Kinetics and mechanism of the oxidation of glycolic and glyoxylic acids by thallium(III) in acidic perchlorate solutions

Abstract

Glycolic and glyoxylic acids react with Tl^III in 1 : 1 mol ratio, the products of oxidation depending on the initial concentration ratio of the reactants. Three complexes [Tl(ga)]²⁺, Tl(ga)·Hga²⁺, and [Tl(ga)₂]⁺ form from Tl^III and Hga, the first and third of these being thought to be reactive. The complete rate law determined from the intial rate of oxidation of glycolic acid in the concentration range 0.001—0.2 mol dm^–3 is complicated, but for large [Hga], limiting rates are obtained and rate law (i) is obeyed where k₂ and K₃ are the rate constant for the decomposition and the stepwise formation –d[Tl^III]/dt=k₂K₃[Tl^III]/([H⁺]+K₃)(i) constant, respectively, of the complex [Tl(ga)₂]⁺ which were found to be (6.9 ± 0.6)× 10^–5 s^–1 and 1.1 ± 0.1 mol dm^–3 respectively at 70 °C. The rate law, again from the initial rates, for the oxidation of glyoxylic acid (Hgox) in the concentration range 0.002—0.06 mol dm^–3 is given by equation (ii) where –d[Tl^III]//_dt=kK_HK[Tl^III][Hgox]//_{[H⁺]+KH+KKH[Hgox]}(ii)K_H, K, and k are the hydrolysis constant of Tl³⁺ and the stability and rate constants for the complex [Tl(gox)]²⁺; K and k were found to be 625 ± 50 dm³ mol^–1 and (1.1 ± 0.1)× 10^–4 s^–1 respectively at 70 °C. The stability constants of the complexes have also been determined spectrophotometrically.