Kinetics and mechanism of the oxidation of phosphinic, phenylphosphinic, and phosphorous acids by pyridinium fluorotrioxochromate(VI)

Abstract

Oxidation of the lower phosphorus oxyacids by [C₅H₅NH][CrO₃F] results in the formation of corresponding oxyacids in the higher valence states. The reaction is first order with respect to the oxidant concentration. A Michaelis–Menten type kinetics was observed with respect to the substrate, indicating the formation of a complex in a pre-equilibrium. The formation constants and the rates of disproportionation of the complexes have been evaluated at different temperatures. The reaction exhibits a substantial primary kinetic isotope effect. The rates in 19 different organic solvents have been analysed using Kamlet-Taft and Swain equations. It has been found that the cation-solvating power of the solvents plays an important role. It is proposed that the ‘inactive’ tautomer of the phosphorus oxyacids is the reactive reductant, and that transfer of a hydride ion from the P–H bond to the oxidant in the rate-determining step occurs.