Which compound is least reactive towards nucleophilic substitution re…

The correct answer is Option (3) → Bromobenzene

Aryl halides such as bromobenzene are far less reactive towards nucleophilic substitution than alkyl halides. Besides the resonance effect, which gives the C–Br bond partial double bond character:

• the carbon carrying the halogen is sp² hybridised and holds the bond electrons more tightly than an sp³ carbon;

• the electron-rich ring repels the nucleophile;

• the phenyl cation is not stabilised by resonance, so an S_N1 route is not possible.

Why the other options are incorrect

• Option (1) Ethyl bromide — Incorrect. Ethyl bromide is a primary alkyl halide and reacts readily by S_N2.

• Option (2) Isopropyl bromide — Incorrect. Isopropyl bromide is a secondary alkyl halide; it reacts by S_N1 or S_N2.

• Option (4) Methyl bromide — Incorrect. Methyl bromide is the most reactive of the four in S_N2 reactions, because it is the least hindered.

NCERT (2026-27 edition):
Nucleophilic Substitution