Stereocenter
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In stereochemistry, a stereocenter (or stereogenic center) of a molecule is an atom (center), axis or plane that is the focus of stereoisomerism; that is, when having at least three different groups bound to the stereocenter, interchanging any two different groups creates a new stereoisomer.[1]
A stereocenter is geometrically defined as a point, or location, in a molecule; a stereocenter is not necessarily an atom.[2] The term stereocenter was introduced in 1984 by Kurt Mislow and Jay Siegel.[3]
Standort
Stereocenters can exist on chiral or achiral molecules. A stereocenter can have either four different attachment groups, or three different attachment groups where one group is connected by a double bond.[1] Since stereocenters can exist on achiral molecules, stereocenters can have either 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.[4] A molecule can have multiple stereocenters, producing 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-superimposable mirror images) and diastereomers (non-superimposable, non-identical molecules).[4] Enantiomers and diastereomers will produce individual stereoisomers that all contribute to the total number of possible stereoisomers for a particular molecule.
However, the stereoisomers produced may also give a meso compound, which is an achiral compound that is superimposable on its mirror image; the presence of a meso compound will reduce the number of stereoisomers.[5] Since the meso compound is superimposable 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.[4]
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.[4] The Cahn-Ingold-Prelog (CIP) system uses R and S designations to define the configuration of atoms about any stereocenter.[6] 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.[6]
Chirality Centers
A chirality center is a type of stereocenter. A chirality center is defined as an atom holding a set of four different ligands (atoms or groups of atoms) in a spatial arrangement which is non-superimposable on its mirror image. Chirality centers must be sp3 hybridized, meaning that a chirality center can only have single bonds.[7] In organic chemistry, a chirality center usually refers to a carbon, phosphorus, or sulfur atom, though it is also possible for other atoms to be chirality centers, especially in areas of organometallic and inorganic chemistry.
The most common chirality center is a carbon atom.[4] The concept of a chirality center generalizes the concept of an asymmetric carbon atom (a carbon atom bonded to four different entities) such that an interchanging of any two 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.
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 amine may be a stereocenter if all three groups attached are different because the electron pair of the amine functions as a fourth group.[9] However, nitrogen inversion, a form of pyramidal inversion, causes racemization which means that both epimers at that nitrogen are present under normal circumstances.[9] Racemization by 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.
See also
- Chirality (chemistry) § Stereogenic centers
- Cahn–Ingold–Prelog priority rules for nomenclature
- Descriptor (chemistry)
References
- ^ a b "5.4: Stereogenic Centers". libretexts.org.
- ^ Solomons, T. W. Graham; Fryhle, Craig (2004). Organic Chemistry (8th ed.). John Wiley & Sons.[page needed]
- ^ Mislow, Kurt; Siegel, Jay (1984). "Stereoisomerism and local chirality". Journal of the American Chemical Society. 106 (11): 3319. doi:10.1021/ja00323a043.
- ^ a b c d e Brown, William; Iverson, Brent; Anslyn, Eric; Foote, Christopher (2018). Organic Chemistry (8th ed.). Boston, MA: Cengage Learning. pp. 117, 137–139. ISBN 978-1-305-58035-0.
- ^ Soderberg, Timothy (July 1, 2019). "Organic Chemistry with a Biological Emphasis Volume I". Chemistry Publications.
- ^ a b Barta, Nancy S.; Stille, John R. (1994). "Grasping the Concepts of Stereochemistry". Journal of Chemical Education. 71 (1): 1. doi:10.1021/ed071p20. ISSN 0021-9584.
- ^ "5.3: Chirality and R/S Naming System". Chemistry LibreTexts. December 15, 2021. Retrieved November 12, 2022.
- ^ "chiral (chirality) center". IUPAC.org.
- ^ a b c Smith, Janice Gorzynski (2011). "Chapter 25 Amines". In Hodge, Tami; Nemmers, Donna; Klein, Jayne (eds.). Organic chemistry (Book) (3rd ed.). New York, NY: McGraw-Hill. pp. 949–993. ISBN 978-0-07-337562-5.