Cyclopropane is the simplest saturated carbon ring: three carbon atoms joined in a triangle, each carrying two hydrogen atoms, giving . The three carbon nuclei necessarily lie in a plane, and the internal carbon–carbon–carbon bond angles are . This is much smaller than the approximately preferred around an ordinary tetrahedral saturated carbon.[ref]
The geometry produces substantial ring strain. The carbon–carbon sigma bonds cannot have the same straight, head-on orbital overlap as those in an unstrained open chain. The arrangement of neighboring carbon–hydrogen bonds also contributes torsional strain.[ref]
Calorimetry connects this structural picture to measurable energy. Comparing the energy released on complete combustion per group across saturated carbon rings reveals an excess for strained rings. Normalizing by the number of carbon atoms matters: a larger molecule generally releases more total combustion energy merely because more material is oxidized. The informative comparison separates molecular size from energy associated with the bonding arrangement.[ref] This is an energetic argument; combustion energy by itself does not specify the rate of an unrelated chemical reaction.
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