Kinetics and Mechanisms of Oxidations by Metal Ions. V. Oxidation of 4-Oxopentanoic Acid by the Aquavanadium(V) Ion

Abstract

Abstract The outer sphere oxidation of 4-oxopentanoic acid(4-OPA), studied at 50 °C by aquavanadium(V) ion, is H+-catalyzed reaction. The reaction has a first-order dependence on each of [Vv], [4-OPA], and [H+]. The H+ catalysis can not be ascribed to keto\ightleftarrowsenol equilibrium because of the knowledge that a γ-keto acid is the least enolized amongst keto acids. Hence V(OH)32+ (aq) ion is the active oxidant. The proposed mechanism, assumed to involve the initial decarboxylation, is supported by the spot test characterization of acetoin as the intermediate oxidation product. Acetoin is further oxidized to two moles of acetic acid which is the final oxidation product. The overall energy of activation(ΔH1=26±3 kJ mol−1) is lower than the normal value(84 kJ mol−1) and therefore the highly negative value of the overall entropy of activation(ΔS1=−268±8 J K−1 mol−1) is considered to be responsible for the observed slowrate of oxidation.