’ll see if I can remember like . . . I think there was something in the

lecture about . . . hydrogen has to be, most likely to be sticking up . . . and

bromine’s most likely to be sticking down (redraws picture) . . . and . . .

I’d guess, CH3, H, CH3 and . . . another H . . . I think that’s the same . . .

yeah (laughs). And, I guess from this one, you’d probably most likely to

get the other one (points at his original product drawing) . . . the trans

isomer . . .

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Interviewer: Ok.

Felix: Because it’s a bit more stable. And . . . I think it’s, just at the same time

because, I don’t know, it’s easier . . . I wouldn’t actually have a . . . um . .

. I don’t know. Just pick one . . . (laughs).

The structures of both geometric isomers of 2-butene are represented in Figure 10.13.

C CCH3

HH

H3CC C

H

CH3H

H3C

(i) (ii)

Figure 10.13: Representations of (i) cis-2-butene and (ii) trans-2-butene.

All of the students redrew the structure shown in question 3 before suggesting a

possible answer. The structural representations that the students drew in response to the

question depended on how they had chosen to redraw the starting material.

Fabian and Felix redrew this structure as a square planar representation, with the focus

on the bond between the second and third carbon in the chain ((i) in Figure 10.14).

Both represented elimination products (Fabian showed cis-2-butene, Felix gave the

answer as trans-2-butene).

Fiona drew a similar type of representation to Fabian and Felix, focussing on the bond

between the first and second carbons ((ii) in Figure 10.14), giving 1-butene ((iii) in

Figure 10.14) as the product.

Fernando redrew the structure as both full square planar representation ((iv) in Figure

10.14) and a three dimensional representation, similar to 1 in Figure 10.11. He chose to

use this three dimensional representation in his mechanism, giving a single enantiomer

of 2-butanol as the product.

Frank and Felicity redrew this structure in a similar manner ((v) in Figure 10.14). The

only difference between their structures was that Felicity did not show the hydrogen

atom on the structure. Frank represented 1-butene as the product of such a reaction.

Felicity suggested the formation of 2-butanol.

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(i) (ii)

C C

CH3

Br

CH3

H

HH

C C C C

H

Br

C C

H

H

H

Br

HCH3CH2 CH3CH2CH CH2

(iii)

(iv) (v)

H C C C C

HH

H

H

H

H

H

HBr

Figure 10.14: Representations of drawings made by (i) Felix and Fabian, (ii) Fiona,

(iv) Fernando and (v) Frank and Felicity in answer to question 3. Felicity did not

draw an H on her structure. (iii) is a representation of 1-butene.

The structural representations used in each of these three questions appear to have

influenced the type of reaction process that the students have predicted. Each of the

three alkyl bromide structures represented in the tasks shown in Figure 10.11 are

different structural representations of the same compound, 2-bromobutane. The

products that the different students represented (R- and S-2-butanol, cis- and trans-2-

butene and 1-butene) are all possible products of the reaction between 2-bromobutane

and hydroxide ions. Some products are more likely than others, depending on the

reaction conditions, but all are possible reaction products.

When the starting material, 2-bromobutane, was represented as a three dimensional type

of structure, as it was in 1, students appear to be cued into thinking about substitution

reaction processes. This can also be seen in Fernando’s response to question 3; he

chose to use his redrawn three-dimensional structure when working through this

question, and gave a substitution product as his response. This three-dimensional

structure is consistent with the types of structural representations their lecturer used

when teaching about substitution reaction processes.

Square planar representations appear to focus students’ attention on possible elimination

processes. As the compound was represented in question 2, with the bond between the

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second and third carbons in the butane chain highlighted, the students were directed

towards giving a particular elimination product; one of the isomers of 2-butene. Again,

this was consistent with the types of structural representations that Associate Professor

Andrews had used when teaching about elimination reaction mechanisms.

Both Felix and Fabian redrew the structure in question 3 in a similar manner to its

representation in question 2, focussing on the bond between carbons two and three.

This led them to give the same elimination product that they had in question 2 (cis-2-

butene). Fiona also redrew the structure as a square planar representation, but with a

focus on carbons one and two. Her represented product, 1-butene, is also consistent

with the claim that the type of structural representation use had suggested the possible

reaction product to her.

Felicity and Frank redrew the structure in question three in a similar way. The only

difference was that Frank represented both a bromine atom on the second carbon and a

hydrogen atom on the first carbon of his structure, whereas Felicity only showed the

bromine. There were differences in their answers; Frank showed an elimination

product, resulting from the loss of both the hydrogen and bromine atoms he had

represented, with a double bond forming between carbons one and two. Felicity

showed direct substitution of OH onto the second carbon. Perhaps the lack of a

represented hydrogen on the adjacent carbon atom(s) (such as Frank, Fiona, Felix and

Fabian all showed) influenced the type of reaction process (substitution) that Felicity

represented in this example.

The observed link between type of structural representation used and reaction outcome

represented suggest that the manner in which the lecturer taught about substitution and

elimination processes has led students to construct understandings about reaction

processes that are linked to the type of structural representation. Students appear to be

operating with the incorrect notion that the type of structural representation determines

the reaction outcome, not that the reaction conditions (and the associated number of

successfully collisions between particles within the reaction mixture) determine the

reaction outcome. This suggests that students are considering only the representation of

a reaction mechanism (what is shown on the paper) when predicting reaction outcomes,

not the multiple particle interactions that are actually occurring in any given reaction

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mixture. Students are operating on the representations as real things, not as models to

explain, rationalise or simplify the real process.

This perceived link between structural representation and type of reaction occurring is

also consistent with a single particle understanding of reaction processes. Students only

see one alkyl halide particle and one nucleophile ion. It is sensible to consider that only

one product will result from such an interactions. Students do not consider that there

may be many particles in the reaction mixture and therefore more than one process

occurring, so linking the type of structural representation used to the reaction outcome

appears (to the students) to be a reasonable thing to do. In addition, students have not

encountered situations in their course where this understanding does not work, and they

have not had to consider the weaknesses of such an understanding.

If students have well developed notions of competing reaction processes and the

multiple particle nature of reaction mixtures, it is felt that they would not be so likely to

use the type of structural representation in a task as a major cue for the type of process

that is more likely to happen. Such understandings would allow students to better filter

information about representations of reaction mechanisms and to form understandings

that allow them to use appropriate structural representations for writing substitution

and/or elimination mechanisms but that do not suggest links between structural

representations and reaction outcomes.

10.5 Summary

Associate Professor Andrews implemented many of his stated intentions in teaching

Chemistry 2XX. He commonly used specific structural representations when teaching

about particular reaction processes. Three-dimensional representations were used when

discussing substitution reaction processes, while square-planar representations were

used in the sections of the course that addressed elimination processes. The majority of

Associate Professor Andrews’ classroom discussion was consistent with a single

particle understanding of reaction processes.

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While some students (such as Damon and Dianne) did demonstrate continued

difficulties in representing reaction mechanisms using curly arrows, students’ overall

abilities in interpreting these representations appeared to improve over the course of

their second year study of organic chemistry.

Students demonstrated more facility with representing substitution reaction mechanisms

than elimination reaction mechanisms. This may be linked to the fact that substitution

reaction mechanisms were covered in a little more detail than elimination processes in

the lecture course.

When students were asked to suggest possible products from the reaction between two

reactants, they were able to identify the possible substitution products. Only two of the

five students interviewed were able to suggest how elimination products may be

formed. In both cases, these students redrew the substrate structure (see Figures 10.4

and 10.5) before attempting to represent a reaction mechanism for these elimination

processes. The other interview volunteers either felt that elimination products would

not be produced (Debra and Damon) or didn’t know how to represent their formation

(Dianne).

Two of the Chemistry 2XX students (Dennis and Debra) demonstrated an unwillingness

to represent an unfamiliar protonated alcohol species as an answer in a set of tasks. Both

of these students added extra curly arrows to the provided representation when they

suggested answers to the tasks (see Figure 10.6). The students added these curly arrows

in an appropriate manner. Their difficulty appeared to be linked to the unfamiliarity of

the expected product structure, and not to the curly arrow representation. This was

consistent with responses from a small number of Chemistry 100 students in an

examination question.

The relative positioning of the nucleophile and the substrate structures in an equation

can influence the how the student represents the product of the reaction (Figure 10.9).

The use of particular structural representations appears to influence the type of reaction

process a student is likely to predict, given particular starting materials. The use of

three-dimensional structures (such as 1 in Figure 10.11) appears to cue students towards

thinking about substitution reaction processes, while square planar structures (2 in

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Figure 10.11 is an example) tend to lead to elimination products being suggested.

These structural representations are consistent with the types used by the lecturer when

discussing substitution and elimination reaction processes. When students redraw

particular structures to enable them to work through problems, the type of structural

representation they choose to draw can impact upon the type of reaction process they

then perceive as occurring.

Additionally, the use of a particular structural representation, such as the three

dimensional structure used in the examination question represented in Appendix 6.25,

might make it difficult for students to consider that more than one reaction process may

be occurring. If a student links the three-dimensional representation to substitution

processes, it may be very difficult for him/her to be able to suggest a mechanism for the

formation of an elimination product.

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11 Conclusions and Pedagogical Implications

11.1 Introduction

This research study was designed to investigate the teaching and learning processes

involved in the study of reaction mechanisms in organic chemistry. The study

addressed several aims (section 1.4), which were translated into six research questions.

In response to these research questions, the researcher put forward several claims

(section 1.6), which are summarised below.

This chapter also discusses the theoretical and pedagogical implications of the research

claims discussed in the previous chapters. Suggestions are made as to how such

implications might be addressed in a teaching situation at university. These suggestions

encompass the three teaching situations that were utilised in the courses studied—

lectures, tutorials and laboratory classes.

11.2 Research Claims and Related Implications

11.2.1 Lecturers’ and Students’ Perceptions

11.2.1.1 Research Questions and Claims

Two research questions addressed the motivations for teaching about reaction

mechanisms in organic chemistry. One question (L1) considered lecturers’ perceptions.

The other (S1) considered students’ perceptions. These research questions asked the

following:

L1: What perceptions do lecturers have of:

a. the purpose and importance of teaching about reaction mechanisms as a

tool for understanding in organic chemistry?

b. the difficulties students have at various levels of their tertiary education


predict reaction outcomes?

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S1: What perceptions do students have of:







From a consideration of the findings, the researcher claims that:

Lecturers’ perceptions of the importance of teaching about reaction mechanisms in

organic chemistry are consistent with those of their students. However, although there

is some consistency between perceptions of the difficulties that students experience

when studying this topic, the perceptions of lecturers and students are not entirely in

agreement.

11.2.1.2 Findings: Perceptions of Importance

The research study identified that participating lecturers had two main motivations for

teaching about reaction mechanisms in their organic chemistry courses. These purposes

were:

• to facilitate the generalising of information about particular reaction

processes, which decreased the volume of information to be memorised;

• as a useful conceptual tool for predictive purposes.

Students perceived representations of reaction mechanisms to have four main uses their

study of organic chemistry. These were:

• to allow generalising about reaction processes;

• to help predict reaction outcomes;

• to explain what was going on in a reaction mixture;

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• as a revision topic in second year courses.

Students and lecturers shared two common perceptions about the usefulness of studying

reaction mechanisms in tertiary level organic chemistry courses. Using reaction

mechanisms to enable the generalisation of information (for example, the propositional

knowledge alkyl halides can undergo substitution reactions with hydroxide ions to

produce alcohol and halide particles) provides students with a means for chunking

information. This reduced the quantity of information that a student might have to

process when dealing with a particular task or problem, therefore making the task easier

to perform (Johnstone, 1997). The ability to use understandings of reaction

mechanisms to predict reaction outcomes is also linked to the generalisability of

reaction mechanisms; students can apply their general knowledge to particular

experimental situations to predict what products might be produced.

Students mentioned two other reasons for learning about reaction mechanisms in

organic chemistry classes. One reason—that mechanisms can be used to explain how a

reaction might proceed—is quite similar to the lecturers’ explanations of what

representations of reaction mechanisms are. The second reason was that reaction

mechanisms are useful for revision purposes. It was also not anticipated that lecturers

would consider the importance and usefulness of these representations in terms of

revision. Two of the courses being investigated were first year courses and the lecturers

of both assumed their students had never seen mechanistic representations before. The

second year lecturer also assumed that his students had not been exposed to

representations of reaction mechanisms before starting his course. He knew that about

half of the students had seen these representations in first year, but introduced the topic

as new material to assist the students who had not.

Students and lecturers were found to share very similar perceptions about the

importance of teaching and learning about reaction mechanisms in organic chemistry.

In the three courses observed, lecturers taught about reaction mechanisms using

strategies that reflected their perceptions that reaction mechanisms are important

generalising and predictive tools in the study of organic chemistry. The fact that

students have constructed corresponding understandings implies that the three

participant lecturers in this study effectively communicated the reasons why they were

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teaching about reaction mechanisms to their students and that the students formed good

understandings of why they were learning about reaction mechanisms. Examples of

how lecturers communicated this importance can be seen in their class presentations.

Dr Anderson commenced his discussion of reaction mechanisms by using general

equations of the form:

R—X + Y- → R—Y + X-

Using such general representations before going into specifics of particular mechanistic

processes appears to be a useful way of indicating that one mechanism (for example, an

SN2 mechanism) can be applied to many (similar) reactions.

Dr Adams used similar general equations in his teaching, as well as giving his students

general outlines of the types of tasks that they should be able to use reaction

mechanisms to do.

Associate Professor Andrews drew attention to the fact that he considered reaction

mechanisms as a way of systemising the study of organic chemistry. His course outline

gave students an example of how one type of reaction mechanism (hydrolysis of a

carboxylic acid derivative with aqueous base, see Figure 10.1) could be applied to a

range of different derivatives (carboxylic acid, oxygen ester, amide, acyl chloride and

anhydride). Giving the students a concrete example of the generalising ability

associated with the representation of reaction mechanisms appeared to be a powerful

tool in showing students why they were learning about the topic.

The fact that lecturers and students share the same perceptions about the usefulness of

teaching and learning about reaction mechanisms suggests that the way in which

lecturers choose to teach about the topic will be well suited to the students who are

studying the topic. All three participant lecturers made conscious choices in their

teaching to emphasise the predictive and generalising nature of representations of

reaction mechanisms, which agrees with students’ understandings of the usefulness of

these representations.

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11.2.1.3 Findings: Perceptions of Difficulties Faced by Students

Lecturers considered representational aspects of reaction mechanisms to be difficult for

their students. This included such things as the correct use of curly arrows and formal

charges and the type of structural representation used in a particular reaction

mechanism. Associate Professor Andrews commented upon a different aspect of

students’ understandings of mechanistic conventions. He stated that he had noticed

some students using correct representations and conventions to represent reaction

mechanisms for processes that were not chemically viable and would not happen.

While their understandings of the representational aspects of reaction mechanisms may

be appropriate, their chemistry understandings are not necessarily appropriate, and it

was these limited understandings of chemistry that was influencing the students’

abilities to represent appropriate reaction mechanisms.

These difficulties have implications for how lecturers teach their courses. A

consideration of the pedagogical content knowledge associated with teaching about a

particular topic involves understanding what parts of the topic students find difficult.

For example, if a lecturer feels that students have difficulty with translating three-

dimensional structures to square planar structures, he/she might design a tutorial in

which students make plastic models of various molecules and then represent them using

different types of structural representation.

However, if the lecturers’ perceptions of the difficulties faced by students are not the

same as those perceived by the students, students may not feel that there is a need to pay

particular attention to those aspects in the lecture or tutorial. For example, lecturers

perceived that students have difficulty representing curly arrows. A lecturer might

decide to start each organic chemistry laboratory class by discussing proposed reaction

mechanisms for the synthetic processes to be carried out, or may require students to

write a reaction mechanism as a prelaboratory exercise. If the lecturers’ students do not

perceive that they have difficulty with representing curly arrows, they may not pay

attention or take notes in the discussion, or may copy a mechanism into a pre-lab from

lecture notes, without realising that their understandings of the representation are

inappropriate or incomplete.

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Students’ difficulties were classified as either perceived (what the students thought was

hard to do) or demonstrated (what the students had difficulty doing in tasks). Students’

perceived difficulty with the topic ranged from the very general; ‘I think it was difficult,

at first, to get a grasp on it’ (Graeme); to the specific; Barbara’s comment on the

information ‘overload’ from the topic, or Benjamin’s difficulty with visualising what

was going on in the reaction mixture. Students also identified chemistry language

(terms such as racemic mixture) as something that caused them difficulty in their study

and commented that mechanistic processes in which more events happen are more

difficult to interpret and represent than those with fewer events.

While some students commented upon the difficulty caused by the language used when

studying reaction mechanisms, this was not something that the lecturers commented

upon as impacting upon the difficulty of studying about reaction mechanisms. It may

be that lecturers do not consider this to be a difficulty that is specific to the study of

reaction mechanisms; scientific language can increase students’ perceived difficulties in

studying all science topics, not just reaction mechanisms (Johnstone, 1991; Millar,

1991).

Many students’ demonstrated difficulties were related to representational aspects of

reaction mechanisms. Students were observed to represent curly arrows incorrectly,

particularly pointing arrows in the wrong direction when H+ was involved in the

representation. This was observed more commonly with first year students, although

some second year students did demonstrate the same incorrect use of the representation.

In addition, it was noticed that students exhibited some difficulty in calculating

appropriate formal charges on structures in mechanistic representations.

Students were also observed to generally perform more poorly on more complex tasks.

This was consistent with students’ comments about the number of events increasing the

difficulty of a task, and with the information processing model that was previously

discussed in section 2.3.4 (Johnstone, 1997).

It was noticed that students generally found elimination reaction mechanisms more

difficult to represent than substitution processes and that they were more likely to make

errors when representing such mechanisms. Generally, elimination reaction

mechanisms contain more bond breakages and formations than substitution processes,

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resulting in a representation with more curly arrows. One curly arrow is used to

represent both breakage of one bond and formation of another. This is not the case in

substitution representations, where each curly arrow represents either bond formation or

bond breakage, not both.

Students and lecturers demonstrated differences in their perceptions of the difficulties

that students faced in studying about reaction mechanisms. It is suggested that these

differences are related to the fact that lecturers base their perceptions on the difficulties

that they observe students exhibiting in written examinations and tests, whereas students

do not necessarily perceive that they are demonstrating poor understandings in their

examinations. Lecturers’ perceptions of students’ difficulties correspond quite well to

the difficulties that the researcher observed them exhibiting during the course of the

research study, which suggests that the lecturers have valid perceptions of the

difficulties that students face in their studies.

11.2.1.4 Pedagogical Implications of Findings

The differences between students’ and lecturers’ perceptions of the difficulties posed in

understanding reaction mechanisms has implications for the way students are examined

on their understandings. Many of these observations were based upon students’

performances in examinations. In each of the courses studied, students sit only one

examination at the completion of the course. Students generally do not receive their

examinations back from their lecturer, and therefore do not see where they have made

errors in the paper. Even in cases where students do receive their examinations back

(some Chemistry 100 students do choose to collect theirs), the examinations aren’t

necessarily marked in a manner that is designed to give students feedback on errors or

misjudgements that they have made in the paper.

For the most part, if a student wants to know how he or she did on the paper (and why

particular answers are wrong or not completely correct), he or she must approach the

lecturer to ask. In the case of Chemistry 100 and 2XX, they can do this at the start of

second semester (approximately four weeks after they have sat the examination).

Chemistry 121/122 students would need to speak with their lecturer about their

November examination either during their end of year break (late December – mid

February) or at the start of semester the next year (late February). This time delay leads

254

to many students not receiving (or actively seeking) any feedback on their

examinations. Many students remain unaware of any errors that they might have made.

If students are unaware of any errors that they might have made on the examination or

test paper, they do not perceive that there is any need to improve or modify their current

understandings. Strike and Posner (1985) suggested that certain conditions needed to be

met for students to appreciate that their understandings might need to change. One of

these conditions was that students must be dissatisfied with their current

understandings. Without any evidence to suggest to them that their current

understandings are inappropriate, students do not have a need to attempt to assimilate or

incorporate different understandings into their current (and, to them, workable)

constructs.

To help students with their perceptions of where they might have incomplete or

inappropriate understandings about reaction mechanisms, it would be useful to provide

them with opportunities to attempt problems and to discuss the worked solutions with

their lecturer. This could assist in identifying difficulties or inappropriate

understandings to students.

This opportunity is provided to the students in tutorials in each of the courses. In

Chemistry 100, the tutorials are not compulsory, and very few students attend. The

Chemistry 121/122 tutorials are compulsory, but these tutorials cover organic, inorganic

and physical chemistry, and the time devoted to discussing mechanistic problems is

limited and involves little interaction. Dr Anderson also not necessarily conducts these

tutorials, as the large class is broken down into tutorials of around 20 students, which

are then taught by other academics and post-graduate students. Associate Professor

Andrews offers a limited number of tutorials in his lectures (lectures are not

compulsory), which do cover mechanistic problems, although time constraints mean

that not all of the mechanistic problems that he sets students in their problems sheets are

addressed in class.

Dr Anderson suggested a method that he had devised to attempt to assist students to

improve their understandings of what he perceived to be a limitation in their

understandings—the curly arrow representation. As was described previously, Dr

Anderson set aside one lecture class to work through arrow representations with his

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students. He allowed the students to work through the problems in small groups and he

walked around the class, questioning students and assisting if required. It is felt that

spending this type of interactive time, where students’ misunderstandings can be

exposed while they are working through problems, working on particularly problematic

aspects of students’ understandings might help students to develop more appropriate

understandings. This could also affect their perceptions of what was difficult and what

was easier about the study of reaction mechanisms, and perhaps influence what they

chose to focus on in future lectures.

In addition to this suggestion made by Dr Anderson, teaching strategies suggested by

Harvey and Hodges (1999) might also be beneficial in helping students to recognise

weaknesses in their understandings about reaction mechanisms and to develop more

appropriate understandings. Harvey and Hodges suggested the use of guided reading

worksheets, dialogues, in-class worksheets and role-playing exercises. Each of these

could be applied to the topic of reaction mechanisms to enable students to focus on the

topic during their study.

The role-playing exercise suggested by Harvey and Hodges would be more suited to a

tutorial situation, rather than a lecture. Lectures generally contain much larger numbers

of students (often between 100 and 200 students in first year), whereas tutorials usually

consist of no less than 20 students, and acting out a reaction process (for example, an

SN2 process, with students playing the roles of nucleophiles, substrates and leaving

groups) would be considerably easier (and more accessible to students) in the smaller

groups. If students were unwilling to attempt a dramatic recreation of a reaction process

in a class, the tutorial situation also provides the opportunity for students to get together

in smaller groups and take it in turns to describe particular reaction processes to each

other. This interaction with their peers would probably be less threatening than

attempting the same process with a lecturer or tutor, and students could also provide a

written explanation (Harvey and Hodges refer to this as a dialogue), which would allow

the lecturer or tutor to check the appropriateness of their students’ understandings.

In terms of the three courses that were investigated in this study, the most appropriate

place for such small group work might be the laboratory, particularly for the first year

students. The laboratory classes are compulsory for all students and the laboratory

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reports are collected and marked each week and returned to students to provide

feedback on their understandings. Although the classes can be large (the first year

laboratory at University B houses approximately ninety students, while the first year

laboratory classes at University A are generally no larger than 20 students), students can

be divided into smaller groups to work on dialogues or role-plays together.

There are, however, several disadvantages to using the laboratory classes for this

purpose. Each of the courses has a limited number of laboratory classes assigned to it.

Chemistry 121/122 students have 12 or 13 laboratory sessions each semester, of which

about one third are devoted to organic chemistry. At the time that this study was

conducted, Chemistry 100 students attended 11 laboratory sessions in first semester,

seven of which were organic chemistry experiments. This number has dropped to eight

laboratory sessions (five of which cover organic chemistry) in the years since the

Chemistry 100 course was studied. Chemistry 2XX students have at least one

laboratory class each week for most of the teaching year, but only six weeks in second

semester are devoted to the study of organic chemistry.

In addition to the limited time available in laboratories, there is also the issue of who is

teaching the class. At both University A and B, laboratory classes are often taught by

honours or post-graduate students. The honours or post-graduate students who teach in

the Chemistry 2XX organic chemistry laboratories are generally carrying out organic

chemistry related honours of PhD studies, or are recognised to have an aptitude for

organic chemistry. The honours and post-graduate students who teach first year courses

might by doing their research in any field of chemistry or biochemistry, and therefore

may not have a particular interest or aptitude for organic chemistry.

At University A, the lecturer spends time in each of the laboratories, but he/she is not

always present in the laboratory. At University B, the lecturer does teach laboratory

classes, but is assisted by honours or post-graduate students (the number of laboratory

demonstrators depends upon the size of the class). Students attend the laboratory

classes that best suited their own timetables. As a result, some students who attended

Dr Anderson’s Chemistry 121/122 lectures did not attend a laboratory that he taught.

