Organic Chemistry: Stereoisomers

Summary: Physical Properties of Stereoisomers

Two chiral objects that are mirror images of each other behave identically in achiral environments. Therefore, enantiomers can only be distinguished in chiral environments. Enantiomers have identical physical properties in almost every regard except one: their ability to rotate plane- polarized light, or optical activity. When plane-polarized light is passed through a solution containing chiral compounds, the plane is rotated by a number of degrees depending on the nature of the molecules in solution. Enantiomers have equal but opposite optical rotations.

A solution that exhibits no optical rotation may, however, contain chiral substances. This occurs when the solution is a racemic mixture: one that contains an equal amount of both enantiomers. In such solutions, the optical rotations of the enantiomers cancel each other out. Racemic mixtures are often formed when achiral substances are converted to chiral ones via a chemical reaction. Since reactions typically occur in achiral environments, reactions typically are not enantioselective: one enantiomer cannot be formed favorably over another.

Racemic mixtures cannot be separated using conventional techniques. Rather, they must be separated, or resolved, in a chiral environment. Approaches include the formation of diastereomeric salts and the use of chiral chromatography. Separation, however, is often an inefficient approach to synthesizing chiral substances in high enantiomeric excess. Instead, chemists try to use reactions that are enantioselective. This can be done using chiral reagents or chiral catalysts.