Chair conformation

The chair conformation is a low-energy nonplanar arrangement of the six-membered cyclohexane ring. Its carbon-carbon bonds are approximately staggered and its bond angles remain close to tetrahedral values, reducing torsional and angle strain.

Each ring carbon has one axial and one equatorial substituent position. A ring flip exchanges these positions while retaining whether each substituent points to the same or opposite side of the ring. Bulky substituents commonly prefer equatorial positions because this reduces unfavorable 1,3-diaxial interactions, making chair analysis useful for predicting conformational populations and reaction geometry.^[ref]

In a chair conformation of cyclohexane, axial bonds point approximately parallel to the ring's threefold axis, alternating up and down around the ring. Equatorial bonds point outward around its perimeter, also alternating in their up/down relationship to the ring.

A ring flip exchanges axial and equatorial positions but preserves a substituent's up or down orientation. Thus axial does not mean up, and equatorial does not mean down. In methylcyclohexane the equatorial methyl conformation is favored because an axial methyl group has unfavorable close contacts with axial substituents on carbons three and five.^[ref]