Kinetics and mechanism of oxidation of hypophosphite by tetrabutylammonium tribromide

Abstract

The reaction between tetrabutylammonium tribromide and hypophosphite was studied in 50% (v/v) acetic acid under second order conditions. The overall order of reaction was found to be two, unity each in both the reactants. The reaction was found to involve direct two electron transfer step as the test for formation of free radicals due to single electron step was negative. The hypophosphorous acid (H3PO2) in aqueous solution dissociates to give H2PO2- and in the present study, as the acetic acid content increases, the rate of the reaction decreases. The plot of 1/ks against [H+] was found to be linear with an intercept. These observations suggest that the undissociated phosphorus acid is the active reductant species whose concentration decreases as the pH of the solution increases, thus resulting in a decrease of the rate constant. The activation parameters for the reaction were also determined by the studying the reaction at different temperature and it was found that there is considerable decrease in the entropy of the reaction. Such a decrease in the entropy is the explained by considering an ordered transition state between the reactants.