Kinetics and Mechanisms of the Oxidation of Hydroxylamine by Aqueous Iodine

Article abstract of DOI:10.1021/ic00128a021

First-order rate constants (kf, 25.0°C, μ = 0.50 M) for the loss of I3(1-)/I2 in the presence of excess NH3OH(1+)/NH2OH are measured from pH 2.0 to 6.8 with variation of the reactant concentrations as wellas the concentrations of I(1-) and various buffers. The kf values rangefrom 5 x 10**-4 to 2 x 10**4 s**-1 and depend on the NH2OH concentration. A multistep mechanism is proposed where I2 and NH2OH react rapidly toform an I2NH2OH adduct (KA = 480 M**-1) that undergoes general-base (B)assisted deprotonation to give INHOH + I(1-) + BH(1+). At higher pH, hydroxylamine acts as a general base as well as a reductant. Rate constants for various bases (H2O, CH3COO(1-), NH2OH, HPO4(2-), and OH(1-)) fit aBroensted β value of 0.58. The rates decrease greatly with increases of H(1+) and I(1-) concentrations due to NH3OH(1+) and I3(1-) formation, loss of general-base assistance, and the reverse reaction of BH(1+) + I(1-) + INHOH to re-form I2NH2OH. The INHOH species is a steady-state intermediate that decays to form I(1-) + HNO + H(1+). Subsequent rapid dehydrative dimerization of HNO gives N2O as the final product. The hydroxylamine oxidation process proceeds entirely by I(1+) transfer to nitrogen followed by I(1-) loss, as opposed to electron-transfer pathways. Kinetic evidence is given for I2NH2OH as an intermediate and for INHOH as asteady-state species.