Solvent effect on the ortho : para ratio in the bromination of phenols. Bromination with bromocyclohexadienones and N-bromosuccinimide

Bromination of phenol with N-bromosuccinimide and 2,4,4,6-tetrabromocyclohexa-2,5-dienone has been studied in various solvents. In contrast to bromination with molecular bromine, when these reagents are used the ortho : para ratio is greatly influenced by the solvent and by the concentration of hydrobromic acid concentration present in the reaction medium. The selectivity of these reagents is not due to their ability to furnish very low bromine concentrations, but to their properties as very efficient hydrobromic acid scavengers. The high ortho : para ratios observed during bromination in chlorinated solvents are better explained by an ionic mechanism rather than a radical one involving bromination of the phenoxyl radical.