Question

The correct order of rate of solvolysis for the following compounds is

(I) Cyclopentadienyl bromide
(II) Cycloheptatrienyl bromide
(III) Benzyl bromide

Hint:

Aromatic intermediates accelerate reactions, while anti-aromatic intermediates act as extreme barriers.

Answer 1

Step 1: Understanding the Concept:
Solvolysis rate in \(S_N1\) reactions is governed by the stability of the intermediate carbocation.
Step 2: Detailed Explanation:
1. Compound II: Forms the tropylium cation, which is cyclic, planar, fully conjugated, and has \(6\pi\) electrons. It is aromatic and extremely stable.
2. Compound III: Forms the benzyl cation, which is stabilized by resonance with the phenyl ring. It is stable but less so than an aromatic cation.
3. Compound I: Forms the cyclopentadienyl cation. It has \(4\pi\) electrons in a cyclic planar system, making it anti-aromatic and highly unstable.
Step 3: Final Answer:
The order is II > III > I.