Introduction
In liquid cyclohexane, the chair conformation is predominantly adopted due to its stability. Six hydrogen atoms are arranged in axial positions while the remaining six are aligned with the molecule’s equatorial plane. Each carbon atom is bonded to one hydrogen atom above and one below, resulting in staggered C–H bonds across successive carbons, thereby minimizing torsional strain. Even when hydrogen atoms are replaced by halogens or other groups, the chair geometry is typically retained. However, larger substituents can induce strain due to diaxial interactions, which are generally repulsive forces between two axial substituents on the cyclohexane ring.
Visualizing a carbon atom as a point with four half-bonds extending towards the vertices of a tetrahedron, one can imagine these atoms arranged on a plane, with one bond pointing upwards. From an overhead view, the remaining three bonds extend outward, forming a hexagon when connected. Reflecting three atoms below this plane results in a structure similar to the chair conformation of cyclohexane. In this model, the vertical half-bonds remain upright, while the ends of the outward-stretching half-bonds align with the equatorial plane. Given that C–H bonds are longer than half the length of a C–C bond, equatorial hydrogen atoms attached to an upper-positioned carbon will sit slightly below the equatorial plane, and vice versa for other substituents. The chair conformation is rigid and cannot deform without altering bond angles or lengths.
Summary:
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Six-membered cyclic alkanes are not flat, they adopt a “chair” conformation
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This is because each carbon atom is sp3-hybridized and has bond angles of 109.5º
Axial and Equatorial Positions
Relative stereochemistry
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Notice that both axial and equatorial positions can be either up or down
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Groups that are “up” with respect to “down”, or vice versa, are said to be “anti” with respect to each other
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Groups that are “up” or “down” with respect to each other are said to be “syn”
Drawing Chair Conformations
Drawing a basic chair:
Drawing axial and equatorial positions:
Drawing Chair Flips
Visualizing what happens during the chair flip:
Drawing the flip the easy way:
Converting 2D Rings to Chairs
Note 1: You can always check your work by determining R/S for the stereocenters and making sure they’re the same both on the 2D ring as well as the 3D chair. Don’t confuse “up” and “down” with axial and equatorial.
Note 2: There is more than one way to do this. I’ve just shown one simple way that can be standardized.
Differences in Cyclohexane Chair Stability
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Axial positions are subject to steric hindrance via interactions with other axial substituents
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These disfavored interactions are called “1,3-diaxial interactions”
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To determine which chair conformation is more stable, look at how many substituents are axial, that should be equatorial, or whether a particularly bulky substituent is axial, even though others would become axial by making the bulkiest one equatorial
Keywords
Cyclohexane chair | Conformation | Conformer | Chair Flip | Diaxial | Axial | Equatorial