Stereocenter
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A stereocenter, or stereogenic centre, is any atom in a molecule bearing groups such that an interchanging of any two groups leads to a stereoisomer [1]. In organic chemistry this usually refers to a carbon, phosphorus or sulfur atom, though it is also possible for other atoms to be stereocenters in organic and inorganic chemistry.
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. Molecules with symmetry frequently have fewer than the maximum possible number of stereoisomers.
The term stereocenter was introduced in 1984 by Mislow and Siegel [2].
The broad term stereocenter is often confused with that of the narrower set of chirality center. It is important to remember that a compound like 2-butene has two stereocenters forming two possible stereoisomers (cis and trans 2-butene) (and not 4!) and is not considered a meso compound [3]
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[edit] Exceptions
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.
[edit] Chiral carbon
A chiral carbon is a carbon atom which is asymmetric. Having a chiral carbon is usually a prerequisite for a molecule to have chirality, though the presence of a chiral carbon does not necessarily make a molecule chiral. A chiral carbon is often denoted by C*.
For the carbon to be chiral, it follows that:
- the carbon atom is 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. The exceptions, probably due to the form of chirality exhibited (Axial chirality), are hardly ever mentioned in normal-level discussions on strereochemistry and form two groups:
- Allenes which are of the form RR'C=C=CRR'
- Spiranes which have asymmetric rings, which can be identical.
[edit] 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. Racemization by Walden inversion may be restricted (such as ammonium or phosphonium cations), or slow. This allows the presence 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.