Chirality (chemistry)
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In
chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, properties ...
, a molecule or ion is called chiral () if it cannot be superposed on its
mirror image A mirror image (in a plane mirror) is a reflected duplication of an object that appears almost identical, but is reversed in the direction perpendicular to the mirror surface. As an optics, optical effect it results from Specular reflection, re ...
by any combination of
rotation Rotation, or spin, is the circular movement of an object around a ''axis of rotation, central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A t ...
s,
translation Translation is the communication of the Meaning (linguistic), meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The ...
s, and some conformational changes. This geometric property is called chirality (). The terms are derived from
Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek Dark ...
χείρ (''cheir'') 'hand'; which is the canonical example of an object with this property. A chiral molecule or ion exists in two
stereoisomer In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in ...
s that are mirror images of each other, called
enantiomer In chemistry, an enantiomer (Help:IPA/English, /ɪˈnænti.əmər, ɛ-, -oʊ-/ Help:Pronunciation respelling key, ''ih-NAN-tee-ə-mər''; from Ancient Greek language, Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''( ...
s; they are often distinguished as either "right-handed" or "left-handed" by their
absolute configuration Absolute configuration refers to the spatial arrangement of atoms within a chirality (chemistry), chiral molecular entity (or group) and its resultant stereochemical description. Absolute configuration is typically relevant in organic molecules, ...
or some other criterion. The two enantiomers have the same chemical properties, except when reacting with other chiral compounds. They also have the same physical properties, except that they often have opposite optical activities. A homogeneous mixture of the two enantiomers in equal parts is said to be
racemic In chemistry, a racemic mixture, or racemate (), is one that has equal Amount of substance, amounts of left- and right-handed enantiomers of a Chirality (chemistry), chiral molecule or salt. Racemic mixtures are rare in nature, but many compounds ...
, and it usually differs chemically and physically from the pure enantiomers. Chiral molecules will usually have a stereogenic element from which chirality arises. The most common type of stereogenic element is a stereogenic center, or stereocenter. In the case of organic compounds, stereocenters most frequently take the form of a carbon atom with four distinct groups attached to it in a tetrahedral geometry. A given stereocenter has two possible configurations, which give rise to stereoisomers (
diastereomers In stereochemistry, diastereomers (sometimes called diastereoisomers) are a type of stereoisomer. Diastereomers are defined as non-mirror image, non-identical stereoisomers. Hence, they occur when two or more stereoisomers of a compound have dif ...
and
enantiomers In chemistry, an enantiomer (Help:IPA/English, /ɪˈnænti.əmər, ɛ-, -oʊ-/ Help:Pronunciation respelling key, ''ih-NAN-tee-ə-mər''; from Ancient Greek language, Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''( ...
) in molecules with one or more stereocenter. For a chiral molecule with one or more stereocenter, the enantiomer corresponds to the stereoisomer in which every stereocenter has the opposite configuration. An organic compound with only one stereogenic carbon is always chiral. On the other hand, an organic compound with multiple stereogenic carbons is typically, but not always, chiral. In particular, if the stereocenters are configured in such a way that the molecule has an internal plane of symmetry, then the molecule is achiral and is known as a ''meso'' compound. Less commonly, other atoms like N, P, S, and Si can also serve as stereocenters, provided they have four distinct substituents (including lone pair electrons) attached to them. Molecules with chirality arising from one or more stereocenters are classified as possessing central chirality. There are two other types of stereogenic elements that can give rise to chirality, a stereogenic axis (
axial chirality Axial may refer to: * one of the Anatomical terms of location#Other directional terms, anatomical directions describing relationships in an animal body * In geometry: :* a geometric term of location :* an axis of rotation * In chemistry, referring ...
) and a stereogenic plane (
planar chirality Planar chirality, also known as 2D chirality, is the 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 ch ...
). Finally, the inherent curvature of a molecule can also give rise to chirality ( inherent chirality). These types of chirality are far less common than central chirality. BINOL is a typical example of an axially chiral molecule, while ''trans''-cyclooctene is a commonly cited example of a planar chiral molecule. Finally,
helicene In organic chemistry, helicenes are aromatic ortho substituent, ortho-condensed Polycyclic compound, polycyclic Aromaticity, aromatic compounds in which Benzene, benzene rings or other aromatics are angularly annulation, annulated to give helix, h ...
possesses helical chirality, which is one type of inherent chirality. Chirality is an important concept for
stereochemistry Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms that form the structure of molecules and their manipulation. The study of stereochemistry focuses on the relationships between stereois ...
and
biochemistry Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology a ...
. Most substances relevant to
biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of Cell (biology), cells that proce ...
are chiral, such as
carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may or may ...
s (
sugars Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides o ...
,
starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diets, ...
, and
cellulose Cellulose is an organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with other ...
), the
amino acid Amino acids are organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with ot ...
s that are the building blocks of
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metabo ...
s, and the
nucleic acid Nucleic acids are biopolymers, macromolecules, essential to all Organism, known forms of life. They are composed of nucleotides, which are the monomers made of three components: a pentose, 5-carbon sugar, a phosphate group and a nitrogenous base. ...
s. In living organisms, one typically finds only one of the two enantiomers of a chiral compound. For that reason, organisms that consume a chiral compound usually can metabolize only one of its enantiomers. For the same reason, the two enantiomers of a chiral
pharmaceutical A medication (also called medicament, medicine, pharmaceutical drug, medicinal drug or simply drug) is a drug used to medical diagnosis, diagnose, cure, treat, or preventive medicine, prevent disease. Drug therapy (pharmacotherapy) is an imp ...
usually have vastly different potencies or effects.


