4.5 Conformations of Cyclohexane

Cyclohexane Conformations

Cyclohexane doesn't sit flat like a regular hexagon. A planar ring would force bond angles to 120°, far from the ideal tetrahedral angle of 109.5°, and every C-H bond would be eclipsed. Instead, cyclohexane adopts puckered shapes called conformations that relieve this strain. The most important of these is the chair conformation, and understanding why it's so stable is central to predicting how substituted cyclohexanes behave.

Chair Conformation of Cyclohexane

The chair conformation gets its name because it roughly resembles a lounge chair. Four carbons form a plane in the middle, while one carbon sits above that plane and one sits below it.

To draw a chair:

1. Draw two parallel, slightly offset lines (like a shallow "Z" or backslash-frontslash pair). These represent four of the six carbons.
2. Add one carbon pointing up above the right end and one pointing down below the left end. These are the "head" and "foot" of the chair.
3. Connect all six carbons in sequence to complete the ring.
4. Add axial hydrogens: these point straight up or straight down, alternating around the ring. If a carbon is the "up" tip of the chair, its axial hydrogen points up; the next carbon's axial hydrogen points down, and so on.
5. Add equatorial hydrogens: these point outward from the ring at a slight angle, roughly following the plane of the ring. Each equatorial bond is parallel to the ring bond one position over.

Every carbon has one axial and one equatorial hydrogen, giving 12 C-H bonds total.

Stability of the Chair Conformation

The chair is the lowest-energy conformation of cyclohexane. Three types of strain are minimized simultaneously:

• Angle strain: Bond angles in the chair are about 111°, very close to the ideal tetrahedral angle of 109.5°. A flat hexagon would force 120° angles, creating significant strain.
• Torsional strain: All adjacent C-H bonds are perfectly staggered. If you sight down any C-C bond in the chair, the substituents are in a staggered arrangement, just like the most stable conformation of ethane.
• Steric strain: The alternating axial-up/axial-down pattern keeps hydrogens far enough apart to avoid van der Waals repulsion.

Because all three types of strain are minimized at once, the chair has the lowest overall ring strain of any cyclohexane conformation.

Chair vs. Twist-Boat Conformations

The boat conformation is another puckered form where two opposite carbons both point up (or both point down). It's significantly less stable than the chair for two reasons:

• Eclipsing interactions: Four pairs of adjacent C-H bonds are eclipsed along the "sides" of the boat, creating substantial torsional strain.
• Flagpole interactions: The two carbons that point in the same direction bring their axial hydrogens close together (the "flagpole" hydrogens), producing steric strain. These flagpole H's are only about 1.8 Å apart.

The boat isn't even an energy minimum. It relaxes into the slightly more stable twist-boat (or skew-boat), which partially relieves both the eclipsing and flagpole strain by twisting the ring. Still, the twist-boat is about 5.5 kcal/mol less stable than the chair. At room temperature, over 99.99% of cyclohexane molecules are in a chair conformation at any given moment.

Conformational Dynamics of Cyclohexane

Even though the chair is strongly preferred, cyclohexane is not locked in one shape. The molecule is constantly flexing.

Ring flip: One chair can convert into the other chair by passing through a half-chair transition state, then a twist-boat intermediate, then another half-chair, and finally arriving at the "flipped" chair. This process is called a ring flip, and it has an energy barrier of about 10.8 kcal/mol. That's low enough that ring flips happen rapidly at room temperature (on the order of hundreds of thousands of times per second).

The key consequence of a ring flip: every axial substituent becomes equatorial, and every equatorial substituent becomes axial. For unsubstituted cyclohexane, the two chairs are identical, so this doesn't matter. But for substituted cyclohexanes, one chair may be strongly preferred over the other because bulky substituents are more stable in the equatorial position, where they avoid 1,3-diaxial interactions (steric clashes with other axial groups on the same face of the ring).

Pseudorotation is a separate, lower-energy process where one twist-boat converts into another twist-boat without passing through the chair. Since molecules spend very little time in twist-boat forms, pseudorotation is less important for predicting chemical behavior, but it does mean the twist-boat is not a single rigid structure.