Which one of the following compounds undergoes predominantly S_N2 rea…

The correct answer is Option (4) → 1-(chloromethyl)-4-nitrobenzene

All four compounds are primary benzylic chlorides, \( \ce{Ar-CH2Cl} \). A benzylic halide can react by S_N1 (through a benzylic carbocation stabilised by resonance) or by S_N2 (direct back-side attack). Which path dominates depends on how well the ring substituent stabilises the carbocation:

• An electron-donating group at the para position (–\( \ce{OCH3} \), –\( \ce{NH2} \)) delocalises the positive charge further and favours S_N1.

• An electron-withdrawing group (–\( \ce{NO2} \)) destabilises the carbocation, so S_N1 is suppressed; it also makes the \( \ce{CH2} \) carbon more positive and open to attack by \( \ce{OH^{-}} \). A polar aprotic solvent also favours S_N2.

So 4-nitrobenzyl chloride reacts predominantly by S_N2.

Why the other options are incorrect

• Option (1) 1-(chloromethyl)-4-methoxybenzene — Incorrect. –\( \ce{OCH3} \) (+R) strongly stabilises the benzylic cation, so this halide prefers S_N1.

• Option (2) 4-(chloromethyl)benzenamine — Incorrect. –\( \ce{NH2} \) is an even stronger +R donor; S_N1 is favoured.

• Option (3) 1-(chloromethyl)benzene — Incorrect. Benzyl chloride forms a resonance-stabilised cation and reacts readily by S_N1 as well; with no withdrawing group it is not the one that goes predominantly by S_N2.