In chemistry, axial chirality is a special case of

chirality Chirality () is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable fro ...

in which a

molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...

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. The axis of chirality (or ''chiral axis'') is usually determined by a chemical bond that is constrained against free rotation either by

steric hindrance Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is generally a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivi ...

of the groups, as in substituted biaryl compounds such as

BINAP BINAP (2,2′-bis(diphenylphosphino)-1,1′-binaphthyl) is an organophosphorus compound. This Optical isomerism, chiral diphosphines, diphosphine ligand is widely used in chiral synthesis, asymmetric synthesis. It consists of a pair of 2-diphe ...

, or by torsional stiffness of the bonds, as in the C=C double bonds in

allenes In organic chemistry, allenes are organic compounds in which one carbon atom has double bonds with each of its two adjacent carbon atoms (, where R is H or some organyl group). Allenes are classified as cumulated dienes. The parent compound o ...

such as

glutinic acid Penta-2,3-dienedioic acid (one of two chemicals called glutinic acid), is an allene-containing dicarboxylic acid. It was the first allene to be synthesized, in 1887, but the structure of it was thought to be a propyne core instead of an allene. ...

. 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 Atropisomers are stereoisomers arising because of hindered rotation about a covalent bond, single bond, where Gibbs free energy, energy differences due to steric strain or other contributors create a barrier to rotation that is high enough to all ...

s, as in various ortho-substituted

biphenyl Biphenyl (also known as diphenyl, phenylbenzene, 1,1′-biphenyl, lemonene or BP) is an organic compound that forms colorless crystals. Particularly in older literature, compounds containing the functional group consisting of biphenyl less one ...

s, and in binaphthyls such as

. Axial chirality differs from 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 Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

. Bonding to asymmetric carbon has the form Cabcd where a, b, c, and d must be distinct groups. Allenes have the form 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 may have some identical groups (), as in BINAP.

Nomenclature

The

enantiomer In chemistry, an enantiomer (Help:IPA/English, /ɪˈnænti.əmər, ɛ-, -oʊ-/ Help:Pronunciation respelling key, ''ih-NAN-tee-ə-mər''), also known as an optical isomer, antipode, or optical antipode, is one of a pair of molecular entities whi ...

s of axially chiral compounds are usually given the stereochemical labels (''R''_a) and (''S''_a), sometimes abbreviated (''R'') and (''S''). The designations are based on the same

Cahn–Ingold–Prelog priority rules In organic chemistry, the Cahn–Ingold–Prelog (CIP) sequence rules (also the CIP priority convention; named after Robert Sidney Cahn, Christopher Kelk Ingold, and Vladimir Prelog) are a standard process to completely and unequivocally nam ...

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, but with the additional rule that the two near substituents have higher priority than the far ones.

Helical chirality

The chirality of a molecule that has a helical, propeller, or screw-shaped geometry is called ''helicity'' or ''helical chirality''. The

screw axis A screw axis (helical axis or twist axis) is a line that is simultaneously the axis of rotation and the line along which translation of a body occurs. Chasles' theorem shows that each Euclidean displacement in three-dimensional space has a screw ...

or the ''D''_''n'', or ''C''_''n'' principle

symmetry axis Axial symmetry is symmetry around an axis or line (geometry). An object is said to be ''axially symmetric'' if its appearance is unchanged if transformed around an axis. The main types of axial symmetry are ''reflection symmetry'' and ''rotatio ...

is considered to be the axis of chirality. Some sources consider helical chirality to be a type of axial chirality, and some do not.

IUPAC The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

does not refer to helicity as a type of 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. 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.

External links

Axial Chirality in 6,6′-Dinitrobiphenyl-2,2′-dicarboxylic acid 3D representation

References

{{DEFAULTSORT:Axial Chirality Stereochemistry