View source for Axial chirality

{{Short description|Type of symmetry in molecules}}
[[File:Axial chirality V.1.svg|thumb|Two types of molecules having axial chirality: [[allenes]] (left) and binaryl [[atropisomer]]s (right)]]
In chemistry, '''axial chirality''' is a special case of [[chirality (chemistry)|chirality]] in which a [[molecule]] contains two pairs of chemical groups in a non-planar arrangement about an '''axis of chirality''' so that the molecule is not superposable on its mirror image.<ref name="GoldBook axial chirality">{{GoldBookRef|file=A00547|title=axial chirality}}</ref><ref name="Eliel 1994">{{cite book | last=Eliel | first=Ernest L. | last2=Wilen | first2=Samuel H. | last3=Mander | first3=Lewis N. | title=Stereochemistry of organic compounds | publisher=Wiley | publication-place=New York | date=1994 | isbn=0-471-01670-5 | oclc=27642721}}</ref> The axis of chirality (or ''chiral axis'') is usually determined by a chemical bond that is constrained against free rotation either by [[steric hindrance]] of the groups, as in substituted [[aryl|biaryl]] compounds such as [[BINAP]], or by [[Torsion constant#Torsional_stiffness|torsional stiffness]] of the bonds, as in the C=C double bonds in [[allenes]] such as [[glutinic acid]]. Axial chirality is most commonly observed in substituted biaryl compounds wherein the rotation about the aryl–aryl bond is restricted so it results in chiral [[atropisomer]]s, as in various ortho-substituted [[biphenyl]]s, and in binaphthyls such as [[BINAP]]. 

Axial chirality differs from [[Stereocenter|central chirality]] (point chirality) in that axial chirality does not require a [[chiral center]] such as an [[asymmetric carbon]] atom, the most common form of chirality in [[organic compounds]]. Bonding to asymmetric carbon has the form Cabcd where a, b, c, and d must be distinct groups. Allenes have the form {{chem2|abC\dC\dCcd}} and the groups need not all be distinct as long as groups in each pair are distinct: abC=C=Cab is sufficient for the compound to be chiral, as in [[penta-2,3-dienedioic acid]]. Similarly, chiral atropisomers of the form {{chem2|abC\sCcd}} may have some identical groups ({{chem2|abC\sCab}}), as in BINAP.

==Nomenclature==
The [[enantiomer]]s of axially chiral compounds are usually given the stereochemical labels (''R''<sub>a</sub>) and (''S''<sub>a</sub>),<ref name="GoldBook axial chirality" /> sometimes abbreviated (''R'') and (''S'').<ref name="IUPAC 1974">{{cite journal | first1= L. C. | last1= Cross | first2= W. | last2= Klyne | title=Rules for the Nomenclature of Organic Chemistry. Section E: Stereochemistry (Recommendations 1974) | journal=Pure Appl. Chem. | volume=45 | pages=11–30 | issue=1 | date=1976 | issn=1365-3075 | doi=10.1351/pac197645010011 | url=http://publications.iupac.org/pac/45/1/0011/index.html| doi-access=free }}</ref>  The designations are based on the same [[Cahn–Ingold–Prelog priority rules]] used for tetrahedral stereocenters.<ref name="IUPAC 1974" /> The chiral axis is viewed end-on and the two "near" and two "far" substituents on the axial unit are ranked, but with the additional rule that the two near substituents have higher priority than the far ones.<ref>{{cite journal |url= http://publications.iupac.org/pac/1976/pdf/4501x0011.pdf |journal= Pure Appl. Chem. |volume= 45 |pages= 11–30 |year= 1976 |title= Rules for the Nomenclature of Organic Chemistry. Section E: Stereochemistry (Recommendations 1974) |first1= L. C. |last1= Cross |first2= W. |last2= Klyne |doi= 10.1351/pac197645010011 |quote= Chiral axis. The structure is regarded as an elongated tetrahedron and viewed along the axis—it is immaterial from which end it is viewed; the nearer pair of ligands receives the first two positions in the order of preference }}</ref>

{{multiple image
| align     = center
| image1    = BINAP axis.svg
| width1    = 250
| image2    = Allenes_chirality_depiction.png
| width2    = 260
| footer    = ''R'' and ''S'' configurations are determined by precedences of the groups attached to the axial section of the molecule when viewed along that axis. 
}}

