The oxidation of hydroxylamine by hypohalites and other halogen -containing species
Abstract
The kinetics and mechanism of the very fast hypobromite/hydroxylamine reaction has been studied (25.0°C, ionic strength 0.5 M). The reaction proceeds by a halogen cation transfer. Two distinct mechanisms account for hydroxylamine reactivity with hypobromite: one mechanism below p[H +] 11 in which HOBr and NH2OH are the reactants ( k1 = (4.24 ± 0.09) × 108 M −1s−1), and another mechanism above p[H +] 12 in which OBr− and NH2O − are the reactants (k2 = (2.0 ± 0.2) × 107 M−1s−1). The latter reaction proceeds due to the strong nucleophilicity of deprotonated hydroxylamine. The reactivity of NH2O− is also demonstrated in reactions with OCl− and NH 2Cl. A refined value pKa = 13.3 has been determined for NH2OH. The initial hypobromite/hydroxylamine reaction is followed by a series of reactions involving intermediates. Subsequent reactions follow two separate mechanisms, stemming from isomeric initial intermediates BrNHOH and BrONH2. Both mechanisms result in the same minor product, N2O2− (5–15% yield) which implies the formation of some nitric oxide (NO·). The formation of NO2− and (in the presence of dioxygen) ONOO− are also observed. The latter implies a nitroxyl (NO−) intermediate above p[H+] 12. A comparison is made of these and other halogen/hydroxylamine systems. The critical parameters of the initial halogen species/hydroxylamine reaction include the nucleophilicity of the hydroxylamine species present and the lability of the halogen species leaving group with respect to X+.
Degree
Ph.D.
Advisors
Margerum, Purdue University.
Subject Area
Chemistry
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