Micellar effect on the reaction of chromium(VI) oxidation of L-sorbose in the presence and absence of picolinic acid in aqueous acid media: A kinetic study

The kinetics and mechanism of chromic acid oxidation of L-sorbose in the presence and absence of picolinic acid (PA) have been studied under the conditions, [L-sorbose](T) much greater than [PA](T) much greater than [Cr(VI)](T), at different temperatures. In the absence of PA, the monomeric chromic acid undergoes esterification with the substrate followed by the acid catalysed redox decomposition of the Cr(VI)-substrate ester through glycol splitting to formaldehyde and the lactone Of C-5-aldonic acid and Cr(IV) which subsequently participates in the faster reactions. In the presence of PA, the Cr(VI)-PA complex produced in a pre-equilibrium step experiences a nucleophilic attack by the substrate to produce a ternary complex which decomposes through glycol splitting giving rise to the organic products and Cr(IV)-PA complex. Both the uncatalysed and PA-catalysed paths show the first-order dependence on [L-sorbose]T and [Cr(VI)](T). The PA-catalysed path is first-order in [PA](T) and it shows a fractional order in [H+]. The uncatalysed path shows a second-order dependence on [H+]. In the presence of the surfactants like N-cetylpyridinium chloride (CPC, a cationic surfactant) and sodium dodecyl sulfate (SDS, an anionic sulfate), the reaction orders remain unchanged. CPC has been found to inhibit both the uncatalysed and PA-catalysed paths while SDS shows the rate accelerating effect for both the uncatalysed and PA-catalysed paths. The observed micellar effects have been rationalised by considering the distribution of the reactants between the micellar and aqueous phases in terms of the proposed reaction mechanism.