Enantiomers have identical chemical and physical properties and are indistinguishable from each other except for the direction of rotation of the plane of polarized light. They are described as optically active. One enantiomer in solution rotates the plane of polarized light to the right, whereas the other rotates it to the left. Enantiomers are two chemically identical molecular species which differ from each other as non-superposable mirror images.

The biological activity can depend strongly on the enantiomer. For instance, R -thalidomide is used to treat nausea, but S -thalidomide can cause birth defects.

Please see also our information on enantioselective organocatalysts.