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

Concentrated sulphuric acid protonates the hydroxyl group of 1-phenylpropan-1-ol, and loss of water gives a benzylic carbocation stabilised by the ring. Removal of a hydrogen from the neighbouring $\mathrm{C}{\mathrm{H}}_2$ then places the double bond next to the ring, so the alkene is conjugated with it. Both geometries can arise, but in the trans isomer the phenyl and methyl groups lie on opposite sides of the double bond, so the ring remains coplanar with it and the crowding present in the cis isomer is avoided. The more stable trans alkene predominates, so compound A will be the major product.