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

The Correct Option is D

To determine which halide will not show an S_N1 reaction, we must consider the stability of the carbocation formed during the reaction. The S_N1 mechanism involves two steps: ionization to form a carbocation, followed by nucleophilic attack. The stability of the carbocation is crucial in determining whether an S_N1 reaction will occur. 

Let's analyze each provided option:

1. (A) H₂C = CH - CH₂Cl: The carbocation formed would be an allylic carbocation, which is relatively stable due to resonance. Thus, this compound can undergo an S_N1 reaction.
2. (B) CH₃ - CH = CH - Cl: The carbocation formed here would be a vinyl carbocation. Vinyl carbocations are highly unstable as they involve a positive charge on an sp² hybridized carbon, which is not favorable. Hence, this compound will not undergo an S_N1 reaction.
3. (C) The compound has a benzyl chloride structure. Benzyl carbocations are stabilized by resonance with the phenyl group, allowing them to easily undergo an S_N1 reaction.
4. (D) The compound forms a secondary carbocation, which is reasonably stable. Such carbocations can undergo S_N1 reactions.

Based on the above analysis, option (B) is the only case where the S_N1 reaction is not feasible due to the instability of the vinyl carbocation. Hence, the correct answer is (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