Axial chirality
Encyclopedia
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 enantiomer
s of axially chiral compounds are usually given the stereochemical labels Ra and Sa. The designations are based on the same Cahn–Ingold–Prelog priority rules used for tetrahedral stereocenters. The chiral axis is viewed end-on and the two "near" and two "far" substituents on the axial unit are ranked.
This property can also be called helicity, since the axis of the structure has a helical, propeller, or screw-shaped geometry. P (plus) or Δ is a right-handed helix, where M (minus) or Λ is a left-handed helix. The P/M or Δ/Λ terminology is used particularly for molecules that actually resemble a helix, such as hexahelicene. It can also be applied to other structures having axial chirality by considering the helical orientation of the "front" vs "back" Cahn–Ingold–Prelog rankings.
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....
in which a molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
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
Aryl
In the context of organic molecules, aryl refers to any functional group or substituent derived from an aromatic ring, be it phenyl, naphthyl, thienyl, indolyl, etc....
compounds wherein the rotation about the aryl-aryl bond is restricted, for example, biphenyl
Biphenyl
Biphenyl is an organic compound that forms colorless crystals. It has a distinctively pleasant smell. Biphenyl is an aromatic hydrocarbon with a molecular formula 2...
, binaphthyls, e.g., 1,1'-bi-2-naphthol
1,1'-Bi-2-naphthol
1,1'-Bi-2-naphthol is an organic compound that is often used as a ligand for transition-metal catalysed asymmetric synthesis. BINOL has axial chirality and the two enantiomers can be readily separated and are stable toward racemisation. The specific rotation of the two enantiomers is +/- 35.5°...
, and certain dihydroanthracenone compounds. Certain allene
Allene
An allene is a compound in which one carbon atom has double bonds with each of its two adjacent carbon centres. Allenes are classified as polyenes with cumulated dienes. The parent compound of allene is propadiene. Compounds with an allene-type structure but with more than three carbon atoms are...
compounds also display axial chirality. The enantiomer
Enantiomer
In chemistry, an enantiomer is one of two stereoisomers that are mirror images of each other that are non-superposable , much as one's left and right hands are the same except for opposite orientation. It can be clearly understood if you try to place your hands one over the other without...
s of axially chiral compounds are usually given the stereochemical labels Ra and Sa. The designations are based on the same Cahn–Ingold–Prelog priority rules used for tetrahedral stereocenters. The chiral axis is viewed end-on and the two "near" and two "far" substituents on the axial unit are ranked.
This property can also be called helicity, since the axis of the structure has a helical, propeller, or screw-shaped geometry. P (plus) or Δ is a right-handed helix, where M (minus) or Λ is a left-handed helix. The P/M or Δ/Λ terminology is used particularly for molecules that actually resemble a helix, such as hexahelicene. It can also be applied to other structures having axial chirality by considering the helical orientation of the "front" vs "back" Cahn–Ingold–Prelog rankings.