Stereocenter

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Two enantiomers of a generic amino acid at the stereocenter

In

stereoisomer.[1][2]
Stereocenters are also referred to as stereogenic centers.

A stereocenter is geometrically defined as a point (location) in a molecule; a stereocenter is usually but not always a specific atom, often carbon.

achiral molecules; stereocenters can contain single bonds or double bonds.[1] The number of hypothetical stereoisomers can be predicted by using 2n, with n being the number of tetrahedral stereocenters; however, exceptions such as meso compounds can reduce the prediction to below the expected 2n.[4]

Chirality centers are a type of stereocenter with four different substituent groups; chirality centers are a specific subset of stereocenters because they can only have sp3 hybridization, meaning that they can only have single bonds.[5]

Location

Stereocenters can exist on

sp3 or sp2 hybridization
.

Possible Number of Stereoisomers

Stereoisomers are compounds that are identical in composition and connectivity but have a different spatial arrangement of atoms around the central atom.[6] A molecule having multiple stereocenters will produce many possible stereoisomers. In compounds whose stereoisomerism is due to tetrahedral (sp3) stereogenic centers, the total number of hypothetically possible stereoisomers will not exceed 2n, where n is the number of tetrahedral stereocenters. However, this is an upper bound because molecules with symmetry frequently have fewer stereoisomers.

The stereoisomers produced by the presence of multiple stereocenters can be defined as enantiomers (non-superposable mirror images) and diastereomers (non-superposable, non-identical, non-mirror image molecules).[6] Enantiomers and diastereomers are produced due to differing stereochemical configurations of molecules containing the same composition and connectivity (bonding); the molecules must have multiple (two or more) stereocenters to be classified as enantiomers or diastereomers. Enantiomers and diastereomers will produce individual stereoisomers that contribute to the total number of possible stereoisomers.

However, the stereoisomers produced may also give a meso compound, which is an achiral compound that is superposable on its mirror image; the presence of a meso compound will reduce the number of possible stereoisomers.[4] Since a meso compound is superposable on its mirror image, the two "stereoisomers" are actually identical. Resultantly, a meso compound will reduce the number of stereoisomers to below the hypothetical 2n amount due to symmetry.[6]

Additionally, 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.

Configuration

Configuration is defined as the arrangement of atoms around a stereocenter.[6] The Cahn-Ingold-Prelog (CIP) system uses R and S designations to define the configuration of atoms about any stereocenter.[7] A designation of R denotes a clockwise direction of substituent priority around the stereocenter, while a designation of S denotes a counter-clockwise direction of substituent priority.[7]

Chirality Centers

A

organometallic and inorganic chemistry
.

The concept of a chirality center generalizes the concept of an asymmetric carbon atom (a carbon atom bonded to four different entities) to a broader definition of any atom with four different attachment groups in which an interchanging of any two attachment groups gives rise to an enantiomer.[8]

Stereogenic on Carbon

A carbon atom that is attached to four different substituent groups is called an asymmetric carbon atom or chiral carbon. Chiral carbons are the most common type of chirality center.[6]

Stereogenic on Other Atoms

Chirality is not limited to carbon atoms, though carbon atoms are often centers of chirality due to their ubiquity in organic chemistry. Nitrogen and phosphorus atoms can also form bonds in a tetrahedral configuration. A nitrogen in an

nitrogen inversion may be restricted (such as quaternary ammonium or phosphonium cations), or slow, which allows the existence of chirality.[9]

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.

Chirality and Stereocenters

As mentioned earlier, the requirement for an atom to be a chirality center is that the atom must be sp3 hybridized with four different attachments.

stereoisomer
. This means that although all chirality centers are stereocenters, not every stereocenter is a chirality center.

Stereocenters are important identifiers for chiral or achiral molecules. As a general rule, if a molecule has no stereocenters, it is considered achiral. If it has at least one stereocenter, the molecule has the potential for chirality. However, there are some exceptions like meso compounds that make molecules with multiple stereocenters considered achiral.[6]

See also

References

  1. ^ a b c "5.4: Stereogenic Centers". libretexts.org. April 24, 2015.
  2. ^
    doi:10.1021/ja00323a043
    .
  3. ^ a b Solomons, T. W. Graham; Fryhle, Craig (2004). Organic Chemistry (8th ed.). John Wiley & Sons.[page needed]
  4. ^ a b Soderberg, Timothy (July 1, 2019). "Organic Chemistry with a Biological Emphasis Volume I". Chemistry Publications: 170, 177.
  5. ^ a b c "5.3: Chirality and R/S Naming System". Chemistry LibreTexts. December 15, 2021. Retrieved November 12, 2022.
  6. ^
    ISBN 978-1-305-58035-0
    .
  7. ^
    ISSN 0021-9584
    .
  8. doi:10.1351/goldbook.C01060
    .
  9. ^
    ISBN 978-0-07-337562-5
    .
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