View source for Biology:Chirality

{{short description|Geometric property of some molecules and ions}}

[[File:Chirality with hands.svg|upright=1.3|thumb|Two enantiomers of a generic amino acid that are chiral]]
[[File:L- and d- alanine scheme and 3d representation.png|thumb|240x240px|(''S'')-Alanine (left) and (''R'')-alanine (right) in zwitterionic form at neutral pH]]
In [[Chemistry:Chemistry|chemistry]], a molecule or ion is called '''chiral''' ({{IPAc-en|ˈ|k|aɪ|r|əl}}) if it cannot be superposed on its [[Mirror image|mirror image]] by any combination of rotations, [[Translation (geometry)|translation]]s, and some [[Chemistry:Conformational isomerism|conformational]] changes. This geometric property is called '''chirality''' ({{IPAc-en|k|aɪ|ˈ|r|æ|l|ɪ|t|i}}).<ref>''Organic Chemistry'' (4th Edition) Paula Y. Bruice. Pearson Educational Books. {{ISBN|9780131407480}}</ref><ref>''Organic Chemistry'' (3rd Edition) Marye Anne Fox, James K. Whitesell Jones & Bartlett Publishers (2004)
{{ISBN|0763721972}}</ref><ref>{{GoldBookRef|title=Chirality|file=C01058}}</ref><ref>{{GoldBookRef|title=Superposability|file=S06144}}</ref> The terms are derived from [[Social:Ancient Greek|Ancient Greek]] {{wikt-lang|grc|χείρ}} (''cheir'') 'hand'; which is the canonical example of an object with this property.

A chiral molecule or ion exists in two stereoisomers that are mirror images of each other, called [[Chemistry:Enantiomer|enantiomer]]s; they are often distinguished as either "right-handed" or "left-handed" by their [[Chemistry:Absolute configuration|absolute configuration]] or some other criterion. The two enantiomers have the same chemical properties, except when reacting with other chiral compounds. They also have the same [[Physics:Physics|physical]] properties, except that they often have opposite optical activities. A homogeneous mixture of the two enantiomers in equal parts is said to be [[Chemistry:Racemic mixture|racemic]], 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 ([[Chemistry:Diastereomer|diastereomers]] and [[Chemistry:Enantiomer|enantiomers]]) 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 can take a conformation having a plane of symmetry or an inversion point, then the molecule is achiral and is known as a [[Chemistry:Meso compound|''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 ([[Chemistry:Axial chirality|axial chirality]]) and a stereogenic plane ([[Physics:Planar chirality|planar chirality]]). Finally, the inherent curvature of a molecule can also give rise to chirality ([[Chemistry:Inherent chirality|inherent chirality]]). These types of chirality are far less common than central chirality. [[Chemistry:1,1′-Bi-2-naphthol|BINOL]] is a typical example of an axially chiral molecule, while [[Chemistry:Trans-Cyclooctene|''trans''-cyclooctene]] is a commonly cited example of a planar chiral molecule. Finally, [[Chemistry:Helicene|helicene]] possesses helical chirality, which is one type of inherent chirality.

Chirality is an important concept for [[Chemistry:Stereochemistry|stereochemistry]] and [[Biology:Biochemistry|biochemistry]]. Most substances relevant to [[Biology:Biology|biology]] are chiral, such as [[Chemistry:Carbohydrate|carbohydrate]]s (sugars, [[Chemistry:Starch|starch]], and [[Chemistry:Cellulose|cellulose]]), all but one of the [[Chemistry:Amino acid|amino acid]]s that are the building blocks of [[Biology:Protein|protein]]s, and the [[Biology:Nucleic acid|nucleic acid]]s. Naturally occurring [[Chemistry:Triglyceride|triglyceride]]s are often chiral, but not always. 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 [[Chemistry:Pharmaceutical|pharmaceutical]] usually have vastly different [[Chemistry:Potency (pharmacology)|potencies]] or effects.

