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Physical Properties of Stereoisomers

Physical Properties of Stereoisomers

Enantiomers are equal in all their physical properties except for their optical rotation, as they rotate the plane of polarized light by equal amounts in opposite directions. In contrast, diastereomers are characteristically different in their physical properties, such as melting and boiling points, densities, solubilities, heats of formation, and Gibbs free energies. If a molecule contains more than one chirality center and therefore consists of several pairs of enantiomers and diastereomers, the physical properties of each diastereomer must be noted. Tartaric acid, for instance, consists of three stereoisomers, a pair of enantiomers and a meso compound.

Tartaric acid
Physical properties of tartaric acid stereoisomers
Stereoisomer Configuration Melting point (°C) Optical rotation [α]D (degree) Density (g/ml) Solubility at 20 °C (g/100 ml H2O)
(+)-Tartaric acid (2R,3R) 168 - 170 + 12 1.7598 139.0
(-)-Tartaric acid (2S,3S) 168 - 170 - 12 1.7598 139.0
meso-Tartaric acid (2R,3S) / (2S,3R) 146 - 148 0 1.6660 125.0
Racemate of tartaric acid (2R,3R) and (2S,3S) 206 0 1.7880 20.6

(+)- and (-)-tartaric acid represent a pair of entantiomers. They differ only in their direction of optical rotation. Meso-tartaric acid, which is diastereomeric to (+)- and (-)-tartaric acid and shows no mirror-image relationship to them, thus has different physical properties. The racemic mixture of (+)- and (-)-tartaric acid behaves like a pure compound. It shows, for instance, a definite melting point. Nevertheless, its physical properties differ from the properties of meso-, (+)-, and (-)-tartaric acid.


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