Definition

The chirality of a molecule is based on the
molecular symmetry Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of these molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can be used to predict or explain m ...
of its conformations. A conformation of a molecule is chiral if and only if it belongs to the ''Cn'', ''Dn'', ''T'', ''O'', ''I''
point groups In geometry, a point group is a group (mathematics), mathematical group of symmetry operations (isometry, isometries in a Euclidean space) that have a Fixed point (mathematics), fixed point in common. The Origin (mathematics), coordinate origin o ...
(the chiral point groups). However, whether the molecule itself is considered to be chiral depends on whether its chiral conformations are persistent isomers that could be isolated as separated enantiomers, at least in principle, or the enantiomeric conformers rapidly interconvert at a given temperature and timescale through low-energy conformational changes (rendering the molecule achiral). For example, despite having chiral ''gauche'' conformers that belong to the ''C''2 point group,
butane Butane () or ''n''-butane is an alkane with the formula C4H10. Butane is a gas at room temperature and atmospheric pressure. Butane is a highly flammable, colorless, easily liquefied gas that quickly vaporizes at room temperature. The name b ...
is considered achiral at room temperature because rotation about the central C–C bond rapidly interconverts the enantiomers (3.4 kcal/mol barrier). Similarly, ''cis''-1,2-dichlorocyclohexane consists of chair conformers that are nonidentical mirror images, but the two can interconvert via the cyclohexane chair flip (~10 kcal/mol barrier). As another example, amines with three distinct substituents (R1R2R3N:) are also regarded as achiral molecules because their enantiomeric pyramidal conformers rapidly invert and interconvert through a planar transition state (~6 kcal/mol barrier). However, if the temperature in question is low enough, the process that interconverts the enantiomeric chiral conformations becomes slow compared to a given timescale. The molecule would then be considered to be chiral at that temperature. The relevant timescale is, to some degree, arbitrarily defined: 1000 seconds is sometimes employed, as this is regarded as the lower limit for the amount of time required for chemical or chromatographic separation of enantiomers in a practical sense. Molecules that are chiral at room temperature due to restricted rotation about a single bond (barrier to rotation ≥ ca. 23 kcal/mol) are said to exhibit atropisomerism. A chiral compound can contain no improper axis of rotation (''Sn''), which includes planes of symmetry and inversion center. Chiral molecules are always dissymmetric (lacking ''Sn'') but not always asymmetric (lacking all symmetry elements except the trivial identity). Asymmetric molecules are always chiral. The following table shows some examples of chiral and achiral molecules, with the
Schoenflies notation The Schoenflies (or Schönflies) notation, named after the Germans, German mathematician Arthur Moritz Schoenflies, is a notation primarily used to specify point groups in three dimensions. Because a point group alone is completely adequate to descr ...
of the
point group In geometry, a point group is a group (mathematics), mathematical group of symmetry operations (isometry, isometries in a Euclidean space) that have a Fixed point (mathematics), fixed point in common. The Origin (mathematics), coordinate origin o ...
of the molecule. In the achiral molecules, X and Y (with no subscript) represent achiral groups, whereas X and X or Y and Y represent
enantiomer In chemistry, an enantiomer (Help:IPA/English, /ɪˈnænti.əmər, ɛ-, -oʊ-/ Help:Pronunciation respelling key, ''ih-NAN-tee-ə-mər''; from Ancient Greek language, Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''( ...
s. Note that there is no meaning to the orientation of an ''S'' axis, which is just an inversion. Any orientation will do, so long as it passes through the center of inversion. Also note that higher symmetries of chiral and achiral molecules also exist, and symmetries that do not include those in the table, such as the chiral ''C'' or the achiral ''S''.


