Allylic and benzylic halides show high reactivity towards the SN1 reaction. Which of the following is the reason?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → The carbocation formed is stabilised through resonance.
The rate of an SN1 reaction is set by the slow step in which the C–X bond breaks to give a carbocation. The more stable this carbocation, the faster it forms. In the allyl and benzyl carbocations the positive charge is spread by resonance over the double bond or the ring, so these cations are stable and form easily.
Option (1) The carbon atom bonded to the halogen is sp2 hybridised. — Incorrect. In allylic and benzylic halides the halogen is on an sp3 carbon next to the C=C or the ring. An sp2 C–X carbon is found in vinylic and aryl halides.
Option (2) The carbon-halogen bond acquires a partial double bond character. — Incorrect. This happens in haloarenes, by resonance of the halogen lone pairs with the ring, and it makes the bond harder to break, so it would lower the reactivity.
Option (3) The carbon atom bonded to the halogen is the least hindered towards attack by the nucleophile. — Incorrect. Low steric hindrance is the reason methyl halides react fastest by SN2, where the nucleophile must reach the carbon. SN1 depends on carbocation stability instead.
NCERT (2026-27 edition):
6.7.1 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.” … “For the same reasons, allylic and benzylic halides show high reactivity towards the SN1 reaction. The carbocation thus formed gets stabilised through resonance … as shown below:”
\( \displaystyle \ce{CH2=CH-\overset{+}{C}H2 <-> \overset{+}{C}H2-CH=CH2} \)