Question

The correct order of the rate of reaction of the following reactants with nucleophile by \( \mathrm{S_N1} \) mechanism is: 
(Given: Structures I and II are rigid) 

• A. \( \text{III}<\text{I}<\text{II}<\text{IV} \)
• B. \( \text{I}<\text{II}<\text{III}<\text{IV} \)
• C. \( \text{II}<\text{I}<\text{III}<\text{IV} \)
• D. \( \text{IV}<\text{III}<\text{II}<\text{I} \)

Hint:

For \( \mathrm{S_N1} \) reactions, carbocation stability governs the rate; resonance stabilization dominates over hyperconjugation.

Answer 1

The Correct Option is C

The question asks for the correct order of the rate of reaction of different reactants with a nucleophile by the \( \text{S}_\text{N}1 \) mechanism. To solve this, we need to consider how the \( \text{S}_\text{N}1 \) mechanism works:

1. The \( \text{S}_\text{N}1 \) mechanism involves two main steps:
   • Formation of a carbocation intermediate.
   • Nucleophilic attack on the carbocation.
2. The rate of the \( \text{S}_\text{N}1 \) reaction is determined by the stability of the carbocation intermediate. More stable carbocations lead to faster reactions.
3. Let us analyze each structure provided:
   • Structure I and Structure II are rigid systems. Their carbocations will be less stable than those from more flexible tertiary systems.
   • Structure III is a secondary system and forms a secondary carbocation, which is more stable than primary but less stable than tertiary carbocations.
   • Structure IV is a tertiary system, which forms a tertiary carbocation, providing the most stable intermediate.

Considering the stability of the carbocations formed from these structures, the order of increasing \( \text{S}_\text{N}1 \) reactivity is:

• Structure II: Least stable carbocation
• Structure I
• Structure III
• Structure IV: Most stable carbocation (tertiary)

Thus, the correct order of the rate of reaction is: \(\text{II} < \text{I} < \text{III} < \text{IV}\).

This matches the correct answer.