JEE Main · 2024 · 27 Jan 2024 · Shift 2 · Haloalkanes and Haloarenes

An $S_{N}1$ reaction requires the substrate to ionise into a reasonably stable carbocation. $H_{2}C=CH\text{-}C{H}_{2}Cl$ (allyl chloride) ionises to an allylic cation stabilised by resonance with the double bond, so it readily undergoes $S_{N}1$. $C{H}_3\text{-}CH=CH\text{-}Cl$ (a vinylic chloride) would have to ionise to a vinyl cation, $C{H}_3\text{-}CH=C{H}^{+}$, in which the positive carbon is sp-hybridised and cannot be stabilised by the adjacent double bond in the same way an allylic or benzylic cation can — vinylic cations are far too unstable to form under normal conditions, so this halide cannot undergo $S_{N}1$. The remaining substrates ionise to a benzylic-type and a secondary-type carbocation respectively, both stable enough for $S_{N}1$ to proceed. Only the vinylic chloride, (B), fails to show $S_{N}1$ behaviour.