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

• Published in: Journal of physical organic chemistry Vol. 28; no. 1; pp. 17 - 24
• Main Authors: Sen', Vasily D., Tikhonov, Ivan V., Borodin, Leonid I., Pliss, Evgeny M., Golubev, Valery A., Syroeshkin, Mikhail A., Rusakov, Alexander I.
• Format: Journal Article
• Language: English
• Published: Bognor Regis Blackwell Publishing Ltd 01-01-2015; Wiley Subscription Services, Inc
• Subjects: comproportionation; Equilibrium; free-energy relationships; hydroxylamines; Kinetics; nitroxyl radicals; oxoammonium salts; reduction potentials; Thermodynamics
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.3392

Kinetics and equilibrium of the acid‐catalyzed disproportionation of cyclic nitroxyl radicals R2NO• to oxoammonium cations R2NO+ and hydroxylamines R2NOH 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 R2NO• in acidic media depends on the basicity of the nitroxyl group. Here, we examined the kinetics of the reverse comproportionation reaction of R2NO+ and R2NOH to R2NO• and found that increasing in –I‐effects of substituents greatly reduces the overall equilibrium constant of the reaction K4. This occurs because of both the increase of acidity constants of hydroxyammonium cations K3H+ and the difference between the reduction potentials of oxoammonium cations ER2NO+/R2NO• and nitroxyl radicals ER2NO•/R2NOH. pH dependences of reduction potentials of nitroxyl radicals to hydroxylamines E1/3Σ and bond dissociation energies D(O–H) for hydroxylamines R2NOH in water were determined. For a wide variety of piperidine‐ and pyrrolidine‐1‐oxyls values of pK3H+ and ER2NO+/R2NO• correlate with each other, as well as with the equilibrium constants K4 and the inductive substituent constants σI. The correlations obtained allow prediction of the acid–base and redox characteristics of redox triads R2NO•–R2NO+–R2NOH. Copyright © 2014 John Wiley & Sons, Ltd.
The overall equilibrium constant K4 of disproportionation of nitroxyl radicals 1 depend on the difference of their pH‐dependent reduction potentials to hydroxyammonium cations 3H+ and reduction potentials of oxoammonium cations 2 to 1: E1/3Σ − E2/1 = (RT / F)ln K4. –I‐Effects of substituents R and heterocycle contraction increase the rate constant of comproportionation k−2, acidity constants of protonated nitroxyl radicals K1H+ and hydroxylamines K3H+, thus reducing the constant K4 = k2 / (k−2 × K1H+ × K3H+) in the studied series of compounds a–i by more than seven orders of magnitude.