The correct order for the reactivity of following halide towards \(S_N1\) reaction is:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → III > IV > II > I
An SN1 reaction goes through a carbocation; its rate depends on (1) the stability of the carbocation formed and (2) how easily the halide ion leaves.
III 1-Bromo-1-methylcyclohexane — a tertiary halide; it gives a tertiary carbocation stabilised by three alkyl groups (+I and hyperconjugation). Fastest.
IV Iodocyclohexane, II Bromocyclohexane, I Chlorocyclohexane — all secondary, forming the same cyclohexyl cation. Among them the leaving-group ability decides: the C–I bond is the weakest and \( \ce{I^{-}} \) the best leaving group, so R–I > R–Br > R–Cl.
NCERT (2026-27 edition):
Substitution Nucleophilic Unimolecular (SN1) – “… 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.”