Micellar effect on the reaction of chromium(VI) oxidation of D‐fructose in the presence and absence of picolinic acid in aqueous media: a kinetic study

Abstract

AbstractThe kinetics and mechanism of the Cr(VI) oxidation of D‐fructose in the presence and absence of picolinic acid (PA) in aqueous acid media were studied under the conditions [D‐fructose]T ≫[Cr(VI)]T at different temperatures. Under the kinetic conditions, the monomeric species of Cr(VI) was found to be kinetically active in the absence of PA whereas in the PA‐catalysed path, the Cr(VI)–PA complex was considered to be the active oxidant. In this path, the Cr(VI)–PA complex undergoes a nucleophilic attack by the substrate to form a ternary complex which subsequently experiences a redox decomposition through glycol splitting leading to the lactone of C5‐aldonic acid along with formaldehyde and the Cr(IV)–PA complex. The primary product formaldehyde undergoes further oxidation (in part) to form formic acid. Then the Cr(IV)–PA complex participates further in the oxidation of D‐fructose and ultimately is converted into the inert Cr(III)–PA complex. In the uncatalysed path, the Cr(VI)–substrate ester experiences an acid‐catalysed redox decomposition (2e transfer) in the rate‐determining step giving rise to the products. The uncatalysed path shows a second‐order dependence on [H+] whereas the PA catalysed path shows a fractional order in [H+]. Both paths show a first‐order dependence on [D‐fructose]T and [Cr(VI)]T. The PA‐catalysed path is first order in [PA]T. All these patterns remain unaltered in the presence of externally added surfactants. The effects of a cationic surfactant, N‐cetylpyridinium chloride (CPC), and an anionic surfactant, sodium dodecyl sulfate (SDS), on both the uncatalysed and PA‐catalysed paths were studied. CPC inhibits both the uncatalysed and PA‐catalysed paths whereas SDS catalyses the reactions. The observed micellar effects are explained by considering a distribution pattern of the reactants between the micellar and aqueous phases. The applicability of different kinetic models, e.g. the pseudo‐phase ion‐exchange model, the Menger–Portnoy model and the Piszkiewicz cooperative model, was tested to explain the observed micellar effects. The effect of [surfactant]T on the activation parameters was explored to rationalize the micellar effect. Copyright © 2001 John Wiley & Sons, Ltd.