Difficulty: Intermediate | Prerequisites: Part 1 of these notes (ring strain types), Chapter 3 conformational analysis (Newman projections, gauche interactions)
Cyclohexane is the most important ring in organic chemistry. Unlike the small rings covered in Part 1, cyclohexane escapes strain entirely by folding into a chair conformation. The catch is that the chair creates two distinct types of position for substituents, axial and equatorial, and this distinction controls the shape, stability, and reactivity of nearly every six-membered ring you will encounter, from glucose to cholesterol to countless drug molecules. This set of notes covers the chair itself, chair flips, axial vs equatorial positions, 1,3-diaxial interactions, and the naming conventions (cis/trans) for multi-substituted cycloalkanes.
TL;DR
Cyclohexane's chair conformation is strain-free. Each carbon carries one axial and one equatorial hydrogen. A chair flip swaps all axial positions to equatorial and vice versa, but "up" and "down" do not change. Substituents prefer equatorial because axial groups suffer 1,3-diaxial (transannular) and gauche strain. For rings with more than one substituent, the favoured chair puts the largest group equatorial.
Chair conformation
The lowest-energy shape of cyclohexane, in which all C–C–C bond angles are 109.5° and all adjacent C–H bonds are perfectly staggered. Think of it as the shape that eliminates every type of ring strain at once.
Axial position
A bond on the cyclohexane chair that points straight up or straight down, roughly parallel to the vertical axis of the ring. In simple terms, axial bonds stick out above and below the ring like antennae.
Equatorial position
A bond on the cyclohexane chair that points outward along the "equator" of the ring, angled slightly up or slightly down. Equatorial bonds fan out around the perimeter, roughly in the plane of the ring.
Chair flip (ring flip)
An interconversion between two chair conformations in which every axial position becomes equatorial and every equatorial position becomes axial. Crucially, substituents that were "up" stay "up," and those that were "down" stay "down." No bonds are broken.
1,3-Diaxial interaction
A type of transannular strain in which an axial substituent on one carbon sterically clashes with the axial hydrogens (or other groups) on the carbons two positions away (the 1,3-positions) on the same face of the ring. This is the main reason large groups prefer the equatorial position.
Gauche interaction
The steric and torsional strain between two groups separated by a 60° dihedral angle. On a cyclohexane, an axial substituent is gauche to the ring bonds on either side. You can see these by drawing a Newman projection through the ring.
A-value
The energy difference (in kcal/mol) between the axial and equatorial conformations of a monosubstituted cyclohexane. A larger A-value means a stronger preference for equatorial. These values are typically provided on exams.
Stereoisomers
Molecules with the same connectivity but different spatial arrangements. In cycloalkanes, this means cis (same face) and trans (opposite faces) relationships between substituents.
Cis isomer
A stereoisomer in which two substituents are on the same face of the ring (both "up" or both "down").
Trans isomer
A stereoisomer in which two substituents are on opposite faces of the ring (one "up," one "down"). Interconversion between cis and trans requires breaking and re-forming bonds.
The chair has zero ring strain: bond angles are 109.5° (no angle strain) and all adjacent C–H bonds are fully staggered (no torsional strain).
The 3D shape matters. A planar hexagon would have 120° angles and six sets of eclipsing C–H bonds, both of which are unfavourable. The chair avoids both.
Each carbon on the chair carries one axial bond and one equatorial bond, alternating up/down around the ring.
On any given carbon, axial and equatorial bonds alternate: if the axial bond points up, the equatorial bond points slightly down (and the next carbon reverses).
Axial bonds on adjacent carbons point in opposite directions (up, down, up, down around the ring).
Equatorial bonds fan out around the ring's perimeter.
A chair flip converts one chair conformation to the other. During the flip the ring passes through higher-energy forms: half-chair, twist-boat, and boat.
The boat conformation has flagpole interactions (steric clash between hydrogens at the 1- and 4-positions) and eclipsing strain along the sides.
The twist-boat partially relieves both, but is still higher in energy than either chair.
After a complete flip, every axial group is now equatorial and vice versa.
"Up" and "down" do not change during a flip. A substituent that was pointing up on the top face of the ring still points up after the flip.
The energy diagram for a chair flip shows two equal-energy chair minima separated by higher-energy half-chair, twist-boat, and boat intermediates.
A substituent in the axial position experiences two kinds of strain:
1,3-diaxial interactions: steric clashing with axial hydrogens at the 1,3-positions on the same face.
Gauche interactions: the axial group is gauche to ring C–C bonds on either side (visible in a Newman projection sighting along the ring).
The equatorial position avoids both: the substituent points away from the ring and into open space.
The energy cost of being axial is measured by the A-value. Larger groups have larger A-values and stronger equatorial preferences. A-values are typically provided on exams.
When a ring carries more than one substituent, put the most and/or largest substituents in equatorial positions.
For disubstituted rings, draw both chair conformations (related by a chair flip) and compare their total strain. The favoured chair is the one that places the largest group equatorial.
Example: in cis-1,4-dimethylcyclohexane, one chair puts both methyl groups equatorial (favoured) and the other puts both axial. In the trans isomer, one methyl must be axial regardless of which chair you draw.
Cycloalkane rings have two faces. Substituents on the same face are cis; substituents on opposite faces are trans.
Cis and trans are stereoisomers: they have identical connectivity but differ in spatial arrangement. You cannot interconvert them without breaking bonds.
Naming convention: the ring is treated like a straight-chain parent, preceded by "cyclo-," and cis/trans is specified as a prefix.
