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Alkanes and Cycloalkanes

Chair Structure

The bond line cyclohexane form represents a 2-D view of a cyclic structure. The chair represents a 2-D representation of the 3-D reality of the molecule.
Drawing the chair form of cycloalkanes (cyclic hydrocarbons) is a way to present a three-dimensional depiction of the molecule. In the chair model of cyclohexane, carbons 1, 3, and 5 are above the plane, and carbons 2, 4, and 6 are below the plane. In this confirmation, all angles are 110.9°—close to the ideal 109.5° bond angle for alkanes, which is the ideal angle of tetrahedral geometry. Each carbon atom in the ring will have its attached hydrogen or substituent in either the equatorial or axial position. Equatorial is the position in which a substituent is perpendicular to the axis of the ring. Axial is the position in which a substituent is parallel to the axis of the ring.

Equatorial and Axial Bonds of Cyclohexane

Cyclohexane can flip continuously between its two chair conformations, but if the constituent groups are different, the molecule spends more time in the more stable conformation, which has the more sterically hindered group or the larger number of groups in the equatorial position.
A flip-chair conformation is the conversion of one cyclohexane chair conformation to another by rotation around the carbon-carbon single bonds. When the chair is flipped, the axial positions become equatorial and vice versa. Cyclohexane can flip between two chair conformations. If there are no substituents (nonhydrogen atoms or groups attached to the ring), the two forms will have equal energy. When substituents are added, the two forms may not have equal energy, and the molecule will spend more time in the more stable chair conformation. Substituents in the equatorial position provide more stability because axial substituents that are closer to each other tend to interact unfavorably. Any nonhydrogen substituent in an axial position on cyclohexane will create steric strain with other groups (including hydrogen) in any other axial position on the same side of the plane. This steric strain is called 1,3-diaxial interactions. The larger a substituent group, the more likely it is to be in the equatorial position to stabilize the chair and avoid 1,3-diaxial interactions.

Equatorial versus Axial Substituent Positioning

Substituents on cycloalkanes may be axial or equatorial. Axial substituents will always have 1,3-diaxial interactions, a type of steric strain, while equatorial substituents do not exhibit steric strain.
Groups on a ring can exhibit cis and trans isomerism. Cis-trans isomerism (or cis-trans) is when one of two or more molecules have the same molecular formula and bond structure but differ in the positions of their substituents around a rigid part of the molecule. Cis-trans isomerism can be indicated in a chair by using axial and equatorial positions of the chair form. Cycloalkanes may bear substituents on two carbons that are either on the same side or on opposite sides of the ring. Substituents on the same side are cis to each other, and substituents on opposite sides are trans to each other. When a ring flips, the axial and equatorial positions will interconvert, but cis and trans will not change. If a group is up in one chair conformation, it will be up in the other chair conformation.
In cis-1,4-dimethylcyclohexane with one group axial and the other equatorial. In trans-1,4-dimethylcyclohexane, both methyl groups are either both axial or both equatorial, with the equatorial conformation being more stable.
The cis-1,4-dimethylcyclohexane conformations are of equal stability, with one axial and one equatorial methyl group in both of the conformers. The two equivalent chair conformations are in equilibrium. In trans-1,4-dimethylcyclohexane, the conformation with both methyl groups in the equatorial position is more stable. While these conformers are in equilibrium too, the more stable form dominates that equilibrium.

The cis/trans designation is not related to the equatorial or axial position of the substituents. The equatorial/axial determination is determined by the position of the carbon atom to which the substituent is bonded. When the chair flips, the axial position will be changed, but the trans/cis conformation will remain the same.