The kinetics and mechanism of the oxidation of pyruvate ion by hypochlorous acid†
The oxidation of pyruvic acid by hypochlorous acid was studied in the pH 1.0–11.0 range. The main path in the redox process is the reaction between the pyruvate ion and HOCl. It was shown that the reaction is second order in both reactants and kHOCl = 2.17 ± 0.04 M−1 s−1 (I = 1.0 M (NaClO4), T = 25.0 °C) with ΔH≠HOCl = 34.6 ± 1.2 kJ mol−1 and ΔS≠HOCl = −120.3 ± 3.6 J mol−1 K−1 for this reaction step. The relatively large negative entropy of activation is consistent with an O atom-transfer mechanism. DFT calculations support this conclusion by predicting a concerted rearrangement of the activated complex which includes the reactants and the solvent water molecule. It was also confirmed that the hydration of pyruvic acid has a significant contribution to the observed kinetic features under acidic conditions. The hydration reaction is a proton catalyzed process and the pseudo-first-order rate constant can be interpreted by considering the protolytic and hydration equilibria of pyruvic acid. The rate constants were determined for the non-catalytic and the proton catalyzed paths: kh0 = 0.24 ± 0.01 s−1 and kh1 = 5.5 ± 0.2 M−1 s−1.