Thermochemical parameters for organic radicals and radical ions. Part 3. The relationship between bond dissociation enthalpy and radical stability in alkyl systems

Abstract

The relationship between radical stability and bond dissociation enthalpy (BDH) is reexamined. It is shown that relative stabilization energies of radicals are not equal to relative BDH values. Net stabilization energies of radicals, SE⁰[R^•, RX] are defined relative to the R components of closed shell species RX (R(RX)). These components are chosen such that they contain the same (or, approximately the same) net charge as that of the radical (R^•). The following results, relative to R = C₂H₅, were obtained: R^•, SE⁰[R^•, RX](kJ mol⁻¹) for X = R (i.e., the dimer RR), CH₃, and H; CH₃^•, 23, 32, 37; n-C₃H₇^•, −2, −2, −3; i-C₃H₇^•, −9, −14, −19; t-C₄H₉^•, −25, −32, −38. These results show that the methyl radical is more destabilized and the n-propyl-, i-propyl-, and tert-butyl radicals are more stabilized than is predicted from the corresponding relative BDH (R—X) values. The intrinsic C—H bond strengths of chosen alkanes are considered. Relative to the C—H bond in ethane, the bond in methane is found to be weaker by 8.12 kJ mol⁻¹ and the primary and secondary bonds in propane and the tertiary bond in methyl propane are stronger by 2.56, 7.98, and 17.12 kJ mol⁻¹ respectively.