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Developing Critical Thinking Problems for Organic

Chemistry

Ray A. Gross, Jr.

Prince George’s Community College

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January 10, 2007

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Thoughts About Students

• They do not spend enough time working problems.

• They need to be taught (learn) how to reason.

• One way to force them to reason is to give them problems they have not seen before.

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Nature of the Problems

• Focused on developing reasoning skills• Solvable by applying course content • The amount of content is limited

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Kinds of Problems Available

• Spectroscopy

• Synthesis

• Degradation*

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Concept of Degradation

A compound of unknown structure is broken down into smaller pieces, which can be reassembled in a logical way to determine the structure.

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Example of Ozonolysis

+CH3CH2CCH3

O

CH3CH2CH

O

Xunknownhydrocarbon

(1) O3

(2) Zn/H+

remove oxygen

CH3CH2CCH3+ CH3CH2CH

anathons

CH3CH2CCH3

HCCH2CH3

analyte

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O3

(R)

C7H14C4H8O C3H6Oketone aldehyde

+

+

C4H8C3H6

anathons

analyte

Cleavage of a double bond yields two carbonyl compounds,aldehydes or ketones.

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Some Aldehydes may be identified by their molecular formulas.

C

O

H HCH2O = C

H H

molecularformula

structuralformula

anathon

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=

molecularformula

structuralformula

anathon

C2H4OC

O

CH3 HC

CH3 H

Some Aldehydes may be identified by their molecular formulas.

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Consider the following reaction.

C3H6

O3

(R)CH2O + C2H4O

Given only molecular formulas.

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Anathons are like building blocks.

C3H6

O3

(R)CH2O + C2H4O

CH H

O+

CH3CH

O

CH H CH3CH

+

CH

HC

H

CH3

propene

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+CH3CH2CCH3

O

CH3CH2CH

O

(1) O3

(2) Ag2O

(1) O3

(2) Zn/H+

CH3CH2CCH3

O

CH3CH2COH

O

+

ketone

ketone

aldehyde

acidCH3CH2C CHCH2CH3

CH3

Ketones remain ketonesAldehydes are oxidized to acids

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(1) O3

(2) Ag2O

(1) O3

(2) Zn/H+

+C7H14

C4H8O C3H6O2

C4H8O C3H6O+

Same reactions using molecular formulas.

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Identify aldehydes and ketones.

(1) O3

(2) Ag2O

(1) O3

(2) Zn/H+

+C7H14

C4H8O C3H6O2

C4H8O C3H6O+

no change

ketone

plus 1 O

aldehyde

Result: A four-carbon ketone and a three-carbon aldehyde.

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Place C atoms in logical locations to find anathons.

Connection of anathons reveals the unknown’s structure.

C7H14C4H8O C3H6O

ketone aldehyde

anathons

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Concept

• Degradation of an unknown leads to products.

• The structure of each degradation product can be determined directly or indirectly from the two kinds of ozonolysis.

• The products can then be converted into anathons, which are connected in a logical way to produce the starting compound.

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Producing Solvable Problems

• Find structures that are deducible from the two kinds of ozonolysis reactions.

• Convert them into anathons.• Join anathons to make unknowns.

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Structures from Molecular Formulas

CH2O CO2 H C H

O

C2H4O C2H4O2 CH3CH

O

C3H4O C3H4OO

(R) (O) R product

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Remove Oxygen to form Anathons

CH2O CO2 H C H

O

C2H4O C2H4O2 CH3CH

O

C3H4O C3H4OO

(R) (O) R product Anathon

H C H

CH3CH

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Join Anathons to Create Unknowns

Anathon

H C H

CH3CH

H2C CHCH3

H2C

CH3CH

Unknown

The process ensures solvable structures are produced.

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Internal Anathons

• When a molecule is cleaved in two places, the middle compound produces an internal anathon.

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Example of an Internal Anathon

H2C CHCH2CH=CHCH3

O3 (O)

O3 (R) CH2O + C3H4O2 + C2H4O

CO2 + C3H4O4 + C2H4O2

HCH HCCH2CH CH3CH

terminal anathon

terminal anathon

internal anathon

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Carbon-Carbon Triple Bonds

• Ozonolysis by either workup gives two carboxylic acids.

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Cleavage of a Triple Bond

CH3C CCH2CH3

CH3CO2H + CH3CH2CO2H

CH3CO2H + CH3CH2CO2HO3 (O)

O3 (R)C5H8

A triple bond in the analyte leads to two new hydrogen atoms and four new oxygen atoms.

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O3 (R)C5H8 C2H4O2 C3H6O2+

2 new H atoms = triple bond two caboxylic acids

CH3C CCH2CH3

Anathons from triple bonds have three bonding sites, andthose from double bonds have two.

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Anathons Readily Determined by their Ozonolysis Products

• 11 Terminal Anathons

• 11 Interior Anathons

• 3 Either Terminal or Interior

306 unique unknown hydrocarbons can be producedfrom the 22 anathons. All of them are solvable by the techniques shown.

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Example of Terminal/Interior Anathon

terminal anathon

A secondterminal anathon is bonded here.

interior anathon

Threeterminal anathons are bonded as indicated.

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Examples of the Unknowns

CH3CH2CH CHCH2CH C(CH3)2

CH3CH2CH C CCH3

CH2CH3

CHC CCH2CH3

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Problems Created that:

• Students have never seen before• Require a minimum amount of course content• Require students to apply logic and reasoning

(synthesis, analysis and evaluation)• Increase in complexity over the semester• Allow the instructor to give each student a

unique problem each week

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The approach described might be of use in other chemistry courses or disciplines.

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Problems of this type are available in the following references.

Chemical Educator 2006, 11, 372-377.

Journal of Chemical Education 2006, 83, 604-609.