GRE Subject Test: Chemistry : Reaction Mechanisms

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

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Example Questions

Example Question #1 : Reaction Mechanisms

When exposed to a good nucleophile, which molecule will most readily undergo an  reaction?

Possible Answers:

Correct answer:

Explanation:

 reactions, also known as unimolecular nucleophilic substitution reactions, occur in two steps. Here, we are concerned with the first and second (rate-determining) steps, in which the leaving group breaks off of the molecule to form a carbocation. Alkanes that form the most stable carbocations are most likely to undergo  reactions. Tertiary carbocations are the most stable, followed by secondary. Primary and methyl carbocations are very unstable and unlikely to form at all. The tertiary alkane, , will form a very stable tertiary carbocation compared to the other answer choices.

Example Question #1412 : Mcat Biological Sciences

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

Possible Answers:

Alkane

Aldehyde

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 #1 : Functional Groups And Properties

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

Possible Answers:

Propanal

Acetic acid

Methyl ethanoate

Ethanamide

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.

Example Question #1 : Properties Of Hydrocarbons

 

  Bromobenzene_labAnisole_labNitrobenzene_lab

Of the aromatic compounds shown above, which would be meta-directing groups for subsequent electrophilic aromatic substitution reactions?

Possible Answers:

Nitrobenzene

Anisole

Nitrobenzene and anisole

Bromobenzene

Correct answer:

Nitrobenzene

Explanation:

Only nitrobenzene would be a meta-directing group for additional electrophilic aromatic substitution (EAS) reactions. Although the bromine of bromobenzene is an electron-withdrawing group, halogens are not meta-directors; therefore, additional EAS reactions with bromobenzene would result in ortho or para attached substituents.

Remember that ortho additions are adjacent to the first substituent, meta additions are two carbons displaced from the first substituent, and para additions are opposite the first substituent.

(Note that these reactions would take place much more slowly than if there was an electron-donating group attached).

Example Question #2 : Reaction Mechanisms

Which of the aromatic compounds (shown above) would undergo electrophilic aromatic substitution most quickly?

Possible Answers:

Bromobenzene

Nitrobenzene

Acetophenone

Anisole

Correct answer:

Anisole

Explanation:

Electrophilic aromatic substitution occurs most rapidly when the aromatic compound has electron-donating groups attached. Due to their electron affinity, halogens are electron-withdrawing groups. Acetophenone and nitrobenzene both bear partial positive charges on the substituent directly attached to the benzene ring, which pulls electron density out of the ring as well, causing the reaction not to occur.

Anisole is the only compound with an electron-donating group, and is the correct answer. The lone pairs on the oxygen atom can be used to initiate new bonds.

Example Question #1 : Electrophilic Addition

What is the product of a hydroboration–oxidation reaction with 1-hexylcyclohexene?

Possible Answers:

Cyclohexane

3-hexylcyclohexanol

2-hexylcyclohexanol

1-hexylcyclohexanol

Hexylcyclohexane

Correct answer:

2-hexylcyclohexanol

Explanation:

This reaction is an electrophilic addition reaction with an alkene. This is one of many alkene addition reactions that can add an -OH group onto your starting material. The key aspect of an hydroboration-oxidation reaction is the anti-Markovinikov addition to the double bond. The -OH group should be on the least substituted of the two carbons that originate from the double bond. In light of this information, the answer is 2-cyclohexanol.

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