Which one of the following is most reactive towards substitution nucleophilic unimolecular \((S_N1)\) reactions?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(C_6H_5C(CH_3)(C_6H_5)Br\)
The rate of an SN1 reaction is decided by the slow first step, the ionisation of the C–Br bond to a carbocation. The more stable the carbocation, the faster the reaction. Benzylic carbocations are stabilised by resonance with the ring, and each extra phenyl or alkyl group on the positive carbon stabilises it further.
\( \ce{C6H5C(CH3)(C6H5)^{+}} \) — tertiary, stabilised by two phenyl rings and a methyl group: most stable.
\( \ce{C6H5CH(C6H5)^{+}} \) — secondary, two phenyl rings.
\( \ce{C6H5CH(CH3)^{+}} \) — secondary, one phenyl ring.
\( \ce{C6H5CH2^{+}} \) — primary, one phenyl ring: least stable.
Order of SN1 reactivity: \( \ce{C6H5C(CH3)(C6H5)Br} \) > \( \ce{C6H5CH(C6H5)Br} \) > \( \ce{C6H5CH(CH3)Br} \) > \( \ce{C6H5CH2Br} \), so Option (3) is the most reactive.
Why the other options are incorrect
Option (1) C6H5CH(C6H5)Br — Incorrect. Gives a secondary carbocation stabilised by two rings, but it lacks the extra methyl group of a tertiary centre, so it is second.
Option (2) C6H5CH(Br)CH3 — Incorrect. A secondary benzylic carbocation with only one ring; less stable than both diphenyl cations.
Option (4) C6H5CH2Br — Incorrect. Gives the primary benzyl cation, the least stable of the four, so it is the slowest in SN1.
NCERT (2026-27 edition):
Reactions of Haloalkanes – “… Predict the order of reactivity of the following compounds in SN1 and SN2 reactions: … (ii) C6H5CH2Br, C6H5CH(C6H5)Br, C6H5CH(CH3)Br, C6H5C(CH3)(C6H5)Br
Solution … (ii) C6H5C(CH3)(C6H5)Br > C6H5CH(C6H5)Br > C6H5CH(CH3)Br > C6H5CH2Br (SN1) …”