Re: ORGLIST:meso and racemic?

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From: Richard Prankerd (richard.prankerd$##$vcp.monash.edu)
Date: Mon Sep 09 2002 - 18:15:06 EDT

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
> ****************
>

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