15. - Organic Chemistry (Mcmurry)

Question

What is the energy difference between the axial and equatorial conformations of cyclohexanol (hydroxycyclohexane)?

Step-by-Step Solution

Answer

The energy difference between cyclohexanol's axial and equatorial conformations is 4.2 kJ/mol.

Chair conformation

Chair conformation structure for a six-membered ring is when atoms 2, 3, 5, and 6 lie in the same plane, atom one lies above the plane, and atom 4 lies below the plane.

Axial and equatorial positions

Axial bonds are the bonds that are perpendicular or vertical to the ring plane. Equatorial bonds are approximately horizontal in the plane of the ring.

Energy difference cause and calculation

The energy difference between axial and equatorial conformations is due to steric strain caused by 1,3-diaxial interactions. The axial hydroxyl group on C1 is too close to the axial hydrogens three carbons away on C3 and C5, resulting in two times the steric strain. 

One 1,3-diaxial interaction results in 2.1 kJ/mol increase in energy of axial cyclohexanol.

Hence, two such interactions give 4.2 kJ/mol.