Kinetics and mechanism of electron-transfer reactions of aqueous and co-ordinated thallium(III). Part IX. Stoicheiometry and kinetics of reduction of hexa-aquathallium(III) by hydroxylamine

Abstract

The Stoicheiometry of the reaction between Tl^III and NH₃OH⁺ depends on the relative concentrations of the reactants, and measurements under different conditions have indicated that the products may be formed by transfer of two, four, and six electrons in stages from hydroxylamine to produce N^I, HNO₂, or NO₃^–, respectively. In the presence of excess of NH₃OH⁺, the Stoicheiometry of the reaction is as in (i) with the corresponding rate law Tl^III+ NH₂OH⁺→ Tl^I+ N^I+ 3H⁺(i)–d[Tl^III]/dt=K_A[Tl^III][NH₂OH⁺]; a mechanism is proposed which relates the observed effect of acidity to the entity K_A=kK_h/([H⁺]+K_h), where k is the rate constant for reaction of [Tl(OH)]²⁺(aq) with NH₃OH⁺ and K_h is the acid-dissociation constant of Tl³⁺(aq). These parameters are 154 ± 8 l mol^–1 s^–1 and 0·078 ± 0·008 respectively at 25 °C and ionic strength 1M. The energy and entropy of activation for the rate-determining step were found to be 10·0 ± 1·0 kcal mol^–1 and –17 ± 4 cal K^–1 mol^–1 respectively; ΔH and ΔS associated with K_h were found to be 16·6 ± 3·3 kcal mol^–1 and 51 ± 14 cal K^–1 mol^–1 respectively. Addition of Tl^I, oxygen, and sulphate or nitrate ions has no observable effect on the rate, but chloride ions strongly inhibit the reaction, which is also inhibited by an increase in ionic strength. Comparison of the observed enthalpy of activation with those for other oxidations by Tl^III and the observation of chloride inhibition suggest that the reaction proceeds via an intermediate complex.