The Bromination of p-Hydroxybenzoic Acid in Aqueous Solution. Reaction via the Minor p-Carboxyphenoxide Anion Tautomer

Article abstract of DOI:10.1021/jo00207a013

The kinetics of bromination of p-hydroxybenzoic acid have been measured in aqueous solution in the pH range 0-6.From the pH-log rate profile the reactive species appear to be the free acid (pH < 1.5), the monoanion (pH 1.5-5), and the dianion (pH > 5).However, the reactivity attributable to the monoanion is about 9 times higher than that predicted by a Hammett plot for the bromination of p-substituted phenols.This deviation is described to reaction via the tautomeric p-carboxyphenoxide monoanion which is present to the extent of 1 in 3100.Support for this proposal comes from related studies on ethyl p-hydroxybenzate, 3-bromo-4-hydroxybenzoic acid, and p-methoxybenzoic acid.In particular, the 3-bromo-4-hydroxybenzoate anion appears to be 5200 times more reactive than predicted and twice as reactive as the p-hydroxybenzoate anion.These anomalies are removed if reaction occurs via the minor tautomeric anion.