How to calculate ring strain energy? Series A and B below can be viewed in two different ways to annulate an ethano-bridge onto four-, five-, and six-membered rings. Using the $\Delta_\mathrm{f}H^\circ$ values given (in kcal/mol), how can one calculate the strain energy for each compound?


Could anyone also explain whether the trend seen in each series is consistent with expectations based on additivity of ring strain?
Typically, ring strain energies (RSE) are calculated as the difference of the heat of formation of cyclic starting materials and less cyclic products in homodesmic reactions.

For cyclopropane, the RSE would be calculated using the following reaction:

$$\ce{(CH2)3 + 3 CH3-CH3 -&gt; 3 CH3-CH2-CH3}$$