Ring Strain in Cyclopropane
As can be observed in the molecular model, cyclopropane has bond angles of 60º, which is a significant deviation from the typical 109.5º of sp3 carbons. Additionally, the hydrogens are eclipsed, three on the upper face and three on the lower face. These two factors, angular strain and eclipsing, make cyclopropane a high-energy molecule.
Cyclopropane is planar and has no mechanism to relieve this strain. In other cycloalkanes, spatial arrangements help minimize hydrogen-hydrogen eclipsing interactions.
Ring Strain in Cyclobutane
Cyclobutane has bond angles of 90º between carbons, causing significant strain, though much less than that in cyclopropane. In its planar form, it has four eclipsed hydrogens on each face. Cyclobutane reduces eclipsing by moving the opposite carbons out of the plane.
Ring Strain in Cyclopentane
The bond angles in cyclopentane are very close to 109º, making this molecule nearly free of angular strain. It has five hydrogen-hydrogen eclipsing interactions on each face, which it minimizes by adopting a spatial arrangement called the envelope form.
