Kinetics and equilibrium of the acid-catalyzed disproportionation of cyclic nitroxyl radicals R 2 NO• to oxoammonium cations R 2 NO + and hydroxylamines R 2 NOH is defined by redox and acid-base properties of these compounds. In a recent work (J. Phys. Org. Chem. 2014, 27, 114-120), we showed that the kinetic stability of R 2 NO• in acidic media depends on the basicity of the nitroxyl group. Here, we examined the kinetics of the reverse comproportionation reaction of R 2 NO + and R 2 NOH to R 2 NO• and found that increasing in -I-effects of substituents greatly reduces the overall equilibrium constant of the reaction K 4 . This occurs because of both the increase of acidity constants of hydroxyammonium cations K 3H+ and the difference between the reduction potentials of oxoammonium cations E R2NO+/R2NO• and nitroxyl radicals E R2NO•/R2NOH . pH dependences of reduction potentials of nitroxyl radicals to hydroxylamines E 1/3Σ and bond dissociation energies D(O-H) for hydroxylamines R 2 NOH in water were determined. For a wide variety of piperidine-and pyrrolidine-1-oxyls values of pK 3H+ and E R2NO+/R2NO• correlate with each other, as well as with the equilibrium constants K 4 and the inductive substituent constants σ I . The correlations obtained allow prediction of the acid-base and redox characteristics of redox triads R 2 NO• -R 2 NO + -R 2 NOH.

Acid‐catalyzed disproportionation of cyclic nitroxyl radicals R2NO• includes the half‐reactions of their oxidation to oxoammonium cations R2NO+ and reduction to hydroxylamines R2NOH. For many nitroxyl radicals, this reaction is characterized by its ~100% reversibility. Quantitative characteristics of acid–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 piperidine‐, pyrroline‐, pyrrolidine‐, and imidazoline nitroxyl radicals in aqueous H2SO4, and interpreted it in terms of the excess acidity function X. The rate‐limiting step of this reaction is R2NO• oxidation by its protonated counterpart R2NOH+•. Kinetic stability of R2NO• 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+• (from −5.8 to −12.0) indicate a very low basicity of nitroxyl groups in all commonly used R2NO•. For the first time, a linear correlation is obtained between the one‐electron reduction potentials of oxoammonium cations and the basicity of nitroxyl groups of related radicals. Copyright © 2013 John Wiley & Sons, Ltd.

ABSTRACT:The technique based on monitoring oxygen consumption was applied to test 18 polyphenols (PP) and model phenolics as a chain-breaking antioxidant during the oxidation of styrene initiated by 2,2 -azobis(2,4-dimethylvaleronitril) at 37 • C. The chain-breaking capability of PP was characterized by two parameters: the rate constant k 1 for the reaction of antioxidants with the peroxy radical produced from styrene and the stoichiometric coefficient of inhibition, f, which shows how many kinetic chains are terminated by one molecule of PP. Rate constants k 1 × 10 5 (in M −1 s −1 ) were found to be 10 (catechol), 27 (pyrogallol), 34 (3,6-di-tert-Bucatechol), 4.3 (protocatechic acid), 12 (gallic acid), 15 (caffeic acid), <0.01 (chrysin), 1.3 (kaempferol), 19 (quercetin), 5.3 (baicalein), 16 (epicatechin), 32 (epigallocatechin), 9.0 (dihydroquercetin), 3.3 (resveratrol), and 16 (nordihydroguaiaretic acid). The value of k 1 increases when going from one to two and three adjacent O-H groups in a benzene ring (catechol and pyrogallol derivatives, respectively). At the same time, two O-H groups in metaposition in a A-ring of flavonoids actually do not participate in the inhibition. For the majority of PP, f is near to 2 independent of the number of OH groups. The correlation of k 1 with the structure of PP and the O H bond dissociation enthalpy has been discussed. C 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 92

A kinetic study (EPR, microvolumometry), quantum chemical analysis (DFT B3LYP/6 31G*), and kinetic simulation of the antioxidant activity of aliphatic stable nitroxyl radicals during styrene oxidation was performed. The key reactions constituting the detailed mechanism of the process were analyzed. It was shown that the inhibiting action of nitroxyl radicals was caused by their reaction not only with alkyl radicals but also with substrate peroxy radicals, which resulted in the regeneration of nitroxyl radicals in chain termi nation steps.

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