Kinetics and Mechanisms of the Reactions of Hypochlorous Acid, Chlorine, and Chlorine Monoxide with Bromite Ion

The reaction between BrO2- and excess HOCl (p[H+] 6-7, 25.0 °C) proceeds through several pathways. The primary path is a multistep oxidation of HOCl by BrO2- to form ClO3- and HOBr (85% of the initial 0.15 mM BrO2-). Another pathway produces ClO2 and HOBr (8%), and a third pathway produces BrO3- and Cl- (7%). With excess HOCl concentrations, Cl2O also is a reactive species. In the proposed mechanism, HOCl and Cl2O react with BrO2- to form steady-state species, HOClOBrO- and ClOClOBrO-. Acid facilitates the conversion of HOClOBrO- and ClOClOBrO- to HOBrOClO-. These reactions require a chainlike connectivity of the intermediates with alternating halogen-oxygen bonding (i.e. HOBrOClO-) as opposed to Y-shaped intermediates with a direct halogen-halogen bond (i.e. HOBrCl(O)O-). The HOBrOClO- species dissociates into HOBr and ClO2- or reacts with general acids to form BrOClO. The distribution of products suggests that BrOClO exists as a BrOClO·HOCl adduct in the presence of excess HOCl. The primary products, ClO3- and HOBr, are formed from the hydrolysis of BrOClO·HOCl. A minor hydrolysis path for BrOClO·HOCl gives BrO3- and Cl-. An induction period in the formation of ClO2 is observed due to the buildup of ClO2-, which reacts with BrOClO·HOCl to give 2 ClO2 and Br-. Second-order rate constants for the reactions of HOCl and Cl2O with BrO2- are k1HOCl = 1.6 × 10^2 M^-1 s^-1 and k1Cl2O = 1.8 × 10^5 M^-1 s^-1. When Cl- is added in large excess, a Cl2 pathway exists in competition with the HOCl and Cl2O pathways for the loss of BrO2-. The proposed Cl2 pathway proceeds by Cl+ transfer to form a steady-state ClOBrO species with a rate constant of k1Cl2 = 8.7 × 10^5 M^-1 s^-1.