Kinetics and thermodynamics of reversible disproportionation–comproportionation in redox triad oxoammonium cations – nitroxyl radicals – hydroxylamines

Abstract

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 p K 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. Copyright © 2014 John Wiley & Sons, Ltd.