Chapter 3: Conformations of Alkanes and Cycloalkanes

Cyclohexane

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

• How to draw cyclohexanes so you can convey your knowledge !
• The most stable conformation is the chair
• There are other less stable conformations such as the boat and the twist boat.
• Ring flipping interconverts chair conformations.
• There are two substituent positions around a chair cyclohexane axial and equatorial.

C6H12 CYCLOHEXANE ΔHc / CH2 = -653 kJ/mol (-156.1 kcal/mol)
	planar	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 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.

© Dr. Ian Hunt, Department of Chemistry