Question

Which of the following is the most reactive towards \( S_N1 \) mechanism?

• A. 1
• B. 2
• C. 3
• D. 4

Hint:

Reactivity for \( S_N1 \propto \text{Stability of Carbocation} \). Look for: 1. Resonance (Benzylic/Allylic) 2. Degree ($3^\circ \textgreater 2^\circ \textgreater 1^\circ$) More phenyl groups attached to the cationic carbon usually mean higher stability.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:

The reactivity of alkyl halides towards \( S_N1 \) reactions depends on the stability of the intermediate carbocation formed. The more stable the carbocation, the faster the reaction. Stability order: \( 3^\circ \textgreater 2^\circ \textgreater 1^\circ \), and resonance stabilization (benzylic/allylic) increases stability significantly.
Step 3: Detailed Explanation:

Let's analyze the carbocations formed by removing \( \text{Br}^- \):

• (A) \( \text{Ph-CH}_2^+ \): Primary benzylic carbocation. Stabilized by resonance with one phenyl ring.
• (B) \( \text{Ph-CH}^+\text{-CH}_3 \): Secondary benzylic carbocation. Stabilized by resonance with one phenyl ring + inductive effect of methyl group. More stable than (A).
• (C) \( \text{Ph-CH}^+\text{-Ph} \): Secondary benzylic carbocation. Stabilized by resonance with two phenyl rings. Very stable.
• (D) \( \text{Ph-C}^+(\text{CH}_3\text{)-Ph} \): Tertiary benzylic carbocation. Stabilized by resonance with two phenyl rings + inductive effect of one methyl group. This is the most stable carbocation among the options.

Since carbocation D is the most stable, compound D is most reactive towards \( S_N1 \).
Step 4: Final Answer:

Option (D).