Pictures of the Day CH320M328M

This is cyclohexane in the chair conformation. This is the most stable conformation for cyclohexane. Chair cyclohexane has no angle strain, as the bond angles around carbon are very close to the ideal 109.5°. Notice also how all the bonds are perfectly staggered, thereby eliminating torsional strain. As can be seen in the space filling model, there is no appreciable non-bonded interaction strain in unsubstituted chair cyclohexane. Non-bonded interactions strain is only introduced when an axial hydrogen atom is replaced by a larger atom or group.

Side (Above) and Top (Below) views of chair cyclohexane. Chair cyclohexane has two distinct types of hydrogen atoms, axial (red) and equitorial (purple). The axial positions point in a direction that is normal to the mean plane of the cyclohexane ring, while the equatorial positions are splayed out around the periphery of the ring. You need to be able to recognize the difference between axial and equatorial positions, and you need to be able to draw chair cyclohexane on a piece of paper.

Here are the two chair conformations of cyclohexane, with the hydrogen atoms colored to indicate the axial and equatorial positions. For the chair on the left, the axial hydrogens are drawn red, while the equatorial hydrogens are drawn purple. After the ring flips to the other chair, shown on the right, notice how the axial and equatorial hydrogens have exchanged.

Groups larger than hydrogen such as the methyl group shown here experience more non-bonded interaction strain (also callled steric strain, i.e. atom "crunching") in the axial position (1,3-diaxial interactions) than in the equatorial position. As a result, the chair on the right with the methyl group equatorial predominates at equilibrium. The larger the group, the greater its preference for being equatorial.