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GCE Chemistry A H034/H434:

Practice problems for unit F324

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Demands of A2 Chemistry (Foreword by the Chief Examiner) An aspect of the new revised specifications is that candidates are required to apply their knowledge and understanding to a much greater extent than in the old legacy specification. When devising examinations, two main assessment objectives, AO1 and AO2, are considered. The weightings for these objectives are shown in the specifications for Chemistry A and they comply with requirements of the national Science Core Criteria that apply to approved Chemistry specifications from all Awarding Bodies. An outline of AO1 and AO2 and their relative weightings for the AS and A2 units are outlined below.

AO1 • Recall • Knowledge and understanding

AO2 • Application • Unfamiliar situations

The greater emphasis on AO2 in the revised specifications means that rote recall of course content is unlikely, on its own, to guarantee a good result. The assessment model provides the opportunity for more demanding stretch and challenge questions that help to inform the new A* grade, assessed in the A2 units only. In order to deliver the required range of grades with the application emphasis, the A2 examinations are significantly more demanding than the AS examinations. The new A2 examinations have a much greater emphasis on application of knowledge and understanding than in the past and novel questions that require application of knowledge and understanding will feature in these examinations. A real challenge for both teachers and their students lies in their preparation for this style of examination. The best candidates usually rise to the challenge but less able candidates can struggle. It is particularly important that all candidates answer what they can in the questions, even if they are unable to see the way forwards to a final solution. It is hoped that this resource will provide a useful stimulus in helping students acquire the skills required to tackle some of the more demanding problem-solving questions within Organic Chemistry. Draf

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Introduction These questions have been designed to help students acquire some of the application skills that are now tested in the A2 examinations, focussing on unit F324, Rings, Polymers and Analysis. Some parts of these questions assess skills that should be accessible to the majority of students, whilst others go beyond the demands of the specification or require skills possessed by only the most able students. These parts are labelled with a ‘**’. The questions may also give ideas to teachers for resources that they may then develop themselves.

Assessing application Questions set in examinations will assess AO1 (Recall of knowledge and understanding) and AO2 (Application of knowledge and understanding).

Application in spectral analysis This type of question is perhaps the easiest to recognise as assessing application. Analysis of spectra has problem-solving at its heart, using important principles for each type of analysis. Questions 1–5 in this resource pack provide examples of this type of question, mainly in the context of NMR spectroscopy.

Application in reaction mechanisms When assessing reaction mechanisms, a question might require candidates to outline a mechanism for nucleophilic addition [4.1.2(d)], exemplified by the reaction of NaBH4 with aldehydes and ketones or electrophilic substitution [4.1.1(d)], exemplified by mononitration and monohalogenation of arenes. By application, a question could require candidates to outline these types of mechanism with unfamiliar nucleophiles or electrophiles, which would be provided in the question. This type of approach is illustrated in Question 6 of this resource pack. The examples in the specification are there to illustrate the principles of nucleophilic addition and electrophilic substitution, which could then be applied to other unfamiliar situations. When teaching, extra examples could be used as extension material but students should be made aware that these are not necessarily to be learnt. A question could also require an unfamiliar type of mechanism to be completed using the principle that a curly arrow represents the movement of an electron pair.

Application in synthesis When assessing synthesis, a question might require a candidate to suggest a multi-step synthesis of a compound using solely reactions that have been encountered during study of F324. Extra unfamiliar reagents and reactions could also be provided within a question and these might also be required in the synthetic route. This approach is also illustrated in Question 6 of this pack.

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Questions 1. Using 13C NMR spectroscopy to distinguish between different substituted arenes. 2. Using 13C NMR spectroscopy to distinguish between alcohols.

This exercise also shows some of the limitations of 13C NMR spectroscopy. 3. Using 1H NMR spectroscopy to distinguish between alcohols.

This exercise shows how numbers of peaks and splitting can be used in analysis. 4. Using 1H NMR spectroscopy to analyse some cyclic compounds.

