Question

Which among the following halide/s will not show S$_N$1 reaction:

Choose the most appropriate answer from the options given below:

• A. (A), (B) and (D) only
• B. (A) and (B) only
• C. (B) and (C) only
• D. (B) only

Answer 1

Answer: (D)

(B) only

Answer 2

The $S_N1$ reaction mechanism proceeds through the formation of a carbocation intermediate. The stability of the carbocation plays a critical role in determining whether the reaction will proceed.

- Halide (A): $\text{H}_2\text{C} = \text{CH} - \text{CH}_2\text{Cl}$ forms an allylic carbocation upon ionization, which is stabilized by resonance. Therefore, it is likely to undergo an $S_N1$ reaction.

- Halide (B): $\text{CH}_3 - \text{CH} = \text{CH} - \text{Cl}$ forms a carbocation that is not stabilized by resonance or inductive effects. This makes it unlikely to undergo an $S_N1$ reaction, as the carbocation formed would be highly unstable.

- Halide (C): This compound forms a benzylic carbocation upon ionization, which is highly stabilized due to resonance with the aromatic ring, making it suitable for an $S_N1$ reaction.

- Halide (D): $\text{H}_3\text{C} - \text{C(Cl)H}_2$ forms a tertiary carbocation, which is stable and favorable for the $S_N1$ reaction due to hyperconjugation and inductive effects.

Therefore, the only halide that will not show an $S_N1$ reaction is (B).

The Correct answer is: (B) only