3.10: Conformations of Cyclohexane
Cyclohexane does not exist in a planar form due to the high angle and torsional strain it would experience in the planar structure. Instead, it adopts non-planar chair and boat conformations.
The chair form is the most stable and derives its name from its resemblance to the “easy chair.” In the chair conformation, two carbon atoms are arranged out-of-plane — one above and one below, minimizing the torsional strain. In the chair form, the bond angle is very close to the ideal tetrahedral value, and hence, this form does not show any angle strain. The boat conformation, which derives its name from the resemblance to a boat, has 30 kJ/mol more strain energy than the stable chair form. Although the boat form is devoid of the angle strain, it has considerable torsional strain due to the upward-facing methylene groups. Additionally, due to the proximity of the hydrogen atoms on these groups, strong van der Waals repulsions, known as “flagpole interactions,” further destabilizes the boat form. Hence, the boat form slightly twists one of its C-C bonds to create the twist-boat conformation. Due to the twist, the flagpole hydrogens are now placed slightly apart, reducing the overall strain by 7 kJ/mol.