Picolinic Acid Assisted Three-Electron Transfer Chromic Acid Oxidation of dl -Mandelic Acid in Aqueous Micellar Media: A Kinetic Study

The three-electron transfer chromic acid oxidations of dl-mandelic acid (MA) in the presence and absence of picolinic acid (PA) have been studied in aqueous micellar media under the kinetic conditions, [dl-mandelic acid] ≫ [PA]_T ≫ [Cr^VI]_T at different temperatures. Under these kinetic conditions, HCrO₄^- has been found to be kinetically active in the absence of picolinic acid while in the PA-catalysed path, a Cr^VI—PA complex has been established as the active oxidant. In the PA-catalysed path, the Cr^VI—PA complex experiences nucleophilic attack by the substrate to form a ternary complex which subsequently undergoes a redox decomposition involving 3e-transfer, leading to oxidative decarboxylation through C—C bond cleavage. This rate-determining step produces CO₂, benzoyl radical and the corresponding Cr^III—PA complex. Benzoyl radical is subsequently oxidized to benzoic acid in later, faster steps. In the uncatalysed path, the Cr^VI—substrate ester also undergoes an acid-catalysed redox decomposition through 3e-transfer as the rate-determining step. Both the catalysed and uncatalysed paths show a first order dependence on [H⁺] and both paths also show a first order dependence on [dl-mandelic acid]_T and [Cr^VI]_T. The PA-catalysed path is first order in [PA]_T. These observations remain unaltered in the presence of externally added surfactants. The effects of a cationic surfactant such as N-cetylpyridinium chloride (CPC) and an anionic surfactant such as sodium dodecyl sulfate (SDS) on both the uncatalysed and PA-catalysed paths have been studied. CPC has been found to accelerate the uncatalysed path and to retard the PA-catalysed path. SDS shows a rate-accelerating effect for both paths in different ways.