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Effect of the structure of nitroxyl radicals on the kinetics of their acid‐catalyzed disproportionation
Author(s) -
Tikhonov Ivan V.,
Sen' Vasily D.,
Borodin Leonid I.,
Pliss Evgeny M.,
Golubev Valery A.,
Rusakov Alexander I.
Publication year - 2014
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.3247
Subject(s) - disproportionation , chemistry , nitroxyl , protonation , radical disproportionation , radical , redox , photochemistry , medicinal chemistry , kinetics , piperidine , deprotonation , catalysis , organic chemistry , polymer chemistry , physics , quantum mechanics , ion
Acid‐catalyzed disproportionation of cyclic nitroxyl radicals R 2 NO • includes the half‐reactions of their oxidation to oxoammonium cations R 2 NO + and reduction to hydroxylamines R 2 NOH. 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 H 2 SO 4 , and interpreted it in terms of the excess acidity function X . The rate‐limiting step of this reaction is R 2 NO • oxidation by its protonated counterpart R 2 NOH +• . Kinetic stability of R 2 NO • 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 p K values for radical cations R 2 NOH +• (from −5.8 to −12.0) indicate a very low basicity of nitroxyl groups in all commonly used R 2 NO • . 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.