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

摘要

The relative stabilization energies of radicals, SE(R center dot), along the simple series methyl/ethyl/isopropyl/tertbutyl 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 C-4 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 center dot) 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 center dot)) and for the parent vinyl, phenyl, and ethynyl radicals (negative SE(R center dot)).