Oxidation of d-glucose in the presence of 2,2′-bipyridine by Cr VI in aqueous micellar media: a kinetic study

Abstract

The kinetics of Cr VI oxidation of d-glucose to the corresponding lactone in the presence and absence of 2,2′-bipyridine (bipy) has been carried out under the conditions, [ d-glucose] T ≫ [Cr VI] T at different temperatures in aqueous micellar media. The monomeric Cr VI species has been found to be kinetically active in the absence of bipy whereas in the bipy-catalysed path, the Cr VI–bipy complex has been found to be the active oxidant. In the bipy-catalysed path, the Cr VI–bipy complex undergoes nucleophilic attack by the substrate to form a ternary complex. The ternary complex spontaneously experiences a redox decomposition (through two-electron transfer) in the rate-determining step leading to the product lactone and Cr IV–bipy complex. The Cr IV–bipy complex then takes part in faster steps in the further oxidation of d-glucose and is ultimately converted into a Cr III–bipy complex. In the uncatalysed path, the Cr VI-substrate ester experiences acid catalysed redox decomposition (two-electron transfer) in the rate-determining step. The uncatalysed path shows a second order dependence on [H +] and a first order dependence on each of the reactants [ d-glucose] T and [Cr VI] T. In contrast, the bipy-catalysed path shows a first order dependence on each of the reactants [H +], [ d-glucose] T and [Cr VI] T. The bipy-catalysed path is first order in [bipy] T. These observations remain unaltered in the presence of externally added surfactants. The effect of the cationic surfactant, N-cetylpyridinium chloride (CPC) and anionic surfactant, sodium dodecyl sulfate (SDS) on both the uncatalysed and bipy-catalysed path has been studied. CPC inhibits both the uncatalysed and bipy-catalysed path, while SDS catalyses these reactions. The observed micellar effects have been explained by considering hydrophobic and electrostatic interactions between the surfactants and reactants.