An enantiomer is one of two stereoisomers that are nonsuperimposable complete mirror images of each other, much as one's left and right hands are "the same" but opposite. Enantiopure compounds refer to a sample having within the limits of detection, molecules of only one chirality.

Enantiomers have, when present in a symmetric environment, ****identical chemical and physical properties except for their ability to rotate plane-polarized light by equal amounts but in opposite directions****. A mixture of equal parts of an optically active isomer and its enantiomer is termed racemic and has a net rotation of plane-polarized light of zero. 

Diastereomers (or diastereoisomers) are stereoisomers that are not enantiomers (nonsuperimposable mirror images of each other). Diastereomers can have ****different physical properties and different reactivity****. In another definition diastereomers are pairs of isomers that have opposite configurations at one or more of the chiral centers but are not mirror images of each other [1]. 

Separation of Enantiomers (Resolution)

Optical activity (=the ability to rotate the plane of polarized light) is a direct result of chirality. Often times, the chirality is based on the existance of at least one chiral center, which has four different ligand attached to it. These four ligands can be oriented in two ways around the center, leading two different molecules that are called enantiomers. These two molecules are mirror images of each other. They are not superimposable.

These enantiomers are identical in most tests like melting point, boiling point, solubility, infrared spectroscopy, NMR spectroscopy, etc. However, they behave differently when it comes to the rotation of plane-polarized light or when placed in a chiral environment.

How can enantiomers be separated from each other?

A common way of separation of enantiomers uses the conversion into diastereomers, that are not mirror images of each other. The following example demonstrates this principle. The two enantiomers 1-phenylethylamine (S-(-), R-(+)) are separated using (L)-(+)-tartaric acid (also known as the (R,R)-form) as resolving agent. The two salts formed possess different cation ions are therefore not enantiomers of each other anymore. The (S)-(1)-phenylethyl ammonium-(R,R)-tartrate salt crystallizes faster than the (R)-(1)-phenylethyl ammonium-(R,R)-tartrate salt.