Transsubstitution and equilibrium of phenols. Part III. Transbromination of phenols in the presence of aluminium phenoxide and other acidic catalysts

Intra- and inter-molecular bromine migrations in bromophenols occur in the presence of catalytic amounts of aluminium phenoxides or proton acids (such as sulfuric and trifluoromethanesulfonic acid) at temperatures between 80 and 180°C. Under these conditions only ortho and para positions are involved; no meta-bromophenols are formed. The equilibrium constants for the ortho-ortho transsubstitution (Kx = [2-bromo-4-X-phenol]2/[2,6-dibromo-4-X-phenol] [4-X-phenol = 15 ± 3) are independent of substituents in the para position. The equilibrium constants for the ortho-para isomerization are calculated (R2/4 = [2-bromophenol]/[4-bromophenol] = 1.9±0.3; R2,6/2,4 = [2,6-dibromophenol]/[2,4-dibromophenol] = 0.34± 0.04). Kinetic data - some of them suggesting second-order kinetics - are given. A mechanism is proposed.