Re: ORGLIST:meso and racemic?

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From: Jacob Zabicky (zabicky$##$bgumail.bgu.ac.il)
Date: Tue Sep 10 2002 - 08:33:58 EDT

Hello,

If I understood well your puzzle, it seems that the method of choice can
be 1H NMR. We have three possible configurations for a molecule of tartaric
acid: the two enantiomers (R,R) and (S,S) and the meso form (R,S), The
chemical shift and splitting constant of the alpha protons of the
enantiomers should be different from those of the meso form, due to
different chemical environments. Furthermore, addition of a suitable
chiral complexing agent may enhance that difference and may even help
resolving the racemic mixture without carrying out an actual separation of
the enantiomers. Some chalenging complications may arise in the particular
case of tartaric acid, such as finding an adecuate solvent, and the
structure of oligomeric aggregates in sulution, all affecting the spectra,
and ways have to be found to overcome them. I leave that to you.

All the best

All the best,

Jacob

At 13:38 9/9/2, Jonathan Brecher wrote:
>>My colleague and I have been discussing a stereochemistry puzzle. Appart
>>from using X-ray , how could you differentiate a meso compound from a
>>racemic mixture of the two enantiomers? For example, tartaric acid. What
>>if only one compound had been isolated?
>
>Well, at a fundamental level, a mixture of any sort (including a racemic
>mixture) can always be separated into its components *somehow*. Tartaric
>acid was originally characterized by crystallizing out single enantiomers
>from a racemate, for example. Chiral chromatography should work well, too.
>Even NMR would work in some cases, if you just want to know the presence of
>the mixture.
>
>Of course, separation is only a good indicator when it succeeds. If you
>can't separate a substance, you don't know if it's truly a meso compound or
>if you simply failed to find the right conditions for a separation...
>
>
>Jonathan Brecher
>CambridgeSoft Corporation
>jsb$##$cambridgesoft.com
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Prof. Jacob Zabicky
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