Stereogenic centers

Many chiral molecules have point chirality, namely a single chiral stereogenic center that coincides with an atom. This stereogenic center usually has four or more bonds to different groups, and may be carbon (as in many biological molecules), phosphorus (as in many
organophosphate In organic chemistry, organophosphates (also known as phosphate esters, or OPEs) are a class of organophosphorus compounds with the general structure , a central phosphate molecule with alkyl or Aryl, aromatic substituents. They can be conside ...
s), silicon, or a metal (as in many chiral coordination compounds). However, a stereogenic center can also be a trivalent atom whose bonds are not in the same plane, such as
phosphorus Phosphorus is a chemical element with the Symbol (chemistry), symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly Reactivity (chemistry), reactive, phosphor ...
in P-chiral phosphines (PRR′R″) and
sulfur Sulfur (or sulphur in British English British English (BrE, en-GB, or BE) is, according to Oxford Dictionaries, " English as used in Great Britain, as distinct from that used elsewhere". More narrowly, it can refer specifically to the ...
in S-chiral sulfoxides (OSRR′), because a lone-pair of electrons is present instead of a fourth bond. Chirality can also arise from isotopic differences between atoms, such as in the deuterated
benzyl alcohol Benzyl alcohol is an aromatic Alcohol (chemistry), alcohol with the formula C6H5CH2OH. The benzyl group is often abbreviated "Bn" (not to be confused with "Bz" which is used for benzoyl), thus benzyl alcohol is denoted as BnOH. Benzyl alcohol is ...
PhCHDOH; which is chiral and optically active ( 'α''sub>D = 0.715°), even though the non-deuterated compound PhCH2OH is not. If two enantiomers easily interconvert, the pure enantiomers may be practically impossible to separate, and only the racemic mixture is observable. This is the case, for example, of most amines with three different substituents (NRR′R″), because of the low energy barrier for
nitrogen inversion In chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structur ...
. While the presence of a stereogenic center describes the great majority of chiral molecules, many variations and exceptions exist. For instance it is not necessary for the chiral substance to have a stereogenic center. Examples include 1-bromo-3-chloro-5-fluoro
adamantane Adamantane is an organic compound with a formula C10H16 or, more descriptively, (CH)4(CH2)6. Adamantane molecules can be described as the fusion of three cyclohexane rings. The molecule is both rigid and virtually stress-free. Adamantane is the m ...
, methylethylphenyl
tetrahedrane Tetrahedrane is a hypothetical platonic hydrocarbon with chemical formula and a tetrahedron, tetrahedral structure. The molecule would be subject to considerable Ring strain, angle strain and has not been synthesized as of 2021. However, a numb ...
, certain
calixarenes A calixarene is a macrocycle Macrocycles are often described as molecules and ions containing a ring of twelve or more atoms. Classical examples include the crown ethers, calixarenes, porphyrins, and cyclodextrins. Macrocycles describe a large, ...
and
fullerenes A fullerene is an allotropes of carbon, allotrope of carbon whose molecule consists of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to seven atoms. The molecule ma ...
, which have inherent chirality. The C2-symmetric species 1,1′-bi-2-naphthol (BINOL), 1,3-dichloro
allene In organic chemistry, allenes are organic compounds in which one carbon atom has double bonds with each of its two adjacent carbon centres (). Allenes are classified as diene#Classes, cumulated dienes. The parent compound of this class is propa ...
have
axial chirality Axial may refer to: * one of the Anatomical terms of location#Other directional terms, anatomical directions describing relationships in an animal body * In geometry: :* a geometric term of location :* an axis of rotation * In chemistry, referring ...
. (''E'')- cyclooctene and many
ferrocene Ferrocene is an organometallic chemistry, organometallic compound with the formula . The molecule is a Cyclopentadienyl complex, complex consisting of two Cyclopentadienyl anion, cyclopentadienyl rings bound to a central iron atom. It is an oran ...
s have
planar chirality Planar chirality, also known as 2D chirality, is the 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 ch ...
. When the optical
rotation Rotation, or spin, is the circular movement of an object around a ''axis of rotation, central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A t ...
for an enantiomer is too low for practical measurement, the species is said to exhibit cryptochirality. Chirality is an intrinsic part of the identity of a molecule, so the
systematic name A systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance, out of a specific population or collection. Systematic names are usually part of a nomenclature. A semisystematic name or semitrivial ...
includes details of the
absolute configuration Absolute configuration refers to the spatial arrangement of atoms within a chirality (chemistry), chiral molecular entity (or group) and its resultant stereochemical description. Absolute configuration is typically relevant in organic molecules, ...
(''R/S'', ''D/L'', or other designations).


