Kinetics and mechanism of oxidation of hexoses by t-butyl hypochlorite in aqueous acetic acid medium

The kinetics of oxidation of D-glucose, D-galactose and D-mannose by t-butyl hypochlorite has been investigated in aqueous acetic acid at 308 K. The reactions show first order kinetics in [t-BuOCl], fractional order in [hexose] and zero order in [H+]. Variation in ionic strength of the medium and addition of reduced products, t-butyl alcohol (0-0.05 mol dm(-3)), have negligible effect on the rate, while lowering of dielectric constant of the medium by the addition of AcOH (20-60 %, v/v) decreases the rate. The rates decrease in the order as follows: D-galactose approximate to D-glucose > D-mannose. Competitive mechanism involving substrate independent and dependent pathways has been suggested to explain the observed results. Coefficients of the rate-limiting steps for both the pathways as function of temperature have been evaluated and the related activation parameters computed. The constancy of the rate coefficient for the substrate independent paths provides evidence for the proposed mechanism. The plots of Delta H* versus Delta S* are linear with isokinetic temperatures of 326 K and 300 K for the substrate independent and dependent pathways in the mechanism.