Hydrocarbons calculation using group equivalents

The heats of formation are not convenient quantities for comparing compounds having different numbers of carbons. Here, the strain energies are much more useful. The simplest definition is the difference between the observed AH and that calculated using a strain-free model. In the case of cyclopropane, AHf = 12.73 kcal mol If cyclohexane is taken as strain-free, the expected AHf for cyclopropane is one-half that for cyclohexane, or —14.75 kcal mol The difference is 27.5 kcal mol and is the strain energy. The strain-free model is frequently derived from Franklin s group equivalents, and for common hydrocarbon groups they are in kcal mol ... 

For many strained hydrocarbons and alkenes, heats of formation are not known. In these cases, computational methodology may be used. For a number of molecules, the total energies determined by ab initio calculations and transformed into heats of formation by special group equivalents may be used (81b). Strain energies have also been obtained via isodesmic reactions, which measure deviations from the additivity of bond energies (224). 

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