Kinetics and mechanisms of nonmetal redox reactions of oxyhalogens

The mechanisms of several oxidation/reduction reactions of oxyhalogen species are presented. Bromine chloride (BrCl) catalyzes the decomposition of hypochlorous acid/hypobromous acid (HOCl/HOBr) mixtures. BrCl reacts with hypochlorite ion (OCl−) to form BrOCl, which hydrolyzes to chlorite (ClO2−) and bromide (Br −) ions. Bromite ion (BrO2−) is formed via HOBr disproportionation. Rapid reactions of HOCl/BrO2 − and HOBr/ClO2− produce bromate (BrO3−) and chlorate (ClO3 −) ions, respectively. The study of the HOCl/HOBr decomposition is enabled by the ion chromatography (IC). The preparation of HOCl/HOBr reaction mixtures for IC requires removal of HOCl and HOBr from the samples. Three dehalogenating species, phenol, 4-hydroxybenzoic acid, and sulfite ion, enable ion chromatographic analysis. The reduction of BrO2− by sulfite ion (k = 3.0 × 107 M−1 s−1) occurs through the OBr + transfer reaction to sulfite ion, with subsequent general-acid catalyzed hydrolysis of OBrSO3−. Conversely, ClO 2− does not react with sulfite, and the reaction is not general-acid catalyzed. Chlorite and S(IV) react via an oxygen-atom transfer where k(ClO2−/SO 2) = 6.26 × 106 M−1 s −1 and k(ClO2−/SO 3H−) = 5.5 M−1 s−1 . The reaction of HOBr with nitrite ion (NO2− ) proceeds by a bromine-atom transfer reaction. HOBr, NO2 −, and H+ are in equilibrium with nitryl bromide, BrNO2 (K = 2.73 × 108 M −2). BrNO2 is detected spectrophotometrically where ϵ(260 nm) = 956 M−1 cm−1. BrNO2 reacts rapidly with NO2− (k = 6.9 × 103 M−1 s−1) to form Br− and N2O 4 and can dissociate to form NO2+ and Br − (k = 18 s−1). Rapid reactions of N2O4 and NO2+ with water produce NO3− as a final product. Nitrite levels above 100 mM cause a suppression of the observed rate constant. This suggests that N2O4 hydrolysis occurs by reversible heterolytic dissociation into NO2+ and NO2 −