A mechanistic study of the ethylenediaminetetraacetic acid-, 2,2′-bipyridyl-, and manganese(II)-assisted one-step two- and three-electron oxidation of lactic acid by chromium(VI)

Abstract

The kinetic results of the oxidation of lactic acid by CrVI in the absence and presence of ethylenediaminetetraacetic acid (EDTA), 2,2′-bipyridyl (bpy), MnII and CeIV are presented. EDTA, bpy and MnII catalyse the reaction, whereas CeIV acts as an inhibitor. Irrespective of the conditions, the order with respect to [lactic acid] was found to lie between one and two, and one in [CrVI]. In the EDTA, bpy and MnII-catalysed paths, CrVI–EDTA, CrVI–bpy and lactic acid–MnII complexes have respectively been suggested as the active oxidant and reductant. The kinetic and cerium(IV) effect studies are consistent with a one-step two-electron transfer mechanism in the absence/presence of EDTA and bpy where a chromate–ester mechanism experiences a redox decomposition (C—H cleavage) in the rate-determining step. On the other hand, the catalytic effect of MnII is described as a one-step three-electron redox decomposition (C—C cleavage) mechanism. Mechanisms in accordance with the experimental data have been proposed for the reactions. Activation parameters have also been evaluated and discussed.