Effect of manganese(II) ions on the oxidation of malic and oxaloethanoic acids by aqueous HCrO4-

The catalytic effect of Mn-II ions on the pseudo-first-order rate constants (k(obs)) for chromic acid oxidations of malic and oxaloethanoic acids (an oxidation product of malic acid) has been studied spectrophotometrically at 25 degreesC. The rates show a first-order dependence on the Cr-VI concentr ation for each reductant. The order with respect to [malic acid] was found to lie between 1 and 2, and 1 for [oxaloethanoic acid]. The rate increased markedly with increasing [Mn-II] in both the cases. The catalytic effects o f Mn-II have been ascribed to a one-step three-electron process in which a termolecular complex is formed between the reductant, Mn-II and HCrO4-. The intermediate Cr-IV is ruled out; details of such a process are discussed. Mechanisms in accordance with the experimental data are proposed for the re actions.