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

• Published in: Journal of physical organic chemistry 2014-02, Vol.27 (2), p.114-120
• Main Authors: Tikhonov, Ivan V., Sen', Vasily D., Borodin, Leonid I., Pliss, Evgeny M., Golubev, Valery A., Rusakov, Alexander I.
• Format: Article
• Language: English
• Subjects: acidity function; disproportionation; kinetics; nitroxyl radicals; reduction potentials
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.3247

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. Acid‐catalyzed disproportionation of cyclic nitroxyl radicals R2NO• includes the half‐reactions of their oxidation to oxoammonium cations R2NO+ and reduction to hydroxylamines R2NOH. Kinetic stability of R2NO• in this reaction depends on pK of corresponding radical cations R2NOH+•, whose values for 20 nitroxyl radicals studied vary from −5.8 to −12.0. A linear correlation between the pK and the one‐electron reduction potentials ER2NO+/R2NO• enabled to calculate the previously unknown values of ER2NO+/R2NO• for nine nitroxyl radicals studied.