Difficulty: Intermediate | Prerequisites: Lewis structures, Newman projections, basic cycloalkane geometry (Chapters 4–5)
Cyclohexane is the most important ring system in organic chemistry, and its chair conformation is the lowest-energy shape it adopts. This topic explains how to draw chairs, identify axial and equatorial positions, and predict which chair conformer is more stable based on substituent size. If you can do this fluently, you can predict physical properties, reactivity, and stereochemical outcomes for a huge number of molecules. You should already be comfortable with staggered vs. eclipsed conformations and gauche interactions from Newman projections.
Cyclohexane flips between two chair conformations. Every axial substituent becomes equatorial (and vice versa) on flipping. Larger substituents prefer equatorial positions because that avoids destabilising 1,3-diaxial interactions, and the A value tells you how strong that preference is in kcal/mol.
Chair conformation
The most stable three-dimensional shape of cyclohexane, in which all C–C bonds are staggered and both angle strain and torsional strain are eliminated. Think of it as the "relaxed" shape the ring naturally settles into.
Axial position
Bonds that point straight up or straight down, roughly parallel to the vertical axis of the ring. In simple terms, these are the positions that stick out above and below the ring.
Equatorial position
Bonds that point outward from the ring, roughly in the plane of the equator. These are the positions that fan out sideways.
Ring flip (conformational interconversion)
The process by which one chair conformation converts to the other. Every axial bond becomes equatorial and every equatorial bond becomes axial. In simple terms, the ring "flips inside out," swapping all up-axial hydrogens to down-equatorial and vice versa. Importantly, ring flip does not break any bonds, so the two chairs are conformers, not isomers.
1,3-Diaxial interaction
Steric repulsion between an axial substituent and the axial hydrogens on the same side of the ring two carbons away (the 1,3-positions). Think of it as the axial group bumping into its neighbours above or below the ring.
A value
A number (in kcal/mol) that quantifies how much a given substituent prefers the equatorial position over the axial position. A larger A value means the group has a stronger preference for equatorial. It is, equivalently, the energy cost of placing that substituent axial.
Gauche interaction
A destabilising interaction that occurs when two large groups are 60° apart on a Newman projection (or equivalently, in a gauche arrangement on adjacent carbons of the ring). In simple terms, the groups are close enough to crowd each other.
Van der Waals repulsion
Repulsive force that arises when the electron clouds of non-bonded atoms overlap because they are too close together. This is the underlying reason that 1,3-diaxial interactions are destabilising.
Boat conformation
A higher-energy conformation of cyclohexane in which four carbons are coplanar and the remaining two point up. It introduces eclipsing interactions along the sides and a flagpole interaction between the two "up" hydrogens.
Twist-boat conformation
A slightly lower-energy variant of the boat, achieved by twisting to relieve some of the eclipsing and flagpole strain. Still much less stable than the chair.
Half-chair conformation
The highest-energy point on the conformational energy surface of cyclohexane, representing the transition state between chair and boat/twist-boat forms.
Cyclohexane in its chair form has six carbons arranged so that three point slightly up ("up carbons") and three point slightly down ("down carbons"), alternating around the ring.
Each carbon carries one axial bond (pointing straight up or straight down) and one equatorial bond (pointing outward at a slight angle).
On the up carbons, the axial bond points up; on the down carbons, the axial bond points down. Equatorial bonds mirror this pattern in reverse.
Equatorial substituents are more stable than axial substituents.
The reason: an axial substituent on C1 sits close to the axial hydrogens on C3 and C5 (the 1,3-diaxial interaction). This produces both gauche interactions and van der Waals repulsion.
An equatorial substituent avoids these close contacts entirely. It points away from the ring and has much more room.
Cyclohexane undergoes a conformational interconversion (ring flip) that swaps every axial position to equatorial and vice versa.
The two resulting chair conformers are not isomers. They are the same molecule in different conformations, interconverting rapidly at room temperature.
For unsubstituted cyclohexane, the two chairs are identical in energy. For substituted cyclohexanes, one chair is more stable than the other.
The chair is the global energy minimum.
To get from one chair to the other, the ring passes through higher-energy intermediates: half-chair (the transition state, roughly 45 kJ/mol or 10.7 kcal/mol above the chair), then boat, then twist-boat (about 20–26 kJ/mol or 4.8–6.2 kcal/mol above the chair), then another half-chair, and finally the second chair.
The boat suffers from eclipsing along four C–C bonds and a "flagpole" interaction where two axial-like hydrogens on opposite ends collide. The twist-boat relieves some of this strain by twisting.
The larger the group, the worse the 1,3-diaxial interactions when it sits axial.
This is quantified by the A value: the energy difference (in kcal/mol) between the axial and equatorial conformers.
A bigger A value means a stronger equatorial preference.
Substituent | A Value (kcal/mol) |
|---|---|
H | 0 |
F | 0.15 |
Cl | 0.4 |
OH | 0.9 |
CH₃ (methyl) | 1.8 |
CH₂CH₃ (ethyl) | 1.8 |
CH(CH₃)₂ (isopropyl) | 2.1 |
C(CH₃)₃ (tert-butyl) | >5 |
Notice that ethyl and methyl share the same A value (1.8). The rest of the ethyl chain extends away from the ring, so it does not increase 1,3-diaxial strain beyond what the first CH₂ causes.
