3.11: Chair Conformation of Cyclohexane

The chair conformation is the most stable form of cyclohexane due to the absence of angle and torsional strain. The absence of angle strain is a result of cyclohexane’s bond angle being very close to the ideal tetrahedral bond angle of 109.5° in its chair conformer. Similarly, the torsional strain is also absent owing to the perfectly staggered arrangement of bonds.

The hydrogen atoms linked to carbons are arranged in two different axial and equatorial orientations to achieve this staggered form. The axial bonds are directed straight up or down, lying parallel to the ring axis, whereas the equatorial bonds are pointed sideways roughly along the equator of the ring. Out of the six axial bonds, three are pointed up, and the remaining three are pointed downward. Similarly, three bonds are slanted upwards among the six equatorial bonds, while the remaining three are slanted downwards. Thus, each carbon atom in the cyclohexane ring has an axial and an equatorial bond, pointing in opposite directions.

A chair conformation of cyclohexane can undergo a conformational change into another chair conformer by the partial rotation of C-C bonds. This chair-chair interconversion that leads to the generation of two equivalent energy forms is known as ring flipping. Upon ring flipping, the axial and equatorial bonds interchange their positions. The axial bonds in one chair conformation get converted to equatorial bonds in the other chair conformation, while equatorial bonds change their position to axial bonds.

The chair conformation, being the most stable and predominant form of cyclohexane, is non-planar, with the “headrest” and “footrest” bonds lying above and below the planar “seat” bonds.

The stability is a consequence of no angle strain — owing to the ‘almost tetrahedral’ C-C-C bond angles — and no torsional strain — due to the perfectly staggered bonds.

The staggered arrangement is achieved when the substituents on each carbon arrange in two distinct orientations: axial and equatorial.

Axial bonds point straight up or straight down, parallel to the vertical axis of the ring.

Among the six axial bonds, three bonds are directed upward, while the remaining three bonds are directed downward, and the direction alternates from one ring carbon to the next.

The equatorial bonds point out sideways —  positioned roughly along the equator of the ring — and are almost perpendicular to the vertical ring axis.

Out of the six equatorial bonds, three bonds have a slight upward slant, whereas the remaining three bonds have a slight downward slant.

Each carbon atom in the cyclohexane ring has an axial and an equatorial bond, pointing in opposite directions.

Moreover, each face of the ring has an alternating arrangement of axial and equatorial bonds.

At room temperature, one chair form conformationally changes into an energetically equivalent  ‘other’ chair form. This chair-chair interconversion is called ring flipping, and the two conformations are in equilibrium.

Ring flipping causes the positions of axial and equatorial bonds to interchange, meaning all axial bonds become equatorial and all equatorial bonds become axial.