Ch 3 : Cyclohexane

Chapter 3: Conformations of Alkanes and Cycloalkanes

Cyclohexane

This is a common and therefore important ring system, what should you know ?

How to draw cyclohexanes so you can convey your knowledge !
The most stable conformation is the chair
There are other less stable conformations such as the boat and the twist boat.
Ring flipping interconverts chair conformations.
There are two types of substituent positions around a chair cyclohexane axial and equatorial.

C₆H₁₂ CYCLOHEXANE ΔHc / CH₂ = -653 kJ/mol (-156.1 kcal/mol)
	planar	most stable structure

Let's investigate in more detail some of the important features of the 3D shape of cyclohexane.

The most stable conformation of cyclohexane is the chair form shown to the right. The C-C-C bonds are very close to 109.5^o, so it is almost free of angle strain. It is also a fully staggered conformation and so is free of torsional strain.
Rotate the molecule in the JSMOL image to show this just like a Newman projection so that you can inspect the staggered C-H and C-C bonds.

The chair conformation is the most stable conformation of cyclohexane.

In chair cyclohexane there are two types of positions, axial and equatorial. The axial positions point perpendicular to the plane of the ring, whereas the equatorial positions are around the plane of the ring. You should notice that adjacent axial postions point in opposite directions. The same is true for the equatorial positions. The axial and equatorial positions can be highlighted in the JSMOL image to the right:

Highlight an equatorial H atom
Highlight all 6 equatorial H atoms
Highlight an axial H atom
Highlight all 6 axial H atoms
Highlight carbon skeleton
Reset colours

axial and equatorial positions

axial and equatorial substituted cyclohexane

A second, much less stable conformer is the boat conformation. This too is almost free of angle strain, but in contrast has torsional strain associated with eclipsed bonds at the four of the C atoms that form the side of the boat. Rotate the molecule in the JSMOL image to show this just like a Newman projection. In addition, a steric interaction of the H atoms inside the "bow" and the "stern", known as the flagpole interaction also destabilises the boat.

Highlight flagpole H atoms
Highlight carbon skeleton
Reset colours

A third conformation is produced by twisting the boat to give the twist or skew-boat conformation. The twist relieves some of the torsional strain of the boat and moves the flagpole H further apart reducing the steric strain. Consequently the twist boat is slightly more stable than the boat.

Highlight flagpole H atoms
Highlight carbon skeleton
Reset colours

Conformational rotation of cyclohexane interconverts the conformations. This proceeds from one chair to twist boat to boat to twist boat to the other chair conformation. This process is often referred to as "ring flipping".

Watch the JSMOL animation carefully and look for the two chair forms, stop and rotate the animation if needed. The animation will pause at the chair conformation before continuing.

An important feature of this process is that the axial and equatorial positions are interchanged. If you use the colour coding you should be able to see that a position that was axial in one chair is equatorial in the other and vice versa.

Highlight all equatorial H atoms
Highlight all axial H atoms
Start/stop ring flip
Reset colours

An important feature of this process is that the axial and equatorial positions switch. If you watch carefully you will see that a position that was axial in one chair is equatorial in the other and vice versa.

The animation below shows how the potential energy of the cyclohexane molecule varies during the ring-flip process.
Use the controls to show the energies of the important conformations. Note that the boat conformation is an unstable conformation (i.e. a maxima) on the pathway.