This is an important ring system, what should you know ?
ΔHc / 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
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
to the plane of the ring, whereas the equatorial
positions are around the plane of the ring. You should notice that
axial postions point in opposite directions. The same is true for the
positions. The axial and equatorial positions can be highlighted in the
diagram below :
|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. Rotate
the molecule in the CHIME image to show this just like a Newman
In addition, a steric interaction of the H atoms inside the "bow" and
"stern", known as the flagpole interaction destabilises
|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
flagpole H further apart reducing the steric strain. Consequently the
boat is slightly more stable than the boat.
rotation (also known as ring-flipping)
of cyclohexane interconverts the conformer. This proceeds from one
to twist boat to boat to twist boat to another chair. Watch the CHIME
carefully and look for the two chair forms, stop and rotate the
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, University of Calgary|