Cyclohexane Chair Conformations: Differences Explained

[MathewsMD]

In the attached file, it shows the two diagrams for a cyclohexane in its chair conformations. I was just wondering if there's any actual differences between these two structures if there were no substituents except H. Like is the top vertex that has an acute angle drawn in the image point up in the actual chair conformation or something like that? The reason I ask is because when you flip from one chair conformation to the other, you get the other image. I just don't understand how you know which image you start off and which one you end with. And by image, I mean the one of the two conformations drawn in the attachment if you did not understand.

I simply don't understand why we use two different base structures when you have essentially the same chair. I don't really see a difference in the actual cyclohexane 3D molecule when I make it using a model so any explanation would be great!

Thank you so much! :)

 

[MathewsMD]

The only observation I've been able to make is that if the slope of the middle two lines of the body is pointing downwards (negative slope) then the bottom right corner carbon will have an axial substituent downwards. Also, if the slope is positive (the middle two lines are pointing upwards) then the bottom right corner substituents will be pointing up. Please correct me if this generalization is incorrect. Also, please point out any other similarities and differences between the two chair conformations if you can. :)

 

[Borek]

As long as there are no substituents these are identical. Rotate the molecule 180 deg.

 

[MathewsMD]

As long as there are no substituents these are identical. Rotate the molecule 180 deg.

So if a question asked, assuming C-1 is the bottom right carbon, which direction is the axial substituent pointing, is there a clear cut answer? Is it correct to assume it is downwards if the chair conformation shown has its middle two lines drawn downwards (the chair on the right)?

 

[Yanick]

Placing artificial labels for the purposes of an exam or something would lead to a unique answer for sure. The positions which the substituents take at any carbon are classified as equatorial or axial, roughly speaking this corresponds to a substituent pointing up/down (axial) versus "out" (equatorial). When chairs flip, the axial positions become equatorial and vice versa.

The point however, is that physically the two conformations you have are equivalent. There is in fact no way to distinguish between the conformers because there is no way to distinguish between the substituents (they are all equivalent hydrogens). Either way you slice it, you'll have 6 equatorial and 6 axial hydrogens. If you exchanged one of the hydrogens for a deuterium or something else, you may be able to use some kind of spectroscopy or other to differentiate between the two conformations, provided the deuterium/substituent is not acidic/exchangeable with the solvent (which is the case here of course).

 

[MathewsMD]

Placing artificial labels for the purposes of an exam or something would lead to a unique answer for sure. The positions which the substituents take at any carbon are classified as equatorial or axial, roughly speaking this corresponds to a substituent pointing up/down (axial) versus "out" (equatorial). When chairs flip, the axial positions become equatorial and vice versa.

The point however, is that physically the two conformations you have are equivalent. There is in fact no way to distinguish between the conformers because there is no way to distinguish between the substituents (they are all equivalent hydrogens). Either way you slice it, you'll have 6 equatorial and 6 axial hydrogens. If you exchanged one of the hydrogens for a deuterium or something else, you may be able to use some kind of spectroscopy or other to differentiate between the two conformations, provided the deuterium/substituent is not acidic/exchangeable with the solvent (which is the case here of course).

Yes, I understand that when the substituents are the same, the conformations are actually indistinguishable. My question is regarding the purposes of an examination like you noted, where there are two drawing and each denotes a specific way to draw the substituent at that carbon, either up or down in that specific point of time. I was wondering if the the right structure (in the attachment) has an axial substituent pointing down on the bottom right carbon all the time and if the left chair conformation has an axial substituent pointing up on the bottom right carbon all the time. Just trying to find any confirmation on the two chair conformations. :)

 

[Yanick]

I think what you said is correct, but I'm having a hard time understanding exactly what it is you mean. I find this kind of concept difficult to learn/explain in purely written word. Perhaps a picture will help.

See the picture here: http://en.m.wikipedia.org/wiki/Ring_flip

It codes the carbons and hydrogens and shows a ring flip. I think this should clarify any confusion.