This question is adapted from one of the questions set on the F324 examination from January 2011.

5. Using 13C NMR spectroscopy and mass spectrometry to identify an organic compound.

This question illustrates some limitations of 13C NMR spectroscopy and shows that, when combined with another analytical technique, a compound may then be identified.

6. Application of mechanisms and organic synthetic routes.

The question requires students to apply their knowledge and understanding of nucleophilic addition to develop an unfamiliar mechanism. The question then asks for students to develop possible synthetic routes using specification content and new information provided in the question.

Useful links 1. Spectral database for organic compounds http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/cre_index.cgi?lang=eng This free site is managed by the National Institute of Advanced Industrial Science and Technology (AIST), Japan. The site contains a range of 13C NMR, 1H NMR, IR and mass spectra for many organic compounds. It is best to search by molecular formula and to have a working knowledge of ‘old’ names. 2. Spectra School https://www.le.ac.uk/spectraschool/ This free site, based at Leicester University, is managed by the RSC as part of Spectroscopy in a Suitcase, a new activity that forms part of the Chemistry for our Future project. This interactive website provides a variety of resource materials for students and teachers including real 13C NMR, 1H NMR, IR and mass spectra. The spectra are high quality and it is possible to zoom in on sections that can prove to be very useful for analysing splitting patterns. Note that chemical drawing packages such as ChemDraw and ChemDoodle also provide facilities to allow simulation of spectra which could be used to create homework tasks. 3. OCR community www.social.ocr.org.uk, a useful place to share teaching resources/ideas. Note that as part of the International Year of Chemistry 2011, OCR is offering a Kindle for the best GCE Chemistry resource material posted to the community, see the community pages for more details.

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1. The three isomers, A, B and C below, can be distinguished by 13C NMR spectroscopy.

NO2

NO2

NO2

NO2

NO2

NO2

NO2O2N

O2N

A B C Spectra 1–3 are the 13C NMR spectra of the three isomers A–C.

Match each isomer to its spectrum and explain how you made your choice. [6]

[Total: 6]

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2. There are four structural isomers of C4H10O that are alcohols, shown as A–D below.

OHOH

A B C D

OHOH

Spectra 1–4 are the 13C NMR spectra of the four isomers A–D.

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(a) Two of the isomers from A–D can be identified from their 13C NMR spectra but the other two isomers cannot easily be identified.

• Match the two isomers that can be identified to the correct spectra. • Explain how you made your choice. • Explain why the other two isomers cannot easily be distinguished for their 13C NMR

spectra.

[10]

(b) The peak at highest chemical shift in each spectrum is likely to be for the C atom bonded to the –OH group. It is difficult to predict which peaks are responsible for the remaining C atoms. It is likely that the closer the C atom to the –OH group, the higher the chemical shift.

• Using this information, predict the C atoms that are responsible for the peaks in

each of the spectra 1–4. • Hence predict which spectrum matches the other two isomers.

**

[8]

[Total: 18]

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3. There are four structural isomers of C4H10O that are alcohols, shown as A–D below. The isomers can be distinguished using 1H NMR spectroscopy.

OHOH

A B C D

OHOH

(a) Spectra 1–3, shown below, are the 1H NMR spectra of the three isomers B, C and D.

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• Match the isomers B–D to the spectra 1–3 and identify the protons responsible for the peaks in each spectrum.

• Explain any splitting patterns. [11]

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(b) The 1H NMR spectrum of isomer A is shown below as Spectrum 4. This spectrum is more difficult to interpret fully than the spectra in (a).

• Analyse this spectrum and explain why it is more difficult to interpret fully. • In your answer, you should identify the protons responsible for each peak and any

splitting patterns. *

[4] [Total: 15]

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4. The three isomers below are members of a family of compounds called dioxanes.

1,3-dioxane 1,4-dioxane 1,2-dioxane

The three dioxanes can be distinguished from their 1H NMR spectra.

(a) The 1H NMR spectrum of 1,4-dioxane is shown below.