The laboratory demonstrators at both universities were required to attend a

demonstrators’ course at least once if they were teaching, in which general teaching

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skills and laboratory safety issues were discussed. In addition, laboratory demonstrators

attend a weekly group discussion with the lecturer, in which the week’s experiment is

discussed. If strategies such as dialogues or role-plays were incorporated into the

laboratory, this pre-laboratory discussion with the demonstrators would be useful to

discuss the understandings that students are expected to display and to help

demonstrators to identify students’ whose understandings of reaction mechanisms are

limited or inappropriate.

Lecturers attempting to use tutorials at University B to implement these types of small

group work would face similar difficulties. Tutorials are compulsory, but they are not

necessarily taught be the lecturer (post-graduate students or other academics also teach

tutorials). In addition, there are only a limited number of tutorials (12 per semester),

with only a third of the tutorials focussing on organic chemistry. At University A, the

lecturer teaches tutorials, but they are either non-compulsory (Chemistry 100) or held

only rarely (Chemistry 2XX).

11.2.2 Students’ Achievements in Their Studies


Two research questions addressed students’ understandings of concepts and skills

related to the study of reaction mechanisms. One question (L2) was lecturer-related.

The other (S2) was focussed on students. These asked:

L2: What do lecturers consider to be the essential knowledge of concepts required by

students at different educational levels to use reaction mechanisms in


S2: What understandings do students have of the important concepts and skills in the

topics of substitution and elimination reaction mechanisms in organic chemistry,

and which factors affect their abilities to demonstrate a facility with these

concepts and skills?

The following claim is made about students’ achievements of lecturers’ aims in their

study of reaction mechanisms in organic chemistry:

258




lecturers in tertiary level courses. These outcomes are rarely articulated by the

lecturers and are generally not achieved by the students.

11.2.2.2 Findings

In their course outlines and their teaching, the three participant lecturers have all made

reference to the explicit aims of their courses and the knowledge that they expect their

students to acquire. These involve such things as being able to represent SN1 and SN2

reaction mechanisms, calculating formal charges and understanding and using curly

arrows. In general, students achieved these explicit aims.

The more implicit aims, such as the ability to competently describe competing reaction

processes in the same reaction mixture resulting in more than one product, are not well

described in the lecture courses, even when they were identified as intended aims in

some courses. Students do not demonstrate good understandings of such aims.


The implications of these observations are related to students’ abilities to use

representations of reaction mechanisms as conceptual tools to generalise and to predict

reaction outcomes. Students have identified these as reasons for teaching and learning

about reaction mechanisms, but if they cannot apply these conceptual tools in anything

more than the most basic cases (simple substitution processes, for example), then they

are not able to fully exploit the usefulness of mechanistic representations.

To improve students’ understandings of some of these implicit aims, it is necessary for

the importance of such understandings to be obvious to the students. In the courses that

were investigated, the focus was teaching students about individual reaction

mechanisms (SN1, SN2, elimination), which Coppola (1996) suggested could lead

students to ‘tend to perceive these as exclusionary predictive choices’, adding that ‘an

understanding of competition between pathways is key to understanding actual

experimental results’. The notion of competition between such processes was discussed

259

OH- H+

only briefly in each course, which did not reinforce the importance of competing

reaction processes to students.

To address this, lecturers first need to consider if developing appropriate understandings

of competing reaction processes is necessary for students in their particular courses.

The researcher argues that understanding that more than one product can be formed in a

particular reaction mixture is important for those students who intend to work as

chemists. Very few (non-enzyme catalysed) reaction processes produce exclusively one

reaction product in 100 % yield. If the notion that reactions are not always 100 %

successful and don’t always result in only one product being formed is discussed with

students from the beginning of their study of chemistry, the researcher believes that this

will have a positive impact on their understandings in many different topics, including

the topic of reaction mechanisms.

An example of how this might be achieved could be to use a particular reaction process

in either the laboratory or the lecture that is known to produce more than one product.

Identifying such a reaction might be difficult, as many organic reactions involve mixing

two colourless liquids together and producing another colourless liquid that needs to be

separated into appropriate fractions for analysis, which will then confirm the presence

of more than one reaction product. In addition, some appropriate reactions might be too

time consuming or too difficult or dangerous for students to perform, so a video of the

reaction process might be more appropriate.

A particular reaction that might be of use in describing the competing reaction processes

that might occur is the Cannizzaro reaction. An example of this type of reaction

involves mixing benzaldehyde (a colourless liquid) with a solution of potassium

hydroxide (also a colourless liquid). Ether extraction and evaporation of the diluted

solution yields benzyl alcohol, a colourless liquid. Acidification of the remaining

aqueous solution yields white crystals of benzoic acid (Furniss, et al., 1989, p. 1028 –

9). While this can be represented as an equation, which shows the formation of two

products:

2PhCHO → PhCH2OH(l) + PhCOO- → PhCOOH(s)

260

students may not necessarily form appropriate understandings of the fact that more than

one product is formed from the same starting material by looking at this equation.

Seeing a video of the experiment being performed, in which two vastly different

products (which can be simply and easily characterised by analytical techniques, such as 1H nmr or IR, that students are generally taught about in first and second year chemistry

courses) are formed from the one reaction mixture is more likely to stick in students’

minds as an example of competing reaction processes. In addition, yields of both

benzyl alcohol and benzoic acid can be calculated for the students, which will show that

not all of the benzaldehyde is converted to the two products, opening the way for the

lecturer and students to discuss the notion of unsuccessful collision of reactant particles.

If the notion of competing reaction processes is discussed and students are shown real

examples of such processes, the researcher feels that students are less likely to consider

reaction mechanisms as exclusive processes and might be more likely to consider the

possibility of more than one reaction process occurring in a particular reaction mixture.

The researcher also feels that competing reaction processes should be discussed

throughout the entire course, not just mentioned at the start of a course and not referred

to again. In this way, students’ perceptions of the importance of competition in reaction

processes are being constantly reinforced during the course.

In addition to such lecture or laboratory presentations, textbooks, notes or CD-ROM

presentations that reinforce the idea of competing reaction processes by using multiple

particle pictures and language would also be useful teaching aids.

11.2.3 Lecturers’ Teaching Strategies and Effect on Students


Lecturers’ teaching strategies and their motivations for using such strategies were the

subject of research question L3. A similar research question (S3) considered the

strategies used by their students. These research questions asked:

L3: When teaching about reaction mechanisms:



261

b. what reasons and motivations do lecturers have for choosing

particular teaching strategies?

S3: When using representations of reaction mechanisms, what strategies do students


The researcher claims that the types of representations that lecturers use has an effect on

the type of process that students consider might be occurring in a particular example:



generally used to when writing substitution reaction mechanisms, while square planar

structures are used to represent elimination reaction processes. Although these

structural representations are useful for representing features of different reaction

processes, the use of a particular structure can cue students into thinking about only

one type of reaction process taking place in a given reaction.



understandings of the multiple processes going on within a reaction mixture. These

strategies are generally consistent with those demonstrated by the lecturers in

representing reaction mechanisms.

11.2.3.2 Findings

An analysis of the individual lectures and courses identified that each of the participant

lecturers generally used language that was consistent with a single particle description

of reaction processes. They also commonly used structural representations of single

molecules when describing reaction processes and representing reaction mechanisms.

This type of language was also commonly used in the textbooks, notes and handouts

used in the courses.

The common use of particular structural representations was also noted. When teaching

about substitution reactions, each of the participating lecturers commonly used three-

dimensional structural representations that focussed students’ attention on one

represented carbon atom, the carbon atom bearing the leaving group. In contrast,

262

structural representations used in descriptions of elimination reactions were more square

planar, showing the bonding around two represented carbon atoms, and often lacking

any indication of shape or stereochemistry.

The representational strategies that students adopted to help them work through

mechanistic explanations generally mirrored those used by their lecturers. Many

students demonstrated a single particle understanding of reaction processes through

both their language and representations. This may be as a result of the language used by

their lecturers and in many textbooks. In some cases, a single particle understanding

did not affect their abilities to correctly interpret specific reaction mechanisms or to

describe particular reaction processes. There do exist some situations, however, for

example, competing reaction processes, where a student requires a multi-particulate

view of the reaction process to allow an understanding of the many different reactions

that may be progressing.

The lecturers’ uses of different types of structural representation when discussing

different reaction processes were reflected in many students’ methods of working

through tasks. The strategies that students employed when working through mechanistic

problems often match those used by their lecturers. Students would use a particular

type of structural representation to represent a reaction mechanism for a substitution

process, and a different representation for an elimination process.

In addition, the type of structural representation used in a question could also play an

important role in assisting students to determine which type of reaction processes is

more likely to occur. It was observed that students were more likely to predict the

formation of a substitution product if shown a three-dimensional structural

representation, whereas they were more likely to show an elimination product if given a

square planar representation. Students did not appear to consider the properties of the

reactant when predicting a reaction outcome—their predictions were based upon

representation alone. This suggests that students have formed inappropriate links (what

Johnstone, 1997, refers to as misfilings) in their minds between structural

representations and reaction outcomes.

263


This finding implies that students are considering the representations that they see on

the page (or the white board) as being the real mechanism. This has significant

implications for students’ abilities to develop useful and workable understandings of

mechanistic processes that they can apply to their study of chemistry. If students

perceive reaction mechanisms to be nothing more than the representations that they

draw, they are not grasping the most useful aspect of reaction mechanisms—that an

understanding of reaction mechanisms can be applied to unfamiliar reaction processes

to determine what products might be produced, or to allow them to design a reaction

process which will favour the production of one (or more) particular products.

To address this difficulty, it is first important to link the real, macroscopic reaction

process and the microscopic, multiple particle model of a mechanistic representation in

an appropriate manner in students’ minds. This aspect might be best addressed in a

laboratory class. Students could be separated into small groups, who each work through

particular experiments. Some examples might be:

• Reaction between 1-butanol, hydrobromic acid and sulfuric acid (which

is already carried out in the Chemistry 121/122 laboratories) to produce

1-bromobutane (and a small amount of 1-butene, which is probably not

able to be separated);

• Oxidation of benzyl alcohol to benzoic acid using potassium

permanganate;

• Preparation of cyclohexene from bromocyclohexane and sulfuric acid.

These types of reactions are simple and can be performed relatively quickly in the

laboratory. They produce products that can be characterised easily by analytical

techniques that students learn about in first and second year chemistry courses.

After completion of their particular reactions, students could discuss the reaction that

they had carried out and consider what sort of product (or products) was formed.

Discussions could be extended to other groups, enabling students to learn more about

the experiments conducted by others in the class. After students have seen and

264

discussed reaction processes, these could then be used as examples in the lectures when

discussing reaction mechanisms, in an attempt to link the real experimental process to

the molecular explanation and the structural representation in the students’ minds,

linking the macro, the sub-micro and the symbol levels (Johnstone, 1982, 1991).

In terms of students using particular representations as cues for the type of reaction that

will occur, lecturers need to consider the appropriateness of using such representations

in their classes. If the use of particular representations is having a detrimental effect

upon students’ understandings (which negates the benefits of using such a

representation), then its use is perhaps inappropriate.

One way of addressing this is for lecturers to use plastic ball-and-stick models when

talking about a reaction process. Using such a model gives students somewhat more of

an understanding of the comparative size of compounds (for example, comparing a

model of 1-bromobutane to a model of OH- will help students to consider that

molecules have size and take up three dimensional space) and can also be used to

explain movement around single bonds.

When lecturers are representing reaction processes in their lectures, there needs to be a

consideration of what type of structural representation is used in representing a

particular mechanism. One suggestion is for a lecturer to show several different

structural representations and ask students to suggest reasons why each particular

representation might (or might not) be useful for representing a given reaction

mechanisms.

Alternatively, lecturers could purposefully choose to use different types of structural

representations when representing mechanisms. For example, if a lecturer alternately

used both square planar and three dimensional representations when writing substitution

mechanisms, students would be less likely to form a link between particular

representations and reaction types, and therefore less likely to use representations as

cues.

Another suggestion is to develop a tutorial exercise that gets students to work through

reaction mechanisms where different structural representations are used. Students can

be given an equation that shows the reaction between (for example) 1-butanol,

265

hydrobromic acid and sulfuric acid to produce 1-bromobutane (a substitution product),

with 1-butanol shown as a square planar structure, and asked to represent a reaction

mechanism to rationalise this process. After doing so, students could be given the same

question again, but with 1-butanol represented as a three dimensional representation.

Students can then discuss which representation was easier to use and talk about why

that representation is easier to use. This focuses students’ attention on aspects of the

representational nature of the structural representation, allowing them to consider it as a

model, not as a real molecule.

11.2.4 Single vs Multiple Particle Descriptions

11.2.4.1 Research Claim Arising From Overall Study

The researcher described an integrated model that incorporated aspects of both

Johnstone’s macro/sub-micro/symbolic triangle (1982, 1991) and Jensen’s levels and

dimensions of chemistry (1998), which was used in guiding the study. In addition to the

levels and dimensions described by these two authors, the researcher incorporated two

new levels into the sub-micro (Johnstone) or molecular (Jensen) classifications. These

levels described the number of reactant particles present in a particular description, and

were labelled single particle and multi-particle (or multiple particle) by the researcher.

As a result of this study, the researcher makes the following claim:

The language used in many instances in lectures is consistent with a consideration of

only individual particles in reactions, and not with multiple particle interactions.


require a consideration of multiple reaction particles. Students tend to display single

particle understandings of reaction processes.

The researcher believes that while expert lecturers understand that many reactant

particles are involved in a reaction process, which can be then simplified to represent

specific interactions between single particles, novice learners (such as the first and

second year students involved in this study) do not (and generally cannot), and consider

only the successful interactions between single particles in a reaction process.

Providing students with the opportunity to develop understandings of the probabilistic,

266

multiple particle nature of reaction processes will assist in improving students’

understandings of reaction mechanisms and their representations.


This claim suggests that students do not consider the real, macroscopic level of what is

going on in a reaction process when they represent reaction mechanisms. This has

implications for their abilities to develop reaction processes in the laboratory, or to

manipulate existing processes to influence the yield of a particular product. Methods to

assist students in forming links between real processes and their mechanistic

representations have already been discussed in section 11.2.3.

In terms of helping students to develop useful understandings of the multiple particle

nature of reaction mechanisms, lecturers can assist by carefully considering the type of

language that they use in their lectures. While using terms that are more consistent with

a consideration of single particles in a reaction process is easier and generally well

understood by other experts, novice learners do not demonstrate the same facility to

form understandings of multiple particle interactions from single particle descriptions.

It is not only the language used by the lecturers themselves, but the level of description

in the textbooks that they use, and the types of representations that are shown on

associated CD-ROMs and other presentations. The ChARMs programme, for example,

is useful for showing particular reaction types, but it does not show much in the way of

many particles in a reaction process. If this programme started with a multiple particle

view of the many molecules and ions in a reaction mixture such as videos by Tasker, et

al. (1996, 1997 and described by Tasker, et al., 2002) before zooming in to the

successful interaction between two reaction particles, this might help students to form

better understandings of what might be going on in a reaction process and to improve

their abilities to link between real, macroscopic processes and the (single particle)

representations that are often used to model such processes.

The probabilistic nature of reaction processes also needs to be discussed in more detail

with students, as this reinforces the consideration of many particles within a reaction

mixture. Students can determine yields of products in particular experiments, and then

267

answer related questions in their laboratory reports, which could then be discussed in

lectures or at the start of the next laboratory. Such questions could include:

• If all of the starting material was converted to product, what mass of

product should have been formed in this reaction? Did you form this

mass?

• If you produced less, why do you think this happened?

• What other products might have been formed in this reaction?

• How might you improve your yield of product? Do you think you could

ever prepare a 100 % yield? Why/why not?

Another way to assist students in developing an appreciation of the multiple particle

nature of reaction processes might be to discuss the model that has been used to guide

this research. This model can be applied to many different topics, not just to the study

of reaction mechanisms. Making students aware of the fact that there are many levels to

consider when learning about reaction mechanisms, as well as learning how to write and

represent reaction mechanisms, can provide them with the ability to make sense of

descriptions at these different levels. In addition, making students aware of the multiple

particle nature of reaction processes will assist them in developing better understandings

about more complex reactions, such as those that result in more than one product.

11.3 Summary

There are many pedagogical implications related to the findings that have been

described in the preceding chapters. Many of these are connected to students’

perceptions of the links between real reactions and mechanistic representations as well

as students’ abilities to consider the interactions of many reaction particles in a given

reaction process.

There is no doubt that representations of reaction mechanisms are a useful conceptual

tool for chemists to use. These representations are also helpful for students studying the

268

information-rich topic of organic chemistry, as they can be utilised for generalisation

and predictive purposes. The problem may be that students simply do not consider

these written representations to be models or representations of a real process, and may

indeed consider a mechanistic representation to show exactly what does happen in a

given reaction process. Helping students to develop an understanding of this aspect of

reaction mechanisms will, I feel, help them to better understand the usefulness and

applications of these representations.

Several strategies to assist students in developing better understandings have been

provided. In general, these strategies involve forming stronger links between work

done in the laboratory and that discussed in the lecture, and helping students to

understand that these two different types of classes are not independent of each other,

but are both important aspects of their chemistry course.

This study has identified that while students can often correctly use mechanistic

symbolism to represent appropriate reaction mechanisms, their understandings may be

limited by a single-particle understanding of these reaction processes. The single

particle understanding can be reinforced both by the types of representations depicted

and the language used when describing these processes.

It is suggested that a consideration of the topic of reaction mechanisms, in terms of the

four-level classification described in Table 5.2, might help to identify areas where

multiple particle understandings would be beneficial, which would assist lecturers in

their consideration of the teaching of the topic. It is also felt that developing students’

understandings of the probabilistic, multiple particle nature of reaction processes will

help them to form richer understandings of mechanisms and their representations.

269

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Silberberg, M.S., (2000). Chemistry: The Molecular Nature of Matter and Change.

Boston: McGraw Hill.

Strike, K.A., and Posner, G.J., (1985). “A Conceptual Change View of Learning and

Understanding”, in West, L.H.T., and Pines, A. L., eds., (1985). Cognitive Structure and

Conceptual Change. Orlando: Academic Press, p. 211 – 231.

Sykes, P., (1986). A Guidebook to Mechanisms in Organic Chemistry, 6th ed. New

York: Longman Scientific and Technical.

275

Tasker, R., Bucat, R., Sleet, R., Chia, W., and Corrigan, D., (1996, 1997). The

Molecular World of Reactions in Water, Part 2: Ionic Equilibrium, Acid/Base and

Oxidation/Reduction Chemistry Video (33 min); The Molecular World of Reactions in

Water Part 1: Dissolving, Precipitation and Complexation Video (25 min); The

Molecular World of Water Video (13 min). Series of three videos, designed for

chemical educators, depicting chemical substances and reactions typically covered in an

introductory chemistry course.

Tasker, R., Bucat, R., Sleet, R., Chia, W., and Corrigan, D., (1997). VisChem Resources

— Learning Chemistry Through Visualisation of the Molecular Level, a resource CD for

chemical educators containing 82 animations in cross-platform QuickTime format

(288Mb).

Tasker, R., Dalton, R., Sleet, R., Bucat, B., Chia, W. and Corrigan, D. (2002).

Description of VisChem: Visualising Chemical Structures and Reactions at the

Molecular Level to Develop a Deep Understanding of Chemistry Concepts. Retrieved

May 23, 2004, from Learning Designs Web site:

http://www.learningdesigns.uow.edu.au/exemplars/info/LD9/index.html

Teo, Y.Y. (2000). Probing Students’ Understandings of Mechanisms Through Activity

Sheets, unpublished third year student project, Curtin University of Technology.

University of Western Australia, Human Research Ethics Committee, (2002).

“Guidelines to assist in the preparation of information sheet and consent form”.

http://www.research.uwa.edu.au/opps/forms/he/guidelinesis-cf.rtf

Weeks, D.P., (1998). Pushing Electrons: A Guide for Students of Organic Chemistry,

3rd ed. Fort Worth: Saunders College Publishing.

Wentland, S.H., (1994). “A New Approach To Teaching Organic Chemistry

Mechanisms”. Journal of Chemical Education, 71(1), 3-8.

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13 Appendices

Appendix 3.1: Course outline for organic chemistry component of Chemistry 2XX. .278

Appendix 4.1: Identified skills and concepts for learning about reaction mechanisms.

............................................................................................................................... 279

Appendix 6.1: Information sheet provided to second year students ............................. 286

Appendix 6.2: Justification for task in Chemistry 2XX questionnaire (Appendix 6.6).

............................................................................................................................... 288

Appendix 6.3: Descriptions of tasks. ............................................................................ 291

Appendix 6.4: Chemistry 121/122 Survey (Semester 2, 2000) .................................... 300

Appendix 6.5: Chemistry 2XX Survey (Semester 1, 2000).......................................... 302


Appendix 6.7: Modified Questions in Chemistry 2XX Survey .................................... 304


Appendix 6.9: Chemistry 121/122 Survey (Semester 1, 2001) .................................... 307

Appendix 6.10: Chemistry 100 Survey (Semester 1, 2001) ......................................... 308

Appendix 6.11: Chemistry 2XX Survey (Semester 1, 2001)........................................ 309

Appendix 6.12: Chemistry 100, Chemistry 121/122 Survey (Semester 1, 2002)......... 311

Appendix 6.13: Five Mechanistic Types used in Chemistry 100, 121/122 Survey...... 313

Appendix 6.14: Tasks Given to Chemistry 100 Students in an Interview in 1999....... 314

Appendix 6.15: Chemistry 100 Interview, Semester 1, 2000 ....................................... 323

Appendix 6.16: Chemistry 2XX Interview, Semester 1, 2000 ..................................... 326

Appendix 6.17: Chemistry 2XX Interview, Semester 2, 2000 ..................................... 330

277

Appendix 6.18: Chemistry 2XX Interview, Semester 1, 2001 .....................................334

Appendix 6.19: Chemistry 2XX Focus Group Questions, Semester 1, 2001 ...............338

Appendix 6.20: Chemistry 121/122 Laboratory Test, Semester 2, 1999......................342

Appendix 6.21: Chemistry 122 Laboratory Test, Semester 2, 2000.............................343

Appendix 6.22: Chemistry 121/122 Examination Question, Semester 2, 1999............344

Appendix 6.23: Chemistry 121/122 Examination Questions, Semester 2, 2000 ..........345

Appendix 6.24: Chemistry 100 Examination Question, Semester 1, 2000...................347

Appendix 6.25: Chemistry 2XX Examination, Semester 1, 2000 ................................349

Appendix 8.1: Description of Dr Anderson’s teaching.................................................350

Appendix 8.2: List of ‘Essential Knowledge’ Prepared by Researcher........................377

Appendix 9.1: Description of Dr Adams’ teaching. .....................................................378

Appendix 10.1: Description of Associate Professor Andrews’ teaching.....................393

278

Appendix 3.1: Course outline for organic chemistry component of Chemistry 2XX.

CHEMISTRY 200/260/270 ORGANIC CHEMISTRY SYLLABUS Semester 1

INTRODUCTION AND REVIEW

Systematic organic chemistry, functional groups, effects and mechanisms. Classes of

reagents, acids-bases, electrophiles-nucleophiles. Chemical transformations, ionic and

radical. Representation of reaction mechanisms. Catalysed reactions.

AROMATIC AND HETEROCYCLIC COMPOUNDS

Delocalization and conjugation. Aromaticity and the Hückel rule. Heteroaromatics:

pyridine, pyrrole, furan and thiophene. Electrophilic aromatic substitution, substituent

effects. Phenols: enhanced acidity relative to alcohols, formation of esters and ethers,

oxidation. Quinone and hydroquinone redox system.

FORMATION OF C-C BONDS

Nucleophilic substitution, SN1 and SN2. Olefin alkylation. Synthesis and biosynthesis

of terpenes. Carbon acids, enols, enolates, enamines. Aldol condensation: chemical and

biological examples. Carboxylic acids and derivatives. Claisen condensation-chemical

and biological examples. Thioesters, oxygen esters. Fatty acid and polyketide

biosynthesis. Industrial synthesis of vitamin A aldehyde.

STEREOCHEMISTRY

Cycloalkanes and ring strain, conformations of cycloalkanes and substituted

cyclohexanes. Configuration, geometric isomerism. Stereochemistry of cyclic

compounds: bicyclic and polycyclic compounds, steroids. Enantiomers,

diastereoisomers, racemic mixtures. Enantiotopic and diastereotopic groups,

heterotopic faces. Consequences of syn and anti- addition to alkenes as illustrated by

addition of bromination, hydrogenation, hydroboration and hydroxylation. Preparation

and reaction of epoxides. Sources of chiral compounds: chiral pool and resolution.

Stereospecificity of biological reactions, addition reactions, reduction, hydration.

