Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: kenny1999 on June 13, 2011, 12:18:47 PM
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For straight-chained stuff, I think it is very easy to determine the whether the molecule has enantiomer by seeing if it has chiral carbon. (but am I correct? have I thought too simply?)
However, how about the cyclohydrocarbons? How to determine whether it has enantiomer? I know there is a method by seeing if the molecule has reflectional symmetry but how to look at cyclo-stuff ?? I really don't think it is that simple by looking only at the 2-dimension polygon. Right? Could anyone help ? I will go crazy if I continue imagining the picture without anyone help. .
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For straight-chained stuff, I think it is very easy to determine the whether the molecule has enantiomer by seeing if it has chiral carbon. (but am I correct? have I thought too simply?)
That depends on how pedantic you want to be. All compounds with only one chiral carbon are chiral compounds and have enantiomers. Most compounds with more than one chiral compound are chiral, although there are a very few examples of compounds with more than one chiral carbon wherein the chiral carbons reflect each other through a plane of symmetry - these are called meso compounds and are achiral. See http://en.wikipedia.org/wiki/Meso_compound (http://en.wikipedia.org/wiki/Meso_compound)
However, how about the cyclohydrocarbons? How to determine whether it has enantiomer? I know there is a method by seeing if the molecule has reflectional symmetry but how to look at cyclo-stuff ?? I really don't think it is that simple by looking only at the 2-dimension polygon. Right? Could anyone help ? I will go crazy if I continue imagining the picture without anyone help. .
You're right, cyclic structures are a little more difficult to visualize if you are drawing flat polygons, and changes of conformation (such as alternating cyclohexane chair structures) can further obscure the issue. BUILD MODELS. The best way to determine if a compound is chiral is to build models of both mirror images of the compound and see if there is any way to overlap them. That being said, however, the statement above still applies - if there is only one chiral carbon in the molecule, it is a chiral compound. If there is more than one chiral center, it is probably chiral, unless there is a plane of symmetry somewhere in the compound.
There are also a very few oddball cases of compounds which are chiral without having a chiral carbon at all. These are compounds such as allenes, o-substituted dibenzenes, spirocycles, or polycyclic aromatics that are locked in either a twisted or spiraled configuration. Some examples are shown here: http://chemistry.umeche.maine.edu/CHY251/Chirality.html (http://chemistry.umeche.maine.edu/CHY251/Chirality.html)
And once you get beyond carbon, you can find all sorts of other atoms that can show chirality, but that is probably far more than you ever wanted to know about stereochemistry.
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That depends on how pedantic you want to be. All compounds with only one chiral carbon are chiral compounds and have enantiomers. Most compounds with more than one chiral compound are chiral, although there are a very few examples of compounds with more than one chiral carbon wherein the chiral carbons reflect each other through a plane of symmetry - these are called meso compounds and are achiral. See http://en.wikipedia.org/wiki/Meso_compound (http://en.wikipedia.org/wiki/Meso_compound)
I would argue that "accurate" would be a more appropriate word than "pedantic".
This is why I find the term "chiral carbon" highly misleading and inappropriate for what it describes - I personally opt for "asymmetric centre". Meso compounds, which contain at least 2 asymmetric centres but are not chiral compounds, are not "very few" in number. They crop up very regularly in exam papers/chemical literature/textbooks etc.
The bottom line is: If the mirror image is superimposable on the original, the compound is achiral. A shourtcut to this is finding a plane (or centre) of symmetry - the presence of which indicates the molecule is achiral. Planes (and centres) of symmetry are more obvious (generally) for cyclic molecules since the conformational rigidity compared to linear molecules limits the number of equivalent pictorial representations you could have on a page for the same structure - so for linear molecules you are more likely to have to rotate bonds and re-orientate the drawing to see symmetry planes/centres. If in doubt, make a model.