Manifestations of chirality

* Flavor: the
artificial sweetener A sugar substitute is a food additive that provides a sweetness like that of sugar while containing significantly less food energy than sugar-based sweeteners, making it a zero-calorie () or low-calorie sweetener. Artificial sweeteners may be d ...
aspartame Aspartame is an chemical synthesis, artificial non-saccharide sweetener 200 times sweeter than sucrose and is commonly used as a sugar substitute in foods and beverages. It is a methyl ester of the aspartic acid/phenylalanine dipeptide w ...
has two enantiomers. L-aspartame tastes sweet whereas D-aspartame is tasteless. * Odor: ''R''-(–)-
carvone Carvone is a member of a family of chemicals called terpenoids. Carvone is found naturally in many essential oils, but is most abundant in the oils from seeds of caraway (''Carum carvi''), spearmint (''Mentha spicata''), and dill. Uses Both carvo ...
smells like
spearmint Spearmint, also known as garden mint, common mint, lamb mint and mackerel mint, is a species of Mentha, mint, ''Mentha spicata'' (, native to Europe and southern temperate Asia, extending from Ireland in the west to southern China in the east. I ...
whereas ''S''-(+)-carvone smells like
caraway Caraway, also known as meridian fennel and Persian cumin (''Carum carvi''), is a biennial plant in the family (biology), family Apiaceae, native to western Asia, Europe, and North Africa. Etymology The etymology of "caraway" is unclear. ...
. * Drug effectiveness: the
antidepressant Antidepressants are a class of medication used to treat major depressive disorder, anxiety disorders, chronic pain conditions, and to help manage addictions. Common Side effect, side-effects of antidepressants include Xerostomia, dry mouth, weig ...
drug
Citalopram Citalopram, sold under the brand name Celexa among others, is an antidepressant of the selective serotonin reuptake inhibitor (SSRI) class. It is used to treat major depressive disorder, obsessive compulsive disorder, panic disorder, and socia ...
is sold as a
racemic In chemistry, a racemic mixture, or racemate (), is one that has equal Amount of substance, amounts of left- and right-handed enantiomers of a Chirality (chemistry), chiral molecule or salt. Racemic mixtures are rare in nature, but many compounds ...
mixture. However, studies have shown that only the (''S'')-(+) enantiomer is responsible for the drug's beneficial effects. * Drug safety: D‑penicillamine is used in
chelation therapy Chelation therapy is a medical procedure that involves the administration of Chelation, chelating agents to remove heavy metals from the body. Chelation therapy has a long history of use in clinical toxicology and remains in use for some very sp ...
and for the treatment of
rheumatoid arthritis Rheumatoid arthritis (RA) is a long-term autoimmune disorder that primarily affects synovial joint, joints. It typically results in warm, swollen, and painful joints. Pain and stiffness often worsen following rest. Most commonly, the wrist and ...
whereas L‑penicillamine is toxic as it inhibits the action of
pyridoxine Pyridoxine, is a form of vitamin B6, vitamin B6 found commonly in food and used as a dietary supplement. As a supplement it is used to treat and prevent pyridoxine deficiency, sideroblastic anaemia, pyridoxine-dependent epilepsy, certain metabol ...
, an essential B vitamin.


