[rleung]

Hi,

I am wondering why there are 4 stereoisomers for 1,2-cyclohexanediol.  I only get 3.  One has 1-R, 2-R, the other has 1-S, 2-S, and the 3rd has 1-R, 2-S (meso compound).  Is the book wrong?

Ryan

[movies]

Look at what happens if you do a ring flip of the 1R,2S isomer.  It's not really meso!!

[rleung]

Hmm, does this only apply to cyclohexanes?

[movies]

This is an example of conformational enantiomers, that is to say that two different conformations of the same molecule are enantiomeric.  It's not just limited to cyclohexanes, but that's probably the most common example.  Another example of this is the gauche conformer of butane.

[rleung]

Hi,

Sorry to resurrect a post from so long ago, but I still don't quite "see" why 1R,2S is not meso.  I was always taught that when you have two chiral centers, one being R, one being S, and if both are bonded to the same 4 substituents, then it is meso.  My book has never talked about conformational enantiomers.  It just talks about conformers of cyclohexane-derived molecules, and it doesn't mention any examples where two conformers are enantiomers - all of the conformers my book has are just different conformers of the same molecule.

Thanks so much

Ryan

PS: are you even able to isolate the two conformational enantiomers of cis-1,2-cyclohexanediol?

[Dan]

Oh yeah! I've never seen this before either, but if you draw the chair forms of the RS and SR, they are in fact enantiomers, I just spent 15 minutes trying to do ring flips and twisting it about - no joy, I'm convinced.

[rleung]

Hmmm, I still can't see.  I was never taught how to tell if chair conformers were enantiomers or not.  According to my lab-book that has 1,2-cyclohexanediol, it only lists 3 isomers. 

Even if they are enantiomers though, can they really even be separated?

Ryan

[Yggdrasil]

Try building models of thes two different molecules and see if you can physically superpose them.

I'm not sure if they can be separated, but I would think that you could possibly separate them at low temperatures when there is not enough thermal energy to allow interconversion between conformers.

[movies]

I'm not sure if these have been separated; in general conformational enantiomers are very difficult to separate.  The classic example that has been resolved is trans-cyclooctene.

These are, however, bona fide enantiomers, they just isomerize between one another very rapidly.  You may have noticed in playing around with models that one of the boat conformers has a plane of symmetry, which indicates that it is a meso conformation.

Here are some perspective pictures that might help:

[rleung]

Thanks so much!  Yea, I guess I never considered it an enantiomer since my book says they are hard to separate.  Thanks again!

Ryan