Question 1

Which of the following best explains why allylic and benzylic carbocations are more stable than typical alkyl carbocations?

Accepted Answer

They are stabilized by resonance delocalization of the positive charge.

Question 2

Consider the following reaction: $$\mathrm{C_6H_5CH_2Cl} ($$benzyl chloride) undergoes solvolysis in water. Which statement best describes the rate of this reaction compared to $$\mathrm{CH_3CH_2Cl} ($$ethyl chloride)?

Accepted Answer

Benzyl chloride reacts much faster due to resonance stabilization of the carbocation.

Question 3

Which of the following is a resonance structure of the allyl cation?

Accepted Answer

$$\mathrm{H_2C^+-CH=CH_2$}$$

Question 4

Why does the presence of a para-methoxy group on a benzyl chloride dramatically increase its rate of solvolysis?

Accepted Answer

The methoxy group is electron-donating and stabilizes the carbocation by resonance.

Question 5

Which of the following best describes the effect of N-bromosuccinimide (NBS) in the bromination of allylic and benzylic positions?

Accepted Answer

NBS slowly generates $$\mathrm{Br_2$}$$, favoring radical substitution at allylic and benzylic positions.

Question 6

Which of the following statements about the resonance structures of the benzyl radical is correct?

Accepted Answer

The unpaired electron is delocalized over the ortho and para positions of the aromatic ring.

Question 7

Which of the following best explains why E2 eliminations are favored over SN2 substitutions at allylic and benzylic positions?

Accepted Answer

The transition state for E2 is stabilized by resonance with the developing carbanion character.

Question 8

Which of the following is a correct statement about the products of NBS bromination of cyclohexene?

Accepted Answer

The major product is 3-bromocyclohexene, formed by allylic bromination.

Question 9

Which of the following best describes the difference between resonance structures and allylic rearrangement in Grignard reagents?

Accepted Answer

Resonance structures are rapidly interconverting, while allylic rearrangement involves distinct compounds in equilibrium.

Question 10

Which of the following statements about the SN2 reactivity of allylic and benzylic halides is correct?

Accepted Answer

Allylic and benzylic halides react faster in SN2 reactions due to stabilization of the transition state by resonance.

Question 11

Which of the following best explains why the pKa of toluene ($$\mathrm{C_6H_5CH_3}) is $$much lower than that of propane ($$\mathrm{C_3H_8}$$)?

Accepted Answer

The benzylic anion is stabilized by resonance with the aromatic ring.

Question 12

Which of the following is a correct statement about regioisomer formation in reactions of allylic and benzylic carbocations?

Accepted Answer

Allylic carbocations can lead to regioisomeric products due to delocalization of the positive charge.

Question 13

Which of the following best describes the effect of branching at the $$\alpha- or$$ $$\beta-$$carbon on elimination versus substitution reactions?

Accepted Answer

Branching at the $$\alpha- or$$ $$\beta-$$carbon favors elimination (E2) over substitution (SN2).

Question 14

Which of the following is a key mechanistic step in the NBS bromination of an allylic position?

Accepted Answer

Homolytic cleavage of the N-Br bond to generate a bromine radical.

Question 15

Which of the following best explains why benzylic hydrogens are more easily abstracted in radical halogenation than non-benzylic hydrogens?

Accepted Answer

The benzylic radical is stabilized by resonance with the aromatic ring.

Question 16

Which of the following is a correct statement about the products of allylic bromination of propene with NBS?

Accepted Answer

A mixture of 3-bromopropene and 1-bromopropene is formed due to resonance stabilization of the allylic radical.

Question 17

Which of the following best explains the enhanced SN1 reactivity of trityl chloride ($$\mathrm{Ph_3CCl}) $$compared to tert-butyl chloride ($$\mathrm{(CH_3$)_3CCl}$$)?

Accepted Answer

The trityl carbocation is stabilized by resonance with three aromatic rings.

Allylic and Benzylic Reactivity: Carbocations, Radicals, Anions, and SN2 Reactions

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