Difficulty: Intermediate | Prerequisites: Part 1 (acyclic conformational analysis), sp3 hybridisation, Newman projections.
Cycloalkanes introduce a new type of strain (angle strain) on top of the torsional and steric strain you already know. Cyclohexane and its chair conformation are the centrepiece of this topic and will appear throughout the rest of organic chemistry. If you are not comfortable with Newman projections and the concepts of torsional and steric strain from Part 1, review those first.
Small rings (3 and 4 carbons) are highly strained because their bond angles are forced far from the ideal 109.5° and their hydrogens are locked in near-eclipsed positions. Cyclohexane escapes all of this by adopting a "chair" shape with perfect bond angles and fully staggered hydrogens. Substituents on cyclohexane can sit in axial (up/down) or equatorial (around the perimeter) positions, and larger groups strongly prefer the equatorial position to avoid 1,3-diaxial steric interactions.
Ring strain
The total excess potential energy a cycloalkane carries compared to a hypothetical strain-free ring. It is the sum of angle strain, torsional strain, and (in some cases) steric strain.
Angle strain (Baeyer strain)
The strain that arises when bond angles in a ring are forced away from the ideal sp3 tetrahedral angle of 109.5°. Smaller rings have more angle strain. Think of it as: bending the bonds away from where they naturally want to point.
Puckering
The bending of a ring out of a flat plane to reduce torsional strain. Cyclobutane and cyclopentane both pucker. The trade-off is that puckering can slightly increase angle strain, but the net effect is lower total strain.
Chair conformation
The lowest-energy conformation of cyclohexane. All C-C-C bond angles are 109.5° (no angle strain) and all adjacent hydrogens are perfectly staggered (no torsional strain). This is the most important single conformation in organic chemistry.
Boat conformation
A higher-energy conformation of cyclohexane where the ring is folded so that two opposite carbons point up, resembling a boat. It has torsional strain from eclipsing and steric strain from "flagpole" interactions.
Twist-boat conformation
A slightly twisted version of the boat that relieves some of the eclipsing and flagpole strain. It is a local energy minimum between the two chair conformations, 23 kJ/mol above the chair.
Half-chair conformation
The highest-energy conformation of cyclohexane, where four carbons are coplanar. It is the transition state between chair and twist-boat. Energy: 42 kJ/mol above the chair.
Axial position
One of 6 positions on the cyclohexane chair that point straight up or straight down, perpendicular to the rough plane of the ring (3 up and 3 down, alternating around the ring). Think of it as: the Earth's axis, pointing to the poles.
Equatorial position
One of 6 positions on the cyclohexane chair that fan out around the perimeter of the ring, roughly in the plane. Think of it as: the equator, around the middle.
Ring flip (chair-chair interconversion)
The process by which one chair conformation of cyclohexane converts to the other. Every axial substituent becomes equatorial, and every equatorial substituent becomes axial. This is fast at room temperature.
1,3-Diaxial interaction
The steric repulsion between an axial substituent and the axial hydrogens (or other axial groups) on the same face of the ring, two carbons away. This is analogous to a gauche interaction in butane. Larger axial groups have larger 1,3-diaxial strain.
Carbon atoms in cyclopropane are sp3 hybridised and want 109.5° bond angles, but the ring geometry forces them to 60°. That is a compression of 49.5°, which severely reduces orbital overlap and weakens the C-C bonds.
Because a three-carbon ring must be planar, all hydrogens on adjacent carbons are locked in a fully eclipsed arrangement. This adds substantial torsional strain on top of the angle strain.
Total ring strain: 115 kJ/mol. This makes cyclopropane highly reactive compared to larger rings.
A perfectly flat square would have 90° bond angles (still 19.5° below the ideal) and fully eclipsed hydrogens. Cyclobutane relieves some of this by puckering: one carbon bends out of the plane, bringing the internal angle to about 88° and pushing the hydrogens into a "not quite eclipsed" arrangement.
Total ring strain: 109 kJ/mol. Nearly as high as cyclopropane, because the angle strain is still severe.
If planar, cyclopentane would have internal angles of 108°, which is almost perfect for sp3. The problem is that a flat five-membered ring locks all hydrogens in eclipsed positions, causing maximum torsional strain.
Cyclopentane solves this by puckering into an "envelope" conformation where one carbon flaps out of the plane. This introduces a small amount of angle strain but dramatically reduces torsional strain.
Total ring strain: only 27 kJ/mol. Cyclopentane is far more stable than cyclopropane or cyclobutane.
Cyclohexane is the key ring system. If it were flat, bond angles would be 120° (too large for sp3) and all hydrogens would be eclipsed.
The chair conformation solves both problems at once:
All C-C-C bond angles are exactly 109.5° (zero angle strain).
All adjacent C-H bonds are perfectly staggered (zero torsional strain).
