4.1: Cyclohexane Ring Conformations

Rings are ubiquitous in organic chemistry. Like their straight-chain cousins, they also have conformations. Due to their cyclic nature, however, they have fewer degrees of freedom because they cannot rotate completely about their \(σ\) bonds.

Let’s start with the conformation of cyclohexane. Even though we draw cyclohexane like a hexagon on a page, cyclohexane itself is not flat. Because of the tetrahedral carbon atoms in the ring, each of the bond angles is 109.5°. This makes the cyclohexane ring pucker to give it three-dimensionality. We say that cyclohexane exists in a chair conformation in its most stable form. There are actually two chair conformations for cyclohexane - I like to arbitrarily call them the "right-handed" and "left-handed" chair conformations - which exist in equilibrium with each other. Since there is no substitution on the ring, the ratio of the left-handed and right-handed chair is equal to one.

Each carbon atom in the ring has what are known as axial and equatorial substituents. Axial substituents point directly above and below the “plane” of the ring, while equatorial substituents form a circle around the “equator” of the ring. Notice that as you move around the ring, axial substituents alternate “up” and “down” if looking from the top (“bird’s eye view”). Likewise for equatorial groups. This is best seen by building a model of cyclohexane and placing it into its chair conformation.

Why does cyclohexane adopt this conformation? Let’s consider what happens when we look down the C₂-C₃ and C₆-C₅ bonds simultaneously. What you might notice is that the chair structure is in the staggered conformation – the dihedral angles are all 60° and there is maximum overlap between the various \(σ_{C-H}\) and \(σ^{*}_{C-H}\)/\(σ^{*}_{C-C}\) orbitals. So, how would you convert from one chair to another, and why does it matter? Just like when converting between alkane conformers, we can draw a potential energy diagram that describes how one would convert between the right- and left-handed chair. At room temperature, an unsubstituted cyclohexane ring has enough energy to overcome several activation barriers, and proceeds through several intermediates and transition states.

Why is the boat conformation so high in energy? It turns out that there is a flagpole interaction between the H atoms on C₁ and C₄ that causes steric strain. If we look closer, however, we can also see an electronic argument that speaks to its higher energy. If we look down the C₂-C₃ or C₆-C₅ bonds, we see that all of the bonds are eclipsed. This increases the torsional strain of the molecule, driving up the energy compared to the chair conformation.

        

 

I want you to practice drawing perfect cyclohexane chair conformations, since we will come back to this conformation again and again throughout the semester.