== Definition ==
The chirality of a molecule is based on the [[Chemistry:Molecular symmetry|molecular symmetry]] of its conformations. A conformation of a molecule is chiral if and only if it belongs to the ''C<sub>n</sub>'', ''D<sub>n</sub>'', ''T'', ''O'', ''I'' [[Point group|point groups]] (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 [[Chemistry:Enantiomer|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''<sub>2</sub> point group, [[Chemistry:Butane|butane]] 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 [[Chemistry:Cyclohexane conformation|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 (R<sup>1</sup>R<sup>2</sup>R<sup>3</sup>N:) 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 [[Chemistry:Atropisomer|atropisomerism]].

A chiral compound can contain no improper axis of rotation (''S<sub>n</sub>''), which includes planes of symmetry and inversion center. Chiral molecules are always dissymmetric (lacking ''S<sub>n</sub>'') but not always asymmetric (lacking all symmetry elements except the trivial identity). Asymmetric molecules are always chiral.<ref>Cotton, F. A., "Chemical Applications of Group Theory," John Wiley & Sons: New York, 1990.</ref>

The following table shows some examples of chiral and achiral molecules, with the [[Physics:Schoenflies notation|Schoenflies notation]] of the [[Point groups in three dimensions|point group]] of the molecule. In the achiral molecules, X and Y (with no subscript) represent achiral groups, whereas X{{sub|R}} and X{{sub|S}} or Y{{sub|R}} and Y{{sub|S}} represent [[Chemistry:Enantiomer|enantiomer]]s. Note that there is no meaning to the orientation of an ''S''{{sub|2}} 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''{{sub|3}} or the achiral ''S''{{sub|4}}.

{| class="wikitable" style="text-align: center"
|+ Molecular symmetry and chirality
|- 
! Rotational<br>axis (''C<sub>n</sub>'') || colspan="3" | Improper rotational elements (''S<sub>n</sub>'')
|- 
| &nbsp;|| '''Chiral'''<br>no ''S<sub>n</sub>'' || '''Achiral''' <br>mirror plane<br>''S''<sub>1</sub> = σ || '''Achiral''' <br>inversion center<br>''S''<sub>2</sub> = ''i''
|- 
| ''C''<sub>1</sub> || [[File:Chiral sym CHXYZ.svg|62px]]<br>''C''{{sub|1}} || 79px<br>''C{{sub|s}}'' || [[File:Chiral sym CCXRYRXSYS.svg|102px]]<br>''C{{sub|i}}''
|- 
| ''C''<sub>2</sub> || [[File:Chiral sym CCCXYXY.svg|154px]]<br>''C''{{sub|2}}<br>(Note: This molecule has only one ''C''{{sub|2}} axis:<br>perpendicular to line of three C, but not in the plane of the figure.)|| 63px<br>''C''{{sub|2''v''}} || [[File:Chiral sym CCXYXY.svg|102px]]<br>''C''{{sub|2h}}<br>Note: This also has a mirror plane.
|}

An example of a molecule that does not have a mirror plane or an inversion and yet would be considered achiral is 1,1-difluoro-2,2-dichlorocyclohexane (or 1,1-difluoro-3,3-dichlorocyclohexane). This may exist in many conformers (conformational isomers), but none of them has a mirror plane. In order to have a mirror plane, the [[Chemistry:Cyclohexane|cyclohexane]] ring would have to be flat, widening the bond angles and giving the conformation a very high energy. This compound would not be considered chiral because the chiral conformers interconvert easily.

An achiral molecule having chiral conformations could theoretically form a mixture of right-handed and left-handed crystals, as often happens with racemic mixtures of chiral molecules (see [[Chemistry:Chiral resolution#Spontaneous resolution and related specialized techniques|Chiral resolution]]), or as when achiral liquid [[Chemistry:Silicon dioxide|silicon dioxide]] is cooled to the point of becoming chiral [[Chemistry:Quartz|quartz]].

