JEE Main · 2023 · 30 Jan 2023 · Shift 2 · Haloalkanes and Haloarenes

An ${\mathrm{S}}_{\mathrm{N}}1$ reaction passes through a benzylic carbocation, so the fastest substrate is the one that gives the most stable cation. A para-methoxy group feeds its oxygen lone pair onto the benzylic centre, stabilising that cation best and making its chloride the most reactive. A para-nitro group does the reverse, draining density from the electron-poor centre and leaving its chloride the least reactive. Plain benzyl chloride carries no ring substituent to help or hinder, while a para-chloro group is mildly deactivating overall, so its cation is a shade less stable than the parent. The reactivity towards substitution therefore falls in the order $b>d>c>a$.