Example: trans-1-cyclopropyl-3-methylcyclohexane.
For molecules where cis/trans is not meaningful (e.g., geminal disubstitution on the same carbon), these prefixes are not used.
Up/down is relative to the position of the carbon on the chair. It stays constant through chair flips.
Axial/equatorial changes with every chair flip.
Cis/trans is relative to the faces of the ring and is permanent (requires bond breaking to change).
Chair cyclohexane bond angle: 109.5° (ideal, strain-free)
Energy order of cyclohexane conformations: chair < twist-boat < boat < half-chair
A-value: the energy penalty (kcal/mol) for placing a given substituent in the axial position. Specific values are provided on exams, but as a reference, A-value for CH₃ ≈ 1.7 kcal/mol.
Nearly every sugar you eat (glucose, fructose) exists as a six-membered ring that adopts a chair conformation, and the axial/equatorial placement of its –OH groups determines its biological properties. Steroid hormones (testosterone, oestrogen, cholesterol) are built from fused six-membered rings locked in chair conformations, and their biological activity depends on the 3D arrangement of substituents in axial and equatorial positions.
"A chair flip changes cis to trans." It does not. A chair flip swaps axial/equatorial, but "up" and "down" (and therefore cis/trans) remain unchanged. If two groups are cis before the flip, they are still cis after.
"Equatorial is always 'up.'" Equatorial bonds alternate between slightly up and slightly down around the ring. Whether a particular equatorial bond points up or down depends on which carbon you are looking at.
"The boat conformation is the other stable form of cyclohexane." The boat is a transition-region geometry, not a true energy minimum. The twist-boat is the local minimum between the two chairs, and even it is significantly higher in energy.
"A bigger substituent always forces the other substituent axial." With multiple substituents, you compare the total strain of each chair. Sometimes the best chair is the one that puts a moderately sized group axial if it means placing a much larger group equatorial.
⚠️ Drawing accurate chair conformations with correctly placed axial and equatorial bonds is a high-frequency exam skill. Practise until it is automatic.
⚠️ Expect to be given a disubstituted cyclohexane and asked to draw both chairs, identify cis or trans, and pick the more stable conformer. This is the classic Chapter 4 problem.
⚠️ Know that up/down stays constant through a chair flip, while axial/equatorial swaps. Confusing these is one of the most common exam errors.
⚠️ A-values will likely be provided. Your job is to use them to determine which chair is lower in energy for a given substitution pattern.
⚠️ Be comfortable drawing Newman projections through the cyclohexane ring to show 1,3-diaxial and gauche interactions. Exam questions sometimes ask you to demonstrate strain this way.
True or false: In a chair flip, a substituent that was "up" becomes "down."
False. Up/down is constant; only axial/equatorial switches.
Fill in the blank: Cyclohexane in its chair conformation has ______ bond-angle strain and ______ torsional strain.
Zero, zero.
True or false: The boat conformation of cyclohexane is a local energy minimum.
False. The twist-boat is the local minimum; the boat is a transition state (or near-transition state) between twist-boats.
Fill in the blank: A substituent prefers the equatorial position because axial placement causes ______ interactions and ______ interactions.
1,3-diaxial, gauche.
True or false: Cis-1,3-dimethylcyclohexane can have both methyl groups equatorial.
True. In cis-1,3, one chair puts both methyls equatorial (and both on the same face).
Q: What makes the chair conformation of cyclohexane strain-free?
A: The chair achieves ideal 109.5° bond angles (no bond-angle strain) and fully staggered C–H bonds on every adjacent pair of carbons (no torsional strain). These two features together eliminate ring strain entirely.
Q: During a chair flip, which property of a substituent changes and which stays the same?
A: Axial/equatorial changes (axial becomes equatorial and vice versa). Up/down stays the same, and therefore cis/trans relationships are preserved.
Q: Why do large substituents on cyclohexane prefer the equatorial position?
A: An axial substituent experiences 1,3-diaxial interactions with axial hydrogens at the 1,3-positions on the same face and gauche interactions with adjacent ring bonds. The equatorial position points the substituent away from the ring into open space, avoiding both types of strain.
Q: For trans-1-fluoro-4-methylcyclohexane, which chair conformation is favoured and why?
A: The favoured chair places the methyl group equatorial (larger A-value) and fluorine axial. Methyl has a larger steric demand than fluorine, so the energy saving from putting methyl equatorial outweighs the cost of putting fluorine axial.
Q: Explain the difference between cis and trans in the context of a disubstituted cycloalkane.
A: Cis means both substituents are on the same face of the ring (both up or both down). Trans means they are on opposite faces (one up, one down). These are stereoisomers, interconvertible only by breaking and re-forming bonds.
Q: Draw or describe the conformations cyclohexane passes through during a chair flip, in order of decreasing stability.
A: Chair (most stable) → half-chair → twist-boat → boat → twist-boat → half-chair → chair (second chair). The half-chair is the highest-energy point; the twist-boat is a shallow local minimum; the boat is slightly higher than the twist-boat.
This material builds directly on Part 1's treatment of ring strain: the reason cyclohexane's chair is so important is that it is the first ring to achieve zero strain. It also connects forward to stereochemistry (Chapters 5–6), where the cis/trans distinctions introduced here evolve into R/S configurational assignments. In biochemistry, the axial/equatorial framework applies to every pyranose sugar and steroid ring system you will encounter.
Related Terms / Search Tags
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