Photooxidation of 2-Hydroxy Acids by Copper(II) Species in Aqueous Solution

Ultraviolet irradiation of aqueous lactic, glycolic, or 2-hydroxybutanoic acid in the presence of copper(II) leads to photoredox reaction to give corresponding α-keto acid, aldehyde with evolution of carbon dioxide, and the precipitate of copper(I). The mole ratio of the carbonyl products to the consumption of copper(II) is close to 0.5. Oxidative decarboxylation dominates at pH>1 while the formation of α-keto acids favors at lower pH. The formation of α-keto acids is selectively suppressed by the addition of free radical scavengers. Free radical oxidations of the acids with bromine atom or benzoyl radicals give exclusively α-keto acids. The reaction mechanism has been discussed.