A Conformation of cyclohexane can refer to many different 3-Dimensional shapes assumed by a cyclohexane molecule without disturbing the integrity of the chemical bonds in it.
A regular hexagon shape contains internal angles of 120o. However, the carbon-carbon bonds belonging to the cyclohexane ring have a tetrahedral symmetry, with the bond angles corresponding to 109.5o.
This is the reason why the cyclohexane ring has a tendency to take up several warped conformations (so that the bond angles are brought closer to the tetrahedral angle (109.5o) and there is reduced overall strain energy).
Examples of common conformations of cyclohexane include the boat, the twist-boat, the chair, and the half-chair conformations, which are named based on the shape that the cyclohexane molecule assumes in them.
These four cyclohexane conformations have been illustrated below along with some insight on their stability.
It can be noted that the cyclohexane molecule has the ability to switch between the conformations listed above and that only the chair and the twist-boat conformations can be isolated into their respective pure forms.
Due to hydrogen-hydrogen interactions in these conformations, the bond length and the bond angle vary slightly from their nominal values.
The chair conformations of cyclohexane have lower energies than the boat forms. However, the rather unstable boat forms of cyclohexane undergo rapid deformation to give twist-boat forms which are the local minima corresponding to the total energy.
The Hydrogen atoms belonging to the carbon-hydrogen bonds that are at a perpendicular angle to the mean plane are called Axial hydrogens, whereas those belonging to the carbon-hydrogen bonds which are parallel to the mean plane are called equatorial hydrogens. These bonds are also referred to as axial and equatorial bonds respectively.
Generally, in the chair shaped conformation of cyclohexane, there are three carbon-hydrogen bonds of each of the following types:
- Axial ‘up’
- Axial ‘down’
- Equatorial ‘up’
- Equatorial ‘down’
This geometry of chair cyclohexane conformations is generally preserved when the hydrogen atoms are replaced by halogen atoms such as fluorine, chlorine, bromine, and iodine. The phenomenon wherein the cyclohexane molecule undergoes a conversion from one chair form to a different chair form is called chair flipping (or ring flipping).
An illustration detailing chair flipping is provided below.
When chair flipping occurs, axial carbon-hydrogen bonds become equatorial and the equatorial carbon-hydrogen bonds become axial. However, they retain the corresponding ‘up’ or ‘down’ positions.
It can be noted that at a temperature of 25o Celsius, 99.99% of the molecules belonging to a given cyclohexane solution would correspond to a chair-type conformation.
The boat conformation of cyclohexane is not a very stable form due to the torsional strain applied to the cyclohexane molecule. The stability of this form is further affected by steric interactions between the hydrogen atoms. Owing to these factors, these conformations are generally converted into twist-boat forms which have a lower torsional strain and steric strain in them.
These twist-boat conformations of cyclohexane are much more stable than their boat-shaped counterparts. This conformation has a concentration of less than 1% in a solution of cyclohexane at 25o. In order to increase the concentration of this conformation, the cyclohexane solution must be heated to 1073K and then cooled to 40K.