==Stereogenic centers==
[[File:Illustrate stereocenter.png|thumb|Here, swapping of the two groups '''a''' and '''b''' leads to a molecule that is a stereoisomer of the original (the enantiomer, assuming there are no other stereogenic elements in the molecule). Hence, the central carbon atom is a stereocenter.]]
A ''stereogenic center'' (or ''stereocenter'') is an atom such that swapping the positions of two ligands (connected groups) on that atom results in a molecule that is stereoisomeric to the original.  For example, a common case is a [[Physics:Tetrahedral molecular geometry|tetrahedral]] carbon bonded to four distinct groups '''a''', '''b''', '''c''', and '''d''' (C'''abcd'''), where swapping any two groups (e.g., C'''bacd''') leads to a stereoisomer of the original, so the central C is a stereocenter.  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 [[Chemistry:Organophosphate|organophosphate]]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 [[Chemistry:Phosphorus|phosphorus]] in [[Chemistry:P-Chiral phosphine|P-chiral phosphines]] (PRR′R″) and [[Chemistry:Sulfur|sulfur]] in [[Chemistry:Sulfoxide#Chirality|S-chiral sulfoxide]]s (OSRR′), because a lone-pair of electrons is present instead of a fourth bond.

[[File:R-BINOL-2D-skeletal.png|thumb|upright|[[Chemistry:1,1'-Bi-2-naphthol|1,1′-Bi-2-naphthol]] is an example of a molecule with a stereogenic axis.]]
Similarly, a stereogenic axis (or plane) is defined as an axis (or plane) in the molecule such that the swapping of any two ligands attached to the axis (or plane) gives rise to a stereoisomer.  For instance, the [[Chemistry:C2-Symmetric ligands|C<sub>2</sub>-symmetric species]] [[Chemistry:1,1'-Bi-2-naphthol|1,1′-bi-2-naphthol]] (BINOL) and 1,3-dichloroallene have stereogenic axes and exhibit [[Chemistry:Axial chirality|axial chirality]], while (''E'')-[[Chemistry:Cyclooctene|cyclooctene]] and many [[Chemistry:Ferrocene|ferrocene]] derivatives bearing two or more substituents have stereogenic planes and exhibit [[Physics:Planar chirality|planar chirality]].

Chirality can also arise from isotopic differences between atoms, such as in the [[Physics:Deuterium|deuterated]] [[Chemistry:Benzyl alcohol|benzyl alcohol]] PhCHDOH; which is chiral and optically active ([''α'']<sub>D</sub> = 0.715°), even though the non-deuterated compound PhCH<sub>2</sub>OH is not.<ref>{{note|Streitwieser}}{{cite journal |author1=Streitwieser, A., Jr. |author2=Wolfe, J. R., Jr. |author3=Schaeffer, W. D. | year = 1959 | title = Stereochemistry of the Primary Carbon. X. Stereochemical Configurations of Some Optically Active Deuterium Compounds | journal = Tetrahedron | volume = 6 | pages = 338–344 | doi = 10.1016/0040-4020(59)80014-4 | issue = 4}}</ref>

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 [[Chemistry:Activation energy|energy barrier]] for [[Chemistry:Nitrogen inversion|nitrogen inversion]].

[[File:Chiral-adamantane-derivative.png|thumb|An adamantane derivative that is a chiral molecule but lacking any stereogenic centers.<ref name="Pashenko 2023" /> The center of chirality, marked by a black dot, is not at any of the atoms themselves.]]
It is not necessary for the chiral substance to have a stereogenic element. Examples include certain [[Chemistry:Helicene|helicenes]], calixarenes and fullerenes, which have [[Chemistry:Inherent chirality|inherent chirality]].  Moreover, it is possible for a molecule to have a center of chirality that sits in a position that does not correspond to an atom. Such a molecule may be chiral without having any stereogenic centers. Examples include 1,3,5(,7)-substituted [[Chemistry:Adamantane|adamantane]]s such as (1''S'',3''R'',5''R'',7''S'')-3-methyl-5-phenyladamantane-1-carboxylic acid (pictured).<ref name="Pashenko 2023">{{cite journal | last=Pashenko | first=Alexander E. | last2=Gaidai | first2=Alexandr | last3=Hryhoriev | first3=Nazar | display-authors=0 | title=Scale-Up Synthesis of 1-Methyladamantane and Its Functionalization as a Key Point for Promising Antiviral Agents | journal=Organic Process Research & Development | volume=27 | issue=3 | date=2023 | issn=1083-6160 | doi=10.1021/acs.oprd.2c00305 | pages=477–487| doi-access=free }}</ref><ref>{{Cite journal |last=Mislow |first=Kurt |last2=Siegel |first2=Jay |date=May 1984 |title=Stereoisomerism and local chirality |url=https://pubs.acs.org/doi/abs/10.1021/ja00323a043 |journal=Journal of the American Chemical Society |language=en |volume=106 |issue=11 |pages=3319–3328 |doi=10.1021/ja00323a043 |issn=0002-7863}}</ref>

