KINETICS AND MECHANISM OF THE OXIDATION OF HYDROXYLAMINE BY AQUEOUS BROMINE

The kinetics, stoichiometry, and mechanism of the reaction between hydroxylamine and bromine are determined from pH 0 to 6. Acid suppresses the rate of reaction because NH3OH+ is not reactive with Br-2. At low pH, nitrous acid is the first observable product and the stoichiometry of the initial reaction is 2:1 [Br-2](T):[NH2OH](T) (where [Br-2](T) = [Br-2] + [Br-3(-)] and [NH2OH](T) = [NH3OH+] + [NH2OH]). A slower subsequent reaction of NH3OH+ with HONO to produce N2O makes the overall stoichiometry 2:2. The proposed reaction sequence corresponds to the following changes in nitrogen oxidation states: N(-I) --> N(I) --> N(III) --> N(I). At pH 0-2 the reaction between [Br-2](T) and excess [NH2OH](T) is measured by stopped-flow spectrophotometry; the rate constant for Br-2 and NH2OH is 1.8 x 10(9) M(-1) s(-1) at 25.0 degrees C, mu = 0.50 M. The activation parameters at 25.0 degrees C are Delta H-double dagger = 15 kJ mol(-1) and Delta S-double dagger = -15 J mol(-1) K-1, but these values are temperature dependent because Delta C-p(double dagger) = -440 J mol(-1) K-1. A parallel Br-3(-) path has a rate constant of 2.4 x 10(8) M(-1) s(-1) at 25.0 degrees C. The first mechanistic step is postulated to be Br+ transfer to form an intermediate, BrNHOH. This intermediate reacts rapidly with a second equivalent of Br-2 to form Br2NOH. Rapid stepwise loss of Br- first gives BrNO and then HONO. At pH 5-6 the [NH2OH](T) reaction with [BT2](T) is much faster and is measured by the pulsed-accelerated-flow method. The initial step appears to consume all the Br-2, but BrNHOH disproportionates to form NH2OH and Br2NOH; further stepwise hydrolysis gives NO2- and Br-.