Cyclohexane

The cyclohexane ring is very special because it lacks tension. This is very likely the reason why cyclohexane is so abundant in natural substances.

Hypothetical planar cyclohexane: 120° angles and all hydrogens eclipsed

If cyclohexane were planar, its internal angles would be 120º, far from ideal for sp³ hybridization. Besides, all hydrogens would be eclipsed.

Space-filling model of planar cyclohexane, which turns into a non-planar shape — Therefore, cyclohexane immediately adopts a non-planar shape.

Cyclohexane is not planar but adopts a so-called chair conformation, where the internal angles are very close to 109.5º, the natural angle of sp³ hybridization. In addition, all hydrogens are perfectly staggered in the chair conformation.

Chair conformation of cyclohexane with its angles (111.4° and 107.5°) and C–C distance (1.536 Å)

Cyclohexane chair with axial (A) and equatorial (E) positions

Can you tell apart two different hydrogen positions in the chair conformer of cyclohexane? Yes: equatorial (E) and axial (A).

Inverted cyclohexane chair: axial (A) and equatorial (E) positions are exchanged

Since cyclohexane is built from single C-C bonds, they can rotate. The combined rotation of several bonds gives rise to a conformational equilibrium in which the chair inverts. Note that chair inversion exchanges the positions: all equatorial hydrogens become axial and vice versa.

Animated space-filling model of cyclohexane chair inversion

Animated stick model of cyclohexane chair inversion

The mechanism of this combined bond torsion is quite complex and passes through 'boat' and 'twist-boat' conformers, with an overall inversion barrier of almost 11 kcal/mol, about four times that of ethane. Even so, the barrier is overcome several thousand times per second at room temperature.

Space-filling model of a cyclohexane chair with the axial hydrogens coloured

Space-filling model of the inverted chair: the coloured hydrogens become equatorial

The two chairs in equilibrium of a cyclohexane ring can be represented in Newman projection.

Newman projection of a cyclohexane chair viewed along two C–C bonds

Newman projection of the inverted cyclohexane chair

It is like drawing two 'Siamese' butane molecules in which the methyl groups have become shared CH₂ groups. Look at how methylcyclohexane is drawn with its methyl group axial.

Animation of methylcyclohexane in Newman projection with the methyl group axial