The relative reactivity order of following halides towards \(S_N1\) reaction is:
(I) 2-Chloro-2-methylpropane
(II) Chloroethane
(III) Chloromethane
(IV) Chlorodiphenylmethane
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → IV > I > II > III
An SN1 reaction goes through a carbocation, formed in the slow first step. The more stable the carbocation, the faster the reaction:
(IV) Chlorodiphenylmethane, \( \ce{(C6H5)2CHCl} \) (first, most reactive) — it gives the benzhydryl cation \( \ce{(C6H5)2CH+} \), whose positive charge is spread by resonance over two benzene rings.
(I) 2-Chloro-2-methylpropane, \( \ce{(CH3)3CCl} \) (second) — it gives the tertiary cation \( \ce{(CH3)3C+} \), stabilised by three methyl groups (+I effect and hyperconjugation).
(II) Chloroethane, \( \ce{CH3CH2Cl} \) (third) — it would give the primary cation \( \ce{CH3CH2+} \), with only one methyl group: much less stable.
(III) Chloromethane, \( \ce{CH3Cl} \) (last, least reactive) — \( \ce{CH3+} \) has no alkyl group to stabilise it.
NCERT (2026-27 edition):
Substitution Nucleophilic Unimolecular (SN1) – “… Since the rate of reaction depends upon the slowest step, the rate of reaction depends only on the concentration of alkyl halide and not on the concentration of hydroxide ion. Further, greater the stability of carbocation, greater will be its ease of formation from alkyl halide and faster will be the rate of reaction.”