When the optical rotation for an enantiomer is too low for practical measurement, the species is said to exhibit [[Chemistry:Cryptochirality|cryptochirality]]. 

Chirality is an intrinsic part of the identity of a molecule, so the systematic name includes details of the [[Chemistry:Absolute configuration|absolute configuration]] (''R/S'', ''<small>D/L</small>'', or [[Chemistry:Descriptor|other designations]]).

==Manifestations of chirality==
* '''Flavor:''' the artificial sweetener [[Chemistry:Aspartame|aspartame]] has two enantiomers. <small>L</small>-aspartame tastes sweet whereas <small>D</small>-aspartame is tasteless.<ref name=gal12>{{cite journal|last=Gal|first=Joseph|title=The Discovery of Stereoselectivity at Biological Receptors: Arnaldo Piutti and the Taste of the Asparagine Enantiomers-History and Analysis on the 125th Anniversary|journal=Chirality|year=2012|volume=24|issue=12|pages=959–976|doi=10.1002/chir.22071|pmid=23034823}}</ref>
* '''Odor:''' ''R''-(–)-[[Chemistry:Carvone|carvone]] smells like [[Biology:Spearmint|spearmint]] whereas ''S''-(+)-carvone smells like [[Biology:Caraway|caraway]].<ref name=leitereg71>{{cite journal | doi = 10.1021/jf60176a035 | title=Chemical and sensory data supporting the difference between the odors of the enantiomeric carvones |author1=Theodore J. Leitereg |author2=Dante G. Guadagni |author3=Jean Harris |author4=Thomas R. Mon |author5=Roy Teranishi | journal=J. Agric. Food Chem. | volume=19 | issue=4 | year=1971 | pages=785–787}}</ref>
* '''Drug effectiveness:''' the [[Chemistry:Antidepressant|antidepressant]] drug [[Chemistry:Citalopram|citalopram]] is sold as a racemic mixture. However, studies have shown that only the (''S'')-(+) enantiomer ([[Chemistry:Escitalopram|escitalopram]]) is responsible for the drug's beneficial effects.<ref name="pmid15107657">{{cite journal |vauthors=Lepola U, Wade A, Andersen HF | title = Do equivalent doses of escitalopram and citalopram have similar efficacy? A pooled analysis of two positive placebo-controlled studies in major depressive disorder | journal = Int Clin Psychopharmacol | volume = 19 | issue = 3 | pages = 149–55 |date=May 2004 | pmid = 15107657 | doi = 10.1097/00004850-200405000-00005 | s2cid = 36768144 }}</ref><ref name=hyttel92>{{cite journal|last=Hyttel|first=J.|author2=Bøgesø, K. P. |author3=Perregaard, J. |author4= Sánchez, C. |title=The pharmacological effect of citalopram resides in the (''S'')-(+)-enantiomer|journal=Journal of Neural Transmission|year=1992|volume=88|issue=2|pages=157–160|doi=10.1007/BF01244820|pmid=1632943|s2cid=20110906}}</ref> 
* '''Drug safety:''' [[Chemistry:Penicillamine|<small>D</small>‑penicillamine]] is used in [[Unsolved:Chelation therapy|chelation therapy]] and for the treatment of [[Medicine:Rheumatoid arthritis|rheumatoid arthritis]] whereas <small>L</small>‑penicillamine is toxic as it inhibits the action of [[Biology:Pyridoxine|pyridoxine]], an essential B vitamin.<ref name=jaffe64>{{cite journal|last=JAFFE|first=IA|author2=ALTMAN, K |author3=MERRYMAN, P |title=The Antipyridoxine Effect of Penicillamine in Man.|journal=The Journal of Clinical Investigation|date=Oct 1964|volume=43|issue=10|pages=1869–73|pmid=14236210|doi=10.1172/JCI105060|pmc=289631}}</ref>