Total ring strain: 0 kJ/mol.
Cyclohexane energy landscape: The chair is the global minimum. To convert between the two chair conformations, the ring must pass through higher-energy forms: chair → half-chair (42 kJ/mol above chair, the transition state) → twist-boat (23 kJ/mol above chair, a local minimum) → boat (29 kJ/mol above chair, a local maximum between twist-boats) → half-chair → chair.
In the chair conformation, the 12 hydrogens of cyclohexane occupy two types of position:
Axial (ax): 6 hydrogens point straight up or straight down, perpendicular to the average plane of the ring. They alternate: up-down-up-down-up-down around the ring.
Equatorial (eq): 6 hydrogens fan out around the perimeter, roughly in the plane of the ring.
When the ring flips to the other chair, every axial position becomes equatorial and every equatorial position becomes axial. Crucially, a substituent that was pointing "up" (above the ring plane) still points "up" after the flip. Wedge vs dash (up vs down) does not change. Only axial vs equatorial changes.
When a substituent (e.g. a methyl group) occupies an axial position, it experiences steric repulsion with the other axial groups on the same face of the ring, specifically those on carbons 1 and 3 positions away. Each such interaction is equivalent to one gauche-butane interaction.
For methylcyclohexane:
Axial methyl has two 1,3-diaxial interactions with axial H atoms, costing 7.6 kJ/mol.
Equatorial methyl has no such interactions.
At room temperature, about 95% of the molecules have methyl in the equatorial position.
Larger substituents have an even stronger preference for the equatorial position. A tert-butyl group, for example, has a 1,3-diaxial strain of 23 kJ/mol and sits equatorial with a ratio of 11,000:1.
trans-1,2-dimethylcyclohexane: In one chair, both methyls are equatorial (only one CH3-CH3 gauche interaction). In the other chair, both are axial (four CH3-H 1,3-diaxial interactions, much more strain). The di-equatorial conformer is strongly favoured.
cis-1,3-dimethylcyclohexane: In one chair, both methyls are equatorial (no strain). In the other, both are axial (two CH3-H 1,3-diaxials plus one severe CH3-CH3 1,3-diaxial). The di-equatorial conformer is strongly favoured.
The general rule: when two or more substituents compete for equatorial positions, the equilibrium favours the chair in which the largest group is equatorial.
In small rings that cannot undergo ring flips, cis/trans isomerism still affects stability. For 1,2-dimethylcyclopropane, the trans isomer is more stable than cis because the cis arrangement forces the two methyl groups into an eclipsed relationship, adding steric strain.
Ring Size | Cycloalkane | Total Ring Strain (kJ/mol) | Main Sources of Strain |
|---|---|---|---|
3 | Cyclopropane | 115 | Severe angle strain + torsional strain |
4 | Cyclobutane | 109 | Angle strain + some torsional strain |
5 | Cyclopentane | 27 | Mostly torsional strain (puckering trades some angle strain) |
6 | Cyclohexane | 0 | None (chair conformation) |
Substituent | 1,3-Diaxial Strain (kJ/mol) | Eq : Ax Ratio at 25 °C |
|---|---|---|
H | 0 | 1 : 1 |
F | 0.8 | 1.4 : 1 |
Cl | 2.1 | 2.3 : 1 |
Br | 2.5 | 2.7 : 1 |
OH | 4.1 | 5.4 : 1 |
Methyl (CH3) | 7.6 | 21 : 1 |
Isopropyl | 8.8 | 35 : 1 |
tert-Butyl | 23 | 11,000 : 1 |
Note: the 1,3-diaxial strain value represents the total cost of placing that substituent axial (i.e. the sum of both 1,3-diaxial interactions). Each individual interaction is half of the listed value.
Conformation | Energy Above Chair (kJ/mol) |
|---|---|
Chair | 0 |
Twist-boat | 23 (5.5 kcal/mol) |
Boat | 29 (6.9 kcal/mol) |
Half-chair | 42 (10 kcal/mol) |
Cyclohexane rings are everywhere in nature and pharmaceuticals. Glucose (blood sugar) exists primarily as a six-membered ring in a chair conformation, and the positions of its hydroxyl groups (axial vs equatorial) directly affect its biological properties. Steroids (cholesterol, testosterone, cortisol) are built from fused six-membered rings, all locked in chair conformations. Understanding which chair is more stable is essential for predicting the shape and reactivity of these molecules.
Students often think that wedge bonds are always equatorial and dash bonds are always axial. This is wrong. Wedge means "above the plane of the ring" and dash means "below the plane," which is independent of axial vs equatorial. Whether a substituent pointing up is axial or equatorial depends on which carbon it is on and which chair you are drawing.
Students sometimes believe that the boat conformation is the transition state for a ring flip. The transition state is the half-chair (42 kJ/mol above the chair). The boat and twist-boat are intermediates along the pathway.
