Observation of Transient Cyclohexadienones during the Aqueous Bromination of Phenols

Article abstract of DOI:10.1021/ja00288a030

Transient 4-bromo-2,5-cyclohexadienone intermediates have been observed during the aqueous bromination of phenol and several methylated derivatives.They enolize to the corresponding p-bromophenols by both acid-catalyzed and water-catalyzed pathways in the pH range 0-6.Studies carried out in buffers also indicate both general acid catalysis (α ca 0.0) and general base catalysis (β=0.54).The latter is ascribed to simple rate-limiting proton abstraction, but the former is not so easily rationalized.The very low α is attributed to a termolecular transition state (water, substrate, and general acid) in which the dienone becomes more anion-like than cation like.This seemingly anomalous behaviour can be explained by the very large enol/keto ratio (ca. 10¹¹) for phenol.