Kinetics and mechanism of picolinic acid promoted chromic acid oxidation of maleic acid in aqueous micellar media

Abstract

The kinetics and mechanism of chromic acid oxidation of maleic acid in the presence and absence of picolinic acid (PA) in aqueous acid media have been studied under the conditions [maleic acid]≫[CrVI]T at different temperatures. Under the kinetic conditions, the monomeric species of CrVI has been found kinetically active in the absence of PA while in the PA-catalysed path, CrVI-PA complex has been suggested as the active oxidant. In the PA-catalysed path, CrVI-PA complex receives a nucleophilic attack by the substrate to form a ternary complex, which subsequently experiences a 2e-transfer redox decomposition leading to the epoxide and CrIV-PA complex at the rate determining step. Subsequently, the epoxide produces pyruvic acid through the decarboxylation of β-keto acid in a faster step. Then CrIV-PA complex participates further in the oxidation of maleic acid in faster steps and ultimately is converted into the inert CrIII-PA complex. In the uncatalysed path, CrVI-substrate ester experiences a redox decomposition through 2e-transfer at the rate determining step in the same way. The uncatalysed path shows a mixed order (i.e. first order+second order) dependence on [H+] while the PA-catalysed path shows a first order dependence on [H+]. Both the uncatalysed path and PA-catalysed path show the first order dependence on both [maleic acid]T and [CrVI]T. The PA-catalysed path is first order in [PA]T. In the presence of surfactants like N-cetylpyridinium chloride (CPC, a cationic surfactant) and sodium dodecyl sulfate (SDS, an anionic surfactant), the reaction orders remain unchanged. CPC has been found to retard both the uncatalysed and PA-catalysed paths but the patterns are different while SDS shows the rate accelerating effect for the both paths in the same pattern. The observed micellar effects have been explained by considering hydrophobic and electrostatic interactions between the reactants and surfactants in terms of the proposed mechanism.