The correct order of reactivity of the following halides towards \(S_N1\) reaction is :
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (IV) > (III) > (II) >(I)
An SN1 reaction goes through a carbocation, so its rate is set by two things: how stable the carbocation is and how easily the C–X bond breaks.
(I) Chlorobenzene — an aryl halide. The C–Cl bond has partial double bond character (resonance of the lone pair of Cl with the ring) and the phenyl cation that would form is not stabilised by resonance. So it does not react by SN1 at all: the least reactive.
(II), (III), (IV) Benzyl halides — all three give the same benzyl carbocation, \( \ce{C6H5CH2^{+}} \), which is stabilised by resonance with the ring. Among them the rate is decided by the leaving group. The C–X bond becomes weaker from Cl to I, so the order is C–I > C–Br > C–Cl.
\( \ce{C6H5CH2-X -> C6H5CH2+ + X-} \) (slow step)
Shortcut: Options (1) and (3) both start with (IV) and end with (I). C–Br breaks more easily than C–Cl, so (III) comes before (II): Option (1).
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. … For the same reasons, allylic and benzylic halides show high reactivity towards the SN1 reaction. … For a given alkyl group, the reactivity of the halide, R-X, follows the same order in both the mechanisms R–I > R–Br > R–Cl >> R–F.”