TEXT: J. MCMURRY "ORGANIC CHEMISTRY" 5th EDITION (1999).

279

Appendix 4.1: Identified skills and concepts for learning about reaction mechanisms.

A4.1 The Concept of Reaction Mechanisms

Concepts

A reaction mechanism is the process by which a reaction takes place; which bonds are

broken and formed, in what order, how many steps are involved and the relative rate of

each step. These processes can be modelled by a pictorial representation that attempts

to predict or explain a reaction outcome in terms of bond breakage and formation. This

model or representation is often referred to as a mechanism.

Skills

To use ‘general’ mechanistic representations to:

• rationalise the outcome of a given reaction;

• predict products given starting materials;

• determine appropriateness of provided mechanism.

To demonstrate an understanding of these two ‘meanings’ of reaction mechanisms.

A4.2 Common Representations

Concepts

A single bond between two nuclei (generally drawn as a single line) represents a pair of

electrons between the two atoms. A bond can be broken in either of the following

ways:

• Heterolytic cleavage, with one atom gaining both bond electrons. The

fragment which gains the electrons becomes negative, the fragment that

does not gain the electrons is positive. There are two possible heterolytic

cleavages; both bond electrons may be gained by either of the fragments

joined together by the bond.

280

• Homolytic cleavage, with the bond breakage resulting in each atom

gaining one of the two bond electrons, resulting in two free radical

fragments.

Formal charge is defined as the charge each atom in a molecule would have if the

bonding electrons were shared equally by the nuclei in a molecule (Silberberg, 2000, p.

367). Formal charges can be positive, negative or 0.

A curly arrow is used to represent the movement of a pair of electrons from the tail of

the arrow towards the head during breakage and/or formation of bonds.

There are many different types of structural representations that may be used to depict

molecules. Each type of representation uses different features/cues to depict

characteristic features of a molecule.

Skills

To represent possible bond breakages using mechanistic conventions.

To use an appropriate calculation method to determine formal charge:

• When a molecule’s overall charge is indicated;

• When a molecule’s overall charge is NOT indicated.

To use an appropriate calculation to determine overall charge from formal charge.

To use curly arrow notation correctly to:

• Rationalise a reaction outcome;

• Predict possible reaction products, given the starting material.

To correctly write structures of molecules of a given compound using appropriate

representations and to be able to identify different structural representations of

molecules of a given compound.

To understand the usefulness of particular representations in reaction mechanisms for

both predictive and explanatory purposes.

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A4.3 Nucleophilic Substitution Reactions

Concepts

Nucleophilic substitution reactions occur when one substituent on a carbon chain is

replaced by a nucleophile. These types of reaction are generally referred to as SN

reactions. A nucleophile is a compound whose molecules have affinity for electron

poor centres.

SN reactions are experimentally observed to occur by two extreme types of process.

One process is observed to be unimolecular (the rate of reaction is dependent upon the

concentration of only one reactant) and is referred to as SN1. The other is bimolecular

(rate of reaction is dependent on the concentration of both reactants) and is called SN2.

An SN1 reaction mechanism has the following characteristics:

• Reaction rate is dependent only on the concentration of the substrate and

not on the concentration of the nucleophile;

• If the starting material is optically active, the product is observed to be a

racemic mixture.

• An SN1 reaction mechanism is modelled as a two-step process. This

model has the following characteristics:

• The first step in the process involves the loss of a leaving group from a

substrate molecule to form a carbocation and an anion and is the rate-

determining step;

• Planar carbocations are formed as intermediates;

• The second step is a reaction between carbocations and nucleophiles to

produce the reaction products;

• The production of racemic mixtures in these types of process is due to the

equal probability of successful collision between nucleophiles and

282

carbocations being from either of two directions (ie; either face of each

planar carbocation).

An SN2 reaction mechanism has the following characteristics:

• Reaction rate is dependent on the concentrations of both the substrate and

the nucleophile;

• If the starting material is optically active, the optical activity of the

product is consistent with stereochemical inversion.

• An SN2 reaction mechanism is modelled as a one-step process. This

model has the following characteristics:

• Both the substrate and the nucleophile are involved in the single step of

the process;

• It involves the simultaneous formation of bonds between substrate

carbons and nucleophiles and breaking of substrate-leaving group bonds;

• A ‘pentavalent’ transition state is formed;

• Stereochemical inversion is due to the formation of the transition state, in

which nucleophilic collision is most likely to be successful at the

‘backside’ of the substrate molecule.

Experimental data for a given reaction may be consistent with only one reaction process

occurring, or it may be consistent with both of the reaction processes happening in the

same reaction mixture. Experimentally observed reaction rates for primary substrates

are consistent with an SN2 reaction process. Experimentally observed reaction rates for

tertiary substrates are consistent with an SN1 reaction process.

Skills

To represent SN1 and SN2 reaction mechanisms for an appropriate reactions.

To use experimental evidence (rate, structure, stereochemistry) to:

283

• suggest an appropriate mechanism for a given reaction;

• predict possible products of a reaction when given the starting materials

using reaction mechanisms.

A4.4 Elimination Reactions

Concepts

Elimination reactions are those in which an alkene is formed from an alkane. This

involves the loss of a hydrogen atom and a leaving group from either side of a single

bond. There may be more than one alkene product produced in this type of reaction.

Elimination reactions are experimentally observed to follow two main reaction

processes. One process is observed to be unimolecular and is referred to as E1. The

other is bimolecular and is referred to as E2. Experimental data for a given reaction

may be consistent with only one reaction process occurring, or it may be consistent with

both happening in the same reaction mixture.

The rate of reaction of an E1 reaction is observed to be dependent only on the

concentration of substrate and is independent of the nucleophile concentration. This

type of reaction mechanism is modelled as a two-step process. This model has the

following characteristics:

• The first, rate-determining, step is analogous with the first step of an SN1

reaction mechanism and involves the loss of a leaving group from a

substrate molecule to form a carbocation and an anion;

• The second step involves reaction between hydrogen atoms and

nucleophiles, with the breakage of carbon-hydrogen bonds and the

formation of nucleophile-hydrogen bonds and carbon-carbon double

bonds.

The rate of reaction of an E2 process is observed to be dependent on the concentration

of both the substrate and the nucleophile. The mechanism is modelled as a one-step

process, which has the following characteristics:

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• The one step process involves the simultaneous formation of nucleophile-

hydrogen bonds and carbon-carbon bonds and breakage of hydrogen-

carbon bonds and carbon-leaving group bonds;

• A transition state is formed as these bonds break and form.

Skills

To use an appropriate reaction mechanism to:

• Predict the outcome of a reaction, given its starting materials;

• Explain a reaction outcome;

• Explain or predict the formation of more than one product in a reaction.

A4.5 Competing Reaction Processes

Concepts

Reactions have been observed in which more than one product is produced. The

production of more than one product may be explained by competing substitution and

elimination processes occurring simultaneously.

There may be products that are theoretically possible but not experimentally observed in

a given reaction.

The structure of a substrate, reaction conditions and solvent type have been observed to

vary the amounts of substitution and elimination products that are produced in a given

reaction.

Skills

To use knowledge of reaction mechanisms to:

• Explain the formation of more than one substitution product in a reaction

mixture;

285

• Explain the formation of more than one product in a reaction mixture

when the products are the result of both substitution and elimination

processes;

• Predict all possible products in a reaction mixture, given the starting

materials.

286

Appendix 6.1: Information sheet provided to second year students

We are conducting a research project in the Department of Chemistry, entitled “An

Investigation into the Teaching and Learning of Reaction Mechanisms in Organic

Chemistry”.

One of the aims of this project is to look at the development of students’ understandings

of reaction mechanisms as they progress through their chemistry course. It is hoped that

the results of the project will give us insight into students’ and lecturers’ understandings

of reaction mechanisms in organic chemistry.

The research has involved your participation in interviews with the research student. As

a further part of the investigation, the research student hopes to study interviewed

students responses to relevant exam questions, to assist in the study of students’

development in their understandings of reaction mechanisms throughout their

Chemistry 200 study. Any research data collected from either the student interviews, or

from your answers to relevant exam questions, will remain confidential, and your

anonymity in any publications of the findings of this study is guaranteed.

If you are willing to allow the research student access to relevant questions on your

exam paper (or papers) for Chemistry 200, please complete the consent form attached.

Your participation (or non-participation) in this study will in no way affect your marks

for any course you are currently studying in the Department of Chemistry. You are free

to withdraw from the study at any time, without prejudice.

The Committee for Human Rights at the University of Western Australia requires that

all participants are informed that, if they have any complaint regarding the manner in

which a research project is conducted, it may be given to the researcher, or alternatively,

to the Secretary, Committee for Human Rights, Registrar’s Office, University of

Western Australia, Nedlands, WA, 6907 (telephone 9380 3703). All study participants

will be provided with a copy of the Information Sheet and Consent Form for their

personal records.

If you have any questions that you would like to raise about the study, you can contact

the project supervisor, Associate Professor Bob Bucat on 9380 3158.

Your co-operation is greatly appreciated.

Associate Professor Bob Bucat Meagan Ladhams Zieba

Project Supervisor PhD Research Student

287

Department of Chemistry, University of Western Australia

Consent Form

“An Investigation into the Teaching and Learning of Reaction Mechanisms in

Organic Chemistry”

I ……………………………………. have read the information sheet and any questions

I have asked have been answered to my satisfaction.

I am willing to participate in the research project conducted by Associate Professor Bob

Bucat and Mrs Meagan Ladhams Zieba, and permit them access to relevant questions

on my exam paper or papers for Chemistry 200. I realise that I may choose to withdraw

from this study at any time without prejudice.

I understand that I can telephone Associate Professor Bucat on 9380 3158 and request

additional information about the study.

I understand that all information provided by me will be treated as strictly confidential.

I agree that research data gathered for the study may be published, provided that names

or other identifying information is not used.

Student signature Date

The Committee for Human Rights at the University of Western Australia requires that all participants are informed

that, if they have any complaint regarding the manner in which a research project is conducted, it may be given to the

researcher, or alternatively, to the Secretary, Committee for Human Rights, Registrar’s Office, University of Western

Australia, Nedlands, WA, 6907 (telephone 9380 3703). All study participants will be provided with a copy of the

Information Sheet and Consent Form for their personal records.

288

Appendix 6.2: Justification for task in Chemistry 2XX questionnaire (Appendix 6.6).

Consider the positively charged protonated alcohol represented in the structure below

[two different structures]:

In the spaces provided above, indicate the formal charge on each atom (spaces for each

individual atom).

Survey 1 representation Survey 2 representation

Hii C O

Hiii

Hi

Hiv

Hv

+

Research question A:1a(ii) (13/07/2000)

Can students correctly deduce the formal charges on atoms in a molecule:

(ii) when the molecule’s overall charge is indicated.

Possible answers

Carbon and all hydrogens have a formal charge of zero. The oxygen has a formal charge

of +1. This answer demonstrates the student understands formal charge applies to all

atoms, not just the atoms with non-zero positive charge. It also shows an understanding

that the sum of the formal charges needs to equal the overall charge on the structure,

and that the student has the ability to identify that the ‘abnormal’ atom (ie the oxygen)

to localise the charge.

Oxygen has a formal charge of +1. No formal charges are mentioned for the carbon and

hydrogens. This answer shows student is able to identify the ‘abnormal’ atom, and

considers that the sum of the non-zero formal charges is equal to the overall charge on

the structure.

Carbon has δ-, as do both hydrogens attached to oxygen. Oxygen has δ+. Answer

indicates that student has limited understanding of the term “formal charge”. However,

student may still have some idea that the oxygen atom is not ‘normal’.

Hii C O

Hiii

Hi

Hiv

Hv

+

289

Oxygen labeled δ+, other atoms not labeled. Answer indicates that student has limited

understanding of the term “formal charge”. However, student may still have some idea

that the oxygen atom is not ‘normal’.

Oxygen labeled δ-. Answer indicates that student has limited understanding of the term

“formal charge”.

Oxygen assigned some formal charge that is not +1. Answer demonstrates limited

understanding of term “formal charge”.

Formal charges in student’s representation do not add up to overall charge. Answer

demonstrates limited understanding of the notion that the sum of formal charge equals

the overall charge.

Why are we asking this question?

The ability to ‘recognise’ areas of localised non-zero formal charge is important in the

study of reaction mechanisms in organic chemistry, as this topic deals with

rationalisations of reaction outcomes, describing bond breakage and formation using

electron movement. Electron movement can result in a ‘transfer of charge’ from one

atom or location to another. If students cannot determine formal charges, they may lose

track of electrons when they are writing reaction mechanisms. The localised formal

charge is also often important to explain why a bond formed between two particular

atoms in a reaction, or to predict where a bond might be formed (ie, between a

nucleophile and an area of localised formal positive charge).

Why this question? What is its usefulness?

This structural representation was selected as it is a relatively simple compound, with

only three types of atoms in it. The structure also includes an atom which has a lone pair

(oxygen) to investigate the effect the presence or absence of the lone pair would have on

students’ abilities to answer the task, and also includes an atom which has a non-zero

formal charge. The represented structure is also simple in that there is only one carbon

atom and one oxygen atom, so a student doesn’t become confused by too many of the

same atom. The inclusion of more than one hydrogen atom couldn’t really be avoided.

290

Comparison of similar task in survey (1) and (2)—Research question A:1d(ii)

Are students’ abilities to perform tasks such as those described above influenced by:

(ii) the inclusion of lone pairs of electrons on the structural representation

For this task:

• Two versions of question (survey 1 and survey 2);

• Difference only in presence of lone pair of electrons on oxygen;

• Compare if this difference effects students’ abilities to answer question.

To answer this question, approximately half the students will be given survey (1), which

includes lone pairs, and half will be given survey (2), which has no lone pairs. The

students’ answers to these questions will then be compared, to see if the inclusion of

lone pairs in the structural representation influences their abilities to answer these

questions.

What do we hope to find out from this?

Can students determine formal charge on the atoms in this represented structure?

Do students consider non-zero formal charge on an atom to still be formal charge?

Do students demonstrate any understanding of the link between formal charge and

overall charge?

Does the inclusion of lone pairs on this structure effect students’ abilities to answer

these tasks?

291

Appendix 6.3: Descriptions of tasks.

Questionnaires Administered in 2000

Chemistry 121/122

The Chemistry 121/122 questionnaire is attached as Appendix 6.4. Both the questions

on this questionnaire are looking at RQ S2. The questionnaire was administered after

the students had been taught about curly arrow notation, but before they specifically

covered substitution and elimination reactions and mechanisms. The questionnaire was

administered just after the students returned from a seven-week break from class. The

task was administered to determine students’ understandings and uses of curly arrows,

and to identify any misconceptions about these representations that the students might

have.

Chemistry 2XX (Semester One)

The Chemistry 2XX questionnaire is attached as Appendix 6.5 and was administered to

the Chemistry 2XX students early in first semester, before they had covered a lot of

chemistry in their lectures. This questionnaire was designed to identify which

Chemistry course the students had studied in first year, and to identify what the students

understood ‘reaction mechanism’ to mean (RQ S2). The questionnaire was also used to

determine whether or not students had studied mechanisms in their first year course.

Chemistry 2XX (Semester Two)

The second semester questionnaire is attached as Appendix 6.6. This is one of two

questionnaires administered to these students in the same class early in semester two.

There were two small variations between the two questionnaires. These variations

involved the presence of lone pairs of electrons in question 1 and the relative position of

the OH- and alkyl halide representations in question 4. The modified questions (1 and 4)

are shown in Appendix 6.5. There were four questions on this questionnaire, each

designed to address a different aspect of the students’ understandings of representations

used in reaction mechanisms (RQ S2).

292

Question 1 was designed to investigate students’ understandings of formal charge and

their ability to determine formal charges on atoms in molecules, particularly when the

molecule is represented with a positive charge. It was also hoped to identify whether or

not students consider ‘0’ to be a formal charge. The modification in this question was

the inclusion of a lone pair of electrons on the O. As formal charge calculations ‘count’

electrons around an atom, the consideration of lone pairs is important in this calculation.

The two different questions were designed to investigate if the inclusion of lone pairs on

this structure affects students’ abilities to answer these tasks.

Questions 2 and 3 are similar types of questions, designed to investigate students’

interpretations of the curly arrow representation in two different situations. In each case,

there is a single curly arrow shown in the representation. In question 2, the curly arrow

is representing the breakage of a single bond. In question 3, it represents both breakage

and formation of bonds. Because of this, question 3 was considered to be slightly more

complex than question 2.

Question 4 was designed to investigate if the relative positions of the nucleophile and

the substrate on the page affect how a student represents the possible products of the

reaction. In both cases, the students were given the same structural representations in

the question. The difference in the representation is that in one example, OH- was

shown on the left-hand side and in the other it was shown on the right. By comparing

students’ answers in the two questionnaires, we can see if there are any common

representations students give, based upon the original representation that they are given.

Chemistry 3XX

The questionnaire is attached as Appendix 6.8. The questionnaire asked students to

detail the courses that they studied in first, second and third year, and to classify

themselves as either an organic, inorganic or physical chemist. Students were asked to

classify their chemistry knowledge as third year students at University A may not have

studied organic chemistry (and therefore reaction mechanisms) beyond first year. The

students were then asked to complete four tasks that looked at their understandings of

curly arrow representations. These four tasks were of varying levels of difficulty. The

intention was to identify if a student’s Chemistry background was linked to their ability

to understand mechanistic tasks.

293

Questionnaires administered in 2001

Chemistry 121/122

The 2001 questionnaire is attached as Appendix 6.9 and was first piloted on a group of

third year students. Based upon their comments, small changes were made. This

questionnaire was administered to investigate Chemistry 121/122 students’ spontaneous

uses of arrows or other pictorial representations when explaining reaction processes.

The students were given four simple equations that they should have recognised from

their secondary school study, and asked to illustrate what was happening in each

equation.

Chemistry 100

The 2001 questionnaire is attached as Appendix 6.10. This questionnaire was designed

after a modification to an original research question upon which the Chemistry 121/122

questionnaire detailed above was based. The Chemistry 121/122 questionnaire results

indicated that the questionnaire was not an appropriate tool for gathering the desired

information, and so the tasks were re-written after consideration of the research

question.

This questionnaire was designed to look at students’ understandings of the curly arrow

symbolism. It comprised of two questions; a multiple choice question which required a

written answer to explain the chosen answer, and a three-part question in which students

were asked to drawn the structure that resulted from a represented electron shift.

Chemistry 2XX

The 2001 questionnaire is attached as Appendix 6.11. This questionnaire is very similar

to that given to the Chemistry 100 students in 2001 and was designed to investigate

students’ understandings of mechanistic representations. A question was also asked to

identify which first year course they had studied before commencing Chemistry 2XX.

The questionnaire consisted of two questions. Question 1 required students to represent

the product of a reaction whose mechanism was provided. Question 2 asked students to

294

select the best mechanistic representation for a represented chemical process and to

explain why he or she had selected that specific mechanism.

Questionnaires Administered in 2002

Chemistry 121/122, Chemistry 123/124 and Chemistry 100

This questionnaire was very similar to that asked of the Chemistry 100 students in 2001.

The questionnaire was modified slightly and included three questions. The first

multiple-choice question was identical to the 2001 questionnaire. The second question

was of a similar style to the 2001 questionnaire, although this question consisted of only

two tasks and the tasks themselves were not the same as those asked in 2001. Question

three was a new addition to the questionnaire. It involved showing students an equation

and asking them to draw arrows on it to explain the reaction outcome.

There were six variations of this questionnaire given out to the three groups of students.

An example is attached as Appendix 6.12. The differences were in the representations

used in questions 2 and 3. The researcher identified five mechanistic ‘types’, which are

similar to those ‘steps’ described by Wentland (1994). These five ‘types’ are shown in

Appendix 6.13. The questionnaires included some combination of types (i), (iii) and (v)

or types (ii), (iii) and (iv). The intention was to identify difficulties that students had

with following and representing curly arrows.

Interviews

Chemistry 100 1999

The tasks students were asked to work through in this exploratory interview are attached

as Appendix 6.14. Nine of the eleven students interviewed answered questions in this

particular order. Some of the questions were removed and reordered before the final two

students (Benjamin and Barbara) were interviewed. The questions were divided into

three sections. The first section was addressing research question S1. The second and

third sections were looking at students’ understandings of a variety of mechanistic

representations and symbols (RQ S2).

295

The questions in section A related to students’ understandings of reaction mechanisms

and why they were taught in the Chemistry 100 course. Section B contained two

questions, both of which required students to use reaction mechanism representations to

rationalise reaction outcomes. Part of question 2 also required students to attempt to

visualise the multiple particulate nature of a reaction mechanism.

Section C consisted of eight questions. Question 1 to 5 required students to use curly

arrows to rationalise represented equations. This question was designed to investigate

students’ abilities to use the curly arrow representations. It was also hoped to identify

students exhibited more difficulty working through one type of reaction process. It was

anticipated that the elimination questions would be slightly more challenging than the

substitution questions.

Question 6 asked them to predict reaction outcomes, given different pairs of starting

materials. This was to investigate students’ perceptions of the possibility of competing

substitution and elimination processes occurring in the same reaction mixture. Question

8, which involved students selecting possible products of reaction between given

starting materials, also investigated students’ perceptions of competing reactions.

Question 8 looked at only competing elimination processes.

Question 7 involved translating a mechanistic representation into an equation to

investigate students’ understandings of two different types of structural representations.

Chemistry 100 2000

The interview questions are attached as Appendix 6.15. Students were asked to

complete six questions in this interview.

The first question was designed to investigate students’ understandings of reaction

mechanisms and the usefulness of such representations in organic chemistry (RQ S1).

The other five questions were designed to study students’ understandings of a variety of

mechanistic representations (RQ S2). Questions 2 and 3 looked at students’ abilities to

calculate formal charges in structures. Questions 4 and 5 investigated students’

understandings of the curly arrow representation. Question 6 was designed to probe

296

students’ understandings of the links between experimental evidence and possible

reaction mechanisms in a given reaction.

Chemistry 2XX 2000

Semester One

Students worked through four questions in this interview, which is attached as

Appendix 6.16.

The first question was designed to probe students’ thoughts on what makes representing

a reaction mechanism difficult and what skills and abilities they might need to help

them answer particular tasks. This contributed to research question S1.

Questions 2, 3 and 4 all address research question S2, and were looking at students’

understandings of various skills and concepts associated with the representation of

reaction mechanisms. Question 2 was designed investigating students’ understandings

of formal charge and their ability to determine charges on atoms in structures. Interview

question 3 is looking at types of bond breakage and students’ understandings of the

outcomes of particular bond breakages. The purpose of question 4 was to examine the

students’ abilities to interpret curly arrows in mechanistic representations.

Semester Two

There were three questions asked in this second semester interview (Appendix 6.17).

All three were addressing research question S2.

Both questions 1 and 2 were looking at students’ abilities to suggest possible

substitution and elimination products from particular starting materials.

Students were also asked to demonstrate their understandings of reaction mechanism

representations by writing mechanisms to explain these reaction outcomes.

Question 3 asked students to repeat a question that was given to them in the mid-year

examination in which they were asked to explain the formation of both a substitution

and an elimination product. Students were then given a selection of sample responses to

each question and asked to comment on the appropriateness of the mechanism.

297

Chemistry 2XX 2001

The four interview questions, which addressed research question S2, are attached as

Appendix 6.18.

Question 1 was designed to investigate students’ abilities to determine appropriateness

of particular mechanistic representations and to suggest improvements where required.

The students were provided with four mechanistic representations to rationalise a

particular reaction process and asked to comment on its appropriateness. Each of the

provided mechanisms was represented with at least one small error.

Questions 2 and 3 looked at students’ understandings of formal charge and their abilities

to determine such charges in both single representations and as a result of a represented

electron shift.

Question 4 probed the link between structural representations and perceived reaction

outcomes to determine whether or not students perceived the type of structural

representation used in a particular question as a cue to suggest the type of reaction

process that was more likely to occur.

Chemistry 2XX Focus Group 2001

Chemistry 2XX students participated in a focus group session (Appendix 6.19), which

was designed to address research question S1b: what did students perceive to be

difficult in the study of reaction mechanisms.

The students were asked to complete a task before coming to the focus group. Their

answers to the task were then discussed with the researcher and the other members of

the focus group.

This discussion included a consideration of any difficulties they had encountered in

answering the represented question. The focus group situation was also used to

investigate students’ understandings of the usefulness of teaching about reaction

mechanisms in organic chemistry (RQ S1a).

298

Examination and Laboratory Tests

Chemistry 121/122 Laboratory Test 1999

A single question (number 4) was prepared for the laboratory test (Appendix 6.20). This

question was designed to probe students’ understandings of competing substitution and

elimination reaction processes happening in the same reaction mixture.

Chemistry 121/122 Lab Test 2000

Two questions (numbers 3 and 4) were included on the laboratory test (Appendix 6.21).