In biochemistry

Many biologically active molecules are chiral, including the naturally occurring
amino acid Amino acids are organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with ot ...
s (the building blocks of
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metabo ...
s) and
sugar Sugar is the generic name for Sweetness, sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides o ...
s. The origin of this
homochirality Homochirality is a uniformity of Chirality (chemistry), chirality, or handedness. Objects are chiral when they cannot be superposed on their mirror images. For example, the left and right hands of a human are approximately mirror images of each oth ...
in
biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of Cell (biology), cells that proce ...
is the subject of much debate. Most scientists believe that Earth life's "choice" of chirality was purely random, and that if carbon-based life forms exist elsewhere in the universe, their chemistry could theoretically have opposite chirality. However, there is some suggestion that early amino acids could have formed in comet dust. In this case, circularly polarised radiation (which makes up 17% of stellar radiation) could have caused the selective destruction of one chirality of amino acids, leading to a selection bias which ultimately resulted in all life on Earth being homochiral.
Enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...
s, which are chiral, often distinguish between the two enantiomers of a chiral substrate. One could imagine an enzyme as having a glove-like cavity that binds a substrate. If this glove is right-handed, then one enantiomer will fit inside and be bound, whereas the other enantiomer will have a poor fit and is unlikely to bind. -forms of amino acids tend to be tasteless, whereas -forms tend to taste sweet.
Spearmint Spearmint, also known as garden mint, common mint, lamb mint and mackerel mint, is a species of Mentha, mint, ''Mentha spicata'' (, native to Europe and southern temperate Asia, extending from Ireland in the west to southern China in the east. I ...
leaves contain the -enantiomer of the chemical
carvone Carvone is a member of a family of chemicals called terpenoids. Carvone is found naturally in many essential oils, but is most abundant in the oils from seeds of caraway (''Carum carvi''), spearmint (''Mentha spicata''), and dill. Uses Both carvo ...
or ''R''-(−)-carvone and
caraway Caraway, also known as meridian fennel and Persian cumin (''Carum carvi''), is a biennial plant in the family (biology), family Apiaceae, native to western Asia, Europe, and North Africa. Etymology The etymology of "caraway" is unclear. ...
seeds contain the -enantiomer or ''S''-(+)-carvone. The two smell different to most people because our olfactory receptors are chiral. Chirality is important in context of ordered phases as well, for example the addition of a small amount of an optically active molecule to a nematic phase (a phase that has long range orientational order of molecules) transforms that phase to a chiral nematic phase (or cholesteric phase). Chirality in context of such phases in polymeric fluids has also been studied in this context.


In inorganic chemistry

Chirality is a symmetry property, not a property of any part of the periodic table. Thus many inorganic materials, molecules, and ions are chiral.
Quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tetrahedra ...
is an example from the mineral kingdom. Such noncentric materials are of interest for applications in
nonlinear optics Nonlinear optics (NLO) is the branch of optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of optical instruments, instruments that use or Pho ...
. In the areas of
coordination chemistry A coordination complex consists of a central atom Every atom is composed of a atomic nucleus, nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common ...
and
organometallic chemistry Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...
, chirality is pervasive and of practical importance. A famous example is tris(bipyridine)ruthenium(II) complex in which the three bipyridine ligands adopt a chiral propeller-like arrangement. The two enantiomers of complexes such as u(2,2′-bipyridine)3sup>2+ may be designated as Λ (capital
lambda Lambda (}, ''lám(b)da'') is the 11th letter of the Greek alphabet The Greek alphabet has been used to write the Greek language since the late 9th or early 8th century BCE. It is derived from the earlier Phoenician alphabet, and was the ear ...
, the Greek version of "L") for a left-handed twist of the propeller described by the ligands, and Δ (capital delta, Greek "D") for a right-handed twist (pictured). Also cf. dextro- and levo- (laevo-). Chiral ligands confer chirality to a metal complex, as illustrated by metal-
amino acid Amino acids are organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with ot ...
complexes. If the metal exhibits catalytic properties, its combination with a chiral ligand is the basis of
asymmetric catalysis Enantioselective synthesis, also called asymmetric synthesis, is a form of chemical synthesis As a topic of chemistry, chemical synthesis (or combination) is the artificial execution of chemical reactions to obtain one or several product (che ...
.


