Effect of the structure of nitroxyl radicals on the kinetics of their acid-catalyzed disproportionation

AcidaEurocatalyzed disproportionation of cyclic nitroxyl radicals R2NOaEuroC includes the halfaEuroreactions of their oxidation to oxoammonium cations R2NO+ and reduction to hydroxylamines R2NOH. For many nitroxyl radicals, this reaction is characterized by itsaEuro?100% reversibility. Quantitative characteristics of acidaEuro'base and redox properties of the whole redox triad may be obtained from research of kinetics and equilibrium of this reaction. Here, we have examined the kinetics for the disproportionation of twenty piperidineaEuro, pyrrolineaEuro, pyrrolidineaEuro, and imidazoline nitroxyl radicals in aqueous H2SO4, and interpreted it in terms of the excess acidity function X. The rateaEurolimiting step of this reaction is R2NOaEuroC oxidation by its protonated counterpart R2NOH+aEuroC. Kinetic stability of R2NOaEuroC in acidic media depends on the basicity of nitroxyl group. This basicity is influenced predominantly by protonation of another, more basic group in radical structure, and its proximity to nitroxyl group. The discovered estimates of pK values for radical cations R2NOH+aEuroC (from a?'5.8 to a?'12.0) indicate a very low basicity of nitroxyl groups in all commonly used R2NOaEuroC. For the first time, a linear correlation is obtained between the oneaEuroelectron reduction potentials of oxoammonium cations and the basicity of nitroxyl groups of related radicals. Copyright a (c) 2013 John Wiley & Sons, Ltd.