II. Bond Polarities, Bond-Dissociation Energies, and Rate Constants for Hydrogen-Abstraction Reactions

Bond polarities, bond-dissociation energies, and rate constants for abstrac­tion of hydrogen atoms bonded to tin, silicon, sulfur, selenium, and carbon all are given in Table 1. Based on Pauling’s electro­nega­tivity values, hydrogen has a small negative charge when bonded to tin or sil­icon and a small positive charge when bonded to sulfur, selenium, or carbon. The infor­mation in Table 2 shows that for each type of bond, the rate constant for hydrogen-atom abstraction by simple pri­mary, secondary, and tertiary, carbon-centered radicals is nearly the same.

When the bond dissociation energies in Table 1 are used to calculate reaction enthalpies, they show that the reaction in eq 4 is more exothermic than that in eq 5. If the Evans-Polanyi relation is obeyed, the first reac­tion (eq 4) should have a lower energy of activation than the second (eq 5), but the opposite appears to be true. The rate constants for these reactions, when related to acti­va­tion energies through the Arrhenius equation (eq 6), show that, unless the frequency factors for these two reactions are quite different, the reaction given in eq 4 actually has a higher energy of acti­vation. Clearly, something in addition to reaction enthalpies must have a significant role in deter­mining energies of activation for these two reactions.

The identification of a likely candidate for this additional factor can be made by returning to the reactions pictured in equations 2 and 3 and recalling that these reactions show some car­bon-centered radicals to be nucleophilic and others electro­philic. If one assumes that the tert-butyl radical is similar in its philicity to the nucleo­philic cyclohexyl radical, then in the reaction in eq 5 there is a polarity match between the nucleophilic radical and the electron-deficient hydrogen atom being abstracted. Since a similar match does not exist in the slower reaction (eq 4), radical philicity becomes a prime candidate for the factor that joins with reaction enthalpy in explaining the rate constants for hydrogen-abstrac­tion reactions.