equatorial bond
Summary
An equatorial bond on a chair cyclohexane points outward around the ring's "equator", roughly in the ring's average plane, rather than straight up or down like an axial bond. Substituents are usually more stable equatorial because that avoids 1,3-diaxial strain, so bulky groups generally prefer it.
Detail
Meaning: Each carbon of a chair cyclohexane carries two substituent positions. The axial bond is parallel to the ring's axis and alternates up and down around the ring. The equatorial bond points outward, tilted slightly opposite to the axial bond on that carbon. An axial substituent lies close to the axial groups on the same face, on the carbons three bonds away, which causes 1,3-diaxial steric strain. An equatorial substituent points away from the ring and avoids this. The equatorial conformer is therefore usually lower in energy, and the larger the group, the stronger the preference (a tert-butyl group is nearly locked equatorial). A ring flip converts every axial position to equatorial and vice versa.
On the MCAT: You may be shown two chair conformers of a substituted cyclohexane and asked which is more stable or which predominates at equilibrium. Place the largest group equatorial. Trans-1,4 and trans-1,2 isomers can be diequatorial. Cis-1,2 and cis-1,4 isomers must have one axial and one equatorial group.
Common confusion: Equatorial does not mean "down" or "up." Wedge or dash (cis or trans) is configuration and does not change in a ring flip, but axial or equatorial does. With several substituents, the best chair may still force a smaller group axial.
Sources
- Organic Chemistry (Clayden)
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.