Conformations of Cyclohexane

In stereochemistry, the study of the configuration of a group of atoms in a molecule and the energy associated with such arrangements is known as conformational analysis. Conformations are the numerous shapes that groupings of atoms assume as they rotate around distinct carbon-carbon axes. The different three-dimensional configurations that cyclohexane molecules can take are known as cyclohexane conformations. For numerous compounds, the structure and dynamics of cyclohexane are crucial prototypes since the six-membered rings of these compounds are structurally similar.

Two of the most essential conformations that cyclohexane can have are boat conformation and chair conformation. The chair conformation is a more stable conformation than the boat position. The skew boat conformation is when the boat conformation is more stable than it usually is due to a small rotation in the C-C bonds. 

Conformation of Cyclohexane: Examples

The variety of 3-dimensional configurations that a cyclohexane molecule may take while maintaining the integrity of its chemical bonds is known as cyclohexane conformation. 

The internal angles of 120 degrees are obtained in a regular hexagon form. However, the carbon-carbon bonds of a cyclohexane ring form a tetrahedral symmetry, with bond angles of 109.5 degrees. For this reason, cyclohexane rings have the tendency to adopt distorted conformations – trying to get the bond angles closer to the tetrahedral angle (109.5 degrees) and reducing overall strain energy.

Some of the examples of common cyclohexane conformations named after the shape cyclohexane molecules may take are: 

• Boat conformations

• Twist-boat conformations

• Chair conformations

• Half-chair conformations 

The chair conformation and twist-boat conformation are the only ones capable of splitting into their pure forms, indicating that cyclohexane molecules can transition between the conformations.

The bond length and bond angle in cyclohexane conformations diverge from their nominal values due to hydrogen-hydrogen interactions. The energy of cyclohexane chair conformations is lower than that of the boat forms. The somewhat unstable boat conformations of cyclohexane deform rapidly to yield twist-boat conformations, which are the local minima corresponding to the total energy.

Hydrogen atoms in carbon-hydrogen bonds that are perpendicular to the mean plane are referred to as axial hydrogens, whereas hydrogen atoms in carbon-hydrogen bonds that are parallel to the mean plane are referred to as equatorial hydrogens. These sorts of bonds are referred to as axial and equatorial bonds.

Principal Conformers

The different conformations are referred to as conformers –  a combination of the words “conformation” and “isomer.”

1. Chair Conformation: The conformer most stable is the chair conformation. In a cyclohexane solution, 99.99% of the molecules take this shape at 298 K (25 °C). D3d is the symmetry. Every carbon centre is the same. Six hydrogen centres are axially aligned, almost parallel to the C3 axis. Near the equator, six hydrogen atoms are found. 

Axial and equatorial H atoms are the two types of H atoms. Each carbon atom has one hydrogen atom up and one hydrogen atom down. As a result, C–H bonds of subsequent carbons are staggered, resulting in reduced torsional strain. When hydrogen atoms are substituted with halogens or other simple groups, the chair geometry is preserved.

2. Boat and twist-boat conformations: Boat conformations have higher energy than chair conformations. The contact between two flagpole hydrogens causes steric strain. Torsion arises between the eclipsed C2–C3 and C5–C6 bonds, which are parallel to one another across the mirror plane (carbon number 1 is one of the two on a mirror plane). 

As a result of the stress, the boat configuration is unstable.  On their own, the boat conformations transform into twist-boat conformations. The symmetry is D2, a purely rotating point group with three twofold axes. The two pairs of eclipsing methylene groups from the boat conformation can be removed by rotating the molecule slightly. The twist-boat shape is chiral, with right- and left-handed variations.

Levels of energy of cyclohexane conformers

• (Ring Strain=108 kcal/mol) Half Chair Form

• (Ring Strain=7.0 kcal/mol) Boat Form

• (Ring Strain=5.5 kcal/mol) Twist Boat

• (Ring Strain=0 kcal/mol) Chair Form

Conclusion

The carbon atoms in the cyclohexane chair are roughly in one place, and an axis can be drawn perpendicular to it. Cyclohexane has two hydrogens linked to each carbon atom. 

Equatorial hydrogen is the hydrogen whose bond is in the rough plane of the ring. This hydrogen atom is known as axial hydrogen because its link to the other hydrogen atom is parallel to the axis. Each of the six carbon atoms in cyclohexane has one equatorial and one axial hydrogen atom. Therefore, there are six equatorial and six axial hydrogens in total. The axial becomes equatorial during the flipping and re-flipping between conformations, and the equatorial becomes axial.