Which of the following structurally distinct haloalkanes is overwhelmingly the most reactive towards — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

The rate of an $S_N1$ reaction is strictly dictated by the stability of the intermediate carbocation. Chloromethane ($1^\circ$) and isopropyl chloride ($2^\circ$) form highly unstable aliphatic carbocations. Both $p$ -nitrobenzyl and $p$ -methoxybenzyl chlorides form resonance-stabilized benzylic carbocations. However, the $-NO_2$ group is strongly electron-withdrawing, heavily destabilizing the carbocation. The $-OCH_3$ group is strongly electron-donating via resonance ($+R$ effect), massively stabilizing the benzylic carbocation and resulting in the fastest $S_N1$ rate.