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 x 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 x 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 x 108 M -2). BrNO2 is detected spectrophotometrically where epsilon(260 nm) = 956 M-1 cm-1. BrNO2 reacts rapidly with NO2- (k = 6.9 x 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 -.