Beginning Organic Chemistry (BOC)
a. Conformational Isomers.
1. Cyclic Compounds.
Return to BOC Index.
As noted on the previous page, cyclohexane is unique amongst the small ring molecules in that it can adopt a conformation which has no strain energy (neither angle nor torsional.)
There are also other conformations with higher energy, most notabe of which is the twist boat conformation.
Other, higher energy, conformations illustrated are the boat conformation, and the half-chair conformation:
|Cyclohexane: chair conformation.
Strain energy: 0 kJ/mol
|Cyclohexane: boat conformation.
Strain energy: 29 kJ/mol
|Cyclohexane: twist-boat conformation.
Strain energy: 23 kJ/mol
|Cyclohexane: half-chair conformation.
This is the energy maximum conformation as the chair and boat forms interconvert.
Strain energy: 45 kJ/mol
Inspection of the chair conformation of the cyclohexane ring laid as horizontal as it can be shows two different arrangement of the non-ring bonds.
|Six bonds point vertically up or down along the axis
of the ring.
The six axial bonds.
These bonds are gauche to the adjacent carbons in the ring, and are more crowded together than the equatorial bonds.
|Six bonds point sideways from the centre of the
The six equatorial bonds.
These bonds are anti to the adjacent ring carbons, and pointing away from the ring have no interaction with each other.
Not surprisingly from the above observations, if an atom or group replaces one of the hydrogens on the cyclohexane ring, the molecule will have a lower energy if that group is equatorial than if it is axial. In the axial position the group will be close to the axial atoms on the third carbon away giving rise to the name: 1,3-diaxial interaction. Thus the equatorial configuration will be favoured over the axial. Calculations as to how favoured it will be can be made as follows:
(In each of the following, converting to the spacefill option will show more clearly the size of the groups)
Eq to Ax (% EQ)
As this table shows, the t-butyl group essentially stops the molecule from inverting its configuration because of the large strain when it is in the axial position.
One unexpected case of a molecule which is more stable in the all axial configuration is that of all trans 1,2,3,4,5,6-hexaisopropylcyclohexane. Here the equatorial isopropyl groups interfere with one-another, and the groups are able to pack together better in the all axial configuration.
Date created: 2005 06 12.