The correct order of reactivity of the following benzyl halides towards reaction with KCN is:

Question

Accepted Answer

B. 1. **Identify the reaction mechanism.** Since benzyl halides reacting with KCN typically proceed via **$S_N1$** (especially when substituents can stabilize a benzylic carbocation),

2. **Determine what controls the rate.** Given the rate depends on the **stability of the carbocation intermediate** formed — more electron-donating substituents on the ring (which stabilize the developing positive charge via resonance/+M effect) increase reactivity, while electron-withdrawing substituents decrease it.

3. **Rank the four given structures** (a-d) by the electron-donating/withdrawing character of their substituents.

[IMAGE: structures a-d — not extracted]

**Hence, based on this stability analysis, the order is b > a > d > c — the answer is option B.**