Radical Stability Paradox: Substituent Effects versus Heats of Formation

Recently, we have shown that alkyl substituents destabilize the carbon radical center of organic radicals. However, seemingly in contradiction with this earlier finding, the series of isomeric n-butyl, s-butyl, and t-butyl radicals shows an increasingly stable (less positive) heat of formation ∆H_f, despite an increasing number of alkyl substituents at the radical center. Herein, we provide a solution to this apparent paradox of contradicting pictures. The crux of the matter is that ∆H_f not only comprises the substituent effect on the radical center but also the intrinsic stability of the substituents. Furthermore, we provide a generalizable framework for extracting the actual intrinsic substituent effects from experimental and computational data, an approach with broad applicability to radical chemistry, thermochemistry, and beyond.