Cyclohexane is a cycloalkane which is an alicyclic hydrocarbon. It is colorless with the molecular formula C6H6, consisting of a ring of six carbon atoms that is flammable and is considered to be a volatile liquid with a detergent-like odor, reminiscent of cleaning products. Cyclohexane has two non-planar puckered conformation and both are completely free from strain. These are called Chair Form and Boat Form because of their shape. There are so many examples of common cyclohexane conformations such as the chair form, boat form, twist boat form, half chair conformations. The naming of the molecules is based on their own shape.
In 1885 Adolf Baeyer explains the relative stability of the first few cycloalkanes. He explained his theory on the fact that the normal angle between any pair of bonds of carbon atom is 109°28'.
In this theory, he has explained that any deviation of bond angles from the normal tetrahedral value would impose a condition of internal strain on the ring.
And the Baeyer Strain theory is not valid for the Cyclohexane ( Which is a cycloalkane).
Sachse and Mohr proposed that seven rings can become free from strain if all the ring carbons are not forced into one plane, as meant by Baeyer. If a ring is assumed to have a 'puckered' or 'folded' condition, then the normal tetrahedral angles of 109°28' are retained and as a result, a strain within the ring is reduced.
Cyclohexane exists as Chair Form and Boat Form because of its shape.
Examination of the chair form of cyclohexane proves that the hydrogen atoms are divided into two categories. Six bonds of the hydrogen atom are found either straight up or down or almost perpendicular to the plane of the molecule. These are called Axial Hydrogen, and the other hydrogens which lie slightly above or slightly below the plane of the Cyclohexane ring, and these are known to us as Equatorial Hydrogen.
The cyclohexane ring can assume many different shapes. A single cyclohexane molecule is in a continuous state of flexing or flipping into different shapes or conformations. Some of these different shapes are given below
Chair Form ( more stable)
Half Chair Form
Boat Form ( less stable)
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Half chair form has some angle strain and some torsional strain but boat form has no significant angle strain and has the torsional strain. In this form hydrogen atoms are attached with the Van Der Waals forces. This interaction is known as flagpole interactions.
Twist boat is twisted in nature and it has a consolation flagpole interaction. Also, it has less angle strain and less torsional strain.
Mostly chair form has no angle strain and here in the chair form all C - C bonds are staggered.
The conformations arise due to rotation around carbon-carbon bonds, but the chair form and the boat form are the two extreme cases.
Energy Levels of the Cyclohexane Conformers are
Half Chair Form ( Ring Strain=108 kcal/mol)
Boat Form ( Ring Strain=7.0 kcal/mol)
Twist Boat ( Ring Strain=5.5 kcal/mol)
Chair Form( Ring Strain=0 kcal/mol)
Stability of Cyclohexane Conformers is
half chair < boat form < twist boat form< Chair form.
Mechanism of cyclohexane ring flip is like
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The carbon atoms of the chair made up of cyclohexane roughly lie in one plane, and an axis can be drawn perpendicular to this plane.
Each carbon atom of cyclohexane is bonded to two hydrogens. The bond to one of these hydrogen lies in the rough plane of the ring; this hydrogen is called Equatorial Hydrogen. The bond to the other hydrogen atom is parallel to the axis; this hydrogen atom is called Axial Hydrogen. Each of the six carbon atoms of cyclohexane has one equatorial and one axial hydrogen atom, we have to remember that there are six equatorial hydrogens and six axial hydrogens. In the flipping and re-flipping between conformations, the axial evolves into equatorial, while equatorial becomes axial.
A methyl group is bulkier than a hydrogen atom. When the methyl group in methylcyclohexane is in the axial position, the methyl group and the m hydrogen of the ring repel each other. These interactions are called Axial-Axial Interactions. When the methyl group is in the equatorial position, the repulsions are minimum. The bulkier the group, the greater is the energy difference between equatorial and axial conformations. In other words, a cyclohexane ring with a bulky substituent (eg:- t-Butyl group) is more likely to have that group in the equatorial position.
1. Can Chair Conformers of Cyclohexane be Less Stable?
No, the Chair conformer of cyclohexane is always more stable than the boat conformed in most of the cases. In between Chair conformer and Boat conformer, Chair conformer is more stable than the boat conformer because boat conformation has more steric strains and torsional strains. And in the chair conformation, there are six axial and six equatorial C-H bonds (out of twelve bonds of cyclohexane). Moreover, the Chair conformers form C-C-C bonds which are too much close to 109.5˚ and that’s why it is free of angle strain and also is free from torsional strain.
2. What is the Difference Between the Boat Form and Chair Form?
Only 1% of cyclohexane is present as a boat form and 99% of cyclohexane is present as a chair form.
The energy of the boat conformer form is high and the energy of the chair conformer form is low.
The symmetry of Boat Conformation is D3d and the symmetry of chair Conformation is C2v
In this case, the Boat conformer form has C-H bonds which are perfectly staggered and on the other hand, the Chair conformer form has all C-H bonds which are perfectly eclipsed.