Cyclohexane Conformation - Interconversions Between Conformations

Interconversions Between Conformations

At room temperature there is a rapid equilibrium between the two chair conformations of cyclohexane. The interconversion of these two conformations has been much debated and still lacks consensus. What is known is that the twist-boat and chair are both energy minima—the twist-boat being a local minimum; the chair being a global minimum (ground state).

The half-chair state (2, below) is the transition state in the interconversion between the chair and twist-boat conformations. Due to 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. Thus, at a minimum, the interconversion between the two chair conformations involves the following sequence: chair - half-chair - twist-boat - half-chair' - chair'. The conformations involve following order of stability: chair form > twist boat form > boat form > half-chair form. The boat conformation (4, below) is also a transition state, allowing the interconversion between two different twist-boat conformations. While the boat conformation is not necessary for interconversion between the two chair conformations of cyclohexane, it is often 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. Thus, there are multiple pathways by which a molecule of cyclohexane in the twist-boat conformation can achieve the chair conformation again.