Isopropyl (2.1) is slightly larger because it branches at the carbon attached to the ring.
tert-Butyl (>5 kcal/mol) has such an enormous preference for equatorial that it essentially locks the ring: the chair with tert-butyl equatorial is the only one observed.
Energy difference between conformers
When a substituent can sit axial or equatorial, the energy gap between the two chairs equals the A value of that substituent. For a monosubstituted cyclohexane:
\Delta G^\circ = G_{\text{axial}} - G_{\text{equatorial}} = A \text{ value (kcal/mol)}A positive value means the equatorial conformer is more stable.
Energy barriers in the chair flip
Conformation | Approximate energy above chair |
|---|---|
Half-chair (transition state) | ~45 kJ/mol (10.7 kcal/mol) |
Boat | ~27 kJ/mol (6.5 kcal/mol) |
Twist-boat | ~20–26 kJ/mol (4.8–6.2 kcal/mol) |
The half-chair is the highest point on the energy surface, making it the transition state for interconversion.
The equatorial preference of large groups directly affects drug design: many pharmaceutical molecules contain cyclohexane rings, and knowing which conformation dominates lets chemists predict how the drug will bind to its target. Steroid hormones (cholesterol, testosterone, cortisol) are built from fused six-membered rings locked in chair conformations, so understanding A values is essential for predicting their 3D shape.
Students often think the two chair conformations of a substituted cyclohexane are different isomers. They are not. They are conformers of the same molecule, interconverting by rotation around C–C bonds without breaking anything.
Students sometimes assume that the axial conformer never exists. Both conformers are present at equilibrium; the equatorial one simply predominates. The ratio depends on the A value.
Students confuse "up" and "down" with "axial" and "equatorial." A substituent that is "up" (on the same face as the reference) can be either axial or equatorial depending on which carbon it sits on.
Students sometimes think a larger A value means the substituent is physically larger. A value measures the energy cost of being axial, which depends on shape and branching pattern, not just atomic size. Fluorine is larger than hydrogen but has a very small A value (0.15) because the C–F bond is short.
⚠️ Drawing chair conformations and correctly placing axial/equatorial substituents is one of the most commonly tested skills in organic chemistry exams.
⚠️ You will be asked to identify the more stable chair conformer. The answer is always the one with the largest substituent(s) in equatorial positions.
⚠️ Ring flip questions test whether you understand that axial and equatorial swap, and that "up" stays "up" (cis/trans relationships are preserved through the flip).
⚠️ Know the A values for common groups, especially methyl (1.8), tert-butyl (>5), and the halogens.
Q: True or false: the two chair conformations of methylcyclohexane are constitutional isomers.
A: False. They are conformers (same connectivity, different spatial arrangement, interconvertible without breaking bonds).
Q: When cyclohexane undergoes a ring flip, axial substituents become ________.
A: Equatorial.
Q: True or false: a substituent that is "up" on one chair will be "down" after a ring flip.
A: False. "Up" stays "up" and "down" stays "down." Only axial/equatorial identity changes.
Q: Which has a larger A value, methyl or tert-butyl?
A: tert-Butyl (>5 kcal/mol vs. 1.8 kcal/mol for methyl).
Q: True or false: the boat conformation of cyclohexane is the transition state for the chair flip.
A: False. The half-chair is the transition state. The boat is a local energy maximum but lower than the half-chair; the twist-boat is a local minimum between two half-chairs.
Q: Draw both chair conformations of chlorocyclohexane and identify which is more stable. Explain your reasoning.
A: In one chair, Cl is axial; in the other, Cl is equatorial. The equatorial conformer is more stable because it avoids 1,3-diaxial interactions between the Cl and the axial H atoms at C3 and C5. The A value for Cl is 0.4 kcal/mol, so the equatorial conformer is favoured by 0.4 kcal/mol.
Q: Why do ethyl and methyl have the same A value (1.8 kcal/mol)?
A: In the axial position, only the atoms directly attached to the ring carbon participate in 1,3-diaxial interactions. The additional CH₃ of ethyl extends away from the ring and does not come close to the C3/C5 axial hydrogens, so it adds no extra strain.
Q: tert-Butylcyclohexane is sometimes described as "conformationally locked." What does this mean?
A: The A value for tert-butyl is so large (>5 kcal/mol) that the equilibrium lies almost entirely on the side with tert-butyl equatorial. The other chair, with tert-butyl axial, is present in negligible amounts. Effectively, the ring does not flip.
Q: Rank the following substituents by increasing equatorial preference: H, Cl, CH₃, C(CH₃)₃.
A: H (0) < Cl (0.4) < CH₃ (1.8) < C(CH₃)₃ (>5). Each successive group has a larger A value and therefore a stronger preference for the equatorial position.
This material connects directly to disubstituted cyclohexanes (Ch. 7 continued), where you must consider the A values of two groups simultaneously to determine the preferred conformer. It also ties back to Newman projections and conformational analysis of butane (Ch. 4–5): the gauche interactions you saw in acyclic systems are the same interactions that destabilise axial substituents. Later, in reactions of cyclohexane derivatives, the equatorial vs. axial position of a leaving group can affect reaction rates and stereochemical outcomes.
Chair conformation, ring flip, axial, equatorial, 1,3-diaxial interaction, A value, conformational analysis, cyclohexane stability, gauche interaction, van der Waals strain, steric strain, boat conformation, twist-boat, half-chair, tert-butyl lock, conformational equilibrium, methylcyclohexane, substituent preference, Newman projection, organic chemistry chapter 7