Axial and equatorial
- Axial bonds (blue in the drawings below) are parallel to the ring's axis, alternating up and down around the ring.
- Equatorial bonds point outwards, roughly along the ring's "equator", also alternating slightly up and down.
The ring flip
The chair can flip into a second chair. When it does, every axial position becomes equatorial and every equatorial becomes axial. Up stays up and down stays down: a group that was axial-up becomes equatorial-up. At room temperature cyclohexane flips about 100,000 times a second.
Why big groups prefer equatorial
An axial group bumps into the other two axial hydrogens on the same face of the ring. This 1,3-diaxial strain costs about 7.3 kJ/mol for a methyl group, so about 95% of methylcyclohexane molecules have the methyl equatorial at room temperature. A tert-butyl group is so large (about 20 kJ/mol) that it is effectively locked equatorial.
Disubstituted cyclohexanes
For two groups, work out which arrangement each isomer allows. The more stable chair has the larger group equatorial, or both groups equatorial if possible.
| Positions | cis | trans |
|---|---|---|
| 1,2 | one axial, one equatorial | both equatorial (or both axial) |
| 1,3 | both equatorial (or both axial) | one axial, one equatorial |
| 1,4 | one axial, one equatorial | both equatorial (or both axial) |
So trans-1,4-dimethylcyclohexane is more stable than cis, because its best chair has both methyls equatorial. For 1,3 the cis isomer is the more stable one.
How to draw a chair
- Draw two parallel lines, slightly offset, sloping down to the right.
- Join the top-left ends with a "V" pointing down-left, and the bottom-right ends with a "V" pointing up-right.
- Draw axial bonds vertically: up from the upward-pointing carbons, down from the downward-pointing ones.
- Draw equatorial bonds parallel to the ring bond once removed from that carbon.
Quick answers
What is the difference between axial and equatorial?
Axial bonds point straight up or down, parallel to the ring's axis. Equatorial bonds point outwards around the ring. Each carbon in a cyclohexane chair has one of each.
Why do large groups prefer the equatorial position?
An axial group clashes with the two other axial hydrogens on the same side of the ring (1,3-diaxial interactions). In the equatorial position it points away from the ring and avoids them.
What happens in a ring flip?
The chair converts into the other chair. Every axial group becomes equatorial and every equatorial group becomes axial, but groups that pointed up still point up.
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