Kinetics and mechanism of oxidations by metal ions. Part 16. Oxidation of 4-oxopentanoic acid by aquomanganese(III) ions

Summary The outer-sphere oxidation of 4-oxopentanoic acid (4-OPA) to MeCO₂H by aquomanganese(III) ions exhibits a first-order dependence on 4-OPA] and Mn(III)](aq). The observed pseudo first-order rate constant k _obs (4-OPA] Mn^III](aq)) is independent of Mn^II] but decreases with increasing H⁺]. The retarding effect of H⁺] on the observed rate could be explained by considering either the reaction between MnOH⁺(aq) and MeCOCH₂— CH₂CO₂H or between Mn³⁺(aq) and MeCOCH₂CH₂-COO^– ions. The rate constant for the latter pair of reactants is much higher than the rate constant for the reaction between the first pair. Since the activation enthalpy for the first pair is about 14 kJ mol^–1 less than that of the second pair, it is concluded that the reactive species are MnOH²⁺(aq) and MeCOCH₂CH₂CO₂H or that the preferred oxidant is MnOH²⁺(aq) ion.