KINETICS AND MECHANISMS OF THE OXIDATION OF HYDROXYLAMINE BY AQUEOUS IODINE

First-order rate constants (k(r), 25.0 degrees C, mu = 0.50 M) for the loss of I-3(-)/I-2 in the presence of excess NH3OH+/ NH2OH are measured from pH 2.0 to 6.8 with variation of the reactant concentrations as well as the concentrations of I- and various buffers. The k(r) values range from 5 x 10(-4) to 2 x 10(4) s(-1) and depend on the NH2OH concentration. A multistep mechanism is proposed where I-2 and NH2OH react rapidly to form an I2NH2OH adduct (K-A = 480 M(-1)) that undergoes general-base(B) assisted deprotonation to give INHOH + I- + BH+. At higher pH, hydroxylamine acts as a general base as well as a reductant. Rate constants for various bases (H2O, CH3COO-, NH2OH, HPO42-, and OH-) fit a Bronsted beta value of 0.58. The rates decrease greatly with increases of H+ and I- concentrations due to NH3OH+ and I-3(-) formation, loss of general-base assistance, and the reverse reaction of BH+ + I- + INHOH to re-form I2NH2OH. The INHOH species is a steady-state intermediate that decays to form I- + HNO + H+. Subsequent rapid dehydrative dimerization of HNO gives N2O as the final product. The hydroxylamine oxidation process proceeds entirely by I+ transfer to nitrogen followed by I- loss, as opposed to electron-transfer pathways. Kinetic evidence is given for I2NH2OH as an intermediate and for INHOH as a steady-state species.