Kinetic and Mechanistic Aspects of the Chromic Acid Oxidation of D-Galactose in the Presence and Absence of Picolinic Acid Catalyst in Aqueous Micellar Acid Media

Abstract

The kinetics and mechanism of the chromic acid oxidation of D-galactose in the presence and absence of picolinic acid (PA) in aqueous acid media have been determined under the conditions, [D — galactose]T≥ [CrVI]Tat different temperatures. Under the kinetic conditions, HCrO4-has been found kinetically active in the absence of PA while in the PA catalysed path, a CrVI- PA complex has been established as the active oxidant. In the PA-catalysed path, the CrVI- PA complex receives a nucleophilic attack by the substrate to form a ternary complex which subsequently undergoes a redox decomposition through a two-electron transfer leading to a lactone (oxidised product) and a CrIV- PA complex. Then the CrIV-PA complex participates further in the oxidation of D-galactose and ultimately is converted into a CrIII- PA complex. In the uncatalysed path, a CrVI- substrate ester experiences an acid catalysed redox decomposition (2e-transfer) at the rate-determining step. The uncatalysed path shows a second-order dependence on [H+]. Under the experimental conditions, both paths show first-order dependences on [D-galactose]Tand [CrVI]T. The PA-catalysed path is first-order in [PA]T. These observations remain unaltered in the presence of externally added surfactants. The effect of surfactants like N-cetylpyridinium chloride (CPC, a cationic surfactant) and sodium dodecyl sulfate (SDS, an anionic surfactant), on both the uncatalysed and PA-catalysed paths have been studied. CPC retards both the uncatalysed and PA-catalysed path, while SDS accelerates the reactions. The observed micellar effects have been explained by considering the hydrophobic and electrostatic interactions between the reactants and surfactants.