Conformations and Cycloalkanes
By James Ashenhurst
The Cyclohexane Chair Flip
Last updated: March 21st, 2019
In our last post, an aerial tour of the cyclohexane chair, we showed that there are two different positions a substituting can occupy on a cyclohexane chair – axial (straight up and down, relative to the ring) and equatorial (off to the side of the ring).
This brings up an interesting thought experiment. Let’s take a really simple substituted version of cyclohexane – 1-methylcyclohexane, for example.
From the “dash wedge” diagram, there is only one way to depict 1-methylcyclohexane. Like this. No matter how you rotate it, or flip it – it’s all superimposable versions of the same thing.
Now let’s draw the cyclohexane chair version. We draw a chair, and then…. hold on!
Can you see how this is a different situation? We could put the methyl group on an axial carbon or an equatorial carbon… and these are not superimposable on each other, unlike the “flat” drawing above.
Same connectivity, different shape – this is a definition of “conformational isomers” if ever there was one.
However, unlike, say, conformations in linear alkanes, which involve rotations about single bonds, it might not be immediately obvious how the chair on the left can be converted to the chair on the right.
The point of this post is to describe how these two conformations can be converted into each other, through a series of bond rotations we call a “chair flip”.
When we first introduced the cyclohexane chair, we mentioned that it was the lowest energy conformation of cyclohexane – but not the only conformation. Here’s a molecular model of cyclohexane, showing all the axial hydrogens (white) and equatorial hydrogens (red). Looked at from one perspective, we see the chair with its “head rest” on the left and “foot rest” on the right.
Now, if we take the “foot rest” of cyclohexane, and rotate several of the bonds, we are able to obtain a structure which looks like it has two “head rests”. This is called the “boat” [To be technical, it will actually be a “twist boat”, where there is a slight offset of the groups on the “prow”]
Now, if we take the “original” head rest (on the left) and pull it down, so that it makes a new “foot rest”, we have the following.
Your first thought might be – that’s pretty useless! Who cares if we just start with a cyclohexane chair and convert it into another cyclohexane chair. That’s like converting a dollar bill into four quarters and then exchanging them back into a dollar bill. It’s almost like nothing happened.
Ahh! You see – it’s not QUITE the same. In this video, watch how this model of a cyclohexane chair is first converted into a boat, and then into a new cyclohexane chair.
Here’s the molecular ballet in motion. Pay close attention to the [white] axial groups and the [red] equatorial groups. What do you see?
During this chair flip, all the axial groups become equatorial… and all the equatorial groups become axial!
This brings us back to 1-methylcyclohexane.
These two conformations can be converted to each other using a chair flip. Note how on C-1, “axial up” becomes “equatorial up”. That’s the right way to do it.
This being organic chemistry, there’s plenty of ways to screw this up. Here’s a few wrong ways to draw a chair flip :
In the next post, we’ll look into this “ring flipping” process in a bit more detail. Stay tuned!