Read the passage carefully and answer the Questions.

Haloalkanes undergo nucelophilic substitution reactions owing to the polarity of C-X bond. The nucleophile reacts with the haloalkane on the carbon possessing a partial positive charge holding the halogen atom. The halogen atom X is replaced by a nucleophile. Depending on the kinetics and mode of bond breaking, the mechanism can be either $S_N1$ or $S_N2$ reaction. The rate of $S_N1$ reaction is governed by the stability of carbocation and in $S_N2$ reaction, the rate of reaction is governed by steric factor. Chirality is the main factor in both $S_N1$ and $S_N2$. In $S_N1$ reaction, the chirality of alkyl halide is accompanied by racemization of the product, while in $S_N2$ reaction, the product is characterized by inversion of configuration. The structure of alkyl halide and the nature of the solvent also governs the mechanism of the substitution.

Which of the following compound will undergo $S_N1$ reaction readily?

$C_6H_5CH_2-X$

The correct answer is Option (4) → $C_6H_5CH_2-X$ **

For SN1 reactions, the rate depends on the stability of the carbocation formed after the halide leaves.

Let’s check carbocation stability:

1. CH₃–X → CH₃⁺ (methyl carbocation — very unstable)
2. (CH₃)₂CH–X → secondary carbocation (moderately stable)
3. CH₃CH₂–X → primary carbocation (unstable)
4. C₆H₅CH₂–X → benzyl carbocation (highly stabilized by resonance)

Most stable carbocation → fastest SN1 reaction

✅ C₆H₅CH₂–X (benzyl halide)