1,4-dioxane

Explain why the spectrum consists of just one peak that is not split.

[1]

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(b) The 1H NMR spectrum of 1,3-dioxane is shown below.

1,3-dioxane

Identify the protons responsible for each peak and explain the splitting patterns.

[3]

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(c) The 1H NMR spectrum of 1,2-dioxane is shown below.

1,2-dioxane

Explain why the spectrum consists of two triplets.

[2]

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5. Xylitol is a compound of C, H and O with the molecular formula C5H12O5.

C5H12O5 has many isomers but initial analysis has limited the possibilities to the four structures below.

The 13C NMR spectrum of xylitol is shown below.

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(a) The 13C NMR spectrum reduces the number of possible structures for xylitol. Explain, with reasoning, which of the structures 1–4 are still possible for xylitol.

• Your answer should refer to all the structures 1–4. • Show how you decided that some structures were not possible.

[6]

(b) The mass spectrum of xylitol contains fragment ions at m/z = 121, m/z = 91, m/z = 61 and m/z = 31.

• Analyse this information and suggest the structure of xylitol. • Explain your reasoning.

**

[5]

[Total: 11]

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6. Potassium cyanide, KCN, contains potassium and cyanide ions. Although the cyanide ion is usually shown as CN–, the negative charge is on the C atom:

C N

Cyanide ions are used in organic synthesis as nucleophiles. Organic compounds containing a CN group are called nitriles. Nitriles can be converted into other functional groups as in the examples below. 1. Reduction with H2/Ni R–CN ⎯→ RCH2NH2 2. Hydrolysis with HCl(aq) under reflux R–CN ⎯→ RCOOH

(a) Carbonyl compounds react with aqueous potassium cyanide by nucleophilic addition.

• Outline a possible mechanism for the reaction of ethanal with KCN(aq). • Use curly arrows and show any relevant dipoles.

[4] (b) For the synthetic route below,

• show the structure of the intermediate • complete the reagents and any essential conditions.

O

COOH

OH

............................ ............................

[3]

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(c) A chemist intended to synthesise 1-aminobutan-2-ol starting from an alcohol.

• Complete the flowchart below to suggest a three-stage synthesis of 1-aminobutan-2-ol starting from an alcohol.

• Show the skeletal formulae of each organic compound and the reagents and essential conditions for each step.

**

.........................................................

.........................................................

.........................................................

1-aminobutan-2-ol [7]

[Total: 14]

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Mark Scheme and guidance

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Qu. Expected answer Mark

1 (a) Spectrum 1 is isomer C as there are two carbon environments giving two peaks in the 13C NMR spectrum

2

(b) Spectrum 2 is isomer A as there are four carbon environments giving four peaks in the 13C NMR spectrum

2

(c) Spectrum 3 is isomer B as there are six carbon environments giving six peaks in the 13C NMR spectrum

2

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KEY POINTS

• The number of peaks give the number of environments • There is no splitting • The area of the peak is not proportional to the number of carbon atoms responsible for

the peak (unlike proton NMR)

2 (a) Spectrum 2 is isomer D There are three carbon environments giving three peaks in the 13C NMR spectrum

OH

D

1

3

32

3

Spectrum 3 is isomer C There are two carbon environments giving two peaks in the 13C NMR spectrum

C

OH

12

2

2

3

Spectrum 1 and Spectrum 4 could be either isomer A or isomer B Isomer A and isomer B both have four carbon environments giving four peaks in the 13C NMR spectrum

OH

B

12

34

OH

A

1

2

3

4

4

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KEY POINTS

• The number of peaks give the number of environments. • Any peak in the region δ ~ 3.3–4.3 ppm shows a proton on a C atom adjacent to the –

OH group: CH–OH. • Splitting gives the number of protons on the adjacent atom. • A broad peak is likely to be from the –OH proton.

