Introduction to Conformation of Cyclohexane
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, and half chair conformations. The naming of the molecules is based on their own shape.
Baeyer Strain Theory
In 1885 Adolf Baeyer explained 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 atoms 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 –Mohr Theory
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
Twist Boat
Boat Form ( less stable)
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Half chair form has some angle strain and some torsional strain but the 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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Conformation in Cyclohexane
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.
FAQs on Conformation of Cyclohexane
1. What are the four main conformations of cyclohexane?
Cyclohexane can exist in several non-planar shapes, or conformations, to minimise strain. The four primary conformations are:
- Chair form: The most stable conformation.
- Boat form: A less stable, higher-energy conformation.
- Twist-boat form: An intermediate conformation that is more stable than the boat form.
- Half-chair form: The least stable conformation, acting as a transition state.
2. Which conformation of cyclohexane is the most stable and why?
The chair conformation is the most stable form of cyclohexane. This stability is due to two key factors: it is completely free from angle strain, as all C-C-C bond angles are approximately 109.5°, which is the ideal tetrahedral angle. Additionally, it has no torsional strain because all the hydrogen atoms on adjacent carbons are in a perfectly staggered arrangement.
3. What is the order of stability for the different conformations of cyclohexane?
The stability of cyclohexane conformations decreases as their internal energy and strain increase. The generally accepted order from most stable to least stable is:
Chair > Twist-boat > Boat > Half-chair
The chair form has the lowest energy, while the half-chair form represents the highest energy peak during conformational changes.
4. What is the difference between the chair and boat forms of cyclohexane?
The main differences between the chair and boat forms relate to their stability and structure:
- Stability: The chair form is significantly more stable (lower in energy) than the boat form.
- Torsional Strain: In the chair form, all C-H bonds are staggered, minimising torsional strain. In the boat form, four pairs of C-H bonds are eclipsed, leading to high torsional strain.
- Steric Strain: The boat form suffers from flagpole interactions, a type of steric strain between the hydrogen atoms on C1 and C4, which are absent in the chair form.
5. What is the difference between axial and equatorial bonds in cyclohexane's chair form?
In the chair conformation, the twelve hydrogen atoms are positioned in two distinct types of bonds:
- Axial bonds: These six bonds are parallel to the main axis of symmetry of the ring. They point straight up or straight down, alternating on adjacent carbons.
- Equatorial bonds: These six bonds point outwards from the 'equator' of the ring, roughly lying in the plane of the ring itself.
Each carbon atom in the chair form has one axial and one equatorial bond.
6. Why is a bulky substituent, like a methyl group, more stable in the equatorial position than the axial position?
A bulky substituent is more stable in the equatorial position to avoid steric hindrance. When placed in an axial position, the bulky group experiences significant repulsive forces from the two other axial atoms on the same side of the ring (at the C3 and C5 positions). This is known as a 1,3-diaxial interaction. Placing the group in the more spacious equatorial position minimises this repulsion, leading to a more stable molecule.
7. How does “ring flipping” affect the axial and equatorial positions in cyclohexane?
Ring flipping is the rapid process where one chair conformation of cyclohexane converts into another. During this flip, a crucial change occurs: all positions that were originally axial become equatorial, and all positions that were equatorial become axial. This interconversion allows the molecule to average out the positions of its substituents, though the conformation with bulky groups in the equatorial position is still heavily favoured.
8. What are flagpole interactions, and in which conformation do they occur?
Flagpole interactions are a specific type of steric strain that occurs in the boat conformation of cyclohexane. They are the repulsive interactions between the two inward-pointing hydrogen atoms on carbon 1 and carbon 4. These atoms are close enough in space to repel each other, similar to two flags on flagpoles, which destabilises the boat conformation relative to the chair form.
9. Which conformation of cyclohexane is chiral?
The twist-boat conformation of cyclohexane is chiral. It lacks both a plane of symmetry and a center of inversion, which are the requirements for a molecule to be chiral. In contrast, the highly symmetric chair conformation and the boat conformation (which has a plane of symmetry) are both achiral.
