Kinetics and Mechanisms of Aqueous Chlorine Reactions with Chlorite Ion in the Presence of Chloride Ion and Acetic Acid/Acetate Buffer

The kinetics and mechanism of the reaction between Cl₂ and ClO₂^- are studied in acetate buffer by stopped-flow spectrometric observation of ClO₂ formation. The reaction is first-order in [Cl₂] and [ClO₂^-], with a rate constant of k₁ = (5.7 ± 0.2) × 10⁵ M^-1 s^-1 at 25.0 °C. Nucleophilic attack by ClO₂^- on Cl₂, with Cl⁺ transfer to form ClOClO and Cl^-, is proposed as the rate-determining step. A possible two-step electron-transfer mechanism for Cl₂ and ClO₂^- is refuted by the lack of ClO₂ suppression. The yield of ClO₂ is much less than 100%, due to the rapid reactions of the metastable ClOClO intermediate via two competing pathways. In one path, ClOClO reacts with ClO₂^- to form 2ClO₂ and Cl^-, while in the other path it hydrolyzes to give ClO₃^- and Cl^-. The observed rate constant also is affected by acetate-assisted hydrolysis of Cl₂. The rate of Cl₂ loss is suppressed as the concentration of Cl^- increases, due to the formation of Cl₃^-. In excess ClO₂^-, a much slower formation of ClO₂ is observed after the initial Cl₂ reaction, due to the presence of HOCl, which reacts with H⁺ and Cl^- to re-form steady-state levels of Cl₂.