Stereocenter



A stereocenter, or stereogenic centre, is any 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 meso compound [3]

Exceptions

Having two chiral centers may give a Planar chirality may also provide for chirality without having an actual chiral center present.

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.

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.

References

  1. ^ Solomons & Fryhle. (2004). Organic Chemistry, 8th ed.
  2. ^ Stereoisomerism and local chirality Kurt Mislow and Jay Siegel J. Am. Chem. Soc.; 1984; 106(11) pp 3319 - 3328; doi:10.1021/ja00323a043
  3. ^ Precision in Stereochemical Terminology Wade, LeRoy G., Jr. J. Chem. Educ. 2006 83 1793 Link.
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