JEE Main · 2023 · 10 Apr 2023 · Shift 1 · Haloalkanes and Haloarenes

In the first pair, aimed at bimolecular substitution, the primary isobutyl bromide is attacked far more readily than the fully substituted tertiary tert-butyl bromide, whose crowded carbon blocks backside attack, so the primary halide is the more reactive. In the second pair, aimed at unimolecular substitution, benzyl bromide ionises to a resonance-stabilised benzylic cation whereas its homologue can give only an ordinary primary cation, so benzyl bromide is the faster. In electrophilic aromatic substitution a ring deactivated by a single chlorine is still richer in electrons than one bearing both a chlorine and a strongly withdrawing nitro group, so chlorobenzene reacts faster than its nitro derivative. In nucleophilic aromatic substitution a nitro group placed para to the leaving group stabilises the negatively charged intermediate, so the nitro-substituted aryl halide reacts faster than the plain one. Taken in their correct sense, all four orders are right.