JEE Main · 2021 · 31 Aug 2021 · Shift 1 · Haloalkanes and Haloarenes

Reaction with acetate in acetic acid proceeds by ionisation of the carbon-chlorine bond ($S_{N}1$ acetolysis), so the rate tracks the stability of the carbocation formed. The allylic chloride whose double bond also carries a methyl group ionises to a tertiary allylic cation and reacts fastest; the two ring-substituted allylic chlorides give secondary allylic cations of intermediate stability; and the exocyclic $-\mathrm{C}{\mathrm{H}}_2\mathrm{Cl}$ gives a primary, non-allylic cation and reacts slowest. Reactivity falls from the methyl-substituted allylic chloride, through the two secondary allylic chlorides, to the primary chloromethyl compound.