The first question was the same as that asked in the 1999 test. The second question

asked students to consider a very similar reaction process to the one they had

investigated in the laboratory and to predict possible reaction products. This task was

designed to investigate students’ abilities to apply their understandings of reaction

mechanisms to similar processes.

Chemistry 121/122 Examination 1999

The Chemistry 122 examination question is attached as Appendix 6.22. The question

was used to investigate students’ abilities to follow the curly arrow representation that is

commonly used in reaction mechanisms. It was also used to identify students’ abilities

to correctly represent formal charges on structures as a result of represented electron

movement.

Chemistry 121/122 Examination 2000

Three questions were designed for the 2000 end-of-year examination (Appendix 6.23).

One question was very similar to that asked in the 1999 examination, but the questions

were reordered and redrawn, based upon the findings from the 1999 examination.

The second question was designed to investigate students’ understandings of the links

between experimental evidence and types of reaction mechanism. The final question

required students to select structures that were possible products from reaction between

a pair of starting materials. This question was designed to investigate students’

understandings of the possibility of more than one product being produced from the

same starting materials.

299

Chemistry 100 Examination 2000

Two questions were asked on the Chemistry 100 examination (Appendix 6.24). The

first question is similar to that asked in the Chemistry 121/122 exams, and was designed

to investigate students’ understandings of the curly arrow representation.

The second question had two parts to it. The first involved determining formal charge

on a simple structure. The second part of the question was designed to investigate

students’ abilities to link structural evidence to the type of reaction mechanism that

might be occurring.

Chemistry 2XX Examination 2000

The examination question written for the Chemistry 2XX is attached as Appendix 6.25.

This question was designed to investigate students’ abilities to explain the production of

more than one reaction product in a given reaction using reaction mechanisms. Students

had seen the question prior to the examination on a problem sheet.

300

Appendix 6.4: Chemistry 121/122 Survey (Semester 2, 2000)

A research project is currently being carried out in the School of Applied Chemistry,

investigating students’ understandings of aspects of the Organic Chemistry course. The

research project also involves brief interviews with student volunteers. If you are

interested in volunteering for this study, please write your name and a contact number

or email below. Your participation is greatly appreciated.

Name: ………………………………………………………………………….

Contact Phone Number: …………………………………………………

Email: ………………………………………………………………………….

Consider the equation represented below:

C Br

CH3

CH3H

CH3

C

HCH3

+ Br

(A) (B) (C)

Describe in words how the products (B) and (C) are formed from starting material (A).

……………………………………………………………………………………………

……………………………………………………………………………………………

……………………………………………………………………………………………

……………………………………………………………………………………………

……………………………………………………………………………………………

How would you represent this description pictorially on the represented equation?

Please turn the page . . .

301

Imagine you can isolate compound (B) from the product mixture , and you reacted it

with hydroxide ions (OH-). This is represented below:

CH3

C

HCH3

(B)

+ OH

Draw the product(s) that would result from the represented reaction.

Describe in words how the product(s) you have drawn would be formed from the

starting materials (B) and hydroxide ions.

……………………………………………………………………………………………

……………………………………………………………………………………………

……………………………………………………………………………………………

……………………………………………………………………………………………

……………………………………………………………………………………………

How would you represent the above description pictorially on the represented equation?

302

Appendix 6.5: Chemistry 2XX Survey (Semester 1, 2000)

Reaction Mechanisms in Organic Chemistry

Which first year Chemistry unit did you study? …………………………………………

Why did you decide to study Organic Chemistry in this second year unit? ……………

……………………………………………………………………………………………

In your first year chemistry course, did you study reaction mechanisms? ………………

In one or two sentences, please explain what you understand by the term ‘reaction

mechanism’.

……………………………………………………………………………………………

……………………………………………………………………………………………

……………………………………………………………………………………………

Do you consider reaction mechanisms to be important in the study of Organic

Chemistry? What is your reason for this opinion? ………………………………………

……………………………………………………………………………………………

……………………………………………………………………………………………

A research project is currently being carried out in the Chemistry Department about

students’ understandings of reaction mechanisms in Organic Chemistry. The research

involves short interviews with Chemistry students at different stages of study. If you are

interested in volunteering to be part of this study, please write your name and a contact

number below. Your participation (or non-participation) in the research project will not

affect your organic chemistry mark.

Name: ……………………………………………………………………………

Phone Number: ……………………………………………………………….

303


Chemistry 200/260/270 Survey—Semester 2 2000

1. Consider the positively charged protonated alcohol represented

in the structure below. In the spaces provided above, indicate

the formal charges on each atom.

Hii C O

Hiii

Hi

Hiv

Hv

+

2. Draw the species that would be produced by the transformation

shown below:

3. Draw the species that would be produced by the transformation

shown below:

BrC

CH3

CH3

H

4. Draw the species that is most likely to be produced from the

reaction represented below. Include curly arrows on the

representation to explain the production of these species.

BrC

CH3CH2

HCH3

OH +

Explain why you have indicated the production of these species in the

reaction represented above.

…………………………………………………………………………

…………………………………………………………………………

…………………………………………………………………………

C C

H

CH3

CH3

CH3

H

C ……………... Hiii ……………. O …………….. Hiv ……………. Hi ……………. Hv ……………. Hii …………….

304

Appendix 6.7: Modified Questions in Chemistry 2XX Survey

1. Consider the positively charged protonated alcohol represented in the

structure below. In the spaces provided above, indicate the formal

charges on each atom.

4. Draw the species that is most likely to be produced from the reaction

represented below. Include curly arrows on the representation to

explain the production of these species.

BrC

CH3CH2

HCH3

+ OH

Explain why the species will be produced in the reaction represented above.

…………………………………………………………………………

…………………………………………………………………………

…………………………………………………………………………

…………………………………………………………………………

…………………………………………………………………………

C …………….. Hiii ……………. O ……………. Hiv ……………. Hi …………… Hv ……………. Hii ……………. Hii C O

Hiii

Hi

Hiv

Hv

+

305


Reaction Mechanisms in Organic Chemistry

My name is Meagan Ladhams Zieba, and I am currently conducting a research study

into students’ understandings of reaction mechanisms in organic chemistry, with the

supervision of Bob Bucat. If you are willing to participate in this study, please complete

the tasks below.

Please note: Your participation in this study is voluntary, and your anonymity is

guaranteed. Your participation (or non-participation) will in no way affect your mark in

the Chemistry Education course.

What chemistry unit(s) did you study in first year? ……………………………………

What chemistry unit(s) did you study in second year? ……………………………

Have you studied any organic chemistry topics in third year? If so, which topics did you

study?

……………………………………………………………………………………………

……………………………………………………………………………………………

……………………………………………………………………………………………

……………………………………………………………………………………………

Do you consider yourself to be: an organic chemist

an inorganic chemist

a physical chemist

Please indicate which category (or categories) you think best describes you as a

chemistry student.

Please turn the page . . .

306

What structure(s) would result from the transformations represented below?

(i)

OC

H

CH3CH2CH2

H H

H

(ii)

OH2

HH2PO4

(iii)

OH2C

H

CH3CH2CH2

HBr

(iv)

C C

H

Cl

CH3

CH3

CH3

H

HO

307

Appendix 6.9: Chemistry 121/122 Survey (Semester 1, 2001)

Consider the following equations. For each equation, illustrate what

is happening.

1.

H Cl H + Cl

2

H + O H HO

H

3.

HO

H+ H

HO

H

H

4.

HN

HH

H

HN

HH + H

308

Appendix 6.10: Chemistry 100 Survey (Semester 1, 2001)

A research project being undertaken in the Department of Chemistry is looking at students’ understandings of aspects

of the Organic Chemistry course. If you are willing to participate in this part of the research project, please

complete the task below. Your participation in this research task in no way affects your marks in the Chemistry 100

course. Your answers will be kept confidential, and any results or conclusions drawn from student responses to this

task will be reported anonymously.

1. The reaction between water molecules and hydrogen ions is represented in the

equation below:

HO

H HO

H

H

+ H+

In your opinion, which of the following is the most correct representation of the

above reaction?

HO

H

H+

HO

H

H+

(a) (b)

What is your reason for your preference?

……………………………………………………………………………………………

2. Draw the species that result from the transformations represented below:

a. H Cl

b. H

NH

H

H

CH2 CH2

Cl H

c.

309


A research project being undertaken in the Department of Chemistry is looking at

students’ understandings of aspects of the Organic Chemistry course. This short task is

designed to investigate your understandings as you commence your second year of

Chemistry study.

If you are willing to participate in this part of the research project, please complete the

task on the reverse of this page.

Please note that your participation in this research task in no way affects your marks in

the Chemistry 200 course. Your answers will be kept confidential, and any results or

conclusions drawn from student responses to this task will be reported anonymously.

The Committee for Human Rights at the University of Western Australia requires that all participants are informed that, if they have

any complaint regarding the manner in which a research project is conducted, it may be given to the researcher, or alternatively, to

the Secretary, Committee for Human Rights, Registrar’s Office, University of Western Australia, Crawley, WA, 6009 (telephone

9380 3703).

Call for Volunteers!

Part of the research project involves one or two short interviews with student

volunteers in semester one. Participants will be interviewed about their perceptions

of certain aspects of the Organic Chemistry course. Each interview will take

approximately 30 minutes.

If you are willing to participate in this part of the research, please fill in the

following details:

Name: ……………………………………………………..

Telephone: ……………………………………………………..

310

Chemistry 200 etc.

What first year chemistry course did you study? ……………………………………

1. Draw the species that result from the transformation represented below:

BrC

CH3

CH3H

HO

2. The reaction between 2-bromo-2-methylbutane (A) and hydroxide ions,

resulting in the formation of 2-methyl-2-butene (B), water and bromide ions is

represented in the equation below:

C C

CH3

CH3

H

HCH3

Br

OH +C C

CH3

H

CH3

CH3

(A) (B)

+ H2O + Br

Which of the following is the most correct representation of the above reaction?

C C

CH3

CH3

H

HCH3

Br

HO

C C

CH3

CH3

H

HCH3

Br

HO

C C

CH3

CH3

H

HCH3

Br

HO

(a) (b) (c)


……………………………………………………………………………………………

……………………………………………………………………………………………

311

Appendix 6.12: Chemistry 100, Chemistry 121/122 Survey (Semester 1, 2002)

A research project being undertaken in the Department of Chemistry is looking at

students’ understandings of aspects of the Organic Chemistry course. If you are willing

to participate in this part of the research project, please complete the task below.

Your participation in this research task in no way affects your marks in the Chemistry

100 course. Your answers will be kept confidential, and any results or conclusions

drawn from student responses to this task will be reported anonymously.

1. The reaction between water molecules and hydrogen ions is represented in the

equation below:

HO

H HO

H

H

+ H

In your opinion, which of the following is the most correct representation of the above

reaction?

HO

H

HH

(a) (b)

HO

H


……………………………………………………………………………………………

……………………………………………………………………………………………

PLEASE TURN THE PAGE

The Committee for Human Rights at the University of Western Australia requires that all participants are

informed that, if they have any complaint regarding the manner in which a research project is conducted,

it may be given to the researcher, or alternatively, to the Secretary, Committee for Human Rights,

Registrar’s Office, University of Western Australia, Crawley, WA, 6009 (telephone 9380 3703).

312

2. Draw the species that result from the transformations represented below:

ClC

CH3

CH3

H

HO

H

H

OH

3. Draw arrows (as are used in questions 1 and 2) to explain the equation represented below:

C C

H

CH3

Br

H

CH3

CH3

OH

C C

H

CH3

H3C

H3C H

O

H

+ + Br

313

Appendix 6.13: Five Mechanistic Types used in Chemistry 100, 121/122 Survey.

Type (i)

ClC

CH3

CH3

H

Type (ii)

Type (iii)

Type (iv)

Type (v)

C C

H

CH3

Br

H

CH3

CH3

OH

CH3

C

CH3CH3

C N

HO

H

H

OH

C C

H

CH3

H

CH3

CH3

OH

314

Appendix 6.14: Tasks Given to Chemistry 100 Students in an Interview in 1999

A: Student Background and Perceptions

1. Why do you intend to study organic chemistry at a second year level?

a) personal interest

b) course requirement

c) unsure of what you want to do, and want to keep your options open

d) other

2. Do you recall studying reaction mechanisms in first semester? Did you find it easy?

Difficult? Of average difficulty? What did you find easy or difficult about the

subject?

3. Do you think reaction mechanisms are important in the study of organic chemistry?

Why or why not?

4. Why do you think the study of reaction mechanisms is included in the organic

chemistry course?

5. How would you define a reaction mechanism?

315

B: Experimental Work

1. A laboratory experiment that you performed in first semester involved the

preparation of cyclohexene from cyclohexanol. This is shown in the reaction below:

H3PO4+ H2O

OH

a) Describe in words what is happening in the reaction.

b) Use a representation of a reaction mechanism to interpret this reaction.

2. 1-Bromobutane can be made in the laboratory by reacting 1-butanol, HBr and

H2SO4. The reaction mixture is heated strongly for about two hours, and the organic

product is separated from the aqueous layer, and purified by distillation. This

reaction is shown below:

+ HBrH2SO4 + H2OOH Br

a) Draw a representation of a reaction mechanism that is consistent with how 1-

bromobutane is made from 1-butanol.

b) Why is the reaction carried out in acidic conditions?

c) The reaction between 1-butanol and HBr in acidic conditions is a

substitution reaction. Which species or group is substituted, and what is it

substituted by?

d) With reference to substitution reactions, it is common to talk about a

nucleophile and a leaving group. In this reaction, what is the nucleophile,

and what is the leaving group?

316

e) An organic product of the reaction between 1-butanol and concentrated

sulfuric acid is 1-butene, produced by a dehydration (or elimination)

reaction. This is represented in the reaction below:

OHH2SO4

+ H2O

With reference to the preparation of 1-bromobutane from 1-butanol, this

alkene is often referred to as a byproduct.

Imagine you have a powerful microscope dipping into the reaction flask.

Both the substitution and elimination reactions are happening in this

reaction. This microscope allows you to see all of the molecules and ions in

the reaction mixture. Draw a picture (or pictures) of what you might see

happening in the reaction mixture through this hypothetical microscope.

317

C: Tasks

1. The benzyl carbocation (A) has four possible resonance structures. Two of these

structures are shown below.

a) Using curved arrows, show how these resonance structures can be

rationalised.

CH2 CH2

+ +

(A)

b) What does the term ‘resonance structure’ mean to you?

2. The reaction of 2-chloro-2-methylpropane (represented as A) with hydroxide ions

produces the alkene represented as B. Using curved arrows, represent a mechanism

which is consistent with B being produced from reaction of A with hydroxide ions.

H

C

H

H C

CH3

Cl

CH3 C C

CH3

CH3

H

H

+H2O + Cl -

(A) (B)

OH - +

3. Ethanol (shown as B) can be produced through the reaction of ethyl chloride (A)

and OH-. Using curved arrows, suggest a mechanism for the production of B from

A.

H C

H

H

C

H

Cl

H

H C

H

H

C

H

OH

H

+ Cl -+ OH -

(A) (B)

318

4. The reaction of 1-bromopropane (A) and hydroxide (OH-) produces 1-propanol (B)

as its major product. Illustrate a mechanism for the production of B from A, using

curved arrows.

C C

H

H

C

H

Br

H

H C

H

H

C

H

C

H

H

H

H

H

OH

H

OH - + + Br-

(A) (B)

5. 2-methylbut-2-ene (B) is produced through the reaction of 2-bromo-2-methylbutane

(A) and sodium hydroxide. Using curved arrows, illustrate a reaction mechanism to

transform A to B.

H3C C

H

H

C

CH3

Br

CH3H3C

CH

C

CH3

CH3+ OH -

(B)(A)

+ H2O + Br-

319

6. Complete the following equations. Some may have more than one possible product.

R

C

XRR2HC

R C

R

H

C

R

X

R

R

C

X

Br

R

+ Nu-

+ B:

R2HC+ B:

+ B:

+ Nu -

Cl

Br

+ Nu -

H

ClH

H

HH

+ Nu -

H

H

H

H

a)

b)

c)

d)

e)

f)

g)

320

7. The reaction between 2-methyl-2-chlorobutane (A) and water is usually represented

as an SN1 reaction, whose mechanism is shown below:

CH3

C

ClH3CH3CH2C

CH3

C+

H3CH2C CH3

CH3

CO+

H

H

CH3

C

OHH3CH3C

H3CH2CH3CH2C + H+

CH3

C+

H3CH2C CH3

+ Cl -

H

O H

followed by:

(A)

The representation shown above is a possible mechanism for the production of B from

A. Can you write a chemical equation that corresponds to what this suggested

mechanism represents?

321

Which of the following (a - d) is a correct representation of what is illustrated in the

mechanism?

Cl OH

Cl OH

Cl OH

+ H2O

+ H2O

+ H2O

+ H2O

+ H+ + Cl -

+ H+ + Cl -

+ H+ + Cl -

+ H+ + Cl -

Cl HO

a)

b)

c)

d)

322

8. An elimination reaction involves the abstraction of a hydrogen atom by a

nucleophile (or Lewis Base), and the loss of a leaving group (often a halide) to form

an alkene from an alkane. For the compound (A) shown below, which of the four

products given (a - d) are possible? Illustrate the reaction mechanisms for each

possible product. Are there any other possible elimination products?

H3CH2C C

Br

CH3

CH(CH3)2

CH3

CH3

H3CH2C

H

CH(CH3)2

H

H3C

CH(CH3)2

H3C

H3C

CH3

H3CH2C

H3C

CH3

H

CH3

(A)

OH -

a)

b)

c)

d)

323

Appendix 6.15: Chemistry 100 Interview, Semester 1, 2000

Task 1:

You have been studying reaction mechanisms in your organic chemistry course. What do you understand the term “reaction mechanism” to mean?

Do you think that knowing about reaction mechanisms is useful in the study of organic chemistry? Under what circumstances do you think they are useful?

Why do you think reaction mechanisms are included in the first year organic chemistry course?

Task 2:

Consider the ion whose structure represented below:

What is the formal charge on:

(i) each of the carbon atoms

(ii) each of the hydrogen atoms

(iii) the oxygen atom

Task 3:

Consider the structure represented below:




(iii) the oxygen atom

Task 4:

Consider the following representation:

H C O

CH3

H

+H

H

H C O

CH3

H

+H

H

324

Explain in words what the curly arrow represents. What

will happen if the change represented by the curly arrow

happens?

Task 5:

In the two examples given below, draw diagrams that represent the compound (or

compounds) that are produced by the transformations shown. What are the formal

charges on each of the atoms in the produced molecule? What is the net charge on the

produced molecule?

H3C C

CH3

O

CH3

H

H+

CH3

CCH3H3C

H O H

CH3

C

BrH3CH

325

Task 6:

Consider the reaction represented in the equation below. The product of this reaction

between the R-enantiomer and methoxide ions is a racemic mixture (ref: Brown, pg

184).

C ClH

CH3O CH

C OCH3H

and

CH3O -

Cl Cl Cl

R-enatiomerS-enantiomer R-enantiomer

racemic mixture

a) If the product of this reaction is a 1:1 mixture of the (R)- and (S)- isomers, what

does that tell you about the reaction mechanism? What type of nucleophilic

substitution reaction might be taking place? Why?

b) Draw a reaction mechanism which rationalises the formation of a racemic mixture

from the starting materials.

c) How does this type of mechanism explain or rationalise the formation of both the

(R)- and (S)- enantiomers of the product?

326

Appendix 6.16: Chemistry 2XX Interview, Semester 1, 2000

Task 1

A substitution reaction can be represented by a generalised equation, where X

represents any halide, Nu- represents a nucleophile, and R represents a hydrogen, or any

alkyl group:

C

R

C

R

+ Nu-+ X -

XR2R1 NuR2

R1

(i) To suggest a mechanism for the represented generalised reaction, what do you

think it is necessary for you to know? Do you think that there are any particular

skills or abilities that you need to have in order to answer part (i)?

(ii) Can you use curly arrows to represent a reaction mechanism which rationalises

the reaction represented above? As you are working through the task, please try

to explain what you are doing (and why you are doing it) and try to ‘think out

loud’.

(iii) Is there anything about the representation that increases the difficulty of this

task? Does the different representation shown below make it easier, more

difficult or no harder or easier for you to suggest a reaction mechanism? What is

it about this representation that changes the level of difficulty for you?

C

R

C

R

+ Nu-+ X -

XR2R1 Nu R2

R1

327

Task 2

Part a

Consider the structure represented below.

a) What is the formal charge on:



b) What is the net charge on the molecule?

Part b

Consider the structure represented below.




Task 3

Consider the simple A-B bond represented in the example below:

A—B

It is common for text books and lecturers to represent single bonds as lines. Each single

line represents a bond formed between two atoms (in this case, atoms A and B), which

is made up of two electrons shared between those atoms.

H3CC

CH3

H

H3CC

CH3

H +

328

If the bond between A and B is broken, there are three possible ways in which this

might happen, resulting in three different types of product molecules. One example of

breaking the A-B bond is illustrated below:

A B A B+

(i) In this example, what has happened to the electrons in the A-B bond to produce

the two fragment ions represented above? Indicate the movement of the bond

electrons with curved arrows.

(ii) There are two other ways in which the A-B bond can be broken. In the case of

the two other possible ways in which the A-B bond can be broken:

a) Draw a picture (using curved arrows) to represent the movement of the

electrons in the bond

b) Indicate what formal charge (if any) would be present on both A and B after

the bond has been broken

Task 4

In the examples given below, part of a reaction mechanism is represented. In

representations of reaction mechanisms, a curved arrow is used to represent electron

movement. In each example, draw the product that results from of the electron

movement which is indicated by the curved arrow. What are the formal charges on each

of the atoms in the produced molecule? What is the net charge on the produced

molecule?

329

H3C C

CH3

O

CH3

H

H+

CH3

CCH3H3C

H O H

H

C

H

H3C C

CH3

Br

CH3

HO -

Example 1

Example 2

Example 3

330


Task 1

Consider the following reaction between (S)-3-bromo-2-methylpentane and hydroxide

ions:

C Br

(CH3)2HC

CH3CH2

H+ OH

Which of the organic products represented below are possible products of this reaction?

Can you illustrate a reaction mechanism for each possible product.

CHO

CH(CH3)2

CH2CH3

H C C

H

CH3

(CH3)2HC

H

C C

CH3

H

(CH3)2HC

H

C C

CH2CH3

H

CH3

CH3

(1) (2)

(3) (4)

Task 2:

If 2-bromo-2-methylpropane (A) reacts with ethoxide ions (B) which of the following

are possible products of the reaction (ref: McMurry, 5th ed, pg 424):

C Br

CH3

CH3

CH3

+ CH3CH2O

331

C OCH2CH3

CH3

CH3

CH3

CCH3CH2O

CH3

CH3

CH3C C

H

H

CH3

CH3

C C

H

H

CH2CH3

CH3

(1) (2)

(3) (4)

For each possible product, illustrate a reaction mechanism that rationalises its

production.

Task 3:

The following question was asked in the first semester exam for Chemistry 200:

Consider the SN2 mechanism represented below:

CH2CH3

C

Br

CH2CH3

CHOH3C CH3

HH OH Br

(A) (B)

b) Suggest a reaction mechanism for the conversion of (A) to (B).

c) The reaction of (A) with hydroxide ions can also lead to some amounts of 2-E-

butene. Draw a reaction mechanism that rationalises this observation.

How would you answer this question?

332

Shown below are two answers to exam question b:

CH2CH3

C

BrH3CH

HO

C

CH2CH3

BrH3C

H O

H

CH2CH3

C

OHH3CH

BrC

CH2CH3

H3C

HHO

C

CH3CH2

CH3

HHO

(i)

(ii)

+ Br

+ Br

Imagine you were marking this exam question answer.

a) Which response do you think best answers the question?

b) What mark (out of ten) would you give each answer?

c) What feedback would you provide to the student who gave each answer?

d) What aspects of each answer are ‘correct’?

e) What aspects of each answer are ‘incorrect’?

f) How would you answer this question?

333

Shown below are two answers to section c) of the exam question:

C C

H

Br

H

CH3

CH3

H

C C

H

Br

CH3

CH3

H

H

HO

C C

H

CH3

CH3

H

+ Br + H2O

OH

C C

H

CH3

CH3

H

+ Br + H2O

(i)

(ii)

Imagine you were marking this exam question answer.

a) Which response do you think best answers the question?

b) What mark (out of ten) would you give each answer?

c) What feedback would you provide to the student who gave each answer?

d) What aspects of each answer are ‘correct’?

e) What aspects of each answer are ‘incorrect’?

f) How would you answer this question?

334


Task #1—Overall Understandings

Consider the following equation:

H3C C

H

CH3

C

CH3

Br

CH3 + OH C C

CH3

CH3

H3C

H3C

+ H2O + Br

Four possible reaction mechanisms to rationalise this reaction are shown. For each

possible reaction mechanism:

- What aspects do you consider to be mechanistically ‘correct’?

