"Cyclopropane is the most strained of all rings, primarily because of the angle strain caused by its 60°C C-C bond angles. In addition, cyclopropane has considerable torsional strain because the C-H bonds on neighboring carbon atoms are eclipsed … Cyclobutane has less angle strain than cyclopropane but has more torsional strain because of its larger number of ring hydrogens. … Cyclopentane was predicted by Baeyer to be nearly strain-free, but it actually has a total strain energy of 26 kJ/mol (6.2 kcal/mol). Although planar cyclopentane has practically no angle strain, it has a large amount of torsional strain. Cyclopentane therefore twists to adopt a puckered, nonplanar conformation that strikes a balance between increased angle strain and decreased torsional strain."