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?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \(C_6H_5CH_2-X\)
An \(S_N1\) reaction goes through a carbocation, and its slow step is the ionisation of the C–X bond. So the halide that gives the most stable carbocation reacts fastest. Alkyl groups stabilise a carbocation by their electron-releasing inductive effect and by hyperconjugation, which gives the order 3° > 2° > 1° > methyl. A carbocation next to a benzene ring (benzylic) or next to a C=C bond (allylic) is stabilised much more, by resonance.
Benzyl halide ionises to the benzyl carbocation:
\( \displaystyle \ce{C6H5CH2-X -> C6H5CH2^+ + X^-} \)
The positive charge of the benzyl cation is delocalised over the ring by resonance, so it is more stable than the isopropyl (secondary) cation. In increasing order of \(S_N1\) reactivity:
\( \displaystyle \ce{CH3-X} \lt \ce{CH3CH2-X} \lt \ce{(CH3)2CH-X} \lt \ce{C6H5CH2-X} \)
So the benzylic halide, \( \ce{C6H5CH2-X} \), undergoes the \(S_N1\) reaction most readily.
Why the other options are incorrect
Option (1) \(CH_3-X\) — Incorrect. The methyl cation has no alkyl group to stabilise it, so methyl halides practically do not react by \(S_N1\); they are the fastest in \(S_N2\) instead.
Option (2) \((CH_3)_2CH- X\) — Incorrect. This secondary halide gives a cation stabilised by two methyl groups through the inductive effect and hyperconjugation only, which is weaker than the resonance in the benzyl cation.
Option (3) \(CH_3CH_2-X\) — Incorrect. This primary halide would give a cation with only one alkyl group; it is poorly stabilised, so ethyl halides react mainly by \(S_N2\).
NCERT (2026-27 edition):
Substitution Nucleophilic Unimolecular (SN1) – …
