LINEAR FREE-ENERGY RELATIONSHIPS IN RADICAL REACTIONS .2. HYDROGEN ABSTRACTION FROM SUBSTITUTED TOLUENES BY TERT-BUTYL, TERT-BUTOXYL AND TERT-BUTYLPEROXYL RADICALS

Calculations were carried out to study the validity of sigma-scales for hydrogen abstraction from substituted toluenes by tert-butyl, tert-butoxyl and tert-butylperoxyl radicals. Rate constants were compiled and evaluated from the literature for meta- and/or para-substituted toluenes. The substituents were characterized by factored ionic sigmas (sigma(I), sigma(R), sigma(R)+), ionic scales (sigma, sigma+) and various radical sigmas (sigma.). The dependence of log k values on these substituent descriptors was investigated using 'stepwise linear regression' and 'all possible regression' methods. The following predictive equations can be recommended: for tert-butyl radicals, at 321 K: log k1 = 1.024 + 0.776sigmaI + 0.653sigmaC. (R = 0.8137) for tert-butoxyl radicals, at 313 K: log k2 = 5.529 - 0.623sigma+ (R = 0.9376) and for tert-butylperoxyl radicals, at 303 K: log k3 = -1.410-0.810sigma + 0.337sigmaC. (R = 0.9628). The results suggest that there is no universal radical scale for hydrogen abstraction reactions, that the rate is primarily influenced by polar factors (inductive, resonance) and that only two radical scales (sigma(C). and sigma(alpha).) are appropriate, showing a small, yet significant, role of radical stabilization.