Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Rutherford on January 17, 2013, 12:22:43 PM
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Cyclohexane-1,2-dicarboxylic acid has only 2 optical isomers because it is a meso compound. My question is how are these isomers called: R,R and R,S or maybe S,S and R,S etc. What system is used for naming the stereocenters of meso compounds?
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As far as I know the Cahn-Ingold-Prelog system is used, as usual.
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Okay, but R,R and S,S are the same compound, they aren't isomers. The same is for R,S and S,R. If I need to write the names of the isomers should I write R,R and R,S or S,S and R,S or R,R and S,R or S,S and S,R?
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In this drawing the top two structures have the plane of symmetry, the bottom two don't. But all are non-superimposable on their mirror images.
Your statement about R,R and S,S being the same compound is not correct, these are enantiomers, or more correctly diastereoisomers.
Qute from Wiki "The two cis stereoisomers of 1,2-substituted cyclohexanes behave like meso compounds at room temperature in most cases. At room temperature, most 1,2-disubstituted cyclohexanes undergo rapid ring flipping (exceptions being rings with bulky substituents), and as a result, the two cis stereoisomers behave chemically identically with chiral reagents.[5] At low temperatures, however, this is not the case, as the activation energy for the ring-flip cannot be overcome, and they therefore behave like enantiomers. Also noteworthy is the fact that when a cyclohexane undergoes a ring flip, the absolute configurations of the sterocenters do not change."
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When looking for symmetry I didn't take into account that one -COOH group is below and that the other is above the plane, therefore there is no symmetry and R,R and S,S aren't meso. Thanks for the explanation.
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therefore there is no symmetry
Careful, there is symmetry, just not a plane of symmetry. The R,R and S,S isomers both have a C2 symmetry axis.
See also: http://en.wikipedia.org/wiki/Rotational_symmetry
I just want to clarify this statement:
Your statement about R,R and S,S being the same compound is not correct, these are enantiomers, or more correctly diastereoisomers.
R,R and S,S are related as enantiomers (it is not correct to say they are diastereoisomers)
R,R and R,S(=S,R) are related as diastereoisomers
S,S and R,S(=S,R) are related as diastereoisomers
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Okay, thanks for the correction.
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Are the attached two compounds the same compound? I would say yes (they should interconvert easily) because the two groups attached to the "pseudo" chiral C atom are same. True or not?
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Consider flattening the cyclobutane ring. Do the substituents have the same geometry?
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Build models!!!!!!!!! Put in all the hydrogen atoms. Do they overlap? Are they the same molecule?
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By looking now, they don't seem as the same compound (one can't be flattened to produce the other), but R/S notation says that there isn't any chiral C atom. How are these two compounds called then? Rotamers?
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By looking now, they don't seem as the same compound (one can't be flattened to produce the other), but R/S notation says that there isn't any chiral C atom.
That is correct - neither has any chiral centres - but that does not mean they are the same.
These compounds can be distinguished by cis/trans nomenclature, or by using the pseudoasymmetric centre descriptors r/s. The latter is quite complicated in this case because it cannot be done directly and requires the use of auxiliary stereodescriptors to assign the pseudoasymmetric centres - see here (http://www.iupac.org/fileadmin/user_upload/publications/recommendations/nomenclature-of-organic-chemistry/Chapter9.pdf) for examples. My advice is to stick with cis/trans unless you specifically need to know how to do the pseudoasymmetric assignment (or you just have a perverse love of stereochemical nomenclature like me).
How are these two compounds called then? Rotamers?
What is the definition of rotamer? Does the definition fit this case?
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Wow, thanks for the link. There are many things I didn't know, but I will need some time to study it. I will stick to cis-trans.
Rotamers arise from the restricted rotation around a double bond. I thought its similar here (restricted rotation because of the ring). Thanks for explaining this.