Which one of the following compounds will undergo unimolecular nucleophilic substitution at the fastest rate?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (C6H5)3CCl
An SN1 reaction goes through a carbocation, and its slow step is the formation of that carbocation. The more stable the carbocation, the faster the reaction. Benzylic carbocations are stabilised by resonance with the benzene ring, and each extra phenyl group spreads the positive charge further:
\( \ce{(C6H5)3C^{+}} \) > \( \ce{(C6H5)2CH^{+}} \) > \( \ce{C6H5CH2^{+}} \)
The triphenylmethyl cation, with the charge delocalised over three rings, is the most stable, so (C6H5)3CCl reacts fastest.
Why the other options are incorrect
Option (1) C6H5Cl — Incorrect. An aryl halide; the C–Cl bond has partial double bond character and the phenyl cation would be very unstable, so it does not undergo SN1 at all.
Option (2) C6H5CH2Cl — Incorrect. Its benzyl cation is stabilised by only one ring.
Option (3) (C6H5)2CHCl — Incorrect. Two rings stabilise the cation, but fewer than in the triphenylmethyl cation.
NCERT (2026-27 edition):
Reactions of Haloalkanes – “… Further, greater the stability of carbocation, greater will be its ease of formation from alkyl halide and faster will be the rate of reaction. In case of alkyl halides, 3° alkyl halides undergo SN1 reaction very fast because of the high stability of 3° carbocations. … For the same reasons, allylic and benzylic halides show high reactivity towards the SN1 reaction. The carbocation thus formed gets stabilised through resonance (Unit 8, Class XI).”