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Cyclohexane's chair has near-tetrahedral angles and all bonds staggered, with six axial and six equatorial positions that a ring flip swaps without changing up or down; an axial group pays two 1,3-diaxial interactions, so substituents prefer equatorial.
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You will count and name chair positions, follow substituents through a ring flip, price an axial group's 1,3-diaxial interactions, and compare the two chairs of a disubstituted ring.
You know that eclipsed bonds cost energy and that sp3 carbons prefer 109.5°. A ring has to reconcile both, and cyclohexane's chair does it perfectly.
The chair is cyclohexane's puckered, lowest-energy shape. Axial positions run straight up or down, parallel to the ring's axis; equatorial positions point out round the ring. A ring flip converts one chair into the other. A 1,3-diaxial interaction is an axial group crowding the axial hydrogens two carbons away.
A flat hexagon would force 120° angles and eclipse every bond. Pucker it — one end up, the other down — and you get the chair: every C–C–C angle close to 111° and every C–C bond perfectly staggered. It has almost no strain.
Each ring carbon has two bonds to fill: one axial, running straight up or down (alternating round the ring), and one equatorial, pointing outwards. So a chair has six axial and six equatorial positions.
The ring flips constantly at room temperature: one chair turns inside out into the other. Every axial position becomes equatorial and every equatorial becomes axial — but a group above the ring stays above it.
Another way: steps
To follow a substituent through a flip:
An axial group points straight up alongside the axial hydrogens on the carbons two along in each direction — carbons 3 and 5 if it sits on carbon 1. It crowds both: two 1,3-diaxial interactions. For a methyl each costs about 3.8 kJ/mol, so axial methylcyclohexane is 7.6 kJ/mol above the equatorial chair, and about 95% of molecules are equatorial at room temperature. Larger groups cost more; a tert-butyl group is so large that its ring is locked with it equatorial.
With two substituents, add up each chair's costs and compare. In trans-1,4-dimethylcyclohexane one chair has both methyls equatorial (no cost) and the other both axial ($4 \times 3.8 = 15.2$ kJ/mol), so the diequatorial chair wins overwhelmingly.
All chair substituents are equatorial. A chair has six axial and six equatorial positions, and flips.
A ring flip turns up into down. It swaps axial and equatorial only.
The chair is flat. It is puckered.
An axial group crowds its neighbour. It crowds the axial hydrogens two carbons away.
The –OH is axial on C1: it crowds the axial hydrogens on C3 and C5.
Two 1,3-diaxial interactions.
At 1.9 kJ/mol each, the axial chair is 3.8 kJ/mol higher.
Smaller than a methyl.
After a ring flip the –OH is equatorial and those interactions vanish.
Flip to relieve strain.
How many 1,3-diaxial interactions?
Two.
How much higher is the axial chair?
$2 \times 4.0 = 8.0$ kJ/mol.
In one chair conformation of cyclohexane, C6H12, how many of the hydrogens are in axial positions?
Answer:
Match each term to its description.
| a bond running straight up or down, parallel to the ring's axis | a bond pointing out round the ring | one chair turning into the other | an axial group crowding axial hydrogens on its own side | |
|---|---|---|---|---|
| axial | ||||
| equatorial | ||||
| ring flip | ||||
| 1,3-diaxial interaction |
Methylcyclohexane has its methyl group axial and pointing up. The ring flips. Where is the methyl now?
In one chair of a disubstituted cyclohexane, a chlorine is axial-up on C1 and a methyl is equatorial-down on C4. After a ring flip, where is each?
| axial or equatorial after the flip | up or down after the flip | |
|---|---|---|
| the chlorine on C1 | ||
| the methyl on C4 |
trans-1,4-dimethylcyclohexane has two chairs: one with both methyls equatorial and one with both axial. Each CH3/H 1,3-diaxial interaction costs 3.8 kJ/mol. How much higher in energy, in kJ/mol, is the diaxial chair?
Answer:
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
A cyclohexane ring carries a chlorine atom, –Cl. When it is axial, each 1,3-diaxial interaction it makes with an axial hydrogen costs $1$ kJ/mol. How much higher in energy, in kJ/mol, is the axial chair than the equatorial one?
The axial chair is higher in energy by a kJ/mol.
You can place groups on a chair and say which chair is favoured. Tell someone why a ring flip does not turn up into down. Next: ranking whole conformers by all their strains together.
8. Your turn: axial ethylcyclohexane, at 4.0 kJ/mol per interaction, step 3