Reaction Mechanisms

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GRE Subject Test: Chemistry › Reaction Mechanisms

Questions 1 - 10
1

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

2-hexylcyclohexanol

1-hexylcyclohexanol

3-hexylcyclohexanol

Hexylcyclohexane

Cyclohexane

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.

2

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

2-hexylcyclohexanol

1-hexylcyclohexanol

3-hexylcyclohexanol

Hexylcyclohexane

Cyclohexane

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.

3

Bromobenzene_labAnisole_labNitrobenzene_lab

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

Nitrobenzene

Bromobenzene

Nitrobenzene and anisole

Anisole

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).

4

Bromobenzene_labAnisole_labNitrobenzene_lab

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

Nitrobenzene

Bromobenzene

Nitrobenzene and anisole

Anisole

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).

5

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

Alkene

Ester

Alkane

Aldehyde

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:

6

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

Alkene

Ester

Alkane

Aldehyde

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:

7

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

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.

8

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

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.

9

Acetophenone_labelled

Anisole_lab

Bromobenzene_lab

Nitrobenzene_lab

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

Anisole

Bromobenzene

Acetophenone

Nitrobenzene

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.

10

Acetophenone_labelled

Anisole_lab

Bromobenzene_lab

Nitrobenzene_lab

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

Anisole

Bromobenzene

Acetophenone

Nitrobenzene

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.

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