GRE Subject Test: Chemistry : Nucleophilic Addition

Study concepts, example questions & explanations for GRE Subject Test: Chemistry

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Example Questions

Example Question #2 : Reaction Mechanisms

What is created when a ketone is reacted with a phosphorus ylide?

Possible Answers:

Aldehyde

Alkane

Ester

Alkene

Correct answer:

Alkene

Explanation:

The Wittig reaction involves a ketone or aldehyde reacting with a phosphorus ylide, a molecule with a negatively charged carbanion. The ketone will undergo nucleophilic addition and form a betaine. This intermediate will then form an alkene with a triphenylphosphine oxide being released. The Wittig reaction will form a mixture of both cis and trans isomers if the carbanion has two different substituents.

Wittig general reaction:

Example Question #202 : Gre Subject Test: Chemistry

Which of the following compounds would you expect to undergo a nucleophilic addition reaction?

Possible Answers:

Ethanamide

Methyl ethanoate

Propanal

Acetic acid

Correct answer:

Propanal

Explanation:

When dealing with carbonyl compounds, remember that a carboxylic acid and all of its derivatives will undergo nucleophilic substitution. Aldehydes and ketones will undergo nucleophilic addition. Propanal is a three-carbon aldehyde, and will thus undergo nucleophilic addition.

Acetic acid is a carboxylic acid, methyl ethanoate is an ether, and ethanamide is an amide; each of these would undergo nucleophilic substitution.

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