3.11 : 环己烷的椅型构象

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.