Direct crystallization of enantiomers and dissociable diastereomers

Molecules whose mirror images cannot be superimposed on each other are identified as being chiral, and a very important branch of separation science has developed around the resolution of compounds having pharmaceutical interest. At one time, scientists associated the phenomenon of optical rotation with the presence of carbon atoms bound to four different molecular fragments, and these asymmetrically substituted carbon atoms became known as "asymmetric carbons." Continued work showed that compounds incapable of rotating the plane of polarized light, but which were known to contain at least one asymmetric carbon atom, could be separated into chemically identical "optical isomers" that now exhibited the phenomenon of optical rotation. Over time, it became clear that optical activity could exist in compounds having no asymmetric atoms, and that other compounds existed that contained two or more asymmetric carbons, but still could not be rendered optically active. These findings necessitated a return to the proposal of Pasteur, who held that optical activity is a consequence of molecular dissymmetry. In other words, a molecule superimposable with its mirror image cannot be optically active, and any molecule not superimposable with its mirror image will exhibit optical activity.