Selected bond enthalpy terms

Despite the previous remarks, organotin compounds are one of the few families of organometallic substances for which the thermochemical data justify the calculation of bond enthalpy terms. Some term values (consistent with the terms recommended by Pilcher and Skinner for hydrocarbons)27 are given in Table 4. A few words on this selection are, however, appropriate. 

There was a clear upper limit in terms of selectivity-conversion beyond which experimental studies have not advanced for many selective oxidation reactions. These limits have been achieved through detailed catalyst design and reactor optimization. This work shows that active sites on oxidation and ammoxidation catalysts are capable of selectively activating, typically, a C-H bond in a reactant, rather than a similar C-H or C-C bond in the product provided that the bond dissociation enthalpy of the weakest bond in the product is no more than 30-40 kJ mole weaker than the bond dissociation enthalpy of the weakest bond in the reactant. When these limits are exceeded selectivity falls drastically. This work also indicates that primary activation of alkanes is through C-H bonds although the corresponding C-C bonds are much weaker. Cleavage of a C-C bond in the primary activation step leads directly to carbon oxide formation, but this step is less favoured because steric Victors make it difficult for the C-C bonds to be accommodated at the active site. 

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