Choice of Bond Dissociation Enthalpies on Which To Base the Stabilization Energies of Simple Radicals: DH(R−H) Is Preferred Because DH(R−Me) and DH(R−R) Are Perturbed by Changes in Chain Branching

Abstract

The relative stabilization energies of radicals, SE(R*), along the simple series methyl/ethyl/isopropyl/tert-butyl are known to vary in spread and even direction dependent on which dissociation enthalpies, DH(R-X), are used for their definition. Using a highly electronegative X is recognized as unwise, but it is not clear that using X = Me or X = R itself might not be preferred over the almost universal use of X = H. The enthalpies of formal isomerization of C4 radical pairs that vary only in the substitution pattern at the radical center but not in carbon skeleton confirm that X = H is indeed the better choice. Comparisons in the context of recent predictive models for alkane and radical stability indicate that, while relative DH(R-H) values highlight the desired difference in substitution pattern at the radical center, relative DH(R-Me) values are perturbed by differences in skeletal branching or protobranching which are well-known to affect thermochemistry. As a result, SE(R*) values derived from relative DH(R-Me) values are consistently too small. The same pattern is illustrated for prim, sec, and tert allylic and benzylic radicals (larger SE(R*)) and for the parent vinyl, phenyl, and ethynyl radicals (negative SE(R*)).