Abstract
Abstract A kinetic study on the oxidation of phosphinic(hypophosphorous) and phenylphosphinic acids by trispolypyridyliron(III) complexes using excess of either phosphinic or phenylphosphinic acid has been made in perchloric acid. The pseudo first-order rate constant kobs, with respect to iron(III) complex, is independent of [H+]. The results indicated that phenylphosphinic acid is more reactive than phosphinic acid towards any given iron(III) complex. The formation of an intermediate complex is indicated by the kinetics and supported by the spectrophotometry of the reaction mixture. The differences in the observed thermodynamic parameters of this intermediate complex, probably an outer-sphere complex, and those predicted by the Fuoss equation are discussed. It is suggested that the ‘inactive’ form of either of the acid is reactive towards the iron(III) complexes. It is also shown that the rate of disproportionation of the intermediate complex is not affected by the form of phosphinic or phenylphosphinic acid entering into the complex formation. The values of β2 and k1, the equilibrium constant for the formation of the oxidant-substrate complex and the rate limiting constant for its disproportionation respectively, are reported together with respective thermodynamic and activation parameters.
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