Kinetics of hydrogen-atom transfer from phenols to galvinoxyl in aprotic solvents

Abstract

The reactions of phenols with the free radical galvinoxyl (which is stable to air and solvents and deeply coloured) constitute a series of H-atom-transfer reactions where the solvent and substituents can conveniently be varied and accurate kinetic measurements can be made. The kinetics for a series of substituted phenols in toluene solution have been studied; we report rate constants (which vary over a range of more than 10⁴ at 25 °C), activation parameters and some deuterium isotope effects (rate ratio ca. 6 at 25 °C). The results can be interpreted in terms of the electronic effects of substituents in stabilising the incipient phenoxy radical in the transition state and the steric effects of o-substituents in reducing the interaction of the phenol with the solvent. The reaction of one of these phenols, 2,4,6-tri-t-butylphenol, has been studied in a series of aprotic solvents of varying polarity and hydrogen-bond-acceptor properties. The rate constant decreases markedly with increasing polarity of the solvent; the isotopic rate ratio also varies appreciably from solvent to solvent. Desolvation of the phenol is apparently involved. Interpretation of the isotope effects may be sought in terms of tunnelling or of transition-state asymmetry.