Methods and practices

The term ''
optical activity Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization Polarization or polarisation may refer to: Mathematics *Polarization of an Abelian variety, in ...
'' is derived from the interaction of chiral materials with polarized light. In a solution, the (−)-form, or
levorotatory Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization (waves), polarization about the optical axis of linear polarization, linearly polarized light as it ...
form, of an optical isomer rotates the plane of a beam of linearly polarized light
counterclockwise Two-dimensional rotation can occur in two possible directions. Clockwise motion (abbreviated CW) proceeds in the same direction as a clock's hands: from the top to the right, then down and then to the left, and back up to the top. The opposite s ...
. The (+)-form, or
dextrorotatory Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization (waves), polarization about the optical axis of linear polarization, linearly polarized light as it ...
form, of an optical isomer does the opposite. The rotation of light is measured using a
polarimeter A polarimeter is a scientific instrument used to measure the angle of rotation caused by passing polarized light through an optically active substance.chiral resolution Chiral resolution, or enantiomeric resolution, is a process in stereochemistry for the separation of racemic compounds into their enantiomers. It is an important tool in the production of optically active compounds, including drugs. Another term wi ...
. This often involves forming crystals of a salt composed of one of the enantiomers and an acid or base from the so-called
chiral pool The chiral pool is a "collection of abundant enantiopure building blocks provided by nature" used in synthesis. In other words, a chiral pool would be a large quantity of common organic enantiomers In chemistry, an enantiomer (Help:IPA/English, ...
of naturally occurring chiral compounds, such as
malic acid Malic acid is an organic compound with the molecular formula . It is a dicarboxylic acid that is made by all living organisms, contributes to the sour taste of fruits, and is used as a food additive. Malic acid has two stereoisomeric forms (L ...
or the amine
brucine Brucine, is an alkaloid closely related to strychnine, most commonly found in the ''Strychnos nux-vomica'' tree. Brucine poisoning is rare, since it is usually ingested with strychnine, and strychnine is more toxic than brucine. In synthetic che ...
. Some racemic mixtures spontaneously crystallize into right-handed and left-handed crystals that can be separated by hand.
Louis Pasteur Louis Pasteur (, ; 27 December 1822 – 28 September 1895) was a French chemist and microbiologist renowned for his discoveries of the principles of vaccination, Fermentation, microbial fermentation and pasteurization, the latter of which wa ...
used this method to separate left-handed and right-handed sodium ammonium tartrate crystals in 1849. Sometimes it is possible to seed a racemic solution with a right-handed and a left-handed crystal so that each will grow into a large crystal. Liquid chromatography (HPLC and TLC) may also used as an analytical method for the direct separation of
enantiomer In chemistry, an enantiomer (Help:IPA/English, /ɪˈnænti.əmər, ɛ-, -oʊ-/ Help:Pronunciation respelling key, ''ih-NAN-tee-ə-mər''; from Ancient Greek language, Ancient Greek ἐνάντιος ''(enántios)'' 'opposite', and μέρος ''( ...
s and the control of enantiomeric purity, e.g. active pharmaceutical ingredients ( APIs) which are chiral.


Miscellaneous nomenclature

* Any non-
racemic In chemistry, a racemic mixture, or racemate (), is one that has equal Amount of substance, amounts of left- and right-handed enantiomers of a Chirality (chemistry), chiral molecule or salt. Racemic mixtures are rare in nature, but many compounds ...
chiral substance is called scalemic. Scalemic materials can be enantiopure or enantioenriched. * A chiral substance is enantiopure when only one of two possible enantiomers is present so that all molecules within a sample have the same chirality sense. Use of homochiral as a synonym is strongly discouraged. * A chiral substance is enantioenriched or heterochiral when its enantiomeric ratio is greater than 50:50 but less than 100:0. *
Enantiomeric excess In stereochemistry, enantiomeric excess (ee) is a measurement of Chemical purity, purity used for chiral substances. It reflects the degree to which a sample contains one enantiomer in greater amounts than the other. A racemic mixture has an ee of ...
or e.e. is the difference between how much of one enantiomer is present compared to the other. For example, a sample with 40% e.e. of ''R'' contains 70% ''R'' and 30% ''S'' (70% − 30% = 40%).