==In biochemistry==
Many biologically active molecules are chiral, including the naturally occurring [[Chemistry:Amino acid|amino acid]]s (the building blocks of [[Biology:Protein|protein]]s) and [[Chemistry:Sugar|sugar]]s. 

The origin of this [[Physics:Homochirality|homochirality]] in [[Biology:Biology|biology]] is the subject of much debate.<ref name=Meierhenrich08>{{cite book | author= Meierhenrich, Uwe J. | year= 2008 | title= Amino acids and the Asymmetry of Life | location = Berlin, GER | publisher = Springer | isbn= 978-3540768852 }}</ref> 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.<ref>{{cite magazine|last=McKee |first=Maggie |url=https://www.newscientist.com/article/dn7895-space-radiation-may-select-amino-acids-for-life.html |title=Space radiation may select amino acids for life |magazine=New Scientist |date=2005-08-24 |access-date=2016-02-05}}</ref><ref>{{cite journal | author = Meierhenrich Uwe J., Nahon Laurent, Alcaraz Christian, Hendrik Bredehöft Jan, Hoffmann Søren V., Barbier Bernard, Brack André | year = 2005 | title = Asymmetric Vacuum UV photolysis of the Amino Acid Leucine in the Solid State | journal = Angew. Chem. Int. Ed. | volume = 44 | issue = 35| pages = 5630–5634 | doi = 10.1002/anie.200501311 | pmid = 16035020 }}</ref>

[[Biology:Enzyme|Enzyme]]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.

{{smallcaps all|L}}-forms of amino acids tend to be tasteless, whereas {{smallcaps all|D}}-forms tend to taste sweet.<ref name=Meierhenrich08/> [[Biology:Spearmint|Spearmint]] leaves contain the {{smallcaps all|L}}-enantiomer of the chemical [[Chemistry:Carvone|carvone]] or ''R''-(−)-carvone and [[Biology:Caraway|caraway]] seeds contain the {{smallcaps all|D}}-enantiomer or ''S''-(+)-carvone.<ref>{{cite journal | doi = 10.1021/jf60176a035 | title=Chemical and sensory data supporting the difference between the odors of the enantiomeric carvones |author1=Theodore J. Leitereg |author2=Dante G. Guadagni |author3=Jean Harris |author4=Thomas R. Mon |author5=Roy Teranishi | journal=J. Agric. Food Chem. | volume=19 | issue=4 | year=1971 | pages=785–787}}</ref> 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.<ref name="Srinivasarao">{{cite journal |author=Srinivasarao, M. |year=1999 |title=Chirality and Polymers |journal=Current Opinion in Colloid & Interface Science |volume=4 |issue=5 |pages=369–376 |doi=10.1016/S1359-0294(99)00024-2}}</ref>

== In inorganic chemistry ==
[[File:Delta-ruthenium-tris(bipyridine)-cation-3D-balls.png|thumb|Delta-ruthenium-tris(bipyridine) cation]]
Chirality is a symmetry property, not a property of any part of the periodic table. Thus many inorganic materials, molecules, and ions are chiral. [[Chemistry:Quartz|Quartz]] is an example from the mineral kingdom. Such noncentric materials are of interest for applications in [[Physics:Nonlinear optics|nonlinear optics]].

