Axial chirality is a special case of
chiralityThe term chiral is used to describe an object that is non-superposable on its mirror image. Achiral objects are objects that are identical to their mirror image....
in which a
moleculeA molecule is defined as an electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds. Molecules are distinguished from polyatomic ions in this strict sense...
does 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 atropisomeric
biarylIn the context of organic molecules, aryl refers to any functional group or substituent derived from a simple aromatic ring, may it be phenyl, thiophene, indolyl, etc . "Aryl" is used for the sake of abbreviation or generalization....
compounds wherein the rotation about the aryl-aryl bond is restricted, for example,
biphenylBiphenyl 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-naphthol1,1'-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 enatiomers is +/- 35.5°...
, and certain dihydroanthracenone compounds
.
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Axial chirality is a special case of
chiralityThe term chiral is used to describe an object that is non-superposable on its mirror image. Achiral objects are objects that are identical to their mirror image....
in which a
moleculeA molecule is defined as an electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds. Molecules are distinguished from polyatomic ions in this strict sense...
does 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 atropisomeric
biarylIn the context of organic molecules, aryl refers to any functional group or substituent derived from a simple aromatic ring, may it be phenyl, thiophene, indolyl, etc . "Aryl" is used for the sake of abbreviation or generalization....
compounds wherein the rotation about the aryl-aryl bond is restricted, for example,
biphenylBiphenyl 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-naphthol1,1'-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 enatiomers is +/- 35.5°...
, and certain dihydroanthracenone compounds
. Certain
alleneAn allene is a hydrocarbon in which one atom of carbon is connected by double bonds with two other atoms of carbon. Allene also is the common name for the parent compound of this series, propadiene....
compounds also display axial chirality. The
enantiomerIn stereochemistry, 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" but opposite....
s of axially chiral compounds are usually given the stereochemical labels R
a and S
a, although the plus (P) or minus (M) notation is occasionally employed. 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.
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.
External links
- Axial Chirality in 6,6'-Dinitrobiphenyl-2,2'-dicarboxylic acid 3D representation Link