The prediction of van’t Hoff and Le Bel that the valencies of the carbon atom are directed towards the comers of a regular tetrahedron was proved in 1848 by L. Pasteur’s resolution of racemic sodium ammonium tartrate into the dextro and laevd forms. This discovery was followed by a period in which a number of compounds were resolved into their optically active forms. In addition, several optically active compounds were converted into one another, thus revealing their stereochemical relationship. Dextro and laevorotatory enantiomorphs are mirror images of one another, but there is no chemical method possible for determining the absolute arrangement of the atoms in space.
With the accumulation of much information it became necessary to adopt an arbitrary standard and relate the then known optically active molecules to it. E. Fischer used D (+) tartaric acid as the standard but later the simpler D (+) glyceral-dehyde was more widely used and given the arbitrary configuration as shown in Fig. 1.
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