In the areas of coordination chemistry and [[Chemistry:Organometallic chemistry|organometallic chemistry]], chirality is pervasive and of practical importance. A famous example is [[Chemistry:Tris(bipyridine)ruthenium(II) chloride|tris(bipyridine)ruthenium(II)]] complex in which the three bipyridine ligands adopt a chiral propeller-like arrangement.<ref>von Zelewsky, A. (1995). ''Stereochemistry of Coordination Compounds.'' Chichester: John Wiley.. {{ISBN|047195599X}}.</ref> The two enantiomers of complexes such as [Ru(2,2′-bipyridine)<sub>3</sub>]<sup>2+</sup> may be designated as Λ (capital lambda, 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. [[Physics:Dextrorotation and levorotation|dextro- and levo- (laevo-)]].

Chiral ligands confer chirality to a metal complex, as illustrated by metal-[[Chemistry:Amino acid|amino acid]] complexes. If the metal exhibits catalytic properties, its combination with a chiral ligand is the basis of asymmetric catalysis.<ref>Hartwig, J. F. Organotransition Metal Chemistry, from Bonding to Catalysis; University Science Books: New York, 2010. {{ISBN|189138953X}}</ref>

==Methods and practices==

The term ''optical activity'' is derived from the interaction of chiral materials with polarized light. In a solution, the (−)-form, or levorotatory form, of an optical isomer rotates the plane of a beam of [[Physics:Linear polarization|linearly polarized light]] counterclockwise. The (+)-form, or dextrorotatory form, of an optical isomer does the opposite. The rotation of light is measured using a [[Physics:Polarimeter|polarimeter]] and is expressed as the optical rotation.

Enantiomers can be separated by [[Chemistry:Chiral resolution|chiral resolution]]. This often involves forming crystals of a salt composed of one of the enantiomers and an acid or base from the so-called [[Chemistry:Chiral pool|chiral pool]] of naturally occurring chiral compounds, such as [[Chemistry:Malic acid|malic acid]] or the amine [[Chemistry:Brucine|brucine]]. Some racemic mixtures spontaneously crystallize into right-handed and left-handed crystals that can be separated by hand. Louis Pasteur used this method to separate left-handed and right-handed [[Chemistry:Tartrate|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 be used as an analytical method for the direct separation of [[Chemistry:Enantiomer|enantiomer]]s and the control of enantiomeric purity, e.g. active  pharmaceutical ingredients (APIs) which are chiral.<ref>Bhushan, R.; Tanwar, S. ''J. Chromatogr. A'' '''2010''',  1395–1398. ({{doi|10.1016/j.chroma.2009.12.071}})</ref><ref>Ravi Bhushan ''Chem. Rec.'' '''2022''', e102100295. ({{doi|10.1002/tcr.202100295}})</ref>

==Miscellaneous nomenclature==
* Any non-racemic chiral substance is called '''scalemic'''. Scalemic materials can be enantiopure or enantioenriched.<ref name=EllielChir97>{{cite journal | author = Eliel, E.L. | year = 1997 | title = Infelicitous Stereochemical Nomenclatures | journal = Chirality | volume = 9 | issue = 56 | pages = 428–430 | url = https://www.uottawa.ca/publications/interscientia/inter.4/eliel/eliel.html | access-date = 5 February 2016 | doi = 10.1002/(sici)1520-636x(1997)9:5/6<428::aid-chir5>3.3.co;2-e | archive-url = https://web.archive.org/web/20160303230750/http://www.uottawa.ca/publications/interscientia/inter.4/eliel/eliel.html | archive-date = 3 March 2016 | url-status = dead }}</ref>
* 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.<ref>{{GoldBookRef|title=asymmetric synthesis| file = E02072}}</ref> 
* A chiral substance is '''enantioenriched''' or '''heterochiral''' when its enantiomeric ratio is greater than 50:50 but less than 100:0.<ref>{{GoldBookRef|title=enantiomerically enriched (enantioenriched)| file = E02071}}</ref> 
* [[Chemistry:Enantiomeric excess|Enantiomeric excess]] 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%).<ref>{{GoldBookRef|title=enantiomer excess (enantiomeric excess)| file = E02070}}</ref>

