Observation of the Cyclohexadienone Intermediate in the Aqueous Bromination of Phenol

Article abstract of DOI:10.1021/jo00153a036

The unstable 4-bromo-2,5-cyclohexadienone intermediate involved in the aqueous bromination of phenol has been observed for the first time by stopped-flow UV spectrophotometry (λ_max ca. 240 nm, ε ca. 10000).In the pH range 0-6 its rearrangement to p-bromophenol occurs by acid-catalyzed and uncatalyzed pathways.The intermediate derived from 2,6-dimethylphenol behaves similarly but rearranges more slowly and so is more easily studied.Starting from p-cresol one can also observe the cyclohexadienone resulting from bromine attack ipso to the p-methyl group.It rearranges relatively slowly by a route that is acid catalyzed and bromide ion catalyzed.However, this route accounts for only about 10percent of the reaction; the major pathway presumably results from bromine attacking at an ortho position.