This is an important ring system, what should you know.....
DHc / CH2 = -653 kJ/mol
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
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
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.
|A third conformation is produced by twisting the boat to give the twist
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.
|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.
In chair cyclohexane there are two types of positions, axial and
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.