3 (a) Expected features of spectra There are many ways of approaching this type of problem. Two approaches are shown below, by a table and by annotated spectra.

spectrum isomer environments splitting 1

H3CC

CH3C

H3C OH

2 broad OH (δ = 3.7 ppm) singlet (δ = 1.2 ppm) 3 × CH3 adjacent to C

2

D

HC

H2C

OHH3C

CH3

4 broad OH (δ = 3.7 ppm) doublet (δ = 3.4 ppm) CH2OH adjacent to CH doublet (δ = 0.9 ppm) 2 × CH3 adjacent to CH multiplet (δ = 1.4 ppm) CH adjacent to 2 × CH3

and 1 CH2

3

H3CCH2

CHCH3

OH

B

5 broad OH (δ = 3.6 ppm) triplet (δ = 0.9 ppm) CH3 adjacent to CH2 doublet (δ = 1.2 ppm) CH3 adjacent to CH multiplet (δ = 4.1 ppm) CHOH adjacent to CH3 and CH2 multiplet (δ = 1.5 ppm) CH2 adjacent to CH3 and CH

2

4

5

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3 (b) There are five proton environments but there appears to be only four peaks. The following peaks can be interpreted: δ = 3.7 ppm: broad OH δ = 3.4 ppm: triplet (δ = 3.4 ppm) for CH2OH adjacent to CH2 δ = 0.8 ppm: triplet for CH3 adjacent to CH2 The peak at δ = 1.4–1.6 ppm is difficult to interpret. The splitting does not appear to be sharp and it is the result of two overlapping multiplets for the two central CH2 groups. The protons in these two CH2 groups have similar, but not identical, environments.

4

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4 (a) All protons are in the same environment.

H2C

H2CO

CH2

CH2

O 11

11

1 singlet peak only at δ = 3.7 ppm

1

(b) Protons are in three environments:

H2C

H2CCH2

O

CH2

O 12

2

3

Protons in environment 1 have no protons on adjacent atoms

singlet at δ = 4.7 ppm

Protons in environment 2 have two protons on adjacent C atoms

triplet at δ = 3.8 ppm

Protons in environment 3 have four protons (2 × CH2) on adjacent C atoms

pentet at δ = 1.7 ppm

3

The protons in environment 1 are on a C atom that is adjacent to an O atom. These protons are shifted to a higher chemical shift. (Data Sheet: δ ~ 3.3–4.3 ppm)

The protons in environments 1 and 2 are on a C atom that is adjacent to an O atom. These protons are shifted to a higher chemical shift. (Data Sheet: δ ~ 3.3–4.3 ppm) Note that the protons at environment 1 are shifted to an even higher chemical shift as they are on a C atom with two adjacent O atoms.

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(c) Protons are in three environments:

H2C

H2CCH2

CH2

OO

1

2

2

1

Protons in environment 1 each have two protons on adjacent C atoms


Protons in environment 2 each have two protons on adjacent C atom


2

Total 6

The protons in environments 1 and 2 are on a C atom that is adjacent to an O atom. These protons are shifted to a higher chemical shift. (Data Sheet: δ ~ 3.3–4.3 ppm)

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5 (a) The C environments in the four structures are shown below:

The only structures with three peaks are those with three C environments structure 1 and structure 3

6

(b) Fragment ions at m/z = 31 = CH2OH+ Remaining fragment ions are at intervals of 30 = additional CHOH Fragment ion at m/z = 61 = CHOHCH2OH+ Fragment ion at m/z = 91 = CHOHCHOHCH2OH+ Fragment ion at m/z = 121 = CHOHCHOHCHOHCH2OH+ The only structure with this sequence is structure 3

5

Total 11

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6 (a)

O

CHH3C

δ+

δ−

CR

O

H

C CR

OH

H

C

HO

H

OH

C N

N N

4 (b)

O

COOH

OH

............................ ............................CN

OH

KCN(aq) HCl(aq)/reflux

3

(c)

K2Cr2O7(aq)/H2SO4(aq), distil

OH

O

CN

OH

OH

NH2

H2/Ni

.........................................................

.........................................................KCN(aq)

.........................................................

7

Total 14

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