==Helical chirality==
The chirality of a molecule that has a helical, propeller, or screw-shaped geometry is called ''helicity''<ref name="GoldBook helicity">{{GoldBookRef|file=H02763|title=helicity}}</ref> or ''helical chirality''.<ref name="Tan 2021">{{cite book | last=Tan | first=Bin | title=Axially Chiral Compounds: Asymmetric Synthesis and Applications | date=2021 | isbn=978-3-527-82517-2 | oclc=1264474520 }}</ref><ref name="Clark 2021">{{cite book | last1=Clark | first1=Andrew | last2=Kitson | first2=Russell R. A. | last3=Mistry | first3=Nimesh | last4=Taylor | first4=Paul | last5=Taylor | first5=Matthew | last6=Lloyd | first6=Michael | last7=Akamune | first7=Caroline | title=Introduction to Stereochemistry | date=2021 | isbn=978-1-78801-315-4 | oclc=1180250839}}</ref> The [[screw axis]] or the ''D''<sub>''n''</sub>, or ''C''<sub>''n''</sub> principle [[Molecular symmetry#Elements|symmetry axis]] is considered to be the axis of chirality. Some sources consider helical chirality to be a type of axial chirality,<ref name="Clark 2021" /> and some do not.<ref name="Tan 2021" /><ref>{{cite journal | last1=Zhang | first1=Dawei | last2=Mulatier | first2=Jean-Christophe | last3=Cochrane | display-authors=etal | title=Helical, Axial, and Central Chirality Combined in a Single Cage: Synthesis, Absolute Configuration, and Recognition Properties | journal=Chemistry - A European Journal | volume=22 | issue=24 | date=2016-05-02 | issn=0947-6539 | doi=10.1002/chem.201600664 | pages=8038–8042| pmid=27037555 }}</ref> [[IUPAC]] does not refer to helicity as a type of axial chirality.<ref name="GoldBook helicity" /><ref name="GoldBook axial chirality" />

Enantiomers having helicity may labeled by using the prefix notation (''P'') ("plus") or Δ (from Latin ''dexter'', "right") for a right-handed helix, and (''M'') ("minus") or Λ (Latin ''levo'', "left") for a left-handed helix.<ref name="GoldBook helicity" /><ref name="IUPAC 1974" /><ref>{{Cite web |url=http://www.chemgapedia.de/vsengine/vlu/vsc/en/ch/12/oc/vlu_organik/stereochemie/weitere_chiralitaetselem.vlu/Page/vsc/en/ch/12/oc/stereochemie/helicale_chiralitaet/helicale_chiralitaet.vscml.html |title=VLU: Additional Chirality Elements - Chemgapedia |access-date=2007-08-06 |archive-url=https://web.archive.org/web/20110718212715/http://www.chemgapedia.de/vsengine/vlu/vsc/en/ch/12/oc/vlu_organik/stereochemie/weitere_chiralitaetselem.vlu/Page/vsc/en/ch/12/oc/stereochemie/helicale_chiralitaet/helicale_chiralitaet.vscml.html |archive-date=2011-07-18 |url-status=dead }}</ref> The ''P''/''M'' or Δ/Λ terminology is used particularly for molecules that actually resemble a helix, such as the [[helicenes]]. This notation can also be applied to non-helical structures having axial chirality by considering the helical orientation of the Cahn–Ingold–Prelog group rankings of the "front" groups compared to the "back", when viewed from either direction along the axis.

{{multiple image
| align     = center
| image1    = P-heptahelicene.svg
| caption1  = ''P'' helicity
| width1    = 150
| image2    = M-heptahelicene.svg
| caption2  = ''M'' helicity
| width2    = 150
| footer    = Configurations of [7][[helicene]]
}}

==External links==
* [https://web.archive.org/web/20050123014310/http://siggy.chem.ucla.edu/VOH/30A/Dinitrobiphenyl_%28COOH%292.pdf#search='axial%20chirality' Axial Chirality in 6,6′-Dinitrobiphenyl-2,2′-dicarboxylic acid 3D representation].

==References==
<references/>

{{chiral synthesis}}

{{DEFAULTSORT:Axial Chirality}}
[[Category:Stereochemistry]]