== History ==
The rotation of plane polarized light by chiral substances was first observed by [[Biography:Jean-Baptiste Biot|Jean-Baptiste Biot]] in 1812,<ref>{{cite journal | last=Frankel |first=Eugene | title=Corpuscular Optics and the Wave Theory of Light: The Science and Politics of a Revolution in Physics |journal=Social Studies of Science |volume=6 |issue=2 |pages=147–154|publisher=Sage Publications Inc. | year=1976|doi=10.1177/030631277600600201 |jstor=284930 |s2cid=122887123 |url=https://www.jstor.org/stable/284930 }}</ref> and gained considerable importance in the sugar industry, analytical chemistry, and pharmaceuticals. Louis Pasteur deduced in 1848 that this phenomenon has a molecular basis.<ref>{{cite book | author= Pasteur, L. | title=Researches on the molecular asymmetry of natural organic products, English translation of French original, published by Alembic Club Reprints (Vol. 14, pp. 1–46) in 1905, facsimile reproduction by SPIE in a 1990 book | year=1848}}</ref><ref>{{cite book |author1= Eliel, Ernest Ludwig |author2=Wilen, Samuel H. |author3=Mander, Lewis N. | year = 1994 | title = Stereochemistry of Organic Compounds | edition = 1st | chapter = Chirality in Molecules Devoid of Chiral Centers (Chapter 14) | location = New York, NY, USA | publisher = Wiley & Sons | chapter-url = https://books.google.com/books?id=IyfwAAAAMAAJ | access-date = 2 February 2016 | isbn =978-0471016700 }}</ref> The term ''chirality'' itself was coined by [[Biography:Lord Kelvin|Lord Kelvin]] in 1894.<ref>{{cite journal | author= Bentley, Ronald | year = 1995 | title = From Optical Activity in Quartz to Chiral Drugs: Molecular Handedness in Biology and Medicine. | journal=Perspect. Biol. Med. | volume=38 |issue=2 |pages=188–229 | doi=10.1353/pbm.1995.0069 | pmid = 7899056 | s2cid = 46514372 }}</ref> Different enantiomers or diastereomers of a compound were formerly called '''optical isomers''' due to their different optical properties.<ref>{{GoldBookRef|title=Optical isomers|file=O04308}}</ref> 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 [[Chemistry:Hexol|hexol]], by [[Biography:Alfred Werner|Alfred Werner]] in 1911.<ref>{{Cite journal|last=Werner|first=A.|date=May 1911|title=Zur Kenntnis des asymmetrischen Kobaltatoms. I|url=http://doi.wiley.com/10.1002/cber.19110440297|journal=Berichte der Deutschen Chemischen Gesellschaft|language=de|volume=44|issue=2|pages=1887–1898|doi=10.1002/cber.19110440297}}</ref>

In the early 1970s, various groups established that the human olfactory organ is capable of distinguishing chiral compounds.<ref name=leitereg71/><ref name=friedman71>{{cite journal |doi=10.1126/science.172.3987.1044|title=Odor Incongruity and Chirality|year=1971|last1=Friedman|first1=L.|last2=Miller|first2=J. G.|journal=Science|volume=172|issue=3987|pages=1044–1046|pmid=5573954|bibcode=1971Sci...172.1044F|s2cid=25725148}}</ref><ref name="ohloff80">{{cite journal |doi=10.1002/hlca.19800630721|title=Stereochemistry-Odor Relationships in Enantiomeric Ambergris Fragrances|year=1980|last1=Ohloff|first1=Günther|last2=Vial|first2=Christian|last3=Wolf|first3=Hans Richard|last4=Job|first4=Kurt|last5=Jégou|first5=Elise|last6=Polonsky|first6=Judith|last7=Lederer|first7=Edgar|journal=Helvetica Chimica Acta|volume=63|issue=7|pages=1932–1946}}</ref>

== See also ==
* [[Physics:Chirality (electromagnetism)|Chirality (electromagnetism)]]
* [[Chirality (mathematics)]]
* [[Physics:Chirality|Chirality]]
* [[Physics:Enantiopure drug|Enantiopure drug]]
* [[Physics:Enantioselective synthesis|Enantioselective synthesis]]
* Handedness
* [[Orientation (vector space)]]
* [[Physics:Pfeiffer effect|Pfeiffer effect]]
* [[Chemistry:Stereochemistry|Stereochemistry]] for overview of stereochemistry in general
* [[Chemistry:Stereoisomerism|Stereoisomerism]]
* [[Physics:Supramolecular chirality|Supramolecular chirality]]

