Isomers and Stereoisomers

Molecules can exist as constitutional isomers or stereoisomers. The stereochemistry of a chirality center can be determined using the Cahn-Ingold-Prelog method.

Stereochemistry is the field of study that is concerned with the structures of molecules in three dimensions. The three-dimensional structure of a molecule is important because often molecules will exist as isomers. An isomer is one of two or more molecules that have the same molecular formula, but different arrangements of atoms and bonds. Isomers can be subdivided into constitutional isomers and stereoisomers. A constitutional isomer is one of two or more molecules that have the same molecular formula but different structural arrangements of atoms and bonds. A stereoisomer (or spatial isomer) is one of two or more molecules that have the same molecular formula and bond structure, but different arrangements of the three-dimensional orientation of the atoms.

Chirality is the configuration, or handedness, of an asymmetric chemical compound. An atom will be a chirality center if the atom, almost always a carbon atom, is bonded to four different atoms or groups. A chiral molecule is a molecule that does not have a plane of symmetry and whose isomers cannot be interconverted by rotation or reflection. An achiral molecule is a molecule that has a plane of symmetry and whose isomers can be interconverted by rotation or reflection.

Comparison of Constitutional Isomers and Stereoisomers

While both constitutional isomer pairs and stereoisomer pairs are characterized by having the same molecular formula, they differ in that constitutional isomer pairs do not have the same atomic connectivity. Stereoisomers are connected in the same way but have different spatial arrangements.
Stereoisomers can be further subdivided into diastereomers and enantiomers. An enantiomer is a stereoisomer that has a mirror image that is not superimposable on itself. A diastereomer is a stereoisomer that is not a mirror image of the original compound and is not superposable on itself. To superpose means to lay something upon another so as to make all like parts coincide. To superimpose means to place or lay over something. The two terms are similar, but superposing is more specific than superimposing. All chirality centers in enantiomers will have different handedness. In diastereomers, at least one chirality center will be the same, and at least one will be different.

Subdivision of Isomers

A flowchart for the subdivision of isomers into categories based on distinct characteristics, such as atom connectivity and superposability, can be used to determine isomerism.
Just as it is possible for a molecule to contain chiral centers and be achiral, it is possible for a molecule to have no chiral centers and still be chiral. The most common example of the phenomenon are cis/trans alkene diastereomers. Conformational isomers are another type of chiral molecule that contain no chiral centers. Rather, the molecules contain a rotational barrier so large that it is possible to isolate individual isomers between conformers. One example of this type of molecule is the enantiomeric forms of BINAP: (S)-BINAP and (R)-BINAP.
BINAP is an example of a chiral compound that does not contain a chiral center, a carbon with four different groups attached.
Enantiomers have the same molecular formulas, they are mirror images of each other, and they have identical physical properties. Since their physical properties are identical, it is impossible to separate enantiomers by conventional methods. Fortunately, diastereomers have different physical properties and can be separated using conventional methods. Separation of enantiomers is more complex and involves nonconventional methods such as resolution. Resolution is the process for the separation of racemic compounds into their enantiomers.

One common method for resolving a racemic mixture or any solution of enantiomers involves converting the enantiomers to diastereomers. Alternatively, resolution of enantiomers is accomplished using chromatography (separation of mixture) with chiral media, a solvent or system that is chiral. A diastereomeric interaction occurs between the media and the enantiomers in the solution, and the distinct enantiomers will move through the chromatography apparatus at different speeds, separating themselves. Additionally, because the media is a solid, there is no need to perform another reaction to separate the enantiomer.