JEE Main · 2010 · 11 Apr 2010 · Offline · Haloalkanes and Haloarenes

${\mathrm{S}}_{\mathrm{N}}1$ rates follow the stability of the carbocation left when bromide departs. The straight-chain bromide gives a primary cation, the least stable; the allylic bromide gives a resonance-stabilised allylic cation, the most stable; and the branched bromide gives a secondary cation of intermediate stability. Ranking the cations allylic above secondary above primary gives the reactivity order $\mathrm{B}>\mathrm{C}>\mathrm{A}$ among the three structures.