The ortho : para ratio in the bromination of phenol. Evidence for a co-ordination effect

Abstract

Bromination of phenol with bromine has been studied in various solvents in the presence of an epoxide as a hydrobromic acid scavenger. From the observed dependence of the orientation of products upon the nature of the electrophile, solvent, phenol, epoxide, and hydrobromic acid concentrations, a mechanism is proposed wherein at very low hydrobromic acid concentrations and in solvents with a small tendency to form hydrogen bonds with phenol, molecular bromine makes a kinetically controlled attack upon phenol to give an o-quinonoid intermediate rather than a p-quinonoid one. At higher concentrations of acid the o-quinonoid intermediate is rapidly isomerized to the more stable p-quinonoid form. This mechanism is confirmed by the similar behaviour of 2,4,6-trimethylphenol. In solvents which can form hydrogen bonds with phenol and hence probably inhibit sterically ortho-attack by the electrophile, the p-quinonoid intermediate is the predominant product. The same is true if bromination is carried out at higher concentrations of phenol and epoxide. Bromination in the presence of epoxide represents a useful and simple method of preparing o- or p-bromophenols.