JEE Main · 2021 · 26 Aug 2021 · Shift 1 · Hydrocarbons

Phenol is strongly activated towards electrophilic substitution by the lone pair of its hydroxyl group, and the solvent decides how far the bromination goes. In water the phenol is partly ionised to phenoxide, which releases electron density into the ring still more effectively, so excess bromine substitutes at both ortho positions and at the para position, and $\mathbf{A}$ is 2,4,6-tribromophenol, which separates as a white solid. Carbon disulphide is non-polar and at low temperature no phenoxide is formed, so the ring is less activated and bromination stops after one substitution, taking the less hindered para position; $\mathbf{B}$ is 4-bromophenol.