Abstract
The kinetics and stoichiometry of the oxidation of hydroxylamine and O-methylhydroxylamine by silver(II) have been determined in acid perchlo'rate media (1.03 M < [HCIOJ < 5.83 M) at 22°. The kinetics of the reaction with iV-methyl- hydroxylamine, but not the stoichiometry, has also been determined. In the presence of excess silver(II), stoichiometric measurements indicated that the product of reaction with NHsOH+ is N03-; with NH3OCH3+ the products are N03~ and C02, the latter directly detected in stoichiometric amounts by gas chromatographic analysis. No dependence of rate on [Ag(I)] or ionic strength was observed, within experimental error, for hydroxylamine; for O-methylhydroxylamine, the rate was inversely dependent on acidity and ionic strength. The acidity variation for both hydroxylamines has been ascribed to the difference in reactivity of Ag2 + and AgOH +, which coexist in rapid equilibrium at these hydrogen ion concentrations. For Kb. = [AgOH +] [H+]/[Ag2+], graphical analysis of the rate data at ionic strength 5.95 M yields Kb = (0.32 ± 0.14) M. With the primed rate constant designating reaction with AgOH+, the rate constants are: for NH3OH+, k < 104 5M~l sec”1 and k' = (1.4 ± 0.5) X 10s Af"1 sec"1; for NH3OCH3+, k < 103 M~l sec"1 and i' ^ (2 ± 1) X 104 Af"1 sec-1. For CH3NH2OH +, the apparent second-order rate constant, &app, determined by the stopped-flow technique is feapp = (1.6 ± 0.6) X 106 M~x sec-1. The mechanisms of these reactions are discussed in terms of free-radical intermediates. Comparison is made with analogous results for oxidation of hydroxylamines by Mn(III), for which the comparable rate constants are approximately one order of magnitude less (NH3OH+), and as much as three orders of magnitude less (NH3OCH3+). In this and other studies the stoichiometric consumption ratio may vary with the ratio of initial moles of reactants present. An explanation is provided for this behavior in terms of competitive intermediate steps.