Conformational Analysis of Cyclohexane | Stereochemistry | Organic Chemistry

Structure of Cyclohexane (C6H12).
Cyclohexane is the most stable alicyclic ring system. The cyclohexane ring exists in the non-planar conformation to avoid the angle strain. The cyclohexane molecule has two important conformations called the chair and boat conformation. In the chair form, there are 4 carbon atoms in one plane and the two opposite carbon atoms are out of plane (one above and one below the plane).
In boat form 4 carbon atoms lie in a plane and two carbon atoms are out of the plane but on the same side of the plane. In Chair form, there are six C—H bonds which are parallel to the axis of symmetry and they are called axial bonds (denoted by a).

The remaining six C—H bonds are directed at an angle of 109.5 deg to the axis of symmetry and they are called equatorial bonds (denoted by e). Thus each carbon atom in the chair form will be attached to one axial and one equatorial bond, and if we consider the adjacent C—H bonds they are all staggered with respect to each other. Thus the chair form is free from torsional strain and bond opposition strain.
In the boat conformation there are 4 boat - axial and 4 boat - equatorial bonds, then there are 2 flag pole bonds and 2 bow spirit bonds at the carbon atom 1 and 4. But if we see along the C—C bond then all the C—H bonds are eclipsed with respect to each other. Hence this configuration has maximum torsional and steric strain. Further, the H atoms at 1, 4 carbon atoms (flagpole bonds) are closer than their Vander Waal's radii. Therefore there will be strong Vader waal repulsion. Thus transannular strain.

In addition to the chair and boat forms, there are two more conformations of cyclohexane. The Half chair and twisted boat forms. The boat form is twisted such that C-C bonds on the plane cross each other and also these atoms go out of the plane and thereby the hydrogen atoms become skew with respect to each other. Thus relieving the torsional and steric strains. Further, the hydrogen atoms at the flag-pole bonds slightly go away from each other and thus, the steric repulsion between them is decreased as compared to the boat form. Thus the trans-annular strain is slightly reduced.
Therefore, the twist-boat form is more stable than the boat form by about 1.6 kcal. Finally, there is the half-chair form which has maximum torsional and angle strains.
The relative stabilities and the energy differences of the different conformations can be represented by the energy profile.

Thus, the order of stabilities of conformations will be chair, twist boat, boat, half-chair.
Thus most of the cyclohexane molecules will be present in the energetically more favorable chair form.


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