Chair and half chair conformation

We will now look at a few of the possible conformations of cyclohexane. The ones we shall describe, are the chair and the half chair conformation, and the boat and the twist boat conformation.

chair conformation

The chair conformation is the conformation with the lowest energy, and is therefore the most stable conformation. Remember what was told in the first exercise about eclipsed and staggered conformations and rotate the molecule above to find out for each single carbon-carbon bond whether the bonds attached to it are either eclipsed or staggered. Check your answer here:

Moreover, all carbons can arrange their bonds in a tetrahedral way, and there are no short distances between non-bonded atoms.

To turn this conformation into the other chair conformation (which is in fact equivalent to it, as we have seen on the previous page), it has to go via a half-chair and twist-boat conformation.

half-chair conformation

This is an energy maximum (transition state) in the chair-chair interconversion process. It is about 11 kcal/mole (~46 kJ/mole) above the chair. The higher potential energy of this conformation is due to the close-to-eclipsed (angles from 11.9° to 7.8°) conformation along three carbon-carbon bonds, combined with angle strain (C-C-C angles in this conformation range from 109.86° to 119.07°).
The structure resembles that of cyclohexene: four connected atoms more or less in a plane, and the two remaining ones above and below this plane.
Select three atoms to measure angle,
or four to measure torsion angle.

The twist-boat conformation

The potential energy of the twist boat conformation lies between that of the chair and the half-chair. It is a relative minimum, an intermediate, in the interconversion. Look closely at the structure and try to figure out why.
Hint: think about this from two points of view:

Why is the energy content lower than in the half-chair?
Why is the energy content higher than in the chair?

Check the next page.