JEE Main · 2021 · 27 Jul 2021 · Shift 2 · Alcohols Phenols and Ethers

The hydroxyl group lies allylic to the $\alpha$-methylene double bond, so protonation by concentrated HBr and loss of water generates an allylic carbocation in which the positive charge is shared between the methyl-bearing carbon and the terminal methylene carbon. Bromide captures the less hindered primary terminus, and the double bond settles between the two inner carbons, where it is conjugated with the ester carbonyl and carries one more alkyl substituent. The trans arrangement keeps the methyl group away from the ester, so the major product P is methyl $(E)$-2-(bromomethyl)but-2-enoate, $\mathrm{C}{\mathrm{H}}_3\mathrm{CH}=\mathrm{C}(\mathrm{C}{\mathrm{H}}_2\mathrm{Br})\mathrm{COOC}{\mathrm{H}}_3$.