Students frequently forget that a ring flip changes axial to equatorial but does not change whether a group points up or down. If a methyl is pointing up (wedge) and axial, after the ring flip it will still point up but will now be equatorial.
Students occasionally think cyclopentane is strain-free because its angles are close to 109.5°. It still has 27 kJ/mol of ring strain, mostly from residual torsional strain even after puckering.
⚠️ Drawing both chair conformers of a substituted cyclohexane and identifying the more stable one is one of the most commonly tested skills in this chapter. Practise until you can do it quickly and accurately.
⚠️ Know the 1,3-diaxial strain values for common substituents, especially methyl and tert-butyl. You may be asked to calculate the energy difference between two chair conformers.
⚠️ For disubstituted cyclohexanes, be able to determine which chair has both groups equatorial, or which chair puts the larger group equatorial when both cannot be equatorial at once.
⚠️ Converting a flat (planar) drawing of a cyclohexane to its chair conformation is a frequently tested procedural skill. Remember: wedge = above the plane, dash = below the plane.
True or false: Cyclohexane in the chair conformation has zero ring strain.
Fill in the blank: The total ring strain of cyclopropane is ______ kJ/mol.
True or false: A ring flip converts axial positions to equatorial and equatorial to axial.
Fill in the blank: An axial methyl group on cyclohexane has ______ 1,3-diaxial interactions with axial hydrogens.
True or false: A tert-butyl group on cyclohexane strongly prefers the axial position.
Answers: 1. True. 2. 115 kJ/mol. 3. True. 4. Two. 5. False (tert-butyl overwhelmingly prefers equatorial, with an eq:ax ratio of 11,000:1).
Q: Draw both chair conformers of trans-1,4-dimethylcyclohexane. Which is more stable and why?
A: In one chair, both methyls are equatorial (trans-1,4 means one is up and one is down, which in a 1,4-relationship corresponds to both equatorial or both axial). In the other chair, both methyls are axial. The di-equatorial conformer is more stable because it avoids 1,3-diaxial interactions.
Q: Why does cyclopropane have more total ring strain than cyclobutane, despite cyclobutane having nearly the same total strain energy?
A: Cyclopropane has 115 kJ/mol and cyclobutane 109 kJ/mol, so cyclopropane is slightly higher. Cyclopropane has worse angle strain (60° vs ideal 109.5°, a compression of 49.5°) and fully eclipsed hydrogens because it must be planar. Cyclobutane puckers to relieve some torsional strain, though it still has substantial angle strain (88° internal angles).
Q: For methylcyclohexane at 25 °C, approximately what percentage of molecules have the methyl group in the equatorial position?
A: About 95%. The 1,3-diaxial strain of an axial methyl is 7.6 kJ/mol, giving an equatorial:axial ratio of roughly 21:1.
Q: In cis-1,3-dimethylcyclohexane, one chair conformer has both methyls equatorial. What interactions are present in the other chair conformer?
A: The other chair has both methyls axial. This creates two CH3-H 1,3-diaxial interactions (one from each methyl with axial hydrogens) plus one particularly severe CH3-CH3 1,3-diaxial interaction where the two methyl groups are directly across from each other in axial positions. This makes the di-axial conformer much higher in energy.
Q: What is the energy barrier for a cyclohexane ring flip (chair to chair), and what is the highest-energy conformation along the pathway?
A: The highest point is the half-chair conformation at 42 kJ/mol (10 kcal/mol) above the chair. The pathway goes: chair → half-chair → twist-boat (23 kJ/mol above chair) → boat (29 kJ/mol) → twist-boat → half-chair → chair.
The 1,3-diaxial interaction is the cyclic equivalent of the gauche interaction from Part 1 (acyclic conformational analysis). If you understand why gauche butane is higher in energy than anti, the same reasoning applies to axial vs equatorial substituents on cyclohexane.
This material connects forward to Part 3 (conformational effects on stereochemistry), where you will see how ring flipping affects chirality and optical activity. It also connects to carbohydrate chemistry, where the axial/equatorial positions of -OH groups on sugar rings determine whether you are looking at glucose or galactose.
Cycloalkane conformations, ring strain, angle strain, Baeyer strain, torsional strain in rings, cyclopropane strain energy, cyclobutane puckering, cyclopentane envelope conformation, cyclohexane chair conformation, boat conformation, twist-boat, half-chair, axial position, equatorial position, ring flip, chair flip, chair-chair interconversion, 1,3-diaxial interaction, 1,3-diaxial strain, gauche-butane analogy, methylcyclohexane, tert-butylcyclohexane, disubstituted cyclohexane, cis trans cyclohexane, diequatorial, diaxial, Klein Ch. 4, organic chemistry CHEM 101