JEE Main · 2020 · 08 Jan 2020 · Shift 1 · Haloalkanes and Haloarenes

An ${\mathrm{E}}_1$ elimination is governed by how readily the substrate forms a carbocation in its rate-determining step, and carbocation stability falls in the order allylic (resonance-stabilized) $>$ tertiary $>$ secondary $>$ primary. The substrate giving the most stable allylic cation reacts fastest, the next a resonance-stabilized allylic cation, the third only a secondary cation and the last a primary one, so the order of reactivity is $\mathrm{D}>\mathrm{B}>\mathrm{C}>\mathrm{A}$.