= Cyclopropane
<Cyclopropane> is the simplest saturated carbon ring: three carbon atoms joined in a triangle, each carrying two hydrogen atoms, giving $\mathrm{C_3H_6}$. The three carbon nuclei necessarily lie in a plane, and the internal carbon–carbon–carbon <bond angles> are $60^\circ$. This is much smaller than the approximately $109.5^\circ$ preferred around an ordinary tetrahedral saturated carbon.<Organic Chemistry by John McMurry>[section 4.3, Stability of Cycloalkanes: Ring Strain]{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>.<Organic Chemistry by John McMurry>[section 4.3, angle strain and torsional strain]{ref}
<Calorimetry> connects this structural picture to measurable energy. Comparing the energy released on complete combustion per $\mathrm{CH_2}$ 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.<Organic Chemistry by John McMurry>[section 4.3, heats of combustion and Figure 4.4]{ref} This is an energetic argument; combustion energy by itself does not specify the rate of an unrelated <chemical reaction>.
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