The most reactive halide towards S_N1 reaction is

The correct answer is Option (1) → (C₆H₅)₂CHCl (diphenylmethyl chloride)

An S_N1 reaction goes through a carbocation; the more stable the carbocation, the faster the reaction. Benzylic and allylic halides are very reactive because their carbocations are stabilised by resonance. In diphenylmethyl chloride the positive charge of \( \ce{(C6H5)2CH^{+}} \) is spread over two benzene rings, so this carbocation is the most stable of the four and the halide is the most reactive.

Why the other options are incorrect

• Option (2) C₆H₅CH₂Cl — Incorrect. The benzyl cation is resonance stabilised by one ring only, so it is less stable than the diphenylmethyl cation.

• Option (3) C₆H₁₁CH₂Cl — Incorrect. The cyclohexyl ring is saturated, so the primary carbocation gets no resonance stabilisation; this is the least reactive.

• Option (4) CH₂=CH–CH₂–Cl — Incorrect. The allyl cation is stabilised by one C=C bond only, less than by two benzene rings.

NCERT (2026-27 edition):
Substitution Nucleophilic Unimolecular (S_N1) – “… 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 S_N1 reaction very fast because of the high stability of 3° carbocations. … For the same reasons, allylic and benzylic halides show high reactivity towards the S_N1 reaction.”