- Axial chirality
Axial chirality is a special case of chirality in which a
moleculedoes not possess a stereogenic center(the most common form of chirality in organic compounds) but an axis of chirality - an axis is about which a set of substituents is held in a spatial arrangement that is not superposable on its mirror image. Axial chirality is most commonly observed in atropisomericbiaryl compounds wherein the rotation about the aryl-aryl bond is restricted, for example, biphenyl, binaphthyls, e.g., 1,1'-bi-2-naphthol, and certain dihydroanthracenone compounds [ "Absolute stereochemistry of fungal metabolites: Icterinoidins A1 and B1, and atrovirins B1 and B2" Melvyn Gill and Peter M. Morgan Arkivoc(RI-1154C)2004 [http://www.arkat-usa.org/ark/journal/2004/I10_Rickards/RI-1154C/RI-1154C.asp Online article] ] . Certain allenecompounds also display axial chirality. The enantiomers of axially chiral compounds are usually given the stereochemical labels Ra and Sa, although the plus (P) or minus (M) notation is occasionally employed. [ Compendium of Chemical Terminology, [http://goldbook.iupac.org/A00547.html axial chirality] ] P/M is used particularly for molecules that resemble a helix, such as hexahelicene, in which case a right-handed helix is denoted P and a left-handed helix is denoted M. [VLU: Additional Chirality Elements - Chemgapedia [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] ]
This can also be called helicity, which is the chirality of a helical, propeller or screw-shape molecular entity. "P" (plus) is a right-handed helix, where "M" (minus) is a left-handed helix. [
Compendium of Chemical Terminology, helicity, [http://old.iupac.org/goldbook/H02763.pdf] ]
* Axial Chirality in 6,6'-Dinitrobiphenyl-2,2'-dicarboxylic acid 3D representation [http://siggy.chem.ucla.edu/VOH/30A/Dinitrobiphenyl_(COOH)2.pdf#search='axial%20chirality' Link]
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