Ring strain

In organic chemistry, ring strain is a type of instability that exists when bonds in a molecule form angles that are abnormal. Strain is most commonly discussed for small rings, such as cyclopropanes and cyclobutanes where the C-C-C angles deviate substantially from the idealized values of approximately 109°. Because of their high strain, the heat of combustion for these small rings is elevated.

Quotes

 * 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.
 * John McMurry, Organic Chemistry 8th ed. (2012), Ch. 4: Organic Compounds: Cycloalkanes and Their Stereochemistry


 * 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.
 * John McMurry, Organic Chemistry 8th ed. (2012), Ch. 4: Organic Compounds: Cycloalkanes and Their Stereochemistry