Abstract
A kinetic study of the oxidative decarboxylation of α-hydroxy acids by bromine water shows that above pH 6 Br2 is a much more significant oxidant for these compounds than is HOBr, and that OBr− is virtually inert. The rate law – d[oxidant]/dt = k[oxidant][substrate] is observed and the oxidation pattern differs from that of alcohols in that the α-hydrogen is not involved in the reaction.All the compounds studied (benzilic, mandelic, and 2-hydroxy-2-methylpropanoic acids) exhibit bell-shaped pH-rate profiles with maxima at about pH 8. A mechanism is proposed involving the attack of Br2 (or HOBr) at the carboxylate site concurrently with base-catalyzed loss of the hydroxylic proton. A steady state treatment based on this mechanism yields calculated rates in good agreement with the observed values. Furthermore, the same parameters predict the rates observed at lower pH in the presence of added excess bromide. The Br2 oxidation rate is greater than the HOBr rate by a factor of 60 for benzilic acid, 37 for 2-hydroxy-2-methylpropanoic acid, and 10 for mandelic acid.
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