1. Cyclopentane
Planar cyclopentane would have no angle strain but very high torsional strain
Actual conformations of cyclopentane are nonplanar, reducing torsional strain
Four carbon atoms are in a plane
The fifth carbon atom is above or below the plane – looks like an envelope

2. Relieve of torsional strain in cyclopentane Envelope (half chair) structure

3. Conformations of Cyclohexane
Substituted cyclohexanes occur widely in nature
The cyclohexane ring is free of angle strain and torsional strain
The conformation is has alternating atoms in a common plane and tetrahedral angles between all carbons
This is called a chair conformation

4. How to Draw Cyclohexane

5. Axial and Equatorial Bonds in Cyclohexane.
Axial and Equatorial Bonds in Cyclohexane
Chair cyclohexane has two types of hydrogens:
axial: C-H axis is “perpendicular” to the “plane of the ring”
equatorial: C-H axis is “parallel” to the “plane of the ring”
Chair cyclohexane has two faces; each face has alternating axial
and equatorial -H’s
axial
equatorial
top face bottom face

6. Axial and Equatorial Positions
Each carbon atom in cyclohexane has one axial and one equatorial hydrogen
Each face of the ring has three axial and three equatorial hydrogens in an alternating arrangement

7. Drawing the Axial and Equatorial Hydrogens

8. Conformational Mobility of Cyclohexane
Chair conformations readily interconvert, resulting in the exchange of axial and equatorial positions by a ring-flip

9. Bromocyclohexane
When bromocyclohexane ring-flips the bromine’s position goes from equatorial to axial and so on

10. Conformations of Monosubstituted Cyclohexanes
The two conformers of a monosubstituted cyclohexane are not equal in energy
The equatorial conformer of methyl cyclohexane is more stable than the axial by 7.6 kJ/mol

12. Example

13. Boat conformation is less stable than the chair
180 pm
All of the bond angles are close to tetrahedral but close contact between flagpole hydrogens causes van der Waals strain in boat. 6

14. Boat conformation is less stable than the chair
Eclipsed bonds gives torsional strain to boat. 6

15. Conformations of cyclohexane