Key structures for this topic — drawn live.
The chair flip: what actually moves
Cyclohexane is not flat. Its lowest-energy shape is the chair conformation, in which all bond angles are close to the ideal tetrahedral 109.5° and every neighboring C–H pair is staggered. Each carbon carries one axial bond (pointing straight up or down, parallel to the ring's vertical axis) and one equatorial bond (pointing outward, roughly along the "equator" of the ring).
A ring flip is the interconversion between two chair forms, passing through higher-energy half-chair, twist-boat, and boat conformations. The key rule to memorize: the ring flip converts every axial position to equatorial and every equatorial position to axial. What it does not change is the up/down orientation of a substituent — a group that points "up" stays "up" after the flip, it simply switches from (say) axial-up to equatorial-up. This is why cis/trans relationships are preserved by a flip even though axial/equatorial labels swap.
1,3-Diaxial strain
Why is one chair usually preferred? An axial substituent points directly toward the two other axial groups on the same face of the ring — the ones on carbons 3 and 5 relative to it. When those axial positions hold hydrogens, an axial substituent bumps into those hydrogens. These repulsive interactions are called 1,3-diaxial interactions, and they are essentially the same gauche/steric strain you see in a Newman projection of butane.
Moving the substituent to the equatorial position points it away into open space, relieving the strain. The larger the group, the worse the axial clash, so the driving force to sit equatorial grows with size.
A-values: quantifying the preference
The A-value of a substituent is the free-energy preference (in kcal/mol) for that group to occupy the equatorial rather than the axial position on a cyclohexane ring. Numerically it equals the energy of the axial conformer minus the equatorial conformer — a bigger A-value means a stronger equatorial preference. Typical values:
- –H: 0 (no preference — hydrogen is the reference)
- –CH3: ~1.7 kcal/mol
- –CH2CH3: ~1.8 kcal/mol
- –CH(CH3)2 (isopropyl): ~2.2 kcal/mol
- –C(CH3)3 (tert-butyl): ~4.9 kcal/mol
- –OH: ~0.9 kcal/mol; –F: ~0.25 kcal/mol
Notice that tert-butyl is enormous. An A-value of ~4.9 kcal/mol is so large that the ring is effectively locked with the tert-butyl group equatorial; the axial conformer is present in only trace amounts at room temperature.
Worked example: methylcyclohexane
Methylcyclohexane has two chair conformers: one with the methyl group axial, one with it equatorial. Using the A-value of methyl (~1.7 kcal/mol), the equatorial chair is favored by 1.7 kcal/mol. Plugging this into ΔG° = –RT ln K at 25 °C gives roughly a 95:5 ratio favoring the equatorial conformer. So at any instant about 95% of methylcyclohexane molecules have the methyl group equatorial.
You can even estimate strain from first principles: each axial methyl/hydrogen 1,3-diaxial interaction costs about 0.9 kcal/mol, and an axial methyl has two such interactions, giving ~1.8 kcal/mol — matching the measured A-value nicely.
Disubstituted rings: add up the A-values
When two substituents are present, the more stable chair is the one that places the larger group (or the greater number of groups) equatorial. If both groups can be equatorial simultaneously, that conformer dominates. If they conflict — one must go axial — compare A-values: the smaller-A-value group takes the axial spot. For a trans-1,2 or cis-1,4 pattern where both cannot be equatorial, subtract the two A-values to estimate the net preference. And when a tert-butyl group is present, it acts as a conformational anchor, forcing every other substituent into whatever position leaves it equatorial.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, curved arrows, wedge/dash bonds and a clickable periodic table built in. No account needed.