The real chair, drawn here — opposite ring bonds are parallel. A flat hexagon can’t show axial vs equatorial; the chair can.
Building on the chair conformation: what happens during a ring flip, and how A-values predict which chair a substituted ring prefers.
Summary figure: the same ring skeleton, but the bigger the substituent, the harder it insists on sitting equatorial.
1. A Ring Flip Interconverts the Two Chairs and Swaps Axial for Equatorial
A ring hops between two equivalent chairs; the flip turns every axial bond equatorial and vice-versa, but never turns the molecule over — so axial-up becomes equatorial-up, and cis/trans is preserved.
2. The Flip Climbs Over Higher-Energy Half-Chair and Twist-Boat Forms
The path runs half-chair → twist-boat → half-chair over a ~10.8 kcal/mol barrier — low enough to flip ~105×/sec, so we see a weighted average of conformers, not one frozen shape.
3. Axial Groups Suffer 1,3-Diaxial Strain, So Substituents Prefer Equatorial
An axial group crowds the two axial H's on carbons 3 and 5 (1,3-diaxial strain); going equatorial swings it into open space, and the bigger the group, the stronger that drive.
4. The A-Value Measures How Strongly a Group Prefers Equatorial
The A-value is the free-energy cost (kcal/mol) of forcing a group axial (H = 0); via ΔG° = –RT ln K, methyl's 1.7 gives a ~95:5 equatorial preference.
5. Bigger Groups Have Bigger A-Values — tert-Butyl Locks the Ring
A-value climbs with bulk (methyl 1.7 → isopropyl 2.1), then tert-butyl jumps to ≈4.7 — under 1:1000 axial, so it locks the ring in its equatorial chair.
6. For Disubstituted Rings, the Favored Chair Minimizes Total Axial Strain
Read cis/trans first (fixed, can't ring-flip), then pick the chair that puts both groups equatorial — or if they conflict, sends the smaller A-value axial.
7. Summary
Ring flip swaps axial ⇄ equatorial, keeps cis/trans · ~10.8 kcal/mol barrier, ~105 flips/sec · axial pays 1,3-diaxial strain · A-values: H 0 · F 0.15 · Cl/Br 0.5 · OH 0.9 · CH3 1.7 · iPr 2.1 · t-Bu 4.7 (locks) · disubstituted: cis/trans first, then maximize equatorial · see chair conformations.
Quiz yourself
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Equatorial-up. The flip swaps axial ⇄ equatorial but does not turn the molecule over, so "up" stays "up." It never becomes equatorial-down — which is why cis/trans relationships survive a ring flip.
It is the free-energy cost of forcing methyl axial, i.e. the equatorial chair is favored by 1.7 kcal/mol. Via ΔG° = –RT ln K that's roughly a 95:5 equatorial:axial population at 25 °C.
Its A-value is ~4.7 kcal/mol — huge. Forcing it axial costs so much energy that the axial conformer is under ~0.1% populated, so the ring stays fixed in whichever chair keeps tert-butyl equatorial. Everything else is then forced into whatever position remains.
The trans isomer, because it can place both methyls equatorial in one chair. The cis isomer is forced to keep one methyl axial no matter which chair it adopts, so it always carries ~1.7 kcal/mol of extra strain.
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.