Micellar effect on the reaction of chromium(VI) oxidation of some representative <img src='/image/spc_char/alpha.gif'>–hydroxy acids in the presence and absence of 2,2´-bipyridyl in aqueous acid media: A kinetic study

Abstract

The kinetics and mechanism of chromic acid oxidation of <img src='/image/spc_char/alpha.gif'>-hydroxy acids in the presence and absence of 2,2´-bipyridyl in aqueous acid media have been studied at different temperatures by quenching technique under the conditions of [<img src='/image/spc_char/alpha.gif'>-hydroxy acid] >> [Cr(VI)]_T and [bpy]_T>>[Cr(VI)]_T. Under the kinetic conditions, the monomeric species of Cr(VI)<b style=""> </b>has been found to be kinetically active in the absence of bpy while<b style=""> </b>in the bpy-catalysed path, Cr(VI)<b style="">–</b>bpy complex is suggested as the active oxidant. In the bpy-catalysed path, Cr(VI)-bpy complex receives a nucleophilic attack by the substrate to form a ternary complex which subsequently experiences a 2e-transfer redox decomposition leading to Cr(IV)-bpy complex and keto acid. Then the Cr(IV)-bpy complex participates in the oxidation of <img src='/image/spc_char/alpha.gif'>-hydroxy acid in faster steps and ultimately is converted into the inert Cr(III)-bpy complex. In the uncatalysed path, Cr(VI)-substrate ester undergoes a redox decomposition through 2e-transfer at the rate determining step. The uncatalysed as well as the bpy-catalysed paths show first order dependence on both [<img src='/image/spc_char/alpha.gif'>-hydroxy acid]_T and [Cr(VI)]_T. The bpy-catalysed path is first order in [bpy]_T. In the presence of surfactants like <i>N</i>-cetylpyridinium chloride and sodium dodecyl sulfate, the reaction orders remain unchanged. The former has been found to retard both the uncatalysed and bpy-catalysed paths while the latter shows rate accelerating effect for both the paths. The observed micellar effects have been explained by considering hydrophobic and electrostatic interactions between the reactants and surfactants in terms of the proposed mechanism.