Abstract
Chemical intuition suggests that the stabilization of a carbon-centered free radical by a substituent X would be the greatest for a prim and least for a more stable tert radical because of “saturation.” However, analysis of a comprehensive recent set of bond dissociation energies computed by Coote and coworkers (Phys. Chem. Chem. Phys. 2010 12 9597) and transformed into radical stabilization energies (RSE) suggests that this supposition is often violated. The RSE for a given X depends not only on the nature of X but also on the ordinality (i.e., prim, sec, or tert) of the radical onto which it is substituted. For substituents that stabilize by electron delocalization but also contain electron-withdrawing centers, such as the carbonyl function, the stabilization of XCMe2• compared with HCMe2• is greater than for XCH2• compared with HCH2•. However, for substituents that stabilize by lone-pair electron donation, such as N or O centers, the order is strongly reversed. This contrast can be qualitatively rationalized by considering charge-separated VB contributors to the radical structure (R2C+–X-• and R2C-–X+•) and the contrasting effects of methyl substituents on them. This conclusion is not dependent on the particular definition used for RSE.