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{{Short description|
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[[File:Chirality with hands.svg|upright=1.3|thumb|Two [[enantiomer]]s of a generic [[amino acid]] at the stereocenter]]
A stereocenter is geometrically defined as a point (location) in a molecule; a stereocenter is usually but not always a specific atom, often carbon.<ref name="Mislow&Siegel 1984" /><ref name="solomons">{{Cite book | first1= T. W. Graham |last1= Solomons| first2= Craig | last2= Fryhle| publisher= John Wiley & Sons| year= 2004 | title = Organic Chemistry | edition = 8th }}{{page needed| date= February 2016}}</ref> Stereocenters can exist on [[Chirality (chemistry)|chiral]] or [[achiral]] molecules; stereocenters can contain single bonds or double bonds.<ref name=":1" /> The number of hypothetical stereoisomers can be predicted by using 2<sup>''n''</sup>, with ''n'' being the number of [[Tetrahedral molecular geometry|tetrahedral]] stereocenters; however, exceptions such as [[Meso compound|meso compounds]] can reduce the prediction to below the expected 2<sup>''n''</sup>.<ref name=":3" />
A '''chirality center''' ('''chirality centre''' in U.K.) is a stereocenter consisting of an atom holding a set of [[ligand|ligands]] (atoms or groups of atoms) in a spatial arrangement which is not superimposable on its mirror image. The concept of a chirality center generalizes the concept of an [[asymmetric carbon|asymmetric carbon atom]] (a carbon atom bonded to four different entities) such that an interchanging of any two groups gives rise to an [[enantiomer]].<ref>{{Cite web | url = http://goldbook.iupac.org/C01060.html | title = chiral (chirality) center | website= [[IUPAC]].org}}</ref> 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]].▼
[[Chirality (chemistry)|'''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 sp<sup>3</sup> hybridization, meaning that they can only have [[Sigma bond|single bonds]].<ref name=":2" />
==Possible number of stereoisomers==▼
== Location ==
==Stereogenic on carbon==▼
Stereocenters can exist on [[Chirality|chiral]] or [[achiral]] molecules. They are defined as a location (point) within a molecule, rather than a particular atom, in which the interchanging of two groups creates a stereoisomer.<ref name="solomons" /> A stereocenter can have either four different attachment groups, or three different attachment groups where one group is connected by a double bond.<ref name=":1" /> Since stereocenters can exist on achiral molecules, stereocenters can have either [[Hybridization (chemistry)|sp<sup>3</sup> or sp<sup>2</sup> hybridization]].
A carbon atom that is attached to four different types of atoms or groups of atoms is called an ''[[Asymmetric carbon|asymmetric carbon atom]]'' or ''chiral carbon''.▼
==Stereogenic on other atoms==▼
Stereoisomers are compounds that are identical in composition and connectivity but have a different spatial arrangement of atoms around the central atom.<ref name=":0">{{Cite book |last1=Brown |first1=William |title=Organic Chemistry |last2=Iverson |first2=Brent |last3=Anslyn |first3=Eric |last4=Foote |first4=Christopher |publisher=Cengage Learning |year=2018 |isbn=978-1-305-58035-0 |edition=8th |location=Boston, MA |pages=117,137–139}}</ref> A molecule having multiple stereocenters will produce many possible stereoisomers. In compounds whose stereoisomerism is due to [[Tetrahedral molecular geometry|tetrahedral]] (sp<sup>3</sup>) stereogenic centers, the total number of hypothetically possible stereoisomers will not exceed 2<sup>''n''</sup>, 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 [[Enantiomer|enantiomers]] (non-superposable mirror images) and [[Diastereomer|diastereomers]] (non-superposable, non-identical, non-mirror image molecules).<ref name=":0" /> Enantiomers and diastereomers are produced due to differing [[Cahn–Ingold–Prelog priority rules|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 [[Chirality (chemistry)|superposable]] on its mirror image; the presence of a meso compound will reduce the number of possible stereoisomers.<ref name=":3">{{Cite journal |last=Soderberg |first=Timothy |date=2019-07-01 |title=Organic Chemistry with a Biological Emphasis Volume I |url=https://digitalcommons.morris.umn.edu/chem_facpubs/1 |journal=Chemistry Publications |pages=170,177}}</ref> 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 2<sup>''n''</sup> amount due to symmetry.<ref name=":0" />
Additionally, certain configurations may not exist due to [[Steric effects|steric]] reasons. [[Cyclic compound|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.<ref name=":0" /> The [[Cahn–Ingold–Prelog priority rules|Cahn-Ingold-Prelog]] (CIP) system uses R and S designations to define the configuration of atoms about any stereocenter.<ref name=":12">{{Cite journal |last1=Barta |first1=Nancy S. |last2=Stille |first2=John R. |date=1994 |title=Grasping the Concepts of Stereochemistry |url=https://pubs.acs.org/doi/abs/10.1021/ed071p20 |journal=Journal of Chemical Education |language= |volume=71 |issue=1 |page=20 |doi=10.1021/ed071p20 |bibcode=1994JChEd..71...20B |issn=0021-9584|url-access=subscription }}</ref> 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.<ref name=":12" />
==Chirality Centers==
▲A
The concept of a chirality center generalizes the concept of an [[asymmetric carbon|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]].<ref>{{Cite journal |title=chiral (chirality) center |url=http://goldbook.iupac.org/C01060.html |website=[[IUPAC]].org|doi=10.1351/goldbook.C01060 |doi-access=free }}</ref>
▲A carbon atom that is attached to four different
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.<ref name=OChemSmith>{{cite book |last1=Smith |first1=Janice Gorzynski |editor1-last=Hodge |editor1-first=Tami |editor2-last=Nemmers |editor2-first=Donna |editor3-last=Klein |editor3-first=Jayne |title=Organic chemistry |date=2011 |publisher=McGraw-Hill |location=New York, NY |isbn=978-0-07-337562-5 |pages=949–993 |edition=3rd |url=http://highered.mheducation.com/sites/007340277x/student_view0/index.html |language=en |format=Book |chapter=Chapter 25 Amines}}</ref> However, [[nitrogen inversion]], a form of [[pyramidal inversion]], causes [[racemization]] which means that both [[epimers]] at that nitrogen are present under normal circumstances.<ref name="OChemSmith" /> Racemization by [[nitrogen inversion]] may be restricted (such as [[quaternary ammonium cation|quaternary ammonium]] or [[phosphonium]] cations), or slow, which allows the existence of chirality.<ref name="OChemSmith" />
Metal atoms with tetrahedral or [[octahedral molecular geometry|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 sp<sup>3</sup> hybridized with four different attachments.<ref name=":2" /> Because of this, all chirality centers are stereocenters. However, only under some conditions is the reverse true. Recall that a point can be considered a sterocenter with a minimum of three attachment points; stereocenters can be either sp<sup>3</sup> or sp<sup>2</sup> hybridized, as long as the interchanging any two different groups creates a new [[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 compound|meso compounds]] that make molecules with multiple stereocenters considered achiral.<ref name=":0" />
==See also==
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