CYCLOHEXANE STEREOCHEMISTRY

CYCLOHEXANE STEREOCHEMISTRY

Anand S. Burange, Wilson College, Mumbai

asgburange@gmail.com

Cyclohexane is an industrially important molecule mainly used for the production of epsilon caprolactum and adipic acid which are precursors to Nylon.

Cyclohexane has a molecular formula C₆H₁₂ and has a cyclic structure. At room temperature it exists as a liquid with boiling point 80.74 ^oC.

Generally, representation of cyclohexane in a line structure form is shown as a hexagon but practically it is not the case. Cyclohexane has six C-C bonds and due to cyclic structure there is a restricted bond rotation. Rotations of bonds lead to the various conformations. The most important conformations of cyclohexane are enlisted below.

Conformations of Cyclohexane

1. Boat

2. Chair

3. Twist boat

4. Twist Chair

Boat Conformation

In a boat conformation, four carbon atoms lie in a plane while remaining two carbons atoms (C1 and C4) are out of plane (above or below) but on same side. Boat conformation is not free from torsional strain. Imagine boat conformation with two carbon atoms above the plane on same side while four others in a plane. Now if we viewed down C-C bonds, then the hydrogen atoms are found eclipsed.

Flagpole Interaction

Hydrogen atoms attached to C1 and C4 carbon in boat conformations are so close to each other to cause van der Waals repulsion which is also called as “flagpole” interaction of boat conformation.

Thus torsional strain as well as flagpole interaction both are responsible factors which increases the energy of the boat conformation.

Chair conformation

In chair conformation, C1 and C4 carbon both are out of plane but on opposite side. Thus in chair form, four carbon atoms lie in a plane while two carbons are out of plane and opposite side to each other.

Chair form is free of torsional strain and all C-C bond angles are 109.5^o. If we view along C-C bonds of any side in chair form, atoms are perfectly staggered to each other. Whereas hydrogen atoms at opposite corners are far away from each other thus there is no flagpole interaction present in chair conformation.

By considering all above aspects, chair form acquires least energy and thus it is the most stable conformation of the cyclehexane.

Twist Boat

Chair conformation is more rigid than boat. In other way, boat form is flexible and flexing of the boat conformation twists the structure and gives twist-boat form which has less energy than boat. Because of twisting , C1 and C4 hydrogen moves away from each other which decreases the repulsion between them and thereby decreases the energy.

Twist Chair               

This conformation is the least stable among all discussed above.

Stability order

Chair >  Twist Boat  >  Boat  > Twist Chair

Because of the highest stability of chair form among all, 99% of molecules are estimated to acquire chair form at any given moment.

Ring Flipping
In cyclohexane, conversion of one chair form into other is called as Ring Flipping. During Ring flip, all axial bonds become equatorial while equatorial bonds become axial.
Axial bonds (C1 and C3) have repulsion between them called as 1,3 diaxial repulsion. Therefore any substituent on cyclohexane at equatorial position is more stable than axial position.

Note: 1. Stereochemistry is well understood with molecular model 2. Blog is especially written for Mumbai Uni. Undergraduate students.