A stereocenter or stereogenic center is an atom bearing groups such that an interchanging of any two groups leads to a stereoisomer. The most common stereocenters are chiral centers (such as asymmetric carbon atoms) and the double-bonded carbon atoms in cis-trans alkenes.
A chiral center consists of an atom holding a set of ligands (atoms or groups of atoms) in a spatial arrangement which is not superposable on its mirror image. A chiral center is a generalized extension of an asymmetric atom. The term stereocenter was introduced in 1984 by Mislow and Siegel.
Number of stereoisomers
A molecule can have multiple stereocenters, giving it many stereoisomers. In compounds whose stereoisomerism is due to chiral centers, the total number of hypothetically possible stereoisomers will not exceed 2n, where n is the number of chiral centers. Molecules with symmetry frequently have fewer than the maximum possible number of stereoisomers.
Species having two equivalent chiral centers, exist as three stereoisomers, RR, SS, and RS, the latter being equivalent to SR. The RS/SR stereoisomer is not chiral, but is called a meso compound. Meso compounds do not exhibit chirality due to the presence of an internal mirror plane of symmetry or a center of symmetry in the structure of the molecule in its most symmetric conformation.
In organic chemistry a chiral center usually refers to a carbon, although it is also possible for other centers to be chiral. A "chiral carbon" or "asymmetric carbon" is jargon for a carbon atom that is a chiral center. Most chiral organic compounds have a chiral carbon, although the presence of a chiral carbon does not require a molecule to be chiral (see meso compound). A chiral carbon is sometimes denoted by C*.
If the carbon is chiral, it follows that:
- the carbon atom is quasi-tetrahedral (sp3-hybridized);
- there are four different groups attached to the carbon atom.
Almost any other configuration for the carbon would produce a center of symmetry. For example, an sp- or sp2-hybridized molecule would be planar, with a mirror plane. Two identical groups would give a mirror plane bisecting the molecule.
Other chiral centers
Chirality is not limited to organic compounds, although carbon atoms are often centers of chirality due to their ubiquity in organic chemistry. Compounds with the formula RR'R"E:, where : is a lone pair are also chiral. When E is N or O, the rates of inversion are so fast, that enantiomers are not typically isolable. Species with tetrahedral or octahedral geometries may also be chiral.
Chirality without stereogenic atoms
Planar chirality provide for chirality without having an actual chiral center present.