- What aspects do you consider to be mechanistically incorrect? What is it

about these aspects that is incorrect?

- If you were to give this representation a mark out of ten (with one mark

subtracted for each incorrect aspect), what mark would you give it, and why?

Where is the representation losing marks?

Of the four reaction mechanisms, which do you think best rationalises the reaction

represented above? What is it about this mechanism that makes it the best one?

H3C C

H

CH3

C

CH3

Br

CH3 H3C C

H

CH3

CCH3

CH3

HO

C C

CH3

CH3

H3C

H3C

+ H2O

(i)

335

H3C C

H

CH3

C

CH3

Br

CH3

HO

C C

CH3

CH3

Br

H3C

H3C

+ H2O C C

CH3

CH3

H3C

H3C

+ Br

(ii)

H3C C

H

CH3

C

CH3

Br

CH3

(iii)

C C

CH3

CH3

H

H3C

CH3

+ Br C C

CH3

CH3

H3C

H3C

+ H2O

C C

CH3

CH3

H3C

H3C

+ Br + H2OH3C C

H

CH3

C

CH3

Br

CH3

(iv)

HOC C

CH3

CH3

BrCH3

H3C

H

HO

Task #2—Formal charges on atoms in molecules

Two chemical structures are represented below.

- Is there an overall charge on either of the molecules? How would you determine whether or not the molecule is charged?

- Would you say that the charge is localised on one or more atoms in the molecule? How would you represent this?

336

C O

H

H3C

H

H

H

NHH

H

(i) (ii)

Task #3—Formal charges on atoms in molecules after electron movement

Consider the following representation of a reaction mechanism.

- What overall charges are present on molecules (i) and (ii)?

- What overall charges are present on molecules (iii) and (iv)?

- In the case of each molecule, would you say that the charge is localised on one or more atoms in the molecule? How would you represent this?

C O

H

H3C

H

H

H

NHH

HN C

H

HCH3

O

H

H

HH

H

+

Task #4—Structural Representations

In each of the following examples, two starting materials, A and B are represented. For each example:

- What product (or products) would be most likely to be formed in this reaction?

- Why do you think these products are the most likely to be formed?

- How would you represent a reaction mechanism to rationalise the formation of these products?

337

C Br

H

H3CH3CH2C

H

C C

CH3

H

BrH

H3C

OH +

OH +

OH + CH3CH2CHBrCH3

338

Appendix 6.19: Chemistry 2XX Focus Group Questions, Semester 1, 2001

Given to students before they attended the focus group:

Before coming to the focus group discussion, please work through the following:

Five equations are shown below. For each equation, write a reaction mechanism that

rationalises the formation of the products from the reactants. If you use a textbook, or

your notes to write a mechanism, please make a note of this. You can write your

answers on this sheet, or rewrite them entirely. How you answer each question is up to

you.

After you have written the reaction mechanisms, please rank these five mechanisms

from 1 (easiest to do) to 5 (most difficult to do). What I would like you to think about is

what is it about these equations that makes it easier (or more difficult) for you to

represent a reaction mechanism?

339

(i)

ClC

H3C

H3CH3C

CH3

C

CH3H3C

+ Cl

(ii)

CC

H H

CH3

CH3

C C

CH3

CH3

H

H3CH3C

+ OH + H2O

(iii)

C C

CH3

CH3

Br

H

CH3

H3C C C

CH3

CH3

H3C

H3C

+ OH + H2O + Br

(iv)

ClC

H

H3C

H3CH2C CNC

H

H3CH3CH2C

NC C

H

CH3CH2CH3

+ CN + Cland

racemic mixture

(v)

C Br

H

H3CH3C

H3CO C

H

CH3

CH3

+ OCH3 + Br

340

Focus group discussion: Once students have arrived

Ask this question to each student individually, concerning the mechanisms they’ve done before coming in:

- What order did you rank these mechanisms in?

- What is it about each question that make the mechanisms easier or harder to do? Are there common ‘hardness’ or ‘easiness’ factors?

- Did you need to consult your textbook or your class notes for any of these questions? If so, did you take this into consideration while ranking the difficulty of the mechanisms?

- Do you think that it is reasonable to expect that you would be able to suggest a mechanism for each of these reactions at this point in the course? Why/why not?

After everyone has had a chance to discuss:

- Would you change any of your mechanisms now, after this discussion?

- Would your order of difficulty change? Why/why not?

Other questions to ask

1. Your lecturer handed out a unit outline at the start of the course, it which he

classified organic chemistry in terms of reaction mechanisms.

- Do you consider this to be an appropriate classification? Why/why not?

- Do you feel that this classification of the organic course has helped you?

May help you in the future?

- Why do you think the lecturer included this classification in your unit

outline?

2. In your second lecture, the lecturer gave out this paper on mechanisms by Roald

Hoffmann.

- Did you read it?

- Do you think it is important for you to read it?

341

- Why do you think that the lecturer gave you this article? What might his

purpose have been?

- Did you find this paper to be useful? Did it enhance your understandings of

reaction mechanisms in organic chemistry?

- Can you think of any changes that there might have been in your

understanding as a result of reading this paper?

3. In the first few lectures of the course, there was also a lot of discussion of

several mechanistic concepts, such as bond breakage, curly arrows, formal

charges and terminology.

- Why do you think this was included?

- What importance (and application) do you think these concepts have to your

course?

- Did you find this to be useful for your understanding of reaction

mechanisms? In what ways?

342

Appendix 6.20: Chemistry 121/122 Laboratory Test, Semester 2, 1999

Chemistry 122 Lab Test 5 NAME:......................................................

Alkyl halides:

1. The reaction between 1-butanol and HBr under acidic conditions is a substitution

reaction. Which species or group is substituted, and what is it substituted by?

2. In a substitution reaction, it is common to talk about a nucleophile and a leaving

group. In this reaction, what is the nucleophile, and what is the leaving group?

3. Bromobutane was less dense than the aqueous layer in the reaction flask (it floated

on top), but was more dense than the aqueous layer during work-up (it was the

lower layer in the separating funnel after distillation). Account for this apparent

change in density.

4. What is a possible organic by-product of the reaction between concentrated sulfuric acid and 1-butanol (prelab Q2 p 145)?

5. You are given three unlabelled bottles each containing a colourless liquid and are

told that each contains a different compound and that the compounds are

bromobenzene, 1-bromobutane and 2-bromobutane. Explain how you would

decide which bottle contained which compound.

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Appendix 6.21: Chemistry 122 Laboratory Test, Semester 2, 2000

Chemistry 122 Lab Test 3 NAME:......................................................

Alkyl halides:

1. In a substitution reaction, it is common to talk about a nucleophile and a leaving

group. In this reaction, what is the nucleophile, and what is the leaving group?

2. Bromobutane was less dense than the aqueous layer in the reaction flask (it floated

on top), but was more dense than the aqueous layer during work-up (it was the

lower layer in the separating funnel after distillation). Account for this apparent

change in density.

3. The reaction between 1-butanol and hydrobromic acid (HBr) in the presence of

sulfuric acid produces mainly 1-bromobutane, a substitution product. Apart from

the 1-bromobutatne, what other organic compound may be produced as a

byproduct of this reaction? (prelab Q2)?

4. Based on the results of this laboratory experiment, what species would you expect to be produced by the reaction of 2-pentanol and hydrobromic acid in the presence of concentrated sulfuric acid? Represent this as a reaction.

4. You are given three unlabelled bottles each containing a colourless liquid and

are told that each contains a different compound and that the compounds are

bromobenzene, 1-bromobutane and 2-bromobutane. Explain how you would

decide which bottle contained which compound.

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Appendix 6.22: Chemistry 121/122 Examination Question, Semester 2, 1999

Give the products of the following transformations:

CH3

C

H

H3C CH2

Br

HO-

CH3CH2CH2 OH2

Br-

H3N: CH3CH2CH2 Br

O-

a)

b)

c)

d)

345

Appendix 6.23: Chemistry 121/122 Examination Questions, Semester 2, 2000

Question 1:

Illustrate the product that would result from the processes indicated by the curved arrows. Include all lone pairs of electrons and formal charges on your answer.

C Br

H

CH3

CH3

HO

C Cl

CH3

HH

H3N

H

C

H

H3C C

CH3

CH3

HO

O

Question 2:

Consider the reaction between (-)-2-bromopentane (A) and cyanide ions that is

represented in the equation below (no stereochemistry is shown in the equation):

346

CH3CH2CH2CHCH3

Br

+ CN CH3CH2CH2CHCH3

OH

How could you determine which mechanism (SN1 or SN2) the reaction has followed?

Imagine that this reaction can occur only by an SN1 reaction mechanism, or only by an

SN2 mechanism. Your answer should include mechanistic explanations, where

appropriate.

Question 3:

Consider the reaction between (+)-3-bromo-2-methylpentane (B) and hydroxide ions.

C Br

(CH3)2CH

HCH3CH2

+ OH

Which of the following represented organic compounds are possible products of this

reaction? There may be more than one possible product of this reaction.

CHO

CH(CH3)2

HCH2CH3

C C

H

CH3

(CH3)2CH

H

C C

CH3

H

(CH3)2CH

H

C C

CH2CH3

H

H3C

H3C

347

Appendix 6.24: Chemistry 100 Examination Question, Semester 1, 2000

Question 1:

Draw diagrams that represent the compound (or compounds) that are produced by the

transformations shown:

a)

CH3

C

OH3C H

H

H

b)

CH2CH3

C

BrH3C

HO

H

c)

C

H

H

C

H

Br

H

HO

Question 2:

Consider the structure (A) represented below. There are no non-bonding electrons on

this structure:

348

CH2CH3

C

CH3H

(A)

c) What formal charge is present on:

(i) each carbon atom

(ii) each hydrogen atom

d) What is the overall charge on (A)?

The major organic products of the reaction of (A) with sodium hydroxide (NaOH) are

represented below:

CH2CH3

C

OH

CH2CH3

C

HOH HCH3H3C

Draw a reaction mechanism (using curly arrows) to rationalise the formation of these

products.

349

Appendix 6.25: Chemistry 2XX Examination, Semester 1, 2000


CH2CH3

C

BrH3C

H

CH2CH3

CHO

CH3

HOH Br

d) Assign the configurations at the stereogenic carbons of (A) and (B)

e) Suggest a reaction mechanism for the conversion of (A) to (B)

The reaction of (A) with hydroxide ions can also lead to some amount of 2E-butene.

Draw a reaction mechanism that rationalises this observation.

350

Appendix 8.1: Description of Dr Anderson’s teaching.

Lectures in 1999

Dr Anderson commenced his discussion of representations of reaction mechanisms in

his second lecture, which introduced students to the chemistry of alkyl halides. This

lecture described a mechanism to rationalise the formation of an alkyl bromide from

reaction between an alcohol and hydrobromic acid (reproduced in Figure A8.1). As he

wrote down this representation, the lecturer commented, ‘I talked about arrows last

semester, just to refresh your memory, the arrow indicates the movement of two

electrons . . . the movement of two electrons’ (lecture 03081999, line 118 – 119).

The lecturer also described to the students how to calculate the represented charges that

he had indicated on the diagram. Dr Anderson asked students to compare the bonds and

lone pairs around the oxygen atom represented in the starting material and to that in the

product (lecture 03081999, line 123 – 129):

Now, notice, the positive charge has now gone from the hydrogen to the oxygen, now if

you sit down and think about it, that make sense, because here [in the alcohol], the oxygen

had two electrons all to itself, it wasn’t sharing them before. In this situation over here [in

the protonated alcohol], now effectively, instead of having two all to itself, it’s still got two

electrons in its outer sphere there, but they’re shared with others, with the hydrogen ion, so

effectively, it’s lost an electron, it had two to itself, and now it’s got two share between two

species, so it’s one each. So, it effectively has lost an electron, and therefore your positive

charge goes onto the oxygen, it’s gone from the hydrogen here.

O H

H+

OH

H

H

C

HR

O

H

H

H

C

H R

Br O

H

H

+Br

Br

redraw

Figure A8.1: A reaction mechanism representing the formation of an alkyl bromide

compound.

351

This lecture led into a discussion of nucleophilic substitution reactions. Dr Anderson

first defined nucleophiles; ‘A nucleophile is the opposite of an electrophile, in some

respects, you expect a nucleophile to be minus or delta minus, and it will be attracted to

positive centres, or delta plus centres’ (lecture 10081999, line 39 – 41); before giving

students examples of a variety of oxygen-, nitrogen-, sulfur- and carbon-based and other

nucleophiles. The lecturer also showed several examples of substitution reactions.

After Dr Anderson had discussed several substitution reactions, he then commenced

talking about reaction mechanisms; the process of how reactions work. This discussion

referred to a general type of nucleophilic substitution reaction:

R—X + Y- → R—Y + X-

The lecturer commenced his discussion of reaction mechanisms by talking to students

about two important factors in reaction mechanisms; stereochemistry and energy of

reactant particles. Dr Anderson used the analogy of clipping a pen lid onto a pen to

discuss this (lecture 10081999, line 201 – 210):

And there are two factors that are important. One is stereochemistry, how they come

together and whether they come together in the right way or not, and the other factor is do

they have sufficient energy for the reaction to occur. And a nice analogy is, you think of a,

um a pen, with a lid. For, to close the pen, the lid and the pen have got to be aligned with

each other, so they go in this sort of form, the pen enters the lid in the appropriate direction.

You’ve got the makings of a, the correct, desired reaction occurring when that happens. It

won’t happen with any other collision, you need the collision in the right direction. The

second thing is, that when they collide, in this form, they need to have sufficient energy to

get over that little lip on the lid so they stay on. So, in an organic reaction, these sort of

things start to play a greater part.

Dr Anderson considered how the reaction might have proceeded to turn its starting

materials into products. He reminded his students that they needed to consider four

points when trying to determine a reaction mechanism:

352

(i) Which bonds break?

(ii) Which bonds form?

(iii) How do the bonds break (heterolytic or homolytic cleavage)?

(iv) In what order do these bond breakages and formations occur?

After a discussion with the students as to how this reaction might be progressing, Dr

Anderson listed three possibilities for how the reaction had occurred. At the conclusion

of the lecture, the students were left to consider if all of these possibilities made sense.

The lecturer described the three possible ways in which the reaction might have

occurred:

• The R—X bond breaks first, then the R—Y bond forms;

• The R—Y bond forms first, then the R—X bond breaks;

• The R—Y bond forms as the R—X bond breaks.

Dr Anderson commenced lecture four by talking about two kinds of reaction

mechanisms in substitution reactions. He then detailed the differences between these

two types of reaction mechanisms in terms of the rates of reaction of both processes.

The lecturer classified these processes as ‘type A’ and ‘type B’.

Dr Anderson then moved on to describe these two types of process. A type A reaction

was described as a reaction that had a rate of reaction proportional only to the

concentration of substrate. Type B reactions were explained as those processes whose

rates of reaction were proportional to the concentrations of both the substrate and the

nucleophile.

He then recalled the three possible reaction processes discussed in the previous lecture.

Questioning lead to students commenting that one of the mechanisms (number two) was

probably unlikely, due to the number of bonds surrounding the central carbon. The

lecturer then followed this with a description of the type of reaction mechanism that

might explain the possible reaction processes. Scenarios one, two and three refer to the

353

reaction types shown on page . . . The associated representations the lecturer drew are

shown in Figure A8.2 (lecture 17081999, line 36 – 46):

[In] scenario number one, you will have . . . one would imagine a two-step process there,

you break the [R—X] bond first, to form an R plus, and then the Y comes in to give you

R—Y, and X takes the negative. That’s scenario number one . . . Scenario number three,

because we’ve kicked out [scenario] two, you would have a situation where, I’m going to

draw this [structure] a little bit different . . . Now, the way I’ve drawn this, is we have

formed the carbon-Y bond as we have broken the carbon-X bond. The dotted line just there

indicates, if you like, part bonds, this bond is being formed, that one is being broken. That’s

what scenario three is essentially . . . I’ll just write underneath there, that the dotted line

signifies partial bonds, not full bonds, so this is Ok. The carbon here, although it’s got five

species around it, two of the species are not bonded formally together, they’re part bonds.

That’s a little bit more reasonable than having a pentavalent carbon with full bonds

associated with it.

Figure A8.2: Suggested reaction processes for two types of substitution processes,

described in Dr Anderson’s lectures.

Dr Anderson commenced his discussion of nucleophilic substitution reaction

mechanisms by describing the characteristics of the SN1 process. When questioned later

by the researcher, Dr Anderson commented that he had commenced his discussion of

nucleophilic substitution reactions with describing SN1-type processes for a reason. He

exp[lained his reasoning for discussing SN1 processes before SN2 mechanisms

(interview 13082002, line 11 – 18):

I think it’s easier to grasp the concept of, pull something off, put something on [in SN1

mechacanisms] . . . [ ] . . .Um . . . rather than this concept of . . . and, and you don’t have to

worry about stereochemistry there. It’s, it’s a simple issue that you take something off,

forming a flat intermediate and then you can react them. The other one [SN2], you got to

354

start thinking about the nucleophile coming in from behind and pushing it, inverting it. The

whole picture of the mechanism seems a lot more complicated.

Dr Anderson felt that a representation of an SN1 reaction mechanism was more

‘intuitive’ and therefore easier for students to understand as a first concept in the study

of mechanistic representations. This was why he had chosen to teach about this process

before he attempted to explain SN2 reaction mechanisms.

In his lectures, Dr Anderson defined the terminology for the students, indicating that the

term ‘unimolecular’ was used to describe this type of process because only one reaction

species (the substrate) was involved in the rate determining step. The lecturer

represented a general mechanism, showing the students that an SN1 process is thought to

be a two step reaction.

Dr Anderson then reminded students that only some reactions follow a rate law which is

consistent with a unimolecular reaction process, asking, ‘What sort of compounds do

you expect to go via an SN1 mechanism?’ (lecture 17081999, line 77 – 78). He drew

students’ attention to the formation of carbocation species in an SN1 process, asking

students to consider which type of carbocations is ‘more stable’. Although a student

made mention of the stability of tertiary carbocations, Dr Anderson did not specifically

state that tertiary alkyl halides are more likely to proceed via an SN1 mechanism.

Type B reactions were described to the students as SN2 reaction mechanisms. Dr

Anderson defined the term ‘bimolecular’ for the students, explaining that it meant two

species are in the rate determining step; the rate of reaction is proportional to the

concentration of both species. The reaction mechanism for this type of process was

described as a one step mechanism, with bonds breaking and forming at the same time.

Dr Anderson then drew a representation of this type of process, indicating the formation

of a transition state. He detailed the differences between an intermediate (as in SN1

reaction mechanisms) and a transition state (in SN2 mechanisms) (lecture 17081999,

line 88 – 90); ‘an intermediate is a species that is isolable, you can isolate it, you can

identify it, whereas a transition state, you generally can’t.’

Dr Anderson used a very general structure to represent an SN1 reaction mechanism ((i)

in Figure A8.3). As can be seen in the diagram, Dr Anderson did not use curly arrows

355

when representing this mechanism. Similarly, his representation of an SN2 reaction

mechanism ((ii) in Figure A8.3) does not use curly arrows. This mechanism used a

different type of structural representation than the SN1 mechanism. This was due to the

need to show the simultaneous breakage and formation of bonds to a carbon atom.

R X R + X

R + Y R Y

slow

fast

(i)

(ii)

H3C X + Y: C XY

_

CH3Y + X:

Figure A8.3: Representations of (i) SN1 and (ii) SN2 reaction mechanisms.

The lecturer then drew what he referred to as ‘energy profiles’ for both reaction

processes (shown in Figure A8.4), and explained these representations. He described

both the activation energy (EA) and the change in energy between starting material and

products, which he referred to as delta H (not shown on energy profile diagrams)

(lecture 17081999, line 92 – 98):

For an SN1 process, we have an energy profile that has two maxima and a hump in the

middle. So, two stages . . . And SN2 only has one hump. Here we have starting materials . . .

in the SN1, this little well here is your carbocation intermediate, that’s energy, the dip

represents the energy of the carbocation. There is no carbocation formed in an SN2. This

point here is your transition state . . . The two humps, or maxima, in the SN1 also represent

transition states. The first one is . . . the bit where you’re in the process of breaking the

carbon-halogen bond, and the second one . . . is where you’re in the process of forming the

carbon-nucleophile bond.

356

Figure A8.4: Energy profile diagrams for SN1 and SN2 reaction mechanisms as drawn

in Dr Anderson’s lecture.

After describing these diagrams to his students, Dr Anderson then detailed how students

could represent SN1 and SN2 reaction mechanisms using curly arrows. He showed

examples of both types of reaction mechanism, using two different sets of reactants (a

tertiary alkyl halide in the SN1 and a primary alkyl halide for SN2) in his explanations

(Figure A8.5).

Figure A8.5: Representations of SN1 and SN2 mechanisms using curly arrows drawn in

Dr Anderson’s lecture.

357

Dr Anderson used 2-chloro-2-methylpropane (a tertiary alkyl chloride) to describe an

SN1 reaction mechanism and ethyl iodide (a primary alkyl iodide) to represent an SN2

mechanism, although he did not explain to students why he chose these different types

of alkyl halides. Discussion with the lecturer after various lectures indicated to the

researcher that he had selected these specific examples to reinforce the notion that

nucleophilic substitution processes involving tertiary alkyl halides were more likely to

proceed via SN1 reaction mechanisms, whereas those involving primary alkyl halides

were more likely to follow an SN2-type reaction pathway. This distinction was not

made explicit to the students at this point of the lecture.

Dr Anderson followed this up with an explanation of a common mistake he has seen

students make in represented reaction mechanisms. This has been previously discussed

in section 7.4.2. The associated diagram is shown in Figure A7.3.

Lecture four was concluded with a demonstration of reaction processes from the

ChARMs CD-ROM (Capon, 1996). This programme enables a user to select the type

of mechanism he or she wishes to view and then to pick the substrate and the

nucleophile that were to be shown in a representation of a reaction process. These may

be either generic structures (for example, R3C-Y and Nu-) or specific structures. The

programme then shows an animated process in which a single structural representation

of the substrate is shown.

Dr Anderson showed his students examples of both SN1 and SN2 mechanisms from the

ChARMs CD-ROM. The lecturer showed both animations twice. The first time, he let

the animation run all the way through without interruption. The second time, he

stopped and started the animation as it proceeded, describing the represented

mechanism to his students. Depending upon the type of reaction mechanism chosen,

the substrate either loses its leaving group, turns into two carbocations and then

interacts with two nucleophiles to form a racemic mixture (SN1) or interacts with a

nucleophile to form a product with inverted stereochemistry (SN2). In both instances,

the structural representations move about the screen and bond formation and breakage

are represented using curly arrows. Bond formation and breakage is shown using

broken lines becoming full lines (formation) or full lines lengthening and becoming

broken before disappearing (breakage).

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The stereochemical implications of both SN1 and SN2 reaction mechanisms were

discussed in lecture five. At the start of the lecturer, Dr Anderson requested that the

students listened to his explanation first, after which he would give them time to write

his notes down.

He commenced his discussion by describing the stereochemistry of SN2 reaction

processes. The lecturer showed students a general example of an SN2 reaction process,

followed by a specific example (Figure A8.6).

C Br

HCH3

C6H13

CH3

C

H

C6H13

BrHO CHO

HCH3

C6H13

OH

fastslow

_

Figure A8.6: Representation of the stereochemical implications of SN2 mechanism. The

stereochemistry of the starting material is inverted.

In the specific example, he detailed the experimental findings for measuring optical

rotation of the starting material and the product. The starting material had a negative

optical rotation, the product had a positive optical rotation. When R and S

configurations are assigned to these two structures, the starting material is (R)-2-

bromooctane and the product is (S)-2-octanol.

In the example used by Dr Anderson, the starting material and product happened to

have both opposite optical rotation and different configurations. However, even when

stereochemical inversion has taken place, these conditions are not necessarily met. Dr

Anderson then discussed the fact that because the starting material and product are

different types of compounds (in Dr Anderson’s example, an alkyl bromide and an

alcohol), they may have the same assigned configuration (R or S) or both have negative

(or positive) optical rotation values even though the starting material has undergone

stereochemical inversion to form the product. At this point, Dr Anderson took a break

and allowed students a few minutes to copy down his notes. He walked around the

lecture theatre, answering questions, while students copied down the notes.

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After describing the stereochemical implications of an SN2 reaction mechanism, Dr

Anderson used a diagram to detail the outcomes of an SN1 mechanism. As he had when

describing the bimolecular pathway, he asked students not to write anything down until

he had finished explaining the process. He reminded them that he would give them

time to write notes after he had gone through his diagram with them. He represented

the carbocation with some three-dimensional character, indicating its planarity and that

it could be attacked from both above and below that plane. He then represented a

racemic mixture whose structures were consistent with this ‘above and below’ attack.

