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.
