The chair conformation of cyclohexane is one of the most stable structures at room temperature, and it is a good standard in lab analysis. It is also used as a benchmark for solvent reference, and for other types of molecular interactions and reactions.
A cyclohexane ring can assume many different conformations, including the chair, boat, and half-chair conformations. However, the most stable is the chair conformation.
When a cyclohexane molecule is in the chair conformation, the carbon atoms of the four-carbon chain are arranged in a dihedral angle (angle between two helices or chains) that is positive. This is because the axial group on this chain of carbon atoms has the same dihedral angle as the equatorial substituent.
In contrast, when a cyclohexane is in the boat conformation, the carbon atoms are arranged in a dihedralangle that is negative. This is because the axial group on the other chain of carbon atoms has the opposite dihedral angle.
The torsional strain and steric hindrance of chair conformation are much lower than those of boat conformation, which is why it is a good structure for use in the laboratory. The torsional strain is minimised by the rotation of the bonds in this conformation.
There are several ways that a cyclohexane can change from a chair conformation to a boat or twist boat, which involves changing the dihedral angles of some of the bonds in the ring. These changes are called ring flips.
Some ring flips can be reversible, but others must be irreversible. If you undergo a ring flip, you will change the location of all of your axial and equatorial substituents.
You can also flip the ring through an interconversion between two chair conformations. The half-chair conformation is the transition state, and it has C2 symmetry. It has a higher energy than the boat or twist boat, so it is less stable.
This type of ring flip is useful in determining the nature and number of substituents in a cyclohexane. Often the axial groups will be equatorial after the ring flip, so it is important to know where those substituteents are located in the cyclohexane before you proceed with the ring flip.
To determine whether a cyclohexane has undergone a chair conformer, you must add up the axial and equatorial energy of all the substituents. For example, if you have an isopropyl group in the axial position and it undergoes a chair conformer, the energy of the isopropyl will be 7.3 kJ/mol.
A cyclohexane that has been substituted with chlorines will not undergo a chair conformer after undergoing a ring flip, because the chlorines in the axial position are equatorial. This explains why the first cyclohexane that undergoes a ring flip is more stable than the second cyclohexane that does not undergo a ring flip.
Studying the chair conformation is probably the most challenging visual topic in organic chemistry, and it can be confusing for students who are not artists. This is because most books show the chair conformation slightly sideways, making it difficult to distinguish which substituents are axial and which are equatorial.