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A stereocenter or stereogenic center is any point in a molecule, though not necessarily an atom, bearing groups, such that an interchanging of any two groups leads to a stereoisomer. The term stereocenter was introduced in 1984 by Mislow and Siegel. A chiral center is a stereocenter consisting of an atom holding a set of ligands (atoms or groups of atoms) in a spatial arrangement which is not superimposable on its mirror image. The concept of a chiral center generalizes the concept of an asymmetric carbon atom, i.e., a carbon atom bonded to four different entities, such that an interchanging of any two groups gives rise to an enantiomer. In organic chemistry, a chiral center usually refers to a carbon, phosphorus, or sulfur atom, though it is also possible for other atoms to be chiral centers, especially in areas of organometallic and inorganic chemistry.
Possible number of stereoisomers
A molecule can have multiple stereocenters, giving it many stereoisomers. In compounds whose stereoisomerism is due to tetrahedral stereogenic centers, the total number of hypothetically possible stereoisomers will not exceed 2n, where n is the number of tetrahedral stereocenters. However, molecules with symmetry frequently have fewer than the maximum possible number of stereoisomers. Having two chiral centers may give a meso compound which is achiral. Certain configurations may not exist due to steric reasons. Cyclic compounds with chiral centers may not exhibit chirality due to the presence of a two-fold rotation axis. Planar chirality may also provide for chirality without having an actual chiral center present.
An obsolete usage describes a carbon atom that is located at a stereocenter of a molecule a chiral carbon or asymmetric carbon. This is confusing as carbon atoms are not chiral - Mislow and Siegel2 clarfied the usage with the introduction of the concept of a stereocenter, which is a stereogenic element. As defined above, a stereocenter is a point in space where interchange of groups within a molecular structure produces a stereoisomeric structure. Further confusion arises from IUPAC's rules that utilize the term "chirality center" in place of "stereocenter" for no apparent reason. This leads to various interpretations in text books.
Other chiral centers
Chirality is not limited to carbon atoms, though carbon atoms are often centers of chirality due to its ubiquity in organic chemistry. Nitrogen and phosphorus atoms are also tetrahedral.[clarification needed] Racemization by Walden inversion may be restricted (such as ammonium or phosphonium cations), or slow, which allows the existence of chirality.
Metal atoms with tetrahedral or octahedral geometries may also be chiral due to having different ligands. For the octahedral case, several chiralities are possible. Having three ligands of two types, the ligands may be lined up along the meridian, giving the mer-isomer, or forming a face—the fac isomer. Having three bidentate ligands of only one type gives a propeller-type structure, with two different enantiomers denoted Λ and Δ.
- Cahn–Ingold–Prelog priority rules for nomenclature