Kinetics of hydrogen isotope exchange reactions. Part XXVII. Catalysis of aromatic tritium exchange between benzyl alcohol and solvent water by potassium tetrachloroplatinate(II)

Abstract

Tritium uptake into the aromatic positions of benzyl alcohol from tritiated solvent water (containing some acid and chloride ions, at 50°) is catalysed by tetrachloroplatinate(II). The reaction shows a somewhat erratic induction period [due to platinum (IV) impurity] which is greatly increased by addition of potassium hexachloroplatinate(IV). Almost no exchange occurs during the induction period, so that for several days the reaction appears to be totally suppressed by Pt^IV. The results are interpreted in terms of the rate-limiting formation of a common intermediate (an adduct between benzyl alcohol and PtCl₃^–) which is oxidised by Pt^IV more rapidly than it can proceed to exchange hydrogen with the solvent. The rate of the exchange reaction (after the induction period) is approximately proportional to the concentration of PtCl₄^2– and inversely proportional to the concentration of chloride ion. The dependence of the exchange rate on hydrogen-ion concentration is consistent with a protolytic equilibrium involving the alcohol group in a benzyl alcohol–PtCl₃^– complex (pK 3·6), with the basic form being more reactive than its conjugate acid.