This general example was followed by a specific example of a reaction that proceeds via

an SN1 mechanism (Figure A8.7).

Figure A8.7: Representations of a general SN1 mechanism and a specific reaction

process, showing the stereochemical implications of the process.

Dr Anderson indicated attack from two faces of a carbocation, resulting in the formation

of a racemic mixture. He also told the students that the optical rotation of the starting

material changed from a value of –34o to approximately 0o. The students were told that

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the product was a racemic mixture. After this, Dr Anderson gave the students a few

minutes to take down notes on the stereochemical implications of an SN1 process. As

he had earlier, he walked around the room and answered questions while they did this.

After detailing the stereochemistry of these two types of mechanisms, the lecturer

explained why understanding and being able to control the stereochemical outcomes of

a reaction process was important (lecture 24081999, line 81 – 90):

[T]he budget for the expenditure on the synthesis of preparation of single enantiomer drugs

. . . [ ] . . . about 70 billion dollars US per year. That much money is spent on the

preparation of single enantiomer drugs. So, if you’re going to be preparing a drug, you

want, you don’t want an SN1 process to be occurring, if you require an optically pure

substance, because in an SN1 process you’re going to get a racemic mixture. An SN2 will

give you an optically pure substance, and if you can control the reaction when required to

go via SN2, so we maintain optical purity, that’s a good thing . . . [ ] . . . we can choose our

materials to enhance the probability of a process going by SN2.

The likelihood of which type of mechanism (SN1 or SN2) is most likely to occur was

then discussed. The lecturer began by describing the key features of each of the reaction

mechanisms, before considering what may be influencing the type of process that is

most likely to occur. Dr Anderson also used a diagram of nucleophilic attack on

differently substituted alkyl halides (bromomethane, bromoethane, 2-bromopropane and

2-bromo-2-methylpropane) to describe how the shape and connectivity of a substrate

can affect the type of mechanism that might occur. This diagram is shown as Figure

A8.8.

Electronic effects were described as governing the likelihood of a reaction proceeding

via an SN1 mechanism. Dr Anderson reminded students that this type of process was

thought to proceed via a carbocation intermediate, and so was more likely to occur

when a substrate produced a more stable carbocation. He identified tertiary, benzylic

and allylic carbocations as the most stable and methyl carbocations as the least stable,

commenting that tertiary alkyl halide reactions were most likely to proceed via an SN1

mechanism.

SN2 mechanisms were described as governed by how accessible the back side of the

carbon bearing the leaving group was. As this reaction process is thought to involve

interaction between nucleophile molecules and carbon atoms in substrate molecules,

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less substituted substrates, such as methyl halides and other primary halides, were

described as more likely to proceed via an SN2 process. In these types of structures,

carbons bearing the leaving group are more ‘accessible’, as they are not surrounded by

large substituent groups.

Figure A8.8: Copies of overhead slides used by Dr Anderson to describe spatial

arrangement around a central carbon in four different bromo-alkanes and how the

degree of substitution can affect the likelihood of particular mechanistic processes

taking place.

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Dr Anderson summarised the likelihood of a reaction proceeding by either reaction

mechanism in a graph, shown as Figure A8.9. The graph is an analogy of the amount of

energy required to ‘make’ a reaction proceed via either SN1 or SN2 reaction

mechanisms. The lower energy process is more likely to be followed.

Figure A8.9: Graph indicating likelihood of reaction proceeding by either SN1 or SN2


When questioned by the researcher at a later date, Dr Anderson commented that this

graphical representation was something he had created himself in an attempt to help

students understand about the likelihood of reactions proceeding by either (or both) SN1

or SN2 reaction mechanisms. He stated (interview 13082002, line 399 – 405):

I think it helps. Um . . . I was trying to summarise um . . . with a rough analogy because I

had no scale. I had no idea of the shape [of the curve]. Whether it was going to be linear or

parabolic or whatever. Just a rough trend type situation, so students could look at it and say,

‘ok, these things have got lower energy, so they’re going to go this way, these have got

higher energy, so they’re not going to go. And there’s that bit in the middle that can

probably go both ways, depending on where the energy is. And, um . . . yeah, I made it up.

In one of his lectures, Dr Anderson then made a brief comment to the students about the

fact that some reactions may proceed by both SN1 and SN2 mechanisms, depending

upon the energy requirements of both processes (lecture 24081999, line 84 – 88):

you may have some molecules in the reaction mixture undergoing an SN1 process . . . [ ] . . .

and some molecules within the reaction mixture undergoing an SN2. The molecules can

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chose to react through the lowest energy pathway, and if both pathways are about the same

energy, some of them might have enough energy to go one way, and others decide to go the

other. It depends on the molecules themselves, the conditions you use.

Dr Anderson finished lecture five by discussing the reactivity of alkyl halides. After

discussion, he wrote down an order of reactivity list, indicating that alkyl fluorides are

much less reactive than alkyl chlorides, which are less reactive than alkyl bromides and

less reactive than alkyl iodides. Aryl halides were described as very unreactive.

Lecture six discussed elimination reactions and their mechanisms. Dr Anderson

commenced his lecture with a discussion of the conditions needed for

dehydrohalogenation, a reaction that generally requires high temperatures and a strong

base. The students were shown a general example, then two specific examples (Figure

A8.10). They were also reminded to look up Zaitsev’s rule from first semester to help

them determine between which pairs of carbon in a molecule elimination is most likely

to occur.

C C

X

H

C C+ KOH + KX + H2O

(i)

Brc. KOH

ethanolhot

(ii)

Cl c. KOH

ethanolhot

+

20 % 80 %

(iii)

Figure A8.10: Equations representing (i) a dehydrohalogenation reaction, (ii) the

formation of cyclohexene from bromocyclohexane and (iii) the formation of 1- and 2-

buetene from 2-chlorobutane.

Dr Anderson described two possible elimination mechanisms to the students. They

were labelled E1 and E2. E1 mechanisms were described as a two step process with

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only one species in the rate determining step; E2 processes were one step mechanisms

with both species involved in the rate determining step. Examples of each were

represented and are shown in Figure A8.11.

C C

X

H

C C

E1

C C

H

+ X

C C

H

HO

+ H2O

E2

C C

X

H

HO

C C

X

H

C C + X + H2O

HO

Figure A8.11: Representations of E1 and E2 mechanisms.

Following this description, the students were then shown appropriate examples of both

types of elimination process from the ChARMS CD-ROM (Capon, 1996). The lecturer

did not talk through these examples to the same extent that he did the substitution

representations in earlier lectures.

After working through these examples, the lecturer then asked the students a question,

drawing their attention to the first step of the E1 process. If the first step of an E1

mechanism is the same as the first step of an SN1 process, why don’t we simply get

substitution happening? His explanation to this was that the type of reaction that

proceeds is dependent upon reaction conditions.

The lecturer described substitution and elimination processes as competing processes

that are not necessarily mutually exclusive. He described three factors that can influence

the pathway the reaction follows:

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• The reaction conditions used;

• The structure of the alkyl halide;

• The nature of the Lewis base used in the reaction.

Harsher reaction conditions were described as favouring elimination processes over

substitution. The lecturer gave students an example of a reaction that produced both

substitution and elimination products. Under milder conditions, the major product was a

substitution product. Under stronger conditions, elimination was favoured (Figure

A8.12). The lecturer added that tertiary alkyl halides were more likely to produce

elimination products than less substituted primary and secondary compounds, and that

stronger bases also favour elimination. This lecture ended the discussion of substitution

and elimination processes in the Chemistry 121/122 course.

Br

CHCH3CH3

OHOH

CHCH3CH3 CH3CH CH2+

Dilute OH-(aq)

at room temp(20oC)

Conc. KOH inethanol (hot)

79 %

18 %

21%

82%

ReactionConditions

Figure A8.12: An equation showing the difference in quantities of substitution and

elimination products under different reaction conditions.

Dr Anderson commented to the researcher that he used these types of real experimental

examples in his lectures as a way of making the examples more real for students. He

stated (interview 13082002, line 183 – 204):

[T]he concept of giving them evidence for theory is becoming more and more where I can,

without turning my syllabus upside down . . . [ ] . . . So, yeah, um. I try and give the

experimental evidence where I can. This is what is observed. Um . . . and where I can

actually give examples where they give yields and proportions I do so, without actually

telling them, telling them that they don’t remember, don’t have to remember these numbers

underneath . . .[ ] . . . They’re just to support the evidence . . . [ ] . . . Yeah. Um, the

numbers, and yields and proportions are not critical . . . [ ] . . . recognising that this

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predominates over that . . . [ ] . . . Like in the elimination reactions, um . . . where I can, I

give them the proportions and yields . . . [ ] . . . But . . . I want them to recognise that that

one’s the favoured one.

He hoped that his students would be able to use these types of examples to help them to

construct useful understandings in relation to the mechanistic representations he taught

about in his course.

Lectures in 2000

Dr Anderson commenced his discussion of nucleophilic substitution reactions at the end

of lecture two. He introduced this type of reaction as a part of the chemistry of alkyl

halides. In lecture three, Dr Anderson identified different types of nucleophiles and

then moved on to discussing nucleophilic substitution reactions in greater detail. He

started by giving students a word equation, followed by a general equation of

nucleophilic substitution before he showed students equations for specific examples

(Figure A8.13).

Figure A8.13: Word equation and general equation.

This introduction was slightly different to how Dr Anderson had commenced his

teaching in 1999, and this was discussed in an interview between the researcher and the

lecturer. When asked why he had chosen to commence his discussion of substitution

reactions in this slightly different way, the lecturer commented that the decision had

been based upon his opinion of how engaging the introductory lecture was for the

students (interview 22082002, line 17 – 42):

Dr Anderson: I don’t know if one’s better than the other. Um . . . I know I’ve got on my

lecture notes, ‘fix, boring. Do something’. Um . . .

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Researcher: With the introduction to the nu, to the sub, sorry, the nucleophilic

reactions?

Dr Anderson: because, yeah, there’s so many examples and I’m just churning through . .

. when an oxygen’s one, you get an alcohol or an ether, and you keep

going through all these examples and I feel like I’m looking at the sea of

faces that are falling to sleep very quickly.

Researcher: They’re all going . . .

Dr Anderson: Um . . . but, but, I don’t know. I’m just never satisfied with that lecture,

it’s one of the lectures that always feel flat. Um . . .

Researcher: What sort of things have you considered doing to make it less flat, or . . .

is that where the problem is?

Dr Anderson: I don’t know. That’s the problem. I can’t think of what I can do to make

them less flat. Um . . . when I overgeneralise, to talk about just something

that’s negative or delta minus, something with a lone pair of electrons that

can act as a nucleophile, reacting with an alkyl halide, because it’s got a

delta plus and a delta minus, I do that now . . . [ ] . . . I don’t, haven’t seen

to have done it here. Um . . . I, I tend to . . . for the bright kids, they pick it

up . . . [ ] . . . but there’s so many who get completely lost. Um . . . when I

start going through examples, they get caught up in the detail and they

lose the message. Um . . . so, I still don’t know how to fix it. Maybe now,

with printed notes, I’ve got a better opportunity to have a lot of them

already there, so I can just skim through them and here’s your examples.

Dr Anderson felt that this introductory lecture was not satisfactory and was attempting

to find an appropriate way to introduce the topic to his students. As his comments

indicated, he was still trying to find an approach that he considered to be suitable for

introducing the topic of nucleophilic substitution reaction mechanisms to his Chemistry

121/122 students in the years after these observations of his classes had taken place.

After this brief introduction, the lecturer then moved on to reaction mechanisms; how

does the reaction actually work? Through discussion with the students and using a

general equation (Figure A8.14), he identified two processes that were happening in

turning the starting materials into the products; the C—X bond was breaking and the

C—Y bond was forming. The lecturer also reminded the students that bonds can break

either heterolytically or homolytically.

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C X + Y: C Y + X:

Figure A8.14: General equation for nucleophilic substitution.

The lecturer outlined three possible ways that the bond breakage and formation could

happen; (i) the C—X bond breaks then the C—Y bond forms, (ii) the C—Y bond forms

then the C—X bond breaks and (iii) the C—X bond breaks as the C—Y bond forms.

Suggestion (ii) was discarded as unlikely. As one student explained, ‘There’s five

bonds to the carbon’ (lecture 08082000, line 192). This left Dr Anderson with two

possibilities to describe to the students.

Lecture four commenced with Dr Anderson describing two possible reaction processes,

which were related to different rates of reaction. Type A was defined as a reaction

whose rate was proportional only to the concentration of the substrate, whereas the rate

of type B reactions was described as proportional to the concentrations of both the

substrate and the nucleophile. After identifying these two feasible reaction types, Dr

Anderson gave students an example of expected experimental rates of reaction and their

dependence of the concentrations of reactants R—X and Y-. This is shown in Table

A8.1.

Table A8.1: Expected experimental data for different types of reaction processes.

[R—X] [Y:-] Relative Rate

Type A 1

2

1

1

1

2

1

2

1

Type B 1

2

1

2

1

1

2

2

1

2

2

4

Dr Anderson then asked his students to consider what type of reaction process would be

consistent with the rate observed for reactions of type A. He described a two-step

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process to the students, before labelling type A reactions SN1 and type B reactions SN2.

Following this, he moved on to discuss SN1 and SN2 reaction mechanisms.

SN1 reaction mechanisms were defined for the students as unimolecular processes, with

only one species involved in the rate determining step. Dr Anderson also told the

students that this was a two step reaction. After questioning, one student commented

that the first step was more likely to be rate determining, due to the fact that it involved

breaking a bond. Dr Anderson reminded the students that it also involved the formation

of a carbocation, which he described as ‘not very stable’ (lecture 15082000, line 92).

He then asked the question, ‘what sort of alkyl halide would be more likely to follow

SN1 processes?’ Discussion following this question led to talk about the types of

carbocations that could be formed. A student identified three types, primary, secondary

and tertiary, and the lecturer repeated the student’s comment that primary carbocations

are difficult to form and primary alkyl halides are therefore less likely to proceed via

SN1 reaction processes. The lecturer also commented that tertiary alkyl halides can form

‘moderately stable carbocations’ (lecture 15082000, line 134 – 135). The lecturer also

discussed the stability of benzylic and allylic carbocations.

The lecturer then drew what he referred to as an ‘energy profile map’ (similar to Figure

A8.9 from 1999). He talked through this diagram with his students, indicating the

species present at various times. Dr Anderson also described the energy of each of the

steps of the reaction in terms of the height of the ‘peaks’ in the diagram. The lecturer

then worked through an example of an SN1 process, with a tertiary alkyl bromide as the

substrate. Dr Anderson represented the equation first, then wrote a two step SN1

mechanisms without curly arrows. Although Dr Anderson represented the structures

correctly in the equation, when he represented the mechanism, he did not include the

central carbon in two places. These carbons are indicated in the equation and

mechanism shown in Figure A8.15.

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(CH3)3C-Br + OH (CH3)3C-OH + Br

(CH3)3C-Br (CH3)3C + Br

(CH3)3C + OH (CH3)3C-OH

Figure A8.15: Equation and mechanism for an SN1 process.

Dr Anderson defined an SN2 reaction mechanism as bimolecular, and showed a simple

example (methyl iodide reacting with hydroxide ions to produce methanol and iodide

ions). The lecturer then asked the students from which direction hydroxide ions would

attack methyl iodide molecules to form methanol; ‘what would that tell you about the

approach of the hydroxide ion . . . ? How would you expect the hydroxide ion to, um,

react with the carbon . . . ?’ (lecture 18082000, line 198 – 199).

The lecturer proceeded to explain the reaction process to the students by indicating

areas of polarity with δ+ (on the C) and δ- (on the I) symbols. He also represented three

pairs of electrons on the I in the representation of methyl iodide. The lecturer repeated

his question; ‘where does the oxygen come from . . . ?’ (lecture 18082000, line 203 –

204); before indicating the possible directions from which nucleophilic attack may

come; ‘from the left, from the right, from the top, from the bottom, from out of the

board, or from behind the board’ (lecture 18082000, line 204 – 205).

After questioning the students, Dr Anderson reminded them that in an SN2 process the

carbon-halide bond was described as breaking as the carbon-nucleophile bond formed,

so the bond must be forming on the other side of the carbon. He uses the analogy of a

car wash to explain this; ‘as one car leaves, a new car can go in, and the car going in is

going in to the back, it’s not coming in from the same direction as the car that’s going

out’ (lecture 18082000, line 218 – 220). He represented this pictorially (Figure A8.16).

371

Figure A8.16: SN2 mechanism for the reaction between methyl iodide and hydroxide

ions.

Dr Anderson followed this by reminding students of the principle features of both SN1

and SN2 reaction mechanisms; ‘in an SN2 process, you’re getting attack from behind . . .

In an SN1 process, you’re forming a carbocation intermediate’ (lecture 18082000, line

226 – 228); before asking the same question he did with regard to SN1 reaction

processes; which types of alkyl halides are more likely to proceed via an SN2 process?

One student makes the suggestion that primary alkyl halides are more likely to follow

an SN2 mechanism because they form the least stable carbocations. Dr Anderson

commented that this is more of a reason why primary alkyl halides don’t proceed via an

SN1 process, adding that this could be part of the explanation; ‘if it can’t go one way,

then it has to go someway, it’s going to take the alternative’ (lecture 18082000, line 238

– 239). Further questioning led to a student commenting that primary alkyl halides are

more likely to follow an SN2 mechanism because, ‘there’s more room available for it,

behind it’ (lecture 18082000, line 246 – 247). He then used a diagram to discuss the

spatial differences around the carbon bearing the leaving group for methyl, primary,

secondary and tertiary alkyl halides. Dr Anderson reminded the students that the

representations weren’t meant to indicate that tertiary alkyl halides wouldn’t react, but

that they wouldn’t follow an SN2 process when they do react.

Dr Anderson then represented an energy diagram for an SN2 reaction mechanism (see

Figure A8.4). He called attention to its single peak, reminding the students that the

process is a one step reaction that results in the formation of a transition state, whose

energy corresponds to the apex of the diagram. He then pointed out the key differences

between SN1 and SN2 processes, referring to the number of steps in each reaction

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process, the relationship between reaction rate and concentration of reactants and

whether the reaction formed an intermediate or a transition state.

In lecture five, Dr Anderson discussed the stereochemical implications of both SN1 and

SN2 reaction mechanisms. He commenced with a discussion of a general SN2 process,

indicating to students that the mechanistic process resulted in the inversion of the

stereochemistry of the carbon bearing the leaving group. In this discussion, he referred

to the general representation of the substrate as an example of an optically active, chiral

compound, reminding students that this meant one carbon (the C bearing the leaving

group, X) was attached to four different groups. The lecturer then used a specific

example of the reaction between (-)-2-bromooctane and hydroxide ions to produce (+)-

2-octanol and bromine ions to describe stereochemical inversion as a result of an SN2

type process. This example was the same as the lecturer had used in 1999 (Figure A8.6).

After discussing the stereochemical outcomes of an SN2 process, Dr Anderson moved

on to discuss the implications of an SN1 process. He used the same diagrams from 1999

(Figure A8.7) to describe the racemic nature of the reaction product from a general SN1

reaction process. Dr Anderson also explained to his students that the optical activity of a

racemic mixture would be zero. He then used a specific example (reaction between 1-

chloro-1-phenylethane and hydroxide ions to produce 1-phenylethanol) to illustrate such

a reaction. The specific example was the same as that used in 1999 (Figure A8.7).

The specific example that the lecturer used in this lecture was described as having a

specific rotation of +1.7o. The lecturer commented that this was very close to the

specific rotation of zero, which was expected of a racemic mixture. The slight deviation

from zero was rationalised with two suggestions. Dr Anderson suggested that the

reaction may not necessarily proceed only by an SN1 process, there may be SN2-type

interactions happening in the mixture as well. He also commented that there could be a

second reason for this slight discrepancy; as the leaving group leaves the substrate, it

may temporarily ‘block’ nucleophilic access to one side of the carbocation.

Following this, the lecturer briefly summarised the stereochemical outcomes of SN1 and

SN2 mechanisms. Dr Anderson then used the ChARMs programme (Capon, 1996) to

show animations of SN1 and SN2 mechanisms. He described the two reaction processes

as he showed these animations to the students.

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Dr Anderson then discussed what conditions favour SN1 processes, and which favour

SN2. The unimolecular process was described as more likely for substrates that form

more stable carbocations, such as tertiary, benzylic and allylic halides. SN2 reaction

mechanisms required access to the carbon bearing the leaving group and were therefore

favoured in less crowded substrates. This was summarised in a graph (Figure A8.17).

Figure A8.17: A graph summarising the energy required for methyl, primary, secondary

and tertiary alkyl halides to proceed via SN1 and SN2 reaction mechanisms.

Dr Anderson briefly discussed the relative reactivities of the various alkyl halides,

describing alkyl iodides as the most reactive and alkyl fluorides (such as Teflon) as the

least reactive. Dr Anderson commented that aryl halides do not normally undergo

nucleophilic substitution reactions.

Dr Anderson discussed elimination reactions in lecture six. He showed students

examples of a dehydrohalogenation process, something they had covered in semester

one. One example had only one alkene product, another (shown as (iii) in Figure A8.10)

had two possible products formed in an 8:2 ratio. This led into a discussion of Zaitsev’s

rule, something the students had also covered in semester one.

After this, the lecturer moved on to talk about the possible mechanisms for elimination

reactions. He commented that there were two mechanistic processes that he called E1

and E2. Both of these processes were described in terms of their rates of reaction, and

the mechanisms compared to those of the substitution processes described earlier in the

course.

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Dr Anderson described an E1 mechanism as being similar to SN1; a two step process.

The first step resulted in a carbocation being formed, the second in the loss of a

‘neighbouring H+ ion’ to form an alkene. The rate of this process was described as

being proportional to the concentration of the substrate. An E2 reaction mechanism was

described as ‘similar to SN2 in that it’s a one step process’. The reaction rate was

expressed as dependent upon the concentrations of both the substrate and the

nucleophile (Figure A8.18).

Figure A8.18: Reaction mechanisms representing (i) E1 and (ii) E2 processes.

Dr Anderson then asked his students why elimination reactions occur. In an E1 process,

where a carbocation is formed, why doesn’t the nucleophile react with the carbocation

(as in SN1) rather than the neighbouring H? In E2, why would a nucleophile abstract a

hydrogen and not attack the polarised carbon (as in SN2)? The lecturer discussed with

the students how both substitution and elimination processes may happen in the same

reaction mixture.

In addition to discussing this competition between substitution and elimination

processes using the specific example shown in Figure A8.8, Dr Anderson also

considered this in terms of a reaction the students had performed in the laboratory the

week before, preparing 1-bromobutane from 1-butanol and hydrobromic acid. Another

375

product produced in this process is 1-butene, an alkene, which is the result of an

elimination reaction occurring in the same reaction mixture.

The discussion of substitution and elimination processes was concluded in this lecture

by considering the factors that can influence which process is favoured in a particular

reaction. Dr Anderson described both processes as possible and influenced by three

important factors:

1) The reaction conditions used;

2) The nature of the nucleophile;

3) The structure of the alkyl halide.

Harsher reaction conditions were described as favouring elimination products over

substitution products. Dr Anderson used the same example as he did in 1999 (Figure

A8.12) to illustrate this point. Elimination was also described as being favoured by the

use of stronger bases, whereas the use of weaker bases favoured substitution. Dr

Anderson commented that elimination occurred more readily in reactions that used

tertiary alkyl halides. Elimination reactions were much less likely to happen in

reactions where the alkyl halide was primary.

The students were then told what they needed to be able to demonstrate an

understanding of in relation to elimination reactions. This included understanding

Zaitsev’s rule, determining the more likely alkene product if more than one was

possible and discussing the factors that favoured substitution processes over elimination

and vice versa.

This was where the discussion of substitution and elimination reaction processes was

essentially ended in Chemistry 121/122 in 2000.

Comparison of Classroom Presentation

While the general presentation of Dr Anderson’s second semester classes did not change

a great deal, there were differences that were noticed between 1999 and 2000. These

differences will be discussed briefly below.

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There was a slight difference in the manner in which Dr Anderson introduced the topic

of nucleophilic substitution reaction mechanisms.

In 2000, the lecturer was observed to use the curly arrow representation less in his

introductory lectures, particularly when discussing substitution reaction processes. For

an example of this, compare Figure A8.1 (from 1999) and Figure A8.13 (from 2000).

While the lecturer is describing similar processes (nucleophilic substitution of alkyl

halides) he has chosen not to use the curly arrow representation in the latter diagram.

As has previously been described in section 7.4.2, Dr Anderson had consciously

decided to use fewer curly arrows when teaching about different reaction mechanisms.

