Kinetics and mechanism of the initial phase of the bromine-chlorite ion reaction in aqueous solution

The kinetics and mechanism of the chlorine(III)-bromine reaction are studie d by the stopped-flow method under acidic conditions in 1.0 M NaClO4 and at 25.0 degrees C. There are two kinetically well-separated phases in this re action. A detailed mechanism is proposed for the first phase of the reactio n, in which Br-2 oxidizes ClO2- to chlorine dioxide. It is confirmed that t he oxidation occurs via competing parallel reaction steps. The autoinhibiti on observed in the reaction is attributed to a backward shift in the revers ible initial step as the oxidation proceeds. On the basis of simultaneous e valuations of the kinetic traces, the following forward rate constants are obtained for the kinetically significant reaction steps: Br-2 + ClO2- rever sible arrow ClO2 + Br-2(-), k(1) = (1.3 +/- 0.2) x 10(3) M-1 s(-1) (k(-1) = 1.1 x 10(9) M-1 s(-1)); Br-2(-) + ClO2- = ClO2 + 2Br(-), k(2) = (4.0 +/- 0 .1) x 10(6) M-1 s(-1); Br + ClO2- = ClO2- + Br-, k(8) (2.3 +/- 0.7) x 10(8) M-1 s(-1); HOBr + HClO2 = BrClO2 + H2O (BrClO2 + ClO2- = Br- + 2ClO(2), ve ry fast), k(9) = (1.9 +/- 0.1) x 10(5) M-1 s(-1). The possible kinetic role of the reactive BrClO2 intermediate is discussed in detail.