At room temperature, there is a rapid equilibrium between the two chair conformations of cyclohexane. The twist-boat and chair are both energy minima—the twist-boat being a local minimum and the chair being a global minimum (ground state). The energy of twist-boat conformation is 5.5 kcal mol−1 (23 kJ mol−1) higher than that of the chair conformation.
The half-chair state (2, below) is the transition state in the interconversion between chair and twist-boat conformations. Because of the D2 symmetry of the twist-boat, there are two energy-equivalent pathways that it can take to two different half-chair conformations, leading to the two different chair conformations of cyclochexane. Therefore, at a minimum, the interconversion between the two chair conformations involves the following sequence: chair - half-chair - twist-boat - half-chair' - chair'. The energies of boat and half-chair states are 6.5kcal mol−1 (27 kJ mol−1) and 10kcal mol−1 (42 kJ mol−1) higher than that of the chair. The conformations involve following stability order: chair form > twist boat form > boat form > half-chair form. …show more content…
While the boat conformation is not necessary for interconversion between the two chair conformations of cyclohexane, it is usually included in the reaction coordinate diagram used to describe this interconversion because its energy is considerably lower than that of the half-chair, so any molecule with enough energy to go from twist-boat to chair also has enough energy to go from twist-boat to boat. Therefore, there are multiple pathways by which a molecule of cyclohexane in the twist-boat conformation can achieve the chair conformation