Kinetic solvent effects on hydrogen-atom abstractions: Reliable, quantitative predictions via a single empirical equation

The rate of hydrogen-atom abstraction from XH by a radical, Y-., can be solvent-dependent. In many cases, the kinetic solvent effect (KSE) is directly related to hydrogen-bonding interactions between XH and the solvent. The relative hydrogen-bond acceptor (HBA) properties of solvents are given by beta (H)(2) constants of Abraham et al. (Abraham, M. H.; Grellier, P. L.; Prior, D. V.; Morris, J. J.; Taylor, P. J. J. Chem. Soc. Perkin Trans. 2 1990, 521-529). Room-temperature rate constants for hydrogen-atom abstraction, k(XH/Y).(S), have been determined in a number of solvents, S, where XH refers to several substituted phenols, tert-butyl hydroperoxide or aniline and Y-. is a tert-alkoxyl radical. In all cases, plots of log(k(XH/Y).(S)/M-1 s(-1)) versus beta (H)(2) gave excellent linear correlations, the slopes of which, M-XH, were found to be proportional to the hydrogenbond-donating (HBD) ability of XH, as scaled with alpha (H)(2) parameters of Abraham et al. (Abraham, M. H.; Grellier, P. L.; Prior, D. V.; Duce, P. P.; Morris, J. J.; Taylor, P. J. J. Chem,. Soc., Perkin Trans. 2 1989, 699-711), with M-XH = - 8.3 alpha (H)(2). This leads to a general empirical equation which quantifies KSEs at room log temperature : k(XH/Y).(S) = log k(XH/Y).(O) - 8.3 alpha (H)(2)beta (H)(2), where k(XH/Y).(O) refers to the rate constant in a non-HBA solvent for which beta (H)(2) = 0, generally a saturated hydrocarbon. Since M-XH depends only on XH, rate constants for hydrogen-atom abstraction from XH by any Y-. can be accurately predicted in any of the several hundred known or measured. HBA solvents can have profound effects on some of the reactions and thermodynamic properties of hydroxylic substrates including: (i) reaction product profiles (ii) antioxidant activities, (iii) Hammett-type correlations, and (iv) O-H bond dissociation enthalpies. Finally, literature data (Nielsen, M. F.; Hammerich, O. Acta Chem. Scand, 1992, 46, 883-896) on KSEs for two proton-transfer reactions are shown to be correlated by the same equation which correlates KSEs for hydrogen-atom transfers.