JEE Chemistry 2024 Question with Solution

Given: We must determine which halide will not show an $S_N1$ reaction.

Find: The correct option among the given choices.

The $S_N1$ reaction mechanism proceeds through formation of a carbocation intermediate, so the key factor is the stability of the carbocation formed after departure of the halide ion.

1. (A) $\mathrm{H_2C = CH - CH_2Cl}$ forms an allylic carbocation. This carbocation is resonance-stabilized, so it can undergo an $S_N1$ reaction.
2. (B) $\mathrm{CH_3 - CH = CH - Cl}$ would form a vinyl carbocation. Vinyl carbocations are highly unstable, so this compound will not undergo an $S_N1$ reaction.
3. (C) forms a benzylic carbocation, which is stabilized by resonance with the phenyl ring. Therefore, it can undergo an $S_N1$ reaction.
4. (D) forms a sufficiently stable carbocation and can undergo an $S_N1$ reaction.

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

Given: $S_N1$ reactions occur through carbocation formation.

Find: Which halide does not undergo $S_N1$ reaction.

Principle used: Greater carbocation stability increases the feasibility of the $S_N1$ mechanism.

For the listed compounds:

• Allylic carbocation from (A) is resonance stabilized.
• Vinylic carbocation from (B) is highly unstable because the positive charge would be on a vinylic carbon.
• Benzylic carbocation from (C) is resonance stabilized.
• Secondary/adequately stabilized carbocation from (D) can also form.

So only (B) fails to undergo $S_N1$ reaction.

Thus, the correct answer is D.