Linear free-energy relationships in radical reactions part V -: Separation of polar and enthalpy effects in radical addition reactions using polar (σ) and radical (σ.) sigma scales

Rate constants (k) of radical addition reactions between 15 carbon- and sulfur-centered radicals and 15 vinyl-type alkenes were collected from the literature. Three descriptor variables {a polar (Hammett) sigma scale [sigma(H)] and two radical sigma scales as defined by Creary and co-workers [sigma(.)(C)] and Dust, Arnold and Wayner [sigma(.)(DAW)]} were chosen to express polar and enthalpy effects. The reactivity data (logk values) of radicals were correlated with the three descriptor variables using stepwise linear regression analysis. The Hammett a can describe the polar effects, whereas the radical sigma(.)(C) and, to a somewhat lesser extent, sigma(.)(DAW) can describe the enthalpy effects in radical addition reactions. The regression equations support the symmetry pattern of correlations proposed in Part IV. Radical reactivity (logk) correlates (i) with Hammett sigma alone for strongly nucleophilic radicals where polar effects dominate, (ii) with Hammett sigma and one of the radical sigma(.)scales for moderately nucleophilic (or electrophilic) radicals and (iii) with one of the radical sigma(.) scales alone for weakly nucleophilic radicals where enthalpy effects dominate. Copyright (C) 2000 John Wiley & Sons, Ltd.