Chair Conformations and Newman Projections: A Step-by-Step Guide with a Free Drawing Tool

The chair conformation

Cyclohexane is not flat. A flat hexagon would have 120° angles and eclipsed C–H bonds. The ring folds into a chair, where every C–C–C angle is about 109.5° and every bond is staggered. The chair has almost no angle or torsional strain, so it is the conformation that nearly all cyclohexane molecules adopt.

Axial and equatorial bonds

• Axial bonds are parallel to the axis of the ring. They point straight up or straight down and alternate up, down, up around the ring.
• Equatorial bonds point outward around the "equator". Each one is parallel to the two ring bonds that are one carbon away.
• On each carbon the axial and equatorial bonds point to opposite faces. If the axial bond is up, the equatorial bond tilts slightly down.

The ring flip

The ring flips between two chairs about 100 000 times a second at room temperature (barrier about 45 kJ/mol). In a ring flip, every axial position becomes equatorial and every equatorial position becomes axial. A group that points up stays up. That is why cis and trans never change on a ring flip.

A-values and the more stable chair

An axial group sits close to the two axial hydrogens three carbons along on the same face. These 1,3-diaxial interactions make the axial chair less stable. The A-value is the free energy difference, ΔG°, between the axial and the equatorial chair of a monosubstituted cyclohexane.

To turn ΔG into a ratio, use ΔG° = −RT ln K. At 298 K, RT = 2.48 kJ/mol. For methyl: K = e^7.3/2.48 = 19, so 19 equatorial molecules for every axial one, or 95%.

Two or more groups

1. Draw both chairs. Keep each group on its face (up or down).
2. For each chair, add the A-value of every axial group.
3. Add about 8 kJ/mol more if two groups (not H) are axial on the same face, 1,3 to each other. Add 3.8 kJ/mol for two carbon groups on neighbouring carbons that are gauche (both equatorial, or one axial and one equatorial).
4. The chair with the lower total is more stable. ΔG is the difference of the totals.

How to use the chair and Newman drawer

1. Open the chair conformation and Newman projection drawer. The Chair conformations tab is open.
2. Press an example such as trans-1,2-Dimethyl, or choose an Up (wedge) and a Down (dash) group for C1 to C6.
3. Read both chairs. Axial groups are drawn in orange, equatorial groups in blue. The more stable chair is marked more stable.
4. Read the strain table: every A-value, 1,3-diaxial clash and gauche interaction, then ΔG, K and the % of each chair at 298 K.
5. Press Animate ring flip to watch the axial groups turn equatorial. Save the drawing with Download SVG or Download PNG.
6. Open the Newman projection tab, choose Butane (C2–C3), then move the slider or press +60°, Anti (180°) or Animate 360°. The plot shows the energy at every angle.

Worked examples

1. trans-1,2-Dimethylcyclohexane

One chair has both methyls equatorial. The two methyls are gauche, so the strain is 3.8 kJ/mol. The flipped chair has both axial: 2 × 7.3 = 14.6 kJ/mol. ΔG = 14.6 − 3.8 = 10.8 kJ/mol, K = 78, so 98.7% diequatorial.

2. cis-1,3-Dimethylcyclohexane

In cis-1,3 the groups are both equatorial or both axial. Diaxial: 2 × 7.3 plus about 8 for the CH₃/CH₃ 1,3-diaxial clash, about 22.6 kJ/mol. Diequatorial: 0. More than 99.9% diequatorial.

3. cis-1-tert-Butyl-4-methylcyclohexane

One group must be axial. tert-Butyl (20.5) costs far more than methyl (7.3), so tert-butyl stays equatorial and methyl goes axial. ΔG = 13.2 kJ/mol, more than 99% of one chair. tert-Butyl "locks" the ring.

4. Menthol

In (−)-menthol the OH, isopropyl and methyl groups can all be equatorial in one chair, so that chair is strongly favoured. The flipped chair would put all three axial.

5. Butane gauche and anti

Gauche butane is 3.8 kJ/mol above anti, and there are two gauche forms. K = 2 × e^−3.8/2.48 = 0.43, so about 70% anti and 30% gauche at 298 K.

Newman projections

A Newman projection looks straight along one C–C bond. The front carbon is a point; its three bonds meet in the centre. The back carbon is a circle; its bonds start at the edge. The angle between a front bond and a back bond is the dihedral angle.

For ethane the three H/H eclipsing interactions give a 12 kJ/mol barrier; for propane (2 H/H + 1 H/CH₃) it is 14 kJ/mol.

Summary table

Common mistakes

• Converting cis to trans during a ring flip. A ring flip never changes cis or trans.
• Thinking cis always means both equatorial. For 1,2 and 1,4, cis is axial plus equatorial. For 1,3, cis is diequatorial or diaxial.
• Drawing axial bonds at an angle. They are vertical.
• Forgetting the gauche interaction in diequatorial trans-1,2-dimethylcyclohexane.
• Using °C in ΔG = −RT ln K. Use kelvin.
• Calling the eclipsed butane at 120° "fully eclipsed". Fully eclipsed means CH₃ behind CH₃ at 0°.

Exam tips

• JEE and NEET: know the order of conformer stability for butane (anti > gauche > eclipsed > fully eclipsed) and that the chair is more stable than the twist-boat, boat and half-chair.
• AP Chemistry and A-level: be able to explain stability with steric strain: bulky groups prefer equatorial.
• University organic: calculate ΔG from A-values and convert it to K and % population. Remember that tert-butyl locks the ring and that cis-1,4-di-tert-butylcyclohexane adopts a twist-boat.
• When you draw a chair in an exam, draw the axial bonds first (straight up and down), then the equatorial bonds parallel to ring bonds.

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