How do cyclohexane chair conformations work?
Cyclohexane folds into a chair shape that keeps its bond angles near the ideal tetrahedral angle and every C-H bond staggered, so it has almost no ring strain. Each carbon has one axial and one equatorial position, and large substituents are more stable equatorial.
Watch it explained
Why is the chair conformation of cyclohexane more stable?
An interactive 3D scene of cyclohexane as a chair and a boat: puckering keeps C–C–C angles near 109.5°, the chair's C–H bonds are all staggered while the boat's sides are eclipsed and its two flagpole hydrogens crowd each other, so the chair, free of angle, torsional and steric strain, sits about 29 kJ/mol lower.
Labeled in the scene:
- Two ring shapes
- Angles near tetrahedral
- Chair: all staggered
- Boat: eclipsed sides
- Flagpole clash
- Chair wins
Axial, equatorial and the ring flip
Each carbon in the chair has two bonds to substituents. The axial bond points straight up or down, parallel to the ring's axis, alternating up and down around the ring. The equatorial bond points outward, roughly along the ring's equator.
At room temperature the ring constantly flips between two chairs. In a ring flip, every axial position becomes equatorial and every equatorial becomes axial, but a group that pointed up still points up. That last part is the one students most often get wrong.
Why equatorial is better
An axial substituent sits close to the axial hydrogens two carbons away on the same face, at carbons 3 and 5 relative to it. These 1,3-diaxial interactions are steric strain. In methylcyclohexane, the chair with methyl equatorial is lower in energy, so most molecules are in that form at any moment. A tert-butyl group is so large that it stays equatorial almost all the time, effectively locking the ring.
For disubstituted rings, these patterns follow from the geometry and are worth knowing:
- 1,2: cis is one axial and one equatorial; trans can be both equatorial.
- 1,3: cis can be both equatorial; trans is one axial and one equatorial.
- 1,4: cis is one axial and one equatorial; trans can be both equatorial.
Worked example
Which chair of cis-1-tert-butyl-4-methylcyclohexane is more stable? Cis-1,4 means one group must be axial and one equatorial. Putting the tert-butyl axial would cost far more strain than putting the methyl axial, so the stable chair has tert-butyl equatorial and methyl axial.
Drawing tips: draw the chair with two parallel sets of lines, put axial bonds straight up at the upper corners and straight down at the lower ones, and draw each equatorial bond parallel to the ring bonds one carbon away. Check your answer by counting up and down groups, not by eye.
Flipping it yourself
A ring flip is hard to follow on paper and easy to follow in three dimensions. Ask Isotoon "What happens to axial and equatorial groups in a chair flip?" and, on Pro, open it as a 3D model you can turn and step through. Related: what makes a molecule chiral, the organic chemistry page, and how to study for organic chemistry exams.
Questions
- Why isn't cyclohexane flat?
- A flat hexagon would force 120 degree bond angles on sp3 carbons and eclipse every neighboring C-H bond. Puckering into a chair relieves both kinds of strain.
- What about the boat conformation?
- The boat is higher in energy than the chair: some C-H bonds are eclipsed and two hydrogens at the ends of the boat point toward each other. Cyclohexane passes through boat-like shapes during a ring flip but spends very little time there.