History

The rotation of plane polarized light by chiral substances was first observed by
Jean-Baptiste Biot Jean-Baptiste Biot (; ; 21 April 1774 – 3 February 1862) was a French physicist, astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They o ...
in 1812, and gained considerable importance in the sugar industry, analytical chemistry, and pharmaceuticals.
Louis Pasteur Louis Pasteur (, ; 27 December 1822 – 28 September 1895) was a French chemist and microbiologist renowned for his discoveries of the principles of vaccination, Fermentation, microbial fermentation and pasteurization, the latter of which wa ...
deduced in 1848 that this phenomenon has a molecular basis. The term ''chirality'' itself was coined by
Lord Kelvin William Thomson, 1st Baron Kelvin, (26 June 182417 December 1907) was a British mathematician, Mathematical physics, mathematical physicist and engineer born in Belfast. Professor of Natural Philosophy (Glasgow), Professor of Natural Philoso ...
in 1894. Different enantiomers or diastereomers of a compound were formerly called optical isomers due to their different optical properties. At one time, chirality was thought to be restricted to organic chemistry, but this misconception was overthrown by the resolution of a purely inorganic compound, a cobalt complex called
hexol In chemistry, hexol is a cation with formula 6+ — a coordination complex consisting of four cobalt cations in oxidation state +3, twelve ammonia molecules , and six hydroxyl, hydroxy anions , with a net charge of +6. The hydroxy groups act as ...
, by
Alfred Werner Alfred Werner (12 December 1866 – 15 November 1919) was a Swiss chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist trained in the study of chemistry. Chemists ...
in 1911. In the early 1970s, various groups established that the human olfactory organ is capable of distinguishing chiral compounds.


See also

* Chirality (electromagnetism) *
Chirality (mathematics) In geometry Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of ...
*
Chirality (physics) A chiral phenomenon is one that is not identical to its mirror image A mirror image (in a plane mirror) is a reflected duplication of an object that appears almost identical, but is reversed in the direction perpendicular to the mirror surface ...
* Enantiopure drug *
Enantioselective synthesis Enantioselective synthesis, also called asymmetric synthesis, is a form of chemical synthesis. It is defined by IUPAC as "a chemical reaction (or reaction sequence) in which one or more new elements of chirality are formed in a substrate mole ...
*
Handedness In human biology Human biology is an interdisciplinary area of academic study that examines humans through the influences and interplay of many diverse fields such as human genetics, genetics, human evolution, evolution, human physiology, ph ...
*
Orientation (vector space) The orientation of a real vector space or simply orientation of a vector space is the arbitrary choice of which ordered bases are "positively" oriented and which are "negatively" oriented. In the three-dimensional Euclidean space Euclidean s ...
* Pfeiffer effect *
Stereochemistry Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms that form the structure of molecules and their manipulation. The study of stereochemistry focuses on the relationships between stereois ...
for overview of stereochemistry in general *
Stereoisomerism In stereochemistry Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms that form the structure of molecules and their manipulation. The study of stereochemistry focuses on the relatio ...
* Supramolecular chirality


References


Further reading

* * * *


External links


21st International Symposium on Chirality




* ttps://web.archive.org/web/20071226005203/http://www.chem.qmul.ac.uk/iupac/AminoAcid/AA3t5.html IUPAC nomenclature for amino acid configurations.
Michigan State University's explanation of R/S nomenclatureChirality & Bioactivity I.: Pharmacology

Chirality and the Search for Extraterrestrial Life
* The Handedness of the Universe by Roger A Hegstrom and Dilip K Kondepudi http://quantummechanics.ucsd.edu/ph87/ScientificAmerican/Sciam/Hegstrom_The_Handedness_of_the_universe.pdf {{DEFAULTSORT:Chirality (Chemistry) Stereochemistry Polarization (waves) Chirality Chemical nomenclature Biochemistry Origin of life Pharmacology