Chapter 3:
Conformations of Alkanes and Cycloalkanes 
Ch 3 contents

Cyclohexane

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

C6H12

CYCLOHEXANE
DHc / 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. You can rotate the molecule in the CHIME image to show this just like a Newman projection. 

The chair is the most stable conformation of cyclohexane.

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. You can rotate the molecule in the 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 also destabilizes the boat. 

Highlight flagpole hydrogen atoms 
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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. 

Highlight flagpole hydrogen atoms 
Reset colors 

Conformational rotation (also known as ring-flipping) of cyclohexane interconverts two chair forms. This proceeds from chair to twist boat to boat to twist boat to chair. Watch the animation carefully looking for the two chair forms.

conformation ring flip chair to chair

Chair 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. These can be highlighted in the diagram below.
 
 

Highlight an equatorial hydrogen atom 

Highlight all equatorial hydrogen atoms

Highlight an axial hydrogen atoms

Highlight all axial hydrogen atoms

Highlight the carbon skeleton 

Reset colors


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