JEE Main · 2019 · 10 Apr 2019 · Shift 1 · Haloalkanes and Haloarenes

The rate of an $S_{N}1$ reaction is controlled by the stability of the benzylic carbocation that forms. A para-methoxy group feeds its oxygen lone pair by resonance straight onto the benzylic centre, giving the most stabilised cation and hence the fastest reaction, whereas a para-methyl group stabilises only weakly through hyperconjugation. A meta-methoxy group cannot donate by resonance to the cationic carbon and instead pulls electron density away inductively, so that compound reacts slowest. The rate therefore increases as meta-methoxy < unsubstituted < para-methyl < para-methoxy.