1. CH 4-5: Cycloalkane Conformations I
Cyclohexane Conformations: similar to alkanes, but:
Very simple “zig-zag” 2-D structure.
Note bonds are all vertical and in same plane.
Cyclic structure limits conformations.
Cyclohexane “chair” conformer is preferred:

2. 3-D Cyclohexane Chair Conformations
The ring is drawn as if it were viewed “on edge” (note “chair” shape):
Bonds outside of the ring are called the Axial and Equatorial positions.
Equatorial bonds (eq) slant “up” and “down”
Axial bonds (ax) are all vertical, and alternate straight “up” and “down”:

3. Cyclohexane Chair Conformations: Geometry of Substituents
1-bromo-4-methyl cyclohexane
What is the Relationship between these two structures?
cis-1-bromo-4-methyl cyclohexane
trans-1-bromo-4-methyl cyclohexane
Cis/Trans isomers are also called Geometric Isomers.

4. Cyclohexane Chair Conformations: Cis-Trans
Draw the two “chair” conformations for trans-1-bromo-4-methyl cyclohexane:
Which is the correct “chair” conformation?

5. Conformational Stability is based on the position of the largest substituent
The most stable “chair” conformer is with the largest substituent in the equatorial position.

6. Conformational Stability is based on the position of the largest substituent
The least stable “chair” conformer is with the largest substituent in the axial position.

7. Cyclohexane Chair Conformations
Axial conformers are less stable due to Steric Hinderance caused by the 1,3-diaxial groups “bumping into” each other.