The kinetics and mechanism of the oxidation of mandelic acid and related compounds with bromine and with hypobromous acid

Abstract

Hypobromous acid is much more effective than bromine in oxidising mandelic acid in weakly acidic solution to benzaldehyde and carbon dioxide. The carboxylate ion is oxidised ca. 9·5 times faster than the undissociated acid and the Hammett reaction constant obtained from rate measurements of six mandelic acids in the carboxylate ion form is –0·43. The reaction is catalysed by silver acetate but does not appear to be general base catalysed. For the oxidation of ethyl p-chloromandelate and β-hydroxy-β-p-chlorophenylpivalic acid, bromine is more effective than hypobromous acid; whereas the latter reagent is more effective in oxidising benzilic acid. Benzoylformic acid is oxidised by hypobromous acid in concurrent first- and second-order processes. The mechanisms of these reactions are discussed.