Kinetics and mechanism of the decomposition of pyridiniomethylthallium(III) species in aqueous solution

Abstract

The first-order rate coefficient of decomposition of the 4-pyridiniomethylthallium(III) ion in aqueous acidic solution decreases markedly on the addition of small amounts of chloride ion, but increases again on the addition of higher concentrations of chloride ion. The organic products of decomposition are the hydroxymethylpyridinium ion in the absence of chloride ion together with increasing amounts of chloromethylpyridinium ion as the chloride-ion concentration is increased. The inorganic products are thallium(I) species in all cases. The equilibrium constants for the complexing of the second and the third chloride ions to the pyridiniomethylthallium(III) ion have been determined and the kinetics of the decomposition have been analysed in terms of the individual reactions of the various pyridiniomethylthallium(III) species present in solution. The results are interpreted in terms of an S_N2 mechanism in which the thallium(I) species is displaced by attack of an appropriate nucleophile on the pyridiniomethyl group.