==References==
{{Reflist|30em}}

== Further reading ==
*{{cite book |author1= Clayden, Jonathan |author2=Greeves, Nick |author3=Warren, Stuart | year = 2012 | title = Organic Chemistry | edition = 2nd | pages = 319f, 432, 604np, 653, 746int, 803ketals, 839, 846f | location = Oxford, UK | publisher = Oxford University Press | url = https://books.google.com/books?id=kQgu2j_ber0C | access-date = 2 February 2016 | isbn =978-0199270293 }} 
*{{cite book |author1= Eliel, Ernest Ludwig |author2=Wilen, Samuel H. |author3=Mander, Lewis N. | year = 1994 | title = Stereochemistry of Organic Compounds | volume = 9 | issue = 5–6 | edition = 1st | chapter = Chirality in Molecules Devoid of Chiral Centers (Chapter 14) | pages =428–430| location = New York, NY, USA | publisher = Wiley & Sons | chapter-url = https://books.google.com/books?id=IyfwAAAAMAAJ | access-date = 2 February 2016 | isbn =  978-0471016700 | doi = 10.1002/(SICI)1520-636X(1997)9:5/6<428::AID-CHIR5>3.0.CO;2-1 }}
*{{cite journal | author = Eliel, E.L. | year = 1997 | title = Infelicitous Stereochemical Nomenclatures | journal = Chirality | volume = 9 | issue = 5–6 | pages = 428–430 | doi = 10.1002/(SICI)1520-636X(1997)9:5/6<428::AID-CHIR5>3.0.CO;2-1 | access-date = 5 February 2016 | url = https://www.uottawa.ca/publications/interscientia/inter.4/eliel/eliel.html | archive-url = https://web.archive.org/web/20160303230750/http://www.uottawa.ca/publications/interscientia/inter.4/eliel/eliel.html | archive-date = 3 March 2016 | url-status = dead }}
*{{cite journal | author = Gal, Joseph | year = 2013 | series = Topics in Current Chemistry | title = Molecular Chirality: Language, History, and Significance | volume = 340 | journal = Chirality | pages =1–20 | doi = 10.1007/128_2013_435 | pmid = 23666078 | isbn = 978-3-319-03238-2 }}

==External links==
* [https://web.archive.org/web/20140517071014/http://chirality2009.org/ 21st International Symposium on Chirality]
* [http://www.chemguide.co.uk/basicorg/isomerism/optical.html#top STEREOISOMERISM - OPTICAL ISOMERISM]
* [http://www.nature.com/horizon/chemicalspace/highlights/s5_nonspec1.html Symposium highlights-Session 5: New technologies for small molecule synthesis]
* [https://web.archive.org/web/20071226005203/http://www.chem.qmul.ac.uk/iupac/AminoAcid/AA3t5.html IUPAC nomenclature for amino acid configurations.]
* [http://arquivo.pt/wayback/20160521132055/http://www.cem.msu.edu/~reusch/VirtualText/sterism3.htm Michigan State University's explanation of R/S nomenclature]
*[http://www.leffingwell.com/chirality/chirality.htm Chirality & Odour Perception at leffingwell.com]
*[http://www.leffingwell.com/download/chirality-phamacology.pdf Chirality & Bioactivity I.: Pharmacology]
* [http://www.eurekalert.org/pub_releases/2009-04/nios-sga042309.php 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

{{chiral synthesis}}

{{DEFAULTSORT:Chirality (Chemistry)}}
[[Category:Stereochemistry]]
[[Category:Polarization (waves)]]
[[Category:Chirality]]
[[Category:Chemical nomenclature]]
[[Category:Biochemistry]]
[[Category:Origin of life]]
[[Category:Pharmacology]]

{{Sourceattribution|Chirality (chemistry)|1}}