Axial chirality is a special case of chirality in which a molecule does not possess a stereogenic center (the most common form of chirality in organic compounds) but an axis of chirality – an axis 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 atropisomeric biaryl 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
. Certain allene compounds 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 (or alternatively Δ / Λ) is occasionally employed. P/M or Δ/Λ is used particularly for molecules that resemble a helix, such as hexahelicene, in which case a right-handed helix is denoted P or Δ and a left-handed helix is denoted M or Λ.
This can also be called helicity, which is the chirality of a helical, propeller or screw-shape molecular entity. P (plus) or Δ is a right-handed helix, where M (minus) or Λ is a left-handed helix.
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