3.9 : シクロアルカンの立体構造

The relative stabilities of cycloalkanes vary with their ring sizes. The variations arise from the combined effects of angle strain and torsional strain, together known as the ring strain of cyclic compounds.

Angle strain is introduced in a cycloalkane when the C-C-C bond angle deviates from the regular tetrahedral bond angle of 109.5°, as predicted for all sp³  hybridized carbons of alkanes.

On the other hand, the torsional strain exists between the eclipsing bonds and is a result of repulsive dispersion forces.

In cyclopropane, the internal angle of 60°, which is significantly smaller than the ideal angle, introduces a severe angle strain in the molecule, forcing the sp³ orbitals to overlap at an angle to give weaker “bent” carbon-carbon bonds.

Cyclopropane, owing to its planar nature, also experiences considerable torsional strain from the six pairs of fully eclipsed C-H bonds.

In essence, cyclopropane is a highly strained molecule with strain energy as high as 116 kJ/mol. For this reason, cyclopropanes are highly reactive.

Unlike cyclopropane, cyclobutane is non-planar and takes up a folded conformation. Folded cyclobutane is more stable than its hypothetical planar form.

Although folding of the ring increases the angle strain slightly, by lowering the bond angle from 90° to 88°, it substantially reduces the torsional strain associated with eight eclipsing C-H bonds, resulting in the net strain energy of 110 kJ/mol.

Like cyclobutane, cyclopentane is also non-planar and assumes an envelope conformation, with one or two atoms bending out of the plane. 

Although a hypothetical planar cyclopentane would have a 108° bond angle — very close to the ideal value, the envelope conformation greatly relieves the torsional strain from ten eclipsing bonds with only a slight rise in the angle strain.

Therefore, the overall ring strain in cyclopentane is as low as 27 kJ/mol.