Cyclohexane
Conformations
Master the chair conformation, understand axial vs. equatorial positions, and predict molecular stability using \( \Delta G^\circ \) and 1,3-diaxial strain.
What is Cyclohexane?
Cyclohexane (\( \text{C}_6\text{H}_{12} \)) is a six-membered carbon ring where every carbon is \( sp^3 \) hybridized. Unlike a flat hexagon drawn on paper, real cyclohexane adopts puckered, three-dimensional shapes called conformations to relieve angle strain.
Why Do Conformations Matter?
Cyclohexane can flex between many shapes, but not all shapes are equal in energy. Understanding conformations lets you predict which form a substituted cyclohexane prefers, which directly affects reactivity and physical properties.
The Chair Conformation
The chair is the most stable conformation of cyclohexane. Four carbons lie in a plane, one carbon points up (the "headrest"), and one points down (the "footrest").
Axial vs. Equatorial Positions
1,3-Diaxial Strain
When a bulky substituent occupies an axial position, it is forced into close proximity with the axial hydrogens on carbons two positions away (the 1,3 relationship). This steric repulsion is called 1,3-diaxial strain.
The Ring Flip
Cyclohexane constantly interconverts between two chair forms via a ring flip. During this process, every axial bond becomes equatorial and every equatorial bond becomes axial. The substituent stays on the same face (up or down) of the ring.
Energy of Conformations
The ring flip passes through higher-energy intermediates. The half-chair is the transition state, and the twist-boat is a local minimum that is still less stable than the chair.
Chair vs. Boat: Key Differences
| Chair | Boat | |
|---|---|---|
| Angle Strain | None | None |
| Torsional Strain | None (all staggered) | Significant (eclipsed H's) |
| Steric Strain | Minimal | Flagpole interaction |
| Relative Energy | \( 0 \text{ kJ/mol} \) | \( \sim 30 \text{ kJ/mol} \) |
| Stability | Most stable | Unstable (twists to twist-boat) |
Equatorial Preference
Bulky substituents strongly prefer the equatorial position because it minimizes 1,3-diaxial interactions. The energy difference between the two chairs (\( \Delta G^\circ \)) depends on the size of the substituent.
| Substituent | \( \Delta G^\circ \) (kJ/mol) | % Equatorial |
|---|---|---|
| \( \text{-F} \) | 1.0 | ~60% |
| \( \text{-CH}_3 \) | 7.6 | ~95% |
| \( \text{-C}_2\text{H}_5 \) | 8.0 | ~96% |
| \( \text{-C(CH}_3)_3 \) | ~23 | >99.9% |
How to Draw the Chair
Example 1: Methylcyclohexane
Draw both chair conformations of methylcyclohexane and determine which is more stable.
Example 2: cis-1,3-Dimethylcyclohexane
In cis-1,3-dimethylcyclohexane, both methyl groups are on the same face (both "up" or both "down"). Let's analyze both chairs.
Example 3: trans-1,4-Dimethylcyclohexane
In trans-1,4, the two methyls are on opposite faces. Analyze which chair is preferred.
C4: \( \text{CH}_3 \) equatorial (down face)
Both equatorial: most stable
C4: \( \text{CH}_3 \) axial (down face)
Both axial: less stable
Common Mistakes
Summary Table
| Concept | Key Fact |
|---|---|
| Chair Conformation | Most stable: no angle, torsional, or steric strain |
| Boat Conformation | ~30 kJ/mol above chair: eclipsed bonds + flagpole clash |
| Axial Position | Vertical bonds: up/down alternating; 1,3-diaxial strain |
| Equatorial Position | Angled outward: more room, less steric strain |
| Ring Flip | All axial ↔ equatorial; face (up/down) stays the same |
| \( t \)-Bu group | Conformational lock: always forces equatorial placement |
Question 1 of 5
What happens to all axial bonds during a cyclohexane ring flip?
Question 2 of 5
Which conformation of cyclohexane has the highest energy?
Question 3 of 5
In the most stable chair of methylcyclohexane, the \( \text{CH}_3 \) group is in which position?
Question 4 of 5
In cis-1,3-dimethylcyclohexane, can both methyl groups be equatorial in one chair conformation?
Question 5 of 5
The tert-butyl group (\( t \)-Bu) on cyclohexane acts as a "conformational lock" because: