3.9: Conformations of Cycloalkanes
Adolf von Baeyer attempted to explain the instabilities of small and large cycloalkane rings using the concept of angle strain — the strain caused by the deviation of bond angles from the ideal 109.5° tetrahedral value for sp3 hybridized carbons. However, while cyclopropane and cyclobutane are strained, as expected from their highly compressed bond angles, cyclopentane is more strained than predicted, and cyclohexane is virtually strain-free. Hence, Baeyer’s theory that was based on the assumption that all cycloalkanes are flat was wrong, and, in reality, most cycloalkanes adopt a non-planar structure.
Cyclopropane, the three-carbon cyclic alkane, has the highest angle strain since its planar structure is highly compressed, deviating by 49.5° from the ideal value. Additionally, cyclopropane has a torsional strain due to the eclipsing interaction between six C-H bonds. Hence, cyclopropane has an overall ring strain of 116 kJ/mol. Unlike cyclopropane, which is planar, cyclobutane takes up a more stable, folded non-planar conformation. Folding causes the angle strain to be slightly elevated compared to the hypothetical planar cyclobutane, but the torsional strain from the ten eclipsing hydrogens is greatly relieved. Cyclobutane has an overall strain of 110 kJ/mol. Cyclopentane also adopts a non-planar conformation known as envelope conformation. Compared to the hypothetical planar form of cyclopentane, the envelope form has its bond angles slightly reduced, which marginally increases the angle strain. However, it significantly alleviates the torsional strain from ten eclipsing C-H bonds. Hence, the overall strain in cyclopentane is 27 kJ/mol.