3.10 : Conformations du cyclohexane

Cyclohexane, in its hypothetical planar form, would have both angle strain and torsional strain.

While angle strain would result from bond angles of 120˚, the torsional strain would arise from eclipsing C-H bonds.

Due to the destabilizing effects of the two strains, the planar form of cyclohexane does not exist. Instead, cyclohexane adopts a non-planar chair and a boat conformation.

The chair conformation is the most stable form, with two carbon atoms bent on opposite sides, one above and one below the ring’s plane.

The chair form has no angle strain since bond angles are very close to 109.5°.

Additionally, the torsional strain is also absent in the chair conformer since all bonds, as viewed down the “seat” bonds, look perfectly staggered.

The boat conformation of cyclohexane, although devoid of the angle strain, has a torsional strain arising from eclipsing bonds of the two upward folded methylene groups at the opposite ends of the “boat”.

Due to the proximity, the two flagpole hydrogens experience a steric strain, also known as the flagpole interaction.

The combined effects of torsional and steric strain make the boat conformer higher in energy than the chair conformer.

The boat conformation partly relieves its strain by flexing to the twist-boat conformation, so the flagpole hydrogens move further apart, making the flexed form more stable.

Hence,  the twisted boat has lower energy than the symmetrical boat.

Due to the low energy barrier between the chair and the boat, the conformations interconvert several times.

The ring of one chair form transitions through several higher-energy conformations to give the other chair form. In the highest-energy, unstable half-chair conformation, the “footrest” becomes coplanar with the “seat” of the molecule.