From: Dr. Stephen J. Simpson (S.J.Simpson$##$salford.ac.uk)
Date: Mon Sep 09 2002 - 18:42:56 EDT
Richard's points are very valid but only applicable if both the meso
compound and the racemic mixture/or one enantiomer are obtainable, as was
the case for tartaric acid. We are interested in the situation where only
a single compound can be obtained which may be the meso compound or a
racemic mixture; i.e an RS(SR) compound versus an RR + SS mixture.
The use of an NMR chiral shift reagent which binds say, to only an R centre
will generate two sets of resonances in either case and so does not solve
the problem. Again if both the meso compound and racemic mixture/or single
enantiomer are available this technique can solve the problem because the
resonances due to the uncomplexed S centre will be different for the two
cases..
Again in principle a chiral chromatography column which binds only to one
centre could work in that a single diastereomer would elute for the meso
compound, for the racemic mixture one diastereomer and one unchanged
enantiomer would elute, and for a single enantiomer only one compound would
elute which could be either a diastereomer or an unchanged enantiomer.
This is close to solving the problem in a perfect case.
>Sylvie and all:
>
>Physicochemical properties of racemates, enantiomers and meso compounds are
>very often different. For example, the melting behaviour of the four
>tartaric acids are:
>
>D-tartaric: m.p. 168-170
>DL-tartaric: m.p. 206
>L-tartaric: m.p. 168-170
>meso-tartaric: m.p. 140 (also reported as 159-160, possibly through
>polymorphism)
>
>The D and L forms must always have the same melting point (and enthalpy of
>fusion, also solubility), but the racemate (and a meso form, if there is
>one) are physically different compounds. Other physical differences will
>persist in the solid state, although to a greater or lesser degree of
>discrimination, e.g., the IR spectrum in a KBr medium, the solubility in
>various solvents and the density. See the discussion by Eliel,
>Stereochemistry of carbon compounds, McGraw-Hill (1962), pp. 26, 43-47,
>which also states that some solution properties of the meso form are
>different from the enantiomers, such as the pKa and the dipole moment.
>
>Regards
>Richard
>
>Richard J. Prankerd, PhD
>Senior Lecturer
>Victorian College of Pharmacy, Monash University
>381 Royal Pde., Parkville VIC 3052
>
>Phone: INT+613-9903-9003
>Phax: INT+613-9903-9583
>
>Drugs need to be designed with delivery components in mind - Takeru Higuchi
>
>----- Original Message -----
>From: "Sylvie Ducki" <sylvie_ducki$##$yahoo.com>
>To: <everybody$##$orglist.net>
>Cc: <s.j.simpson$##$salford.ac.uk>
>Sent: Tuesday, September 10, 2002 2:13 AM
>Subject: ORGLIST:meso and racemic?
>
>
>>
>> Dear All,
>>
>> 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?
>>
>> Dr Sylvie Ducki (and Dr. Steve Simpson)
>>
>>
>> ***************
>> Dr. Sylvie Ducki
>> email sylvie_ducki$##$yahoo.com
>> http://www.sciences.salford.ac.uk/chemist/Staff/sdres.htm
>> ****************
>>
Dr. Stephen J. Simpson Senior Admissions Tutor
Department of Chemistry Senior Lecturer in Chemistry
University of Salford Tel: (44) 161 295 5978
Salford. M5 4WT Fax: (44) 161 295 5111
United Kingdom. E-mail: S.J.Simpson$##$salford.ac.uk
http://www.sciences.salford.ac.uk/chemist//sjsres.html
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