Answer to Question #119681 in Organic Chemistry for balaji

Question

Explain the stability of axial and equatorial substitution of cyclohexane

Answer 1

In a monosubstituted cyclohexane, there can exist two different chair conformers: one with the substituent axial, the other with it equatorial. The two chair conformers will be in rapid equilibrium (by the process we have just described) but they will not have the same energy. In almost all cases, the conformer with the substituent axial is higher in energy, which means there will be less of this form present at equilibrium.

For example, in methylcylcohexane (X = CH3), the conformer with the methyl group axial is 7.3 kJ mol–1 higher in energy than the conformer with the methyl group equatorial. This energy difference corresponds to a 20:1 ratio of equatorial:axial conformers at 25 °C.

There are two reasons why the axial conformer is higher in energy than the equatorial conformer. The first is that the axial conformer is destabilized by the repulsion between the axial group X and the two axial hydrogen atoms on the same side of the ring. This interaction is known as the 1,3-diaxial interaction. As the group X gets larger, this interaction becomes more severe and there is less of the conformer with the group axial. The second reason is that in the equatorial conformer the C–X bond is anti-periplanar to

two C–C bonds, while, for the axial conformer, the C–X bond is synclinal (gauche) to two C–C bonds.

Clayden J., Greeves N., Warren S., Organic Chemistry, OUP Oxford, 2001