He felt that the curly arrows may be confusing to the students and was attempting to

improve the students’ understandings of mechanistic representations without using curly

arrows. However, as the lecturer himself later commented, after attempting this

technique, he decided that it was more appropriate to teach using curly arrows and

spend more time on helping his students understand what the representation was

attempting to show.

A second difference between 1999 and 2000 was the use of the table shown as Table

A8.1. In 1999, the lecturer did describe the differences between SN1 and SN2 processes

in terms of their rates of reaction. In 2000, he used a table that compared reactant

concentrations to reaction rates, as a simple way of describing this difference in reaction

rates. The numbers used in the table were simple, whole number integers that changed

by easily recognisable amounts as concentrations of both starting materials were varied

to enable students to see how the rate of each reaction had been affected by the change

in concentrations.

377

Appendix 8.2: List of ‘Essential Knowledge’ Prepared by Researcher

For rationalisation:

• Understanding of structural representation and its usefulness for the

specific type of reaction mechanism;

• Understanding of curly arrow representation—what does it ‘mean’ and

what is it used for?

• Understanding (or recognition) of presence of lone pairs, and their

importance/usefulness;

• Understanding of implications of electron ‘movement’ (ie, associated

charge transfer);

• Understanding of formal charge representation, what it tells us about

structure, what it means in relation to overall charge, how it relates to

electron movement and how to work it out;

• Understanding of common terminology, chiral, stereochemistry,

inversion, backside attack, nucleophile, substrate, etc;

• Understanding of implications of experimental evidence; reaction rates,

stereochemistry of products, etc;

• Understanding of feasibility of particular electron movement.

For prediction

All of the above, plus;

• Understanding of different structural implications from possible from

various mechanisms.

378

Appendix 9.1: Description of Dr Adams’ teaching.

Presentation of Coursework in 2000

The concept of representations of reaction mechanisms was introduced to the students at

the end of the fifth lecture of the organic chemistry component of the course.

Representations of reaction mechanisms were first shown in the Chemistry 100 course

to rationalise addition reactions as Dr Adams discussed the chemistry of alkenes and

alkynes. Dr Adams commenced his introduction to the topic by giving his students the

following description of a reaction mechanism (lecture 11042000, line 195 – 202):

[M]y working definition for a reaction mechanism is, sort of the details of what is

happening during a reaction when you’re thinking at the molecular level. So if you’re going

to describe a reaction at the molecular level, the important things you need to know about

are what bonds are forming… and what bonds are breaking. And if you’re thinking about

taking bonds and breaking them, what you’re doing is you’re moving some electrons

around. If you’re forming a bond, you’re moving electrons around.

The lecturer also used this introductory lecture to point out to the students that

mechanistic representations are not necessarily ‘real’ things that can be proven ‘right’

(lecture 11042000, line 217 – 220):

You can never prove a mechanism. What you can do is you can disprove mechanisms that

are ridiculous. You can gather evidence in support of a particular mechanism, but you can

never prove a mechanism. That’s an important point. We’re talking about best guesses, but

they’re guesses are based on lots and lots of experimental results.

In this, Dr Adams is drawing students’ attention to the nature of the representative

model he will be using to teach about reaction processes. He has also emphasised an

important consideration: that mechanistic representations are models that are based

upon explanations of ‘best fit’ to observed experimental data. Mechanisms are fixed—

they are fluid descriptions that can be disproved by new experimental evidence. This

echoes the comments of Laszlo (2002), who also considered it very important that

students and teachers of organic chemistry be aware of the limitations of mechanistic

representations:

[L]et us remind ourselves that study of any reaction mechanism is experimental in nature.

It is hard work . . . after we have done all of this work, . . . we allow ourselves to couch the

379

results in the language of reaction mechanisms . . . But before making use of the formalism,

we conducted a study of reality.

After describing mechanistic representations and some of their limitations, Dr Adams

moved on to detailing some of the language associated with these representations. The

lecturer used a very simple example (the ionisation of hydrogen chloride to produce

hydrogen ions and chloride ions) to explain some common mechanistic representations,

such as formal charges and curly arrows.

In this lecture, Dr Adams did not describe the determination of formal charges to the

students. The lecturer had mentioned to the researcher in a previous interview that he

assumed that the students had been taught about formal charge earlier in the semester.

This assumption was true: the general chemistry lecturer had introduced the topic just

before Dr Adams commenced his organic chemistry lectures.

Formal charge was discussed again in Dr Adams’ introduction to reaction mechanisms

and their representations. In lecture five, he described these charges to students,

explaining the commonly used method of representing formal charges in circles (lecture

11042000, line 237 – 243):

I’ve put charges there on each of the ions, so there’s the formal charge. Remember how to

work out formal charge . . . [ ] . . . Now I’ve stuck the charges in circles. It’s something

that you’ll often see when people are talking about mechanisms. And the reason they do

that is because there’ll be bits all over the blackboard, plus signs in other places. So you

can see that the sign is actually meant to be a charge, I’ll stick it in a circle.

In his description and discussion of formal charge representations, Dr Adams described

to students how the charges were represented in mechanisms and the reasoning for

representing them in this manner (in a circle). In this, he is drawing his students’

attention to linguistic aspects of the representations of reaction mechanisms to make

them aware of the conventions associated with these representations. As was previously

described, the highly symbolic nature of mechanistic representations is one aspect that

can affect the complexity associated with teaching and learning about reaction

mechanisms.

Dr Adams commented to the researcher that he anticipated that students had not seen

curly arrows before they commenced their study of first year Chemistry at university.

380

This assumption was consistent with the level of understanding that was expected of

students who had completed Year 12 Chemistry in Western Australia. As previously

mentioned, reaction mechanisms are not formally covered in high school chemistry.

Dr Adams described these curly arrow representations to his students as a ‘conceptual

tool’ (lecture 11042000, line 263 – 264). He explained to students ‘that no one has ever

seen this sort of process happening. So no one can say “oh, the electrons get up and

move over there”’ (lecture 11042000, line 262 – 263), reinforcing the notion of

mechanisms as a representation of reality. Dr Adams also commented upon the curly

arrow representation and its implied meaning (lecture 11042000, line 253 – 258):

[W]hat that arrow means is that you’ve moved two electrons from the tail of the arrow to

the head of the arrow . . . [ ] . . . in using that arrow, we’ve moved some electrons. And at

the same time as we’ve moved those electrons, we’ve broken the hydrogen-chlorine bond.

So when we think about bond breakage and bond formation in reactions, we use these

curved arrows to help show that sort of process.

In lecture six, Dr Adams described to students how to determine formal charges on

atoms in molecules. His approach differed from that of the general Chemistry lecturer

who had taught the first few weeks of the course. This lecturer had provided the

students with a handout that referred to a formula-based calculation from a textbook

(Silberberg, 2000, p. 367). Dr Adams did not use this method to determine formal

charge on atoms in molecules. He described his method of calculating formal charge in

the following manner (lecture notes 12042000):

Formal charge → no of e- in valence shell compared to the number in the valence shell of

an atom floating in space.

Students were also shown a simple example of calculating formal charge by this

comparison method (lecture 120400, lines 22 – 27). The structural representation that

the lecturer was working though when demonstrating the calculation of formal charge is

shown as Figure A9.1.

Imagine you’ve got an atom of chlorine just floating around in space. So an atom of

chlorine has a valence shell of seven electrons, and it’s neutral, because it’s got the right

number of protons in the nucleus to balance out the charge. Okay, if you look at a chloride

ion … it’s got eight electrons in it’s valence shell, which is one more than a neutral atom in

space, so the charge on the chloride ion has got to be minus one.

381

Cl Cl

Figure A9.1: Examples from a Chemistry 100 lecture in which Dr Adams was

demonstrating how to calculate formal charge.

As was commonly observed in Dr Adams’ lectures, the language used to describe

formal charge is single particle in nature. The lecturer is asking students to consider

only one atom of chlorine to assist them in working through a formal charge calculation.

In this case, a single particle approach can be considered appropriate, as the formalism

associated with formal charge calculations does apply only to single atoms within

molecules.

Due to the functional group presentation order that he had decided to use in his course,

the lecturer did not specifically cover substitution mechanisms until lecture 13. Dr

Adams commenced his discussion of the chemistry of alkyl halides and alcohols by

describing their possible classification as primary, secondary and tertiary compounds.

He then went on to describe the ‘dominant reaction of alkyl halides, and to a lesser

extent, alcohols’ (lecture 10052000, line 112 – 113), nucleophilic substitution reactions.

Dr Adams used a general equation to describe nucleophilic substitution reactions to

students before giving students a specific example of a nucleophilic substitution

reaction. He then detailed possible reaction outcomes using different nucleophiles,

before discussing possible reaction mechanisms (lecture 10052000, line 150 – 153):

[I]t turns out that there, there are lots and lots of different ways the reaction can go,

depending on the details of the structure of the halide and the structure of the, the attacking

nucleophile, and also the solvent, but there are two extreme cases.

Dr Adams commenced his discussion of one of these two ‘extreme cases’ by describing

the meaning of the term ‘SN1’ to his students. In this description, he introduced the

concept of the rate of reaction being proportional to the concentration of only one

reactant. He then gave students a general example of this type of reaction mechanism,

which he followed with an example of a reaction coordinate diagram, which ‘represents

the energy of the species as the reaction is going’ (lecture 10052000, line 178) for this

type of reaction process (Figure A9.2). He used this diagram to again comment on the

382

‘unimolecular’ nature of the reaction, referring to the size of the activation energy of the

first reaction step, as represented in the diagram.

Figure A9.2: Representations of (i) a general mechanism and (ii) a reaction coordinate

diagram for an SN1 process.

The lecturer discussed a specific example of an SN1 type process, one that resulted in

the formation of a racemic mixture. This allowed him to describe the planar nature of

the carbocation intermediate and to introduce the notion of nucleophilic attack from

many directions around the carbocation to produce a racemic mixture. Dr Adams

concluded, ‘the overall result of the SN1 reaction, with a stereocentre, is it takes that

stereocentre and it racemises it’ (lecture 10052000, line 216 – 217).

Dr Adams commenced his discussion of the second ‘extreme’ type of substitution

reaction mechanism at the end of lecture 13. He described an SN2 mechanism in a

similar manner to that in which he introduced SN1 processes, by detailing the

relationship between the concentrations of starting materials and the rate of reaction. Dr

Adams wrote a rate equation for a general SN2 process, indicating that the rate was

dependent on the concentrations of both the substrate and the nucleophile. He then

detailed a general mechanism for this type of process (Figure A9.3).

(i)

(ii)

383

Figure A9.3: A representation of an SN2 reaction mechanism.

Dr Adams described this reaction process to his students in terms of bonds breaking and

forming (lecture 10052000, line 227 – 240):

We have . . . some sort of alkyl halide, and we have . . . an attacking nucleophile floating

around the reaction mixture. What happens in the SN2 reaction, is the attacking nucleophile

starts to attack . . . the carbon that has the halogen attached . . . and so you start to form a

bond between the nucleophile and the carbon . . . Now, as that bond’s forming, your

carbon’s getting too many electrons, so what it does is it starts to break the carbon-halogen

bond . . . and the formation of this bond and the breaking of that bond occur at the same

time. So, what you end up with . . . is this funny looking thing . . . which I’ll put in big

square brackets. And what that thing there is, is it’s a transition state . . . [ ] . . . here we’ve

got a transition state, which doesn’t have normal sorts of bonds, so it’s got these funny sorts

of partial bonds there.

Dr Adams focused his students’ attention on another important feature of an SN2

mechanism: the stereochemical implications of such a process. He commented that he

had deliberately chosen to represent the general SN2 mechanism that he had written

(Figure A9.3) using a particular type of structure, so as to emphasise the stereochemical

implications of the process. Dr Adams then used a large, moving model to demonstrate

the change in molecular shape as he discussed an SN2 mechanism (Figure A9.4).

The lecturer used this model to represent nucleophilic attack on a substrate molecule (i).

The ball on the far left-hand-side of the photograph represents a nucleophile. As it

approached the central black ball (representing a carbon atom), Dr Adams showed the

students how the shape of the structure changed, representing the formation of a

transition state (ii). Finally, the ball representing a leaving group was moved away from

the structure, and Dr Adams showed students the shape inversion that had occurred (iii).

The lecturer introduced students to some common terminology; for example, ‘inversion

of stereochemistry’ and ‘backside attack’; while using this model.

384

(i) (ii) (iii)

Figure A9.4: A model used to describe the stereochemistry of an SN2 mechanism.

The lecturer had commented in an earlier interview that he used models in his lectures

to help his students visualise the different concepts he teaches about in Chemistry 100

(interview 23032000, line 330 – 344):

Interviewer: Yeah. Do you use molecular models at all in the course of the lectures?

Dr Adams: Yeah. I used to. I use the big floppy ball-and-stick models.

Interviewer: The push-through one, or . . . the one, there’s one that sitting behind the

lectures on the . . .

Dr Adams: Oh, that’s for SN2 reaction.

Interviewer: Yep.

Dr Adams: Yeah, I use that.

Interviewer: Ok.

Dr Adams: Although it’s pretty small. I wonder what you can see when you look at

that from up the back of the lecture theatre.

Interviewer: But you use the very large kind of ball-and-stick kind of . . .

Dr Adams: Yeah, yeah.

Interviewer: . . . molecular models.

Dr Adams: Again, I wonder what they can see. You can make up and ethane with

that, and if I’m trying to show a staggered or an eclipsed conformation of

ethane, I can hold it up and try and get the students to look along the

carbon-carbon bond as I’m waving it around.

385

Later discussions with the lecturer as to the purpose of using this large model indicated

that he had ceased to use it in his lectures (interview 16102002, line 165 – 173):

Interviewer: [D]o you still use that now? The big plastic model.

Dr Adams: No I don’t anymore.

Interviewer: Ok.

Dr Adams: Um . . . yeah, I don’t know that it makes any, that it helps . . .

Interivewer: Mmm hmm.

Dr Adams: Like, to me, if I look at this, personally, it doesn’t do anything for me. I

can visualise the . . . chalkboard structures and that works for me. I don’t

know, maybe the students . . . can’t visualise the chalk board, and maybe

they would do well with this, but I, just don’t use it.

Dr Adams’ decision to no longer use the model was linked to his own feelings about its

usefulness. He felt that it did not help him construct any better understanding than the

diagrams on the chalk board and had therefore chosen not to use it in later years of

teaching Chemistry 100.

Dr Adams started teaching in lecture 14 by briefly recapping the previous lecture’s

discussion of SN1 reaction mechanisms. He redrew a reaction coordinate diagram

(similar to (ii) in Figure A9.2) and described the first, slow step of the process and the

relationship between reaction rate and the concentration of the substrate in an SN1

process. He then compared this to an SN2 reaction mechanism, detailing the

stereochemical implications of that type of process. After this introduction, Dr Adams

represented a reaction coordinate diagram for an SN2 reaction process (Figure A9.5),

referring to the rate of reaction and its dependence on the concentrations of both the

substrate and the nucleophile. Students were then shown an equation for a specific

reaction that proceeded via an SN2 process.

386

Figure A9.5: Reaction coordinate diagram for an SN2 reaction mechanism.

The lecturer followed this discussion with a question for the students, ‘what determines

which mechanism operates?’ In response to a student’s question in the previous lecture,

Dr Adams had touched on the notion of both processes occurring in the same reaction

mixture. In this lecture, he discussed the implications of substrate type (primary,

secondary or tertiary) and solvent type on whether the reaction was more likely to

proceed via an SN1 process or an SN2 mechanism. Dr Adams explained that the steric

hindrance around the carbon bearing the leaving group in a tertiary alkyl halide,

combined with the relative stability of a tertiary carbocation, would favour SN1

processes occurring. In comparison, the uncrowded nature of a carbon bearing a

leaving group in a primary alkyl halide and the relative instability of its primary

carbocation would lead to SN2 processes being the most probable for these reactions.

Secondary alkyl halides were described as generally following an SN2 process, unless

reactions were performed in very polar solvents.

The lecturer also briefly discussed different types of leaving group at this point in the

lecture, but did not link this to favouring either of the mechanisms previously discussed.

Dr Adams was asked about his expectations of students’ abilities to determine which

type of reaction process was more likely to occur, based upon the structure of the

starting material. He commented that he did not expect students to have anything more

than a general understanding of this concept at a first year level (interview 16102002,

line 116 – 130):

387

Dr Adams: I’ll tend to have wishy-washy things, like I’ll have a summary at the end

of a lecture saying, ‘bulky substituents on the carbon favour elimination

rather than substitution’ . . .


Dr Adams: . . . and that’s the level that I would go to.

Interviewer: Yep. So it’s more a case of which way’s it more likely to go . . .

Dr Adams: Yeah.

Interviewer: . . . not how much of each, or . . .

Dr Adams: And also, when I’m talking about . . . those sort of trends . . . to try and

simplify it a bit, I concentrate on primary and tertiary . . .


Dr Adams: . . . and I say for the secondary, it’s hard to tell.

Interviewer: Mmm.

Dr Adams: And that’s, that’s basically how I would leave it. Um . . . I don’t expect

students to look at say a secondary case and work out what’s going to

happen.

Dr Adams did not expect his Chemistry 100 students to have anything more than a

general idea of how the structure of a starting material might affect the type of reaction

mechanism a reaction was likely to follow. He felt that this was an appropriate level for

first year students to reach. He only intended to teach students these basic

generalisations and for this reason did not cover SN1/SN2 competition in any more

depth.

In lecture 15, Dr Adams described substitution reactions for alcohols. This involved a

discussion of a protonation step in the reaction mechanism. He described this

protonation as necessary to turn the ‘terrible leaving group’ hydroxide into water, which

is a ‘very good leaving group’. He then represented substitution mechanisms for these

protonated alcohol species. The mechanisms discussed by the lecturer were very

similar to those he discussed in lectures 13 and 14.

388

Elimination reactions were introduced in lecture 15 as ‘[t]he other very important

reaction for alcohols and alkyl halides’ (lecture 16052000, line 167 – 168). Dr Adams

described them as the reverse of the addition reaction that students had encountered in

lecture six. He discussed both dehydration (formation of alkenes from alcohols) and

dehydrogenhalogenation (formation of alkenes from alkyl halides) in his lecture. The

lecturer represented two examples of elimination reactions for which he wrote reaction

mechanisms. The first of these was a representation of the formation of cyclohexene

from cyclohexanol (Figure A9.6). Dr Adams also represented the formation of

cyclohexene from bromocyclohexane (Figure A9.7).

H2C

H2CCH2

C

C

H2C

O

H

H

H

H H O P

O

OH

OH

C

C

C

C

C

C

O

H

H

HH

H

H

H

H

H

H

O P

O

OH

OH+

:B

+ B H

Figure A9.6: Representation of a reaction mechanism for the formation of cyclohexene

from cyclohexanol.

389

Br

+ Br + H2O

H

HH

OH

hot conc.

KOH

Figure A9.7: Representation of a mechanism for the formation of cyclohexene from

bromocyclohexane.

There were some differences in the reaction mechanisms written to explain these two

processes. The formation of a carbocation was described for the cyclohexanol reaction

mechanism. The bromocyclohexane mechanism, however, was shown as a concerted

mechanism. Although there were differences in the reaction mechanisms that he

represented, Dr Adams did not discuss two different types of reaction mechanism with

the students while explaining elimination reactions.

Later discussion with the lecturer indicated that he had deliberately chosen not to

describe two types of elimination reaction mechanisms (E1 and E2 processes, as

described in section 4.5) to his Chemistry 100 students. Dr Adams stated that while he

felt that the distinction between E1 and E2 processes was important, it was not

something that was important to first year students.

In the same lecture, Dr Adams also described two types of competition in reaction

processes. The first was a description of competing elimination processes with different

reaction outcomes. He gave students an example of a reaction in which more than one

elimination product was formed. This example was the reaction of 2-bromo-2-

methylbutane with hot, concentrated potassium hydroxide solution ((i) in Figure A9.8),

which can result in the formation of both 3-methyl-1-butene (a) and 2-methyl-2-butene

(b). These compounds are structural isomers.

The lecturer added that ‘there’ll be the possibility of getting . . . [ ] . . . cis/trans isomers

as well’ (line 9 – 10) in some elimination reactions. However, there is no possibility of

geometric (cis/trans) isomerism in the specific example represented as (i) in Figure

A9.8. Both possible structural isomers ((a) and (b)) are shown in this example. Dr

390

Adams gave the students another example of a reaction in which there is competing

elimination processes, the reaction of 1-methylcyclopentanol with hot, concentrated

sulfuric acid ((ii) in Figure A7.45). Although two products were formed in this

reaction, the example did not depict the formation of cis/trans isomers as competing

elimination products. The products represented in this equation were structural isomers;

methylenecyclopentane (c) and 1-methylcyclopentene (d).

OHCH3

c. H2SO4

heat

CH2CH3

major

CH3 C

CH3

Br

C

CH3

CH3

H

hot conc.

KOH

CH2 CHCH(CH3)2

CH3 C

H

C(CH3)2

(a)

(b)

(c) (d)

(i)

(ii)

Figure A9.8: Representations of (i) the reaction of 2-bromo-2,3-dimethylbutane with

hot, concentrated potassium hydroxide solution and (ii) the reaction of 1-

methylcyclopentanol with hot, concentrated sulfuric acid. Neither example represents

the formation of geometric isomers.

The lecturer then moved on to discuss competition between elimination and substitution

processes, using the example of reactions using similar starting materials; an alcohol,

sodium metal and an alkyl halide; which resulted in different products. He described

the reaction between tertiary butanol, sodium metal and methyl iodide as a substitution

process (specifically an SN2 process), resulting in the formation of methoxy tertiary

butane, an ether. The reaction between methanol, sodium metal and tertiary butyl

iodide was described as resulting in 2-methylpropene, an elimination product (Figure

A9.9).

391

The lecturer commented on these different outcomes for similar reaction conditions

(lecture 17052000, line 56 – 59), pointing out to the students that:

we’ve got a bit of a problem here, we’ve got a set of conditions . . .ah . . . an alkoxide sort

of ion, or a hydroxide, if you add those things to alkyl halide, you can get either

elimination, or you can get nucleophilic substitution.

C

CH3

CH3

OHCH3 C

CH3

CH3

OCH3

CH3 OH CH3 O

CCH3

CH3

ICH3

CH3 C

CH3

CH2

(i)Na

NaCH3I

(SN2)

alkoxide ion (nucleophilic, basic)

(ii)Na

Naelimination

strong base

C

CH3

CH3

OCH3CH3

Figure A9.9: Equations representing reaction of (i) tertiary butanol, sodium metal and

methyl iodide to form an ether (a substitution product) and (ii) methanol, sodium metal

and t-butyl iodide, forming 2-methylpropene (an elimination product).

Dr Adams then used these examples to summarise the reaction conditions required to

favour each type of reaction process. Substitution reactions, such as that represented as

(i), are favoured by mild conditions, using warm, dilute solutions of nucleophiles such

as hydroxide and alkoxide. Elimination processes like (ii) are favoured under strong

conditions, using hot, concentrated base. The use of tertiary alkyl halides, such as

tertiary butyl iodide in (ii), as a substrate also favours elimination over substitution.

This general level of understanding was all that he intended for his students to achieve

in first year chemistry.

In these examples (shown in Figure A9.9), Dr Adams is not discussing competition

between substitution and elimination in the same reaction process. He acknowledged

this in a later interview (interview 16102002 line 112 – 114):

392

I don’t ever do anything where I say, ‘there’s this reaction and . . . ten percent of the

product is a substitution product, and ninety percent is elimination’ . . .

acknowledging that this is a more specific level than he intended to teach to his

students. As with competition between SN1 and SN2 processes, Dr Adams wanted his

students to have developed general understandings of the competition between

elimination and substitution reaction processes in their first year course.

393

Appendix 10.1: Description of Associate Professor Andrews’ teaching.

Lectures in 2000

In first semester, students attended 19 organic chemistry lectures in Chemistry 2XX.

Problem sheets were handed out in lectures six, 13 and 17. The lecturer worked through

the first two problem sheets in lectures eight and 17. The lecturer did not work through

all of problem sheet three with the students. He used a single question on this problem

sheet (one designed in conjunction with the researcher, which is shown in Appendix

6.23) to illustrate a point in one of the later lectures in first semester. There were 19

organic chemistry lectures in second semester, with problem sheets given out in lectures

five, ten and 18. The first two problem sheets were worked through in lectures six and

12. The third problem sheet was not worked through in the lectures.

Associate Professor Andrews introduced the concept of reaction mechanisms in his first

lecture. Mechanisms were described in both the course outline and the first class as a

way of classifying information in organic chemistry. The lecturer described reaction

mechanisms to the class as the ‘sequence by which the starting material is converted to

the product’. He then commented on the fact that (lecture 02032000, line 64 – 66):

[i]n some cases, the mechanism is known in great detail, in other cases, there’s still many

reactions in organic chemistry for which there is no reasonable mechanism.

