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

Abstract

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≫ [CrVI]Tat 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 CrVI—PA complex has been established as the active oxidant. In the PA-catalysed path, the CrVI—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 CrIII—PA complex. Benzoyl radical is subsequently oxidized to benzoic acid in later, faster steps. In the uncatalysed path, the CrVI—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]Tand [CrVI]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.