Sodium rubidium tartrate

The method, also called heavy atom method, consists in introducing a heavy atom in the molecule. Then X-rays with a wave length close to the X-ray absorption of the heavy atom is introduced. As a result a phase shift is superimposed on the ordinary diffraction pattern and configuration is then deduced. The method was first employed in 1951 by Bijvoet et al. to examine sodium rubidium tartrate who concluded that it is possible to differentiate between the two optically active forms. In other words it was possible to determine the absolute configuration of the enantiomers. Since then the absolute configurations of about two hundred optically active compounds have been elucidated by their correlation with other substances of known configuration. 

In 1951, it became possible to determine whether Rosanoff s guess was right. Ordinary X-ray crystallography cannot distinguish between a d and a l isomer, but by use of a special technique, Bijvoet was able to examine sodium rubidium tartrate and found that Rosanoff had made the correct choice. It was perhaps historically fitting that the first true absolute configuration should have been determined on a salt of tartaric acid, since Pasteur made his great discoveries on another salt of this acid. 

Figure 1.6. Stereoisomers of sodium rubidium tartrate. (R,R) is the enantiomer of S,S). Top Newman projeetion Bottom Fiseher projeetion. (R,S) is the enantiomer of (S,R). Both (R,R) and (S,S) are diastereomers of (R,S) and (S,R). For disodium tartrate or for tartarie acid, there exists only one nonchiral mesoform instead of the enantiomers (R, S) and (S, R).

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