Homolytic and Heterolytic Bond Dissociation Energies

Heats of reaction are important values for processes that involve reactive intermediates. For example, the standard homol)dic bond dissociation enthalpy of compoimd A-B, denoted DW A-B) or DH29s(A-B), is the heat of reaction AH°) at 298 K for the gas phase dissociation reaction in equation 1.14. 

Here AHf (A ) is the heat of formation of radical A, AHf (B ) is the heat of formation of radical B, and AHf (A-B) is the heat of formation of A-B. DH° (A-B) is also called the bond dissociation energy of A-B. Table 1.10 gives a list of standard bond dissociation enthalpies for bonds involving hydrogen atoms, and Table 1.11 gives a list of DH° values for bonds between carbon atoms in various alkyl groups and a number of common organic substituents.  

A standard bond dissociation energy is different from an average bond dissociation energy. The latter is just the value obtained by calculating the heat of atomization of a compound (the enthalpy change on converting the molecule to individual atoms) divided by the number of bonds from one atom to another in the molecule. For more details on this distinction, see reference 67. Blanksby, S. J. Ellison, G. B. Acc. Chem. Res., 2003, 36, 255. This reference provides the uncertainties for the values in Tables 1.10 and 1.11. 

Substituent R CH3 CH3CH2 (CH3)2CH (CH3)3C CH2=CH CH2=CHCH2 CeHs CeHgCHj CH3O HC(0) CH3CH(0) 

Note Means a redundant entry. — Means not available. Source Reference 69. 

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