Standard homolytic bond dissociation enthalpy

Carbon-centered radicals play an important role in organic synthesis, biological chemistry, and polymer chemistry. The radical chemistry observed in these areas can, to a good part, be rationalized by the thermodynamic stability of the open shell species involved. Challenges associated with the experimental determination of homolytic bond dissociation energies (BDEs) have lead to the widespread use of theoretically calculated values. These can be presented either directly as the enthalpy for the C-H bond dissociation reaction described in Equation 5.1, the gas-phase thermodynamic values at the standard state of 298.15K and 1 bar pressure being the most commonly reported values. 

The standard enthalpy change for the reverse of equation 1 is the heterolytic bond dissociation energy for BH (equal by convention to the proton affinity of B). The homolytic bond dissociation energy for BH", i.e. the standard enthalpy change for reaction 5, has been obtained [17,18] for some bases from the proton affinity and the... 

Stability in chemistry is not an absolute, but a relative concept. Let us consider the standard heats of reaction AH0 of the homolytic dissociation reaction R—H —> R + H. It reflects, on the one hand, the strength of this C—H bond and, on the other hand, the stability of the radical R produced. So the dissociation enthalpy of the R—H bond depends in many ways on the structure of R. But it is not possible to tell clearly whether this is due to an effect on the bond energy of the broken R—H bond and/or an effect on the stability of the radical R that is formed. 

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