JEE Main · 2023 · 11 Apr 2023 · Shift 2 · Haloalkanes and Haloarenes

Protonation of the terminal double bond of $\mathrm{C}{\mathrm{H}}_2=\mathrm{CH}-\mathrm{CHCl}-\mathrm{C}{\mathrm{H}}_3$ places the charge on the secondary carbon, giving $\mathrm{C}{\mathrm{H}}_3-\stackrel{+}{\mathrm{C}}\mathrm{H}-\mathrm{CHCl}-\mathrm{C}{\mathrm{H}}_3$. Capture of chloride here yields $2,3$-dichlorobutane, which has two equivalent stereocentres and so exists as a $d$, an $l$ and a meso form, that is three stereoisomers. The same cation can undergo a hydride shift onto the chlorine-bearing carbon, where the charge is stabilised by a chlorine lone pair; chloride capture there gives $2,2$-dichlorobutane as one further product. Altogether the number of isomeric products is four.