Franklin group equivalents, strain energy

As an example, the heat of formation of cyclopentane in the gas phase is 18.4 kcal/mol. The Franklin group equivalent for CH2 is —4.93 kcaFmol, and thus an unstrained model for cyclopentane is five times this quantity or —24.6 kcaF mol. The difference between the two values gives the strain energy as 6.2 kcal/ mol. The strain energies given in Figure 15.1 were obtained in this way. 

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 ... 

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