Associate Professor Andrews used the example of the similarity between the reactions

of carboxylic acid derivatives and hydroxide ions to explain how reaction mechanisms

can be used to classify information (lecture 02032000, line 71 – 73):

Now, all of these [derivatives], on reaction with aqueous base, sodium hydroxide in water,

undergo . . . a reaction, which leads initially to the same compound.

The diagram he used in this example is shown as Figure A10.1.

394

Figure A10.1: Representation of a reaction mechanism for the reaction between

carboxylic acid derivatives and hydroxide ions.

In lecture one, Associate Professor Andrews also explained a commonly used

mechanistic representation; the curly arrow. He commented that (lecture 02032000, line

85 – 87):

we only use arrows of that type to indicate apparent movement of two electrons from the

reagent which is electron rich, to the carbon, which is essentially electron poor.

Students were provided with a handout of an article about reaction mechanisms at the

start of lecture two. The lecturer recommended the students’ read the article (Hoffmann,

1995, p. 143 – 149), but did not discuss it in great detail in the lecture or later on in the

course. When asked by the researcher why he had provided the students with this

reference, Associate Professor Andrews commented upon its usefulness in describing

scientific thought processes in regards to the mechanistic possibilities in a simple

reaction (interview 16102002, line 73 – 86):

[The article is] very nicely written in a very simple conversational style. Secondly, it deals

with . . . a simple . . . set of mechanistic events . . . looking at one reaction, looking at the

possible interpret . . . mechanistic interpretations . . . and then it provides in a very logic

way, the . . . the approach that was taken to eliminate two possibilities and suggested a third

was the likely mechanism. And it’s very simple. Simple structures, simple mechanism. And

the other important thing is essentially the final message that he has in the final . . . couple

of paragraphs, is that this cannot be proven, you can only eliminate alternatives,

395

alternatives, and Carl Popper’s statement of the scientific method. So I think that’s useful as

well. Because he then goes on to say, but of course, chemists, who want to feel as though

they’ve done something, so they’ll say ‘we have proven the mechanism’. There’s a bit, a bit

of humour in there as well. So I think that’s . . . it’s quite a good summary of . . . the . . . ah,

the process in chemistry, the thought process.

In lecture two, Associate Professor Andrews introduced the students to the concept of

breaking bonds in compounds. The example that he chose to use involved representing

the breaking of a carbon-hydrogen bond. The lecturer felt that these examples were a

useful way of introducing students to the language associated with mechanistic

representations, noting that, ‘it gives one an opportunity to . . . define certain terms’

(interview 16102002, line 118 – 119). He made similar comments in the course of

working through the examples with his students (lecture 03032000, line 3 – 5):

I’m going to introduce a number of words and definition that probably most of which may

be new to you, and I just want everybody to be at the same level so that when we talk in

chemical terms we understand what we mean.

The lecturer then proceeded to describe the three ways in which a C—H bond in a

compound could be broken, supplementing his explanation with diagrams (Figure

A10.2).

Structural representation (1) was described by the lecturer as the removal of H+ by a

‘nucleus seeking’ base (or nucleophile). The electrons that were in the carbon-hydrogen

bond were both transferred onto the carbon. Representation (2) is an example of the

removal of H- by an electrophile. In this case, the electrons in the C—H bond are

represented as forming a new bond between H and A. Example (3) shows reaction with

a radical species, resulting in the heterolytic cleavage of the C—H bond. In each case,

the lecturer has described the curly arrows that represent electron movement. Associate

Professor Andrews added, ‘when we look at organic chemistry, about 90 to 95 percent

of all the reactions that occur are mediated by effects which I’ve shown here’ (lecture

03032000, line 59 – 60).

396

Figure A10.2: Representations of the breaking of a carbon-hydrogen bond; (1) and (2)

are both examples of homolytic bond breakage and formation, (3) is an example of

heterolytic bond breakage and formation.

The lecturer then demonstrated the calculation of formal charge to his students. He

provided students with a formula for working out formal charges on atoms (Figure

A10.3), before demonstrating how to calculate formal charges on the products shown in

Figure A10.2.

Formal

Charge

= Valence

electrons

- ½Bonding

electrons

- Non-bonding

electrons

Figure A10.3: Equation for calculating formal charge provided to Chemistry 2XX

students.

397

Associate Professor Andrews worked through a simple mechanistic representation

(reaction between water and hydrogen chloride) before giving students a list of some

Lewis bases (nucleophiles) and Lewis acids (electrophiles). He then moved on to

discuss a general reaction that he represented as:

A—B + C → B—C + A

The lecturer then commented (lecture 03032000, line 106 – 116):

If we have two molecules, which are going to react with each other, then one of the

imperatives, one of the things that must happen is that they must collide. It’s got to come

close together to interact. So if we have compound A—B reacting with a reagent C . . . you

might get this general reaction . . . but before this has (indistinct) has any chance of

occurring, this molecule has to collide with that molecule, and of course, not all the

collisions will result in a collision, because they have to collide with the right energy and

the right geometry. So not every collision will lead to a reaction. In fact, only a very small

proportion of collisions lead to reaction . . . [ ] . . . and essentially this gives us the

probability of a reaction occurring.

This description is one of the few times where Associate Professor Andrews uses a

multiple particle explanation of a mechanistic process. He describes the probabilistic

nature of reaction processes and how not all collisions between particles in a reaction

mixture will be successful. This type of explanation is useful to help students to

develop an understanding of the probability of a reaction occurring and requires them to

consider multiple reaction particles, not just one interaction between individual reaction

particles.

Following this general example, Associate Professor Andrews moved on to discuss a

specific example of a substitution reaction. In comparison to Dr Adams and Dr

Anderson, Associate Professor Andrews did not discuss two different types of

substitution reaction mechanism in this introductory lecture with his students. He chose

to show only a single type of mechanistic representation, and did not label it as SN1 or

SN2. The lecturer referred to the representation simply as a substitution reaction

mechanism.

The substitution reaction that the lecturer chose to talk about was between methyl

chloride and hydroxide ions, producing methanol and chloride ions. He indicated the

398

polarity of the C—Cl bond by representing δ+ and δ- on the structure as he represented

a reaction mechanism for the process ((i) in Figure 10.4). Experimental details were

then discussed; ‘the reaction is dependent on the two aspects, the concentration of the

base, and the concentration of reactant, starting material’ (lecture 03032000, line 125 –

126); before the lecturer represented an energy diagram for this reaction ((ii) in Figure

10.4).

Associate Professor Andrews commented that he commenced his discussion using this

simple substitution reaction mechanism to help his students build links between the very

simple bond breaking/bond forming he had covered in their second lecture (see Figure

10.2) and representations of mechanisms for actual chemical processes, such as the

substitution reaction between methyl chloride and hydroxide ions.

Figure A10.4: Representations of (i) mechanism to rationalise the production of

methanol from reaction of methyl chloride and hydroxide ions and (ii) an energy profile

diagram for this reaction.

After representing the energy profile diagram, Associate Professor Andrews discussed

the specific orientation of collisions between particles in the reaction mixture (lecture

03032000, line 140 – 156):

(i)

(ii)

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[I]t’s a little bit more than a simple substitution reaction, because, if you look, now, this is a

very rough representation of a tetrahedral carbon. If you just imagine the tetrahedral carbon,

and let’s look at the geometry of the carbon, where do you think the OH minus, it has to

form a bond to the carbon (indistinct) carbon, and kick out the chloride, chlorine atom,

which, from which direction would the, would that hydroxide need to come to more

efficiently kick out the Cl minus? Well, if it came from the same side as the chlorine,

there’s going to be an interaction of electrons, of the oxygen, there’s a lot of electrons

around the chlorine as well, so they’re going to be, OH minus comes close to the chlorine

they’re going to essentially repel each other, and the attack, if you think about it, the most

reasonable direction of approach is for the OH minus to come in from the side directly

opposite. As far away from that chlorine as possible. That’s a geometry effect that needs to

occur before the reaction can go. But again, just leaving you with, we’re going to represent

a reaction like this, but the organic chemist is not very happy with that, he or she wants to

show . . . the movement of these two electrons onto the carbon . . . pushing off the chloride.

That’s more satisfying representation of what happens, and it does take into account the

geometry of the interaction between the organic reagents between the start and the finish.

Although Associate Professor Andrews commonly used mechanistic representations in

his lectures, he did not specifically discuss substitution and elimination reaction

processes until lecture 18, the second last lecture of semester. In this lecture, he

discussed the stereochemical aspects of reactions. He gave the students an example of a

general substitution reaction (Figure A10.5), before discussing experimental data for

these types of processes (lecture 25052000, line 12 – 19):

[T]here are two limiting cases that physical organic chemists have identified. Now, let’s

just look at the rate of reaction . . . The first case . . . is where the rate of reaction depends . .

. both on the concentration of A and on the concentration of B . . [ ] . . . But, there are other

cases, and . . . a second case . . . where the rate of reaction depends only on the

concentration of B . . . And there are several cases in between, that go from one extreme to

the other one.

400

Figure A10.5: General substitution reaction mechanism. Note that ‘A’ and ‘B’ in the

quote above refer to this diagram.

The lecturer commenced his discussion of these two ‘extreme’ types of observations by

considering cases in which the rate of reaction was proportional to the concentrations of

both the substrate and the nucleophile. He commented that, ‘the concentration of A and

B are important in determining the rate of the reaction, they must somehow be involved,

both of them must be involved in the rate determining step’ (lecture 25052000, line 21 –

23), before using a diagram (Figure A10.6) to begin to explain the mechanism to his

students (lecture 25052000, line 27 – 34):

[I]n the slowest step of the reaction, the nucleophile . . . and the starting material are both

involved in the rate determining step, and therefore, it probably involves a complex, we’ll

call it transition state, where the nucleophile is just beginning to attack the central carbon,

and the leaving group is just beginning to go . . . So, eventually, the nucleophile drives the

leaving group out. And, of course, because in this . . . high energy . . . transition state, both

reagents are involved, that’s how you get the, this dependency on the rate of reaction.

Figure A10.6: Diagram used to explain a reaction mechanism that is consistent with

cases whose rate of reaction is proportional to the concentrations of both starting

materials.

401

The lecturer then used an example from the third problem sheet (Figure A10.7) to

demonstrate the stereochemical implications of this type of reaction process; ‘You get

clean inversion of these, of the stereogenic carbon’ (lecture 25052000, line 58).

CH2CH3

BrH3CH

CH2CH3

HO CH3H

- OH- Br

(A) (B)


(a) Assign the configurations at the stereogenic carbons of A and B.(b) Suggest a reaction mechanism for the conversion of A to B.(c) The reaction of (A) with hydroxide ions can also lead to some amount of 2-E-butene. Draw areaction mechanism that rationalises this observation.

Figure A10.7: Question from problem sheet three, semester one, which the lecturer used

as an example in his discussion of stereochemical inversion in nucleophilic substitution.

Following this example, the lecturer asked his students a question; what happens when

the substituent groups on the carbon bearing the leaving group get bigger? Associate

Professor Andrews represents this using a general structure (Figure A20.8) as he works

through a representation of a reaction mechanism (lecture 25052000, line 59 – 76):

[T]he nucleophile is blocked, it’s hindered from reaching the carbon . . . and . . . carrying

out the substitution reaction . . . the reaction will still be made to go . . . but now the rate of

reaction depends only on the concentration of B, the starting material . . . And what’s

happening here . . . [ ] . . . is that the carbon-bromine bond can be broken, if you provide the

right amount of energy . . . and what we get . . . is an intermediate . . . [ ] . . . and now, this

is the slow step, the rate determining step . . . [ ] . . . the rate of the reaction depends entirely

on the concentration of B . . . not the nucleophile. So, it can generate this intermediate, and

of course, remember it’s present in solution, so it’s surrounded by the nucleophile . . . and

the nucleophile, of which there is an abundance in solution . . . once this is formed, because

this is now planar, remember, it’s a carbonium ion, so these R1, R2 and R3 are in the same

plane . . .and this face and the opposite, the other face from the back of the board, are

available to any nucleophile . . . So, the nucleophile can come in, equally likely from one

side . . . or from the other side . . . and these should occur with the same frequency.

402

Figure A10.8: General representation of a substitution process whose rate of reaction is

dependent only upon the concentration of the substrate.

The stereochemical outcomes of a reaction of this type were then detailed for the

students. Associate Professor Andrews commented that if the starting material was

chiral, there was an equal likelihood of nucleophilic attack from either face of the

carbocations produced in the slow, rate-determining step of the reaction, resulting in the

formation of a racemic mixture whose optical rotation can be measured as zero.

Associate Professor Andrews labelled these two processes SN2 and SN1. The lecturer

also defined the meaning of SN2 as bimolecular nucleophilic substitution, bimolecular

‘because there are two species involved in the rate determining step’ (lecture 25052000,

line 101 – 102). He then added:

those are the two extremes, a number of cases occur with a certain amount of SN1 and SN2

character. But, there are quite distinct cases . . . which can be identified . . . as either SN1 or

SN2.

Follow this discussion of substitution processes, Associate Professor Andrews briefly

talked about what he referred to as a ‘competing’ reaction process; elimination. The

lecturer began by describing the requirements for this type of reaction; that the carbon

(or carbons) next to the carbon bearing the leaving group has one or more hydrogens

attached to it. The lecturer referred to these as ‘α-hydrogens’. In reactions where the

substrate has this type of structure, Associate Professor Andrews commented that the

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‘nucleophile has a definite probability of . . . plucking that hydrogen off’ (lecture

25052000, line 118 – 119).

Following this description, he showed a representation of an elimination mechanism,

comparing the direction of attack of a nucleophile in an SN2 process and in elimination

processes (Figure A10.9). He described this as an E2 reaction mechanism, a process

whose rate of reaction would depend on the concentration of both the substrate and the

nucleophile. He then reminded students of the SN1 reaction mechanism, which results

in the formation of a carbocation. This intermediate can react to form an alkene (lecture

25052000, line 131 – 137):

[I]f this carbon has an α carbon, one or more α carbons . . . which have a hydrogen on the

α carbon, then again, normal process would be nucleophile attacking that carbon from

either side, but there’s nucleophile molecules all over, or surrounding this central

intermediate, and of course, what can happen is . . . nucleophile can again pluck that hy . . .

proton off, the two electrons that are there now go between the two carbons, and again we

get . . . the double bond compound.

Figure A10.9: Representations of E1 and E2 reaction mechanisms, including

comparisons to SN1 and SN2 reaction mechanisms.

404

A mechanistic representation was added to the original E2 diagram. Associate

Professor Andrews labelled this mechanistic process E1, commenting that its reaction

rate was proportional only to the concentration of substrate. This is also shown in

Figure A10.9.

Associate Professor Andrews concluded his discussion of substitution and elimination

reactions by reminding students that these processes very often occurred together in the

same reaction mixture. He also drew students’ attention to what he referred to as the

‘stereochemical condition’ for elimination to occur. Although he discussed this

‘condition’ briefly, the lecturer did not supplement this statement with a diagram for the

students to give any type of pictorial representation to assist them in constructing any

understandings related to this (lecture 25052000, line 145 – 149):

H and X have to be anti- to each other, in other words, pointing in opposite directions.

That’s demanded for by, if you move electrons, there’s much less hindrance to, if electrons

move in such a way that that hydrogen and that X are . . . trans- or anti- to each other.

Associate Professor Andrews had very specific reasons for teaching his students about

the observable differences between SN1 and SN2 reaction processes. He wanted his

students to develop understandings about how proposed reaction mechanisms must be

consistent with experimentally observed data, with particular reference to

stereochemical changes that may occur in a reaction. Rates of reactions were also

described as an experimentally observed factor that could also be used to determine a

more appropriate reaction mechanism.

In addition, the lecturer only expected the students to learn about elimination as a

reaction that could (and often did) compete with substitution processes in reaction

mixtures. He commented that this was due to the often complicated nature of

elimination processes, when several different compounds might be formed as a result of

reaction between particular starting materials, and it was therefore somewhat confusing

for students to develop understandings of it. The lecturer noted, ‘I, essentially don’t

emphasise elimination’, ‘elimination does not play a great . . . part in the course’

(interview 16102002, line 264, 276) for these reasons.

One of the examination questions given to the students in the first semester examination

was the problem sheet question represented in Figure A10.7. Due to the fact that the

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lecturer felt the students answered this poorly in the examination, he gave a brief

discussion of the answer he had expected to the question in the first lecture in second

semester. In this lecture, he reminded students about what curly arrows represented and

also discussed that the direction of nucleophilic attack is considered to be from the

backside of the compound in SN2 reaction processes.

Lectures in 2001

In 2001, only first semester was observed. There were 19 lectures given. Problem sheets

were given out in lectures eight, 13 and 18. Problem sheet one was worked through in

lecture nine. Answers to problem sheet 2 were given as a handout in lecture 14.

Problem sheet three was not worked through in class, nor were answers provided to the

students.

The lecturer commenced his course in 2001 in a very similar way to his class in 2000.

In lecture one, Associate Professor Andrews introduced the notion of reaction

mechanisms as a means of classifying information in organic chemistry. As he had in

2000, the lecturer used the example of the reactions between carboxylic acid derivatives

and hydroxide ions (Figure A10.1) to illustrate this point, representing a general

mechanism for the students ((i) in Figure A10.10). He then asked the students, would

this type of reaction occur for acetone ((ii) in Figure A10.10)? After working through

the example on the board, the lecturer indicated to his students that CH3- (circled) was

not likely to leave the molecule, adding that acetone would therefore not react by the

same pathway.

406

Figure A10.10: Representations of (i) general mechanism of reaction between

carboxylic acid derivative and hydroxide ions and (ii) possible reaction between

acetone and hydroxide ions.

The handout the students were given in the second lecture was the same one provided in

2000 (Hoffmann, 1995, p. 143 – 149). Associate Professor Andrews also used this

lecture to talk through some of the terminology that would be common to the course. As

he had done in 2000, the lecturer described heterolytic and homolytic cleavage of C—H

bonds (Figure A10.2), formal charge, using the same formula shown in Figure A10.3

and types of nucleophiles (Lewis bases) and electrophiles (Lewis acids) that the

students were likely to encounter in Chemistry 2XX.

Associate Professor Andrews used lecture three to discuss describing a reaction process

with his students. Unlike a similar lecture in 2000, the lecturer did not use curly arrows

in describing reaction processes. He did represent a transition state for the reaction

between water and hydrogen chloride, as well us for the reaction between methyl

chloride and hydroxide ions. He also drew energy profile diagrams for both of his

example reactions, as well as discussing two important considerations of the methyl

chloride/hydroxide ions reaction; the importance of the orientation of molecules of

methyl chloride and hydroxide to each other, and the dependence of the rate of reaction

on the concentrations of both the methyl chloride and the nucleophile.

(ii)

(i)

407

Although the lecturer didn’t use curly arrows in his discussion of reaction processes in

lecture three, at the start of lecture four he commenced the class by discussing the curly

arrow representation and using two examples (Figure A10.11) to demonstrate the

symbolism.

Figure A10.11: Examples used by Associate Professor Andrews in a description of the

curly arrow symbolism.

Substitution reaction mechanisms were not discussed explicitly in lectures again until

lecture eight, when the lecturer discussed nucleophilic aromatic substitution. He began

by reminding students about nucleophilic substitution, before describing nucleophilic

aromatic substitution. This was a brief discussion only, with nucleophilic substitution

reactions mentioned only as a comparison to nucleophilic aromatic substitution

reactions.

In lecture nine, Associate Professor Andrews worked through questions from the

problem sheet he had given to the students earlier in semester. The first question he

discussed involved a substitution reaction mechanism. The question is shown in Figure

A10.12. Associate Professor Andrews discussed the answer to the question in the

lecture with the students.

408

H

C Cl

HH

HO -

H

CHO

HH

- Cl

(a) Draw the free energy diagram for this reaction(b) What factors determine the rate of the reaction?(c) Draw a possible structure for the species at the transition state.(d) What is the significance of the curly arrow?(e) What factors would the "ideal" catalyst need to increase the rate of reaction?

Figure A10.12: Question 1 from problem sheet 1.

Associate Professor Andrews discussed the chemistry of nucleophilic substitution and

elimination reaction processes in lecture 15. The lecturer commenced by representing a

general equation on the board (Figure A10.13). He accompanied this diagram with

examples of nucleophiles (Nu- in Figure A10.13) and leaving groups (X in Figure

A10.13).

Figure A10.13: General nucleophilic substitution equation.

He then discussed ‘two extreme cases’ of substitution reactions. He differentiated

between these two ‘extremes’ by referring to their rates of reaction. These were

represented as (a), an SN2 process, and (b) as an SN1 mechanism.

(a) rate = k[A][B] (b) rate = k[B]

The lecturer then proceeded to discuss the stereochemical outcomes of an SN2 reaction

mechanism. The diagram he used to explain this to his students is shown in Figure

A10.14.

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Figure A10.14: Representation of SN2 reaction process.

Associate Professor Andrews described the inversion of the structure’s stereochemistry,

representing the structure of a transition state in his explanation. The lecturer did not

use curly arrows in his representation, perhaps not wanting to confuse a diagram that

already included a large amount of information. He then discussed the ‘critical factors’

that influence if a reaction would proceed via an SN2 pathway, commenting on the size

of the substituent groups on the carbon bearing the leaving group. The lecturer

commented that small substituent groups were favourable, as larger groups could retard

the reaction proceeding by an SN2 mechanism. He also reiterated that if the substituents

attached to the carbon were all different, the carbon was labelled ‘stereogenic’ or

‘chiral’ and that the configuration of the product would be different to the starting

material, due to stereochemical inversion.

Associate Professor Andrews then represented a general reaction process for an SN1

reaction mechanism. This is shown in Figure A10.15. Associate Professor Andrews

represented curly arrows in the second step of this process, but did not use them in the

first step. The representation showed the formation of two products, which the lecturer

labelled a ‘racemic mixture’, described as an equal mixture of both enantiomers.

Associate Professor Andrews then commented that SN2 processes could be precluded

from occurring due to lack of space around the central carbon. In these cases, C—X

bonds can break (if provided with sufficient energy), resulting in the formation of planar

410

carbocations that have two faces for attack, resulting in the formation of two possible

products that are enantiomers.

Figure A10.15: Representation of an SN1reaction mechanism.

The lecturer then briefly discussed how students could tell which reaction process had

occurred in a particular example. He commented that an SN1 reaction process would

result in two enantiomers, while an SN2 reaction would give a single enantiomer. The

lecturer did not comment that this was only true if the starting material was chiral or

suggest experimental methods of determining the enantiomeric composition of a

reaction mixture.

Following this, Associate Professor Andrews moved on to discuss elimination reaction

processes. He redrew two separate structures, one carbocation and one general alkyl

halide (Figure A10.16), using different structural representations to those he’d used in

his discussion of substitution reactions. He then used curly arrows on these diagrams to

compare SN1 processes with E1 processes, and SN2 with E2.

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Figure A10.16: Representations of (i) E1 and (ii) E2 reaction mechanisms, including

comparisons to SN1 and SN2 reaction mechanisms.

The lecturer commented on the competition between substitution and elimination

processes being affected by the solvent and nucleophile used in the reaction process, the

reaction temperature and the reaction conditions. He did not give written examples of

the effects of varying reaction conditions.

Associate Professor Andrews also commented on the relative positions of H and X

within an individual molecule for elimination to occur. He told the students that the

substituents needed to be ‘anti’ to each other for the process to occur. To reinforce this,

he represented his reaction mechanism using a sawhorse projection (Figure A10.16), a

representation that is commonly used to represent configurations of molecules. The

lecturer also mentioned that it was possible to stop this configuration occurring, thereby

interfering with elimination processes, but he did not mention how this could be done

experimentally.

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Comparison of Chemistry 2XX in 2000 and 2002

In general, Associate Professor Andrews used the same teaching strategies when

teaching Chemistry 2XX in both 2000 and 2001. The most significant difference was

the order in which part of the course was presented. In 2000, SN1 and SN2 processes

were not covered until lecture 18, whereas in 2001, these processes were discussed in

lecture eight. This was due to time constraints in 2000 (the introductory lectures took a

little longer). Associate Professor Andrews commented that he needed to move on to

covering enzyme chemistry and would therefore move his coverage of nucleophilic

substitution reaction mechanisms to later in semester.

Additionally, when Associate Professor Andrews discussed the notion of mechanisms

as a classification tool in organic chemistry, he took a slightly different approach. In

2000, he showed the students Figure A10.1, discussing the reaction between carboxylic

acid derivatives and hydroxide ions. In 2001, he also showed the reaction between

acetone (a compound which contains a C=O bond like a carboxylic acid derivative but

is not a carboxylic acid derivative), demonstrating an example that cannot be classified

by the same simple process as acid derivatives. The use of such a non-example defines

the limitations of the classification and demonstrates to students that they must

understand how to apply the demonstrated classifications for these classifications to be

helpful in their study of reaction mechanisms in organic chemistry.

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