Ring strain

"To measure the amount of strain in a compound, we have to measure the total energy of the compound and then subtract the energy of a strain-free reference compound. The difference between the two values should represent the amount of extra energy in the molecule due to strain. The simplest experimental way to do this for a cycloalkane is to measure its heat of combustion, the amount of heat released when the compound burns completely with oxygen. … Thus, three kinds of strain contribute to the overall energy of a cycloalkane. … Angle strain—the strain due to expansion or compression of bond angles … Torsional strain—the strain due to eclipsing of bonds on neighboring atoms … Steric strain—the strain due to repulsive interactions when atoms approach each other too closely."

"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."