Oxidations of hydrazine and substituted hydrazines by hexachloroiridate(IV) in aqueous solution: Kinetic and mechanistic analyses

Abstract

Oxidations of hydrazine (H2NNH2) and substituted hydrazines (t-C4H9HNNH2 and c-C6H11HNNH2) by [IrCl6]2− are found to obey overall second-order kinetics: −d[IrCl62−]/dt=k′[RHNNH2]tot[IrCl62−], where k′ pertains to the observed second-order rate constants and [RHNNH2]tot stands for the total concentration of hydrazines. The k′ versus pH profiles have been established over a pH range from 3 to 10. Rapid scan spectra recorded for the reaction courses indicate that there are no fast substitutions on [IrCl6]2− and/or strong associations between [IrCl6]2− and hydrazines during the reactions. The determined ratios of Δ[Ir(IV)]/Δ[RHNNH2]tot clearly endow a 4:1 stoichiometry for all the hydrazines studied. The oxidation product has been identified by NMR spectroscopy in the case of t-C4H9HNNH2. The proposed reaction mechanisms involve parallel attack of [IrCl6]2− on RHNNH3+ and RHNNH2, forming the rate-determining steps and producing hydrazyl free radicals; these are followed by several fast reactions. Rate equations have been derived, and the rate constants of the rate-determining steps at 25.0°C and 1.0M ionic strength have been calculated. It is demonstrated that our kinetic analysis is a good approach for determination of the pKa values of hydrazines. Monoalkyl substitutions on hydrazine enhance the reactivity of substituted hydrazines significantly, but the reactivity has no correlation with the pKa values. All the kinetic and spectral attributes bestow that an outer-sphere electron transfer takes place in the rate-determining steps. Moreover, all the kinetic and mechanistic discrepancies for the hydrazine-[IrCl6]2− reaction remained in literature have been clarified in the present work.