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Absolute Configuration

The connectivity of the atoms of a molecule is called constitution and their spatial arrangement configuration. The arrangement in space of the substituents of a stereogenic unit, for instance a chirality center, a chirality plane or a chirality axis, is called absolute configuration. Two enantiomers always have the opposite absolute configuration and show opposite optical activities, that is, they rotate the plane of polarized light by equal amounts in opposite directions.

D-(+)-glyceraldehyde. L-(-)-glyceraldehyde.

Before 1951, no method to determine the absolute configuration of a chiral molecule was known. It had been possible to distinguish two pure enantiomers by their optical activity but the optical activity could not be correlated with an absolute configuration. In 1906, a New York University chemist, M. A. Rosanoff, therefore proposed the D,L system and arbitrarily assigned the dextrorotatory (from the Latin dextro, right) (+)-glyceraldehyde to the D and the levorotatory (from the Latin levo, left) (-)-glyceraldehyde to the L configuration. Thereafter, glyceraldehyde was used as a standard and many thousands of compounds were related to it by chemical reactions that did not change any bond at a stereogenic unit.

For instance, lactic acid can be correlated to glyceraldehyde by the reaction sequence shown in the illustration below. In this reaction sequence, (+)-isoserine on the one hand is converted into (-)-lactic acid and on the other hand into (-)-glyceric acid without changing any bond at the chirality center. In addition, D-(+)-glyceraldehyde is oxidized with retention of the absolute configuration to (-)-glyceric acid. Therefore, (-)-lactic acid must have the same absolute configuration as D-(+)-glyceraldehyde and can be assigned the D configuration.

As one can see in this case, no simple correlation between the rotation of the plane of polarized light and the absolute configuration of a chiral molecule exists because D-glyceraldehyde and D-lactic acid rotate the plane of polarized light in opposite directions.

In 1951, a new special x-ray cristallography technique, called anomalous dispersion, enabled J. M. Bijvoet to determine the absolute configuration of a chemical compound, sodium rubidium tartrate, for the first time. By this analysis, Bijvoet showed that dextrorotatory (+)-tartaric acid has L configuration.

L-(+)-tartaric acid

By comparison with L-(+)-tartaric acid, (+)-glyceraldehyde could be definitely assigned to the D configuration. Thus, M. A. Rosanoff had made the correct choice when he arbitrarily assigned the D configuration to (+)-glyceraldehyde. With the definite determination of the absolute configuration of (+)-glyceraldehyde, the absolute configuration of thousands of compounds was suddenly known because they had been already correlated to (+)-glyceraldehyde by chemical reactions before.

In the early sixties, R. S. Cahn, C. K. Ingold, and V. Prelog introduced a new system, the (R,S) system, to name the absolute configuration because the D,L system is not unequivocal in all cases and cannot easily be applied to all families of compounds. According to this system, (+)-tartaric acid has (2R,3R) configuration.


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