Separation of polar and enthalpy effects in radical addition reactions using polar (σ) and radical (σ ·) sigma scales
✍ Scribed by Károly Héberger; Antal Lopata; J. Cs. Jászberényi
- Publisher
- John Wiley and Sons
- Year
- 2000
- Tongue
- English
- Weight
- 87 KB
- Volume
- 13
- Category
- Article
- ISSN
- 0894-3230
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✦ Synopsis
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 [s(H)] and two radical sigma scales as defined by Creary and co-workers [s Á (C)] and Dust, Arnold and Wayner [s Á (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 s can describe the polar effects, whereas the radical s Á (C) and, to a somewhat lesser extent, s Á (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 s alone for strongly nucleophilic radicals where polar effects dominate, (ii) with Hammett s and one of the radical s Á scales for moderately nucleophilic (or electrophilic) radicals and (iii) with one of the radical s Á scales alone for weakly nucleophilic radicals where enthalpy effects dominate.