Question

Why is half chair conformation of cyclohexane less stable than boat conformation? Actually in boat configuration there is repulsion between flag hydrogen ,but in half chair conformation there is no such repulsions. Then why is boat conformation more stable?

Answer 1

The half-chair state 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 energies of the boat and half-chair states are 6.6 kcal mol−1 (28 kJ mol−1) and 10.8 kcal mol−1 (45 kJ mol−1) higher than that of the chair. The conformations involve following order of stability: chair form > twist boat form > boat form > half-chair form.