What are the chair boat and twist conformations of cyclohexane and why is chair most stable
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 and Its Chair and Boat Structures
1. What is the conformation of cyclohexane?
The conformation of cyclohexane refers to the different three-dimensional shapes that cyclohexane (C6H12) can adopt due to rotation about its C–C single bonds without breaking any bonds. These conformations minimize angle and torsional strain in the ring.
- The most important conformations are chair, boat, twist-boat, and half-chair.
- Among them, the chair conformation is the most stable.
- Conformational changes occur through bond rotation, not chemical reactions.
2. Why is the chair conformation of cyclohexane the most stable?
The chair conformation of cyclohexane is the most stable because it has almost no angle strain and minimal torsional strain. In this conformation:
- All C–C–C bond angles are close to the ideal 109.5° of sp3 hybridized carbon.
- All adjacent C–H bonds are staggered, reducing torsional strain.
- There are no eclipsing interactions between bonds.
3. What are the different conformations of cyclohexane?
The main conformations of cyclohexane are chair, boat, twist-boat, and half-chair. These differ in stability and spatial arrangement.
- Chair: Most stable, no angle or torsional strain.
- Boat: Less stable due to eclipsing interactions and flagpole repulsion.
- Twist-boat: More stable than boat but less stable than chair.
- Half-chair: Least stable, highest energy intermediate.
4. What is ring flipping in cyclohexane?
Ring flipping in cyclohexane is the process by which one chair conformation converts into another chair conformation through bond rotation. During a ring flip:
- Axial hydrogens become equatorial.
- Equatorial hydrogens become axial.
- The molecule passes through higher-energy forms like the half-chair and boat conformations.
5. What is the difference between axial and equatorial positions in cyclohexane?
In the chair conformation of cyclohexane, axial positions are perpendicular to the ring plane, while equatorial positions are roughly parallel to the ring plane. Specifically:
- There are 6 axial and 6 equatorial positions.
- Axial bonds alternate up and down around the ring.
- Equatorial bonds extend outward around the ring’s equator.
6. Why do substituents prefer the equatorial position in cyclohexane?
Substituents prefer the equatorial position in cyclohexane because it minimizes steric strain caused by 1,3-diaxial interactions. In detail:
- An axial substituent interacts with axial hydrogens on C-3 and C-5 (called 1,3-diaxial interactions).
- These interactions increase steric repulsion and energy.
- In the equatorial position, the substituent experiences much less crowding.
7. What is 1,3-diaxial interaction in cyclohexane?
A 1,3-diaxial interaction is a steric repulsion between an axial substituent on carbon 1 and the axial hydrogens on carbons 3 and 5 of cyclohexane. This occurs only when the substituent is in the axial position.
- It increases steric strain and raises the molecule’s energy.
- It is similar to gauche interactions in open-chain alkanes.
- Larger substituents cause stronger 1,3-diaxial repulsion.
8. What is the energy difference between chair and boat conformations of cyclohexane?
The boat conformation of cyclohexane is approximately 6–7 kcal mol-1 higher in energy than the chair conformation. This energy difference arises because:
- The boat form has eclipsing interactions between adjacent C–H bonds.
- It experiences flagpole interactions between the two axial hydrogens at C-1 and C-4.
9. How do you draw the chair conformation of cyclohexane?
To draw the chair conformation of cyclohexane, sketch a zigzag structure with alternating up and down carbons that forms a distorted hexagon. Follow these steps:
- Draw a slightly tilted parallelogram shape.
- Add one carbon above the left end and one below the right end to form a six-membered ring.
- Label axial bonds vertically (up and down).
- Draw equatorial bonds slanting outward from each carbon.
10. Why doesn’t cyclohexane exist as a flat hexagon?
Cyclohexane does not exist as a flat hexagon because a planar structure would create significant angle and torsional strain. If cyclohexane were flat:
- The C–C–C bond angles would still be 120°, not the ideal 109.5° for sp3 carbons.
- All adjacent C–H bonds would be eclipsed, causing high torsional strain.
