axial bond

Meaning: In the chair form of cyclohexane, each carbon has two substituent positions, axial and equatorial. An axial bond points straight up or down, parallel to the ring's central axis, and axial bonds alternate up and down from one carbon to the next. An axial substituent crowds the two other axial groups on the same face (1,3-diaxial interactions), which adds steric strain, so bulky groups usually prefer the equatorial position.

On the MCAT: Questions ask you to label axial and equatorial positions on a chair, or to pick the more stable chair after a ring flip. The larger group (tert-butyl is the classic case) usually goes equatorial. For disubstituted rings, cis-1,4 has one axial and one equatorial group, while trans-1,4 can be diequatorial or diaxial, and the diequatorial chair is usually favored.

Common confusion: Axial does not mean up. Up and down (wedge and dash) describe which face of the ring a group is on, while axial and equatorial describe position in the chair. A ring flip converts every axial bond to equatorial and vice versa, but a group that was up stays up.