Return to Contents
Chapter 3:
Conformations of Alkanes and Cycloalkanes 
Ch 3 contents


This is an important ring system, what should you know ?


ΔHc / CH2 =
-653 kJ/mol
-156.1 kcal/mol)

most stable structure

Let's investigate in more detail some of the important features of the 3D shape of cyclohexane.

The most stable conformation of cyclohexane is the chair form shown to the right. The C-C-C bonds are very close to 109.5o, so it is almost free of angle strain. It is also a staggered conformation and so is free of torsional strain. 
Rotate the molecule in the CHIME image to show this just like a Newman projection so that you can see the staggered bonds. 

The chair conformation is the most stable conformation of cyclohexane.

In chair cyclohexane there are two types of positions, axial and equatorial. The axial positions point perpendicular to the plane of the ring, whereas the equatorial positions are around the plane of the ring. You should notice that adjacent axial postions point in opposite directions. The same is true for the equatorial positions. The axial and equatorial positions can be highlighted in the diagram below :
Show an equatorial hydrogen atom
Show all equatorial hydrogen atoms
Show the carbon skeleton
Show an axial hydrogen atom
Show all axial hydrogen atoms
Reset colors
A second, much less stable conformer is the boat conformation. This too is almost free of angle strain, but in contrast has torsional strain associated with eclipsed bonds at four of the C atoms. Rotate the molecule in the CHIME image to show this just like a Newman projection. In addition, a steric interaction of the H atoms inside the "bow" and the "stern", known as the flagpole interaction destabilises the boat. 

Show flagpole hydrogen atoms 
Reset colors 
A third conformation is produced by twisting the boat to give the twist or skew-boat conformation. The twist relieves some of the torsional strain of the boat and moves the flagpole H further apart reducing the steric strain. Consequently the twist boat is slightly more stable than the boat

Show flagpole hydrogen atoms 
Reset colors 
Conformational rotation (also known as ring-flipping) of cyclohexane interconverts the conformer. This proceeds from one chair to twist boat to boat to twist boat to another chair. Watch the CHIME animation carefully and look for the two chair forms, stop and rotate the animation if needed. 
Show equatorial hydrogen atoms 
Show axial hydrogen atoms 
Show carbon skeleton 
Reset colors 
Start ring-flip 
Stop ring-flip
conformation ring flip chair to chair
An important feature of this process is that the axial and equatorial positions switch.  If you watch carefully you will see that a position that was axial in one chair is equatorial in the other and vice versa.

The animation below shows how the potential energy of the cyclohexane molecule varies during the ring-flip process.
Use the controls to show the energies of the important conformations.  Note that the boat conformation is an unstable conformation (i.e. a maxima) on the pathway.

previous page
next page
 © Dr. Ian Hunt, Department of Chemistry, University of Calgary