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Author: Subject: Racemic mixtures, enantiomers
Hermes_Trismegistus
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[*] posted on 21-2-2005 at 08:20
Racemic mixtures, enantiomers


Sorry if this is a silly question but my textbooks are theory heavy and practical light.

How are the r and s enantiomers of a racemic mixture customarily seperated from each other?




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sparkgap
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[*] posted on 21-2-2005 at 09:04


If the chiral compound in question is a base, it is reacted with (optically pure, of course) a chiral acid, and vice versa.

The diastereomers thus produced behave differently enough that HPLC can be used to separate them.

hope this helps.

sparky(^_^)




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Kanem
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[*] posted on 21-2-2005 at 09:45


When they were first discovered, enantiomeres were separated by evaporating the solvent and separating the remaining crystals, which consisted of only one enantiomer, by hand under the microscope.

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sparkgap
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[*] posted on 22-2-2005 at 06:29


Pasteur-style enantiomer separation would not be a practical method of resolving a racemic mixture, though. :(

This site may be of interest to everybody interested in resolution.

sparky (^_^)




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JohnWW
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[*] posted on 22-2-2005 at 09:16


One possibility could be separation by means of ion-exchange, by passing the mixture (if it has ionizable carboxylic acid or ammonium salt groups) through an ion-exchange material consisting of a polymer containing chiral carbon (or other e.g. nitrogen) atoms. This is just an idea; I am uncertain whether anyone has thought of or tried it.
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unionised
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[*] posted on 22-2-2005 at 11:36


Broadly the same idea has been tried.
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&a...

You can also use a nonchiral stationary phase and a chiral solvent, but that's rather expensive.

I seem to remember that you can produce chiral products by photochemical reactions using circularly polarised light. (lousy yield, but therotically elegant.)

Pastuer's method is a pain in the neck, but it's the only way that I can think of that doesn't start with something chiral.

[Edited on 22-2-2005 by unionised]
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