The term chiral (pronounced /ˈkaɪɹ(ə)l̩/) is used to describe an object which is non-superimposable on its mirror image. In terms of chemistry, these objects are usually molecules and the study of chiral molecules and associated phenomena is a very active area.

A molecule is chiral when it cannot be superimposed on its mirror image (see diagram) with the two mirror image forms referred to as enantiomers. A mixture of equal amounts of the two enantiomers is said to be a racemic mixture. Chirality is of interest because of its application to stereochemistry in inorganic chemistry, organic chemistry, physical chemistry and biochemistry. The study of chirality falls in the domain of stereochemistry. In chemistry, enantiomers are stereoisomers that are mirror images of each other. ... In chemistry, a racemate is a mixture of equal amounts of left- and right-handed stereoisomers of a chiral molecule. ... The different types of isomers. ... Inorganic chemistry is the branch of chemistry concerned with the properties and reactions of inorganic compounds. ... Organic chemistry is a specific discipline within the subject of chemistry. ... Physical Chemistry is the combined science of physics, chemistry, thermodynamics, and quantum mechanics which functions to provide molecular-level interpretations of observed macroscopic phenomena. ... Biochemistry is the study of the chemical processes and transformations in living organisms. ... The different types of isomers. ...

The term non-superimposable distinguishes mirror images which are superimposable, such as the letter "A" and its mirror image, from those that are not. The classic example of this are human hands. The left hand is a non-superposable mirror image of the right hand: No matter how the two hands are oriented relative to one another, one cannot line up all the major features of one hand with the other, whereas such an operation is trivial for a non-chiral mirror image (e.g., the letter "A").

The two "hands" (enantiomers) of a chiral molecule are sometimes referred to as optical isomers.

The two enantiomers of bromochlorofluoromethane

It is the symmetry of a molecule (or any other object) that determines whether it is chiral or not. Technically, a molecule is achiral (not chiral) if and only if it has an axis of improper rotation; that is, an n-fold rotation (rotation by 360°/n) followed by a reflection in the plane perpendicular to this axis which maps the molecule onto itself. (See chirality (mathematics).) A simplified rule applies to tetrahedrally-bonded carbon, as shown in the illustration: if all four substituents are different, the molecule is chiral. A chiral molecule is not necessarily asymmetric, that is completely devoid of any symmetry elements, as it can have, for example, rotational symmetry. The two chiral isomers of bromochlorofluoromethane. ... The two chiral isomers of bromochlorofluoromethane. ... Sphere symmetry group o. ... In geometry, an improper rotation is the combination of an ordinary rotation of three-dimensional Euclidean space, that keeps the origin fixed, with a coordinate inversion (a vector x goes to −x). ... In geometry, a figure is chiral (and said to have chirality) if it is not identical to its mirror image, or more particularly if it cannot be mapped to its mirror image by rotations and translations alone. ... In a tetrahedral molecular geometry a central atom is located at the center with four substituents located at the corners of a tetrahedron. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Atomic mass 12. ... In organic chemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon. ...

History

The term optical activity derives from the interaction of chiral materials with polarised light. A solution of the (−)-form of an optical isomer rotates the plane of polarization of a beam of plane polarized light in a counterclockwise direction, vice-versa for the (+) optical isomer. The property was first observed by Jean-Baptiste Biot in 1815 ^[1], and gained considerable importance in the sugar industry, analytical chemistry, and pharmaceuticals. Louis Pasteur deduced in 1848 that this phenomenon has a molecular basis^[2]. Artificial composite materials displaying the analog of optical activity but in the microwave regime were introduced by J.C. Bose in 1898 ^[3], and gained considerable attention from the mid-1980s ^[4]. A sphere rotating around its axis. ... This article treats polarization in electrodynamics. ... A clockwise motion is one that proceeds like the clocks hands: from the top to the right, then down and then to the left, and back to the top. ... Categories: People stubs | 1774 births | 1862 deaths | French physicists | French mathematicians | Members of the Acad mie fran aise ... Louis Pasteur (December 27, 1822 – September 28, 1895) was a French microbiologist and chemist. ... 1848 is a leap year starting on Saturday of the Gregorian calendar. ... {{otheruses4|the radiation|the appliance|microwaves equency = 1 GHz) to 1 mm (300 GHz). ...

The word “racemic” is derived from the Latin word for grape; the term having its origins in the work of Louis Pasteur who isolated racemic tartaric acid from wine. Louis Pasteur (December 27, 1822 – September 28, 1895) was a French microbiologist and chemist. ... Tartaric acid or H2C4H4O6 is a white crystalline organic acid. ...

Naming conventions

By optical activity: (+)- and (−)-

An enantiomer can be named by the direction in which it rotates the plane of plane polarized light. If it rotates the light clockwise (as seen by a viewer towards whom the light is traveling), that enantiomer is labeled (+). Its mirror-image is labeled (−). The (+) and (−) isomers have also been termed d- and l-, respectively (for dextrorotatory and levorotatory). This labeling is easy to confuse with D- and L-. Dextrorotation is the property of rotating plane polarized light to the right. ... Levorotation (also spelled laevorotation) is the counterclockwise rotation of plane polarized light. ...

By configuration: D- and L-

An optical isomer can be named by the spatial configuration of its atoms. The D/L system does this by relating the molecule to glyceraldehyde. Glyceraldehyde is chiral itself, and its two isomers are labeled D and L. Certain chemical manipulations can be performed on glyceraldehyde without affecting its configuration, and its historical use for this purpose (possibly combined with its convenience as one of the smallest commonly-used chiral molecules) has resulted in its use for nomenclature. In this system, compounds are named by analogy to glyceraldehyde, which generally produces unambiguous designations, but is easiest to see in the small biomolecules similar to glyceraldehyde. One example is the amino acid alanine: alanine has two optical isomers, and they are labeled according to which isomer of glyceraldehyde they come from. Glycine, the amino acid derived from glyceraldehyde, incidentally, has no optical activity as it is not chiral (achiral). Alanine, however, is chiral. Fischer projection of D-glyceraldehyde Glyceraldehyde is a triose monosaccharide with chemical formula C3H6O3. ... The general structure of an amino acid molecule, with the amine group on the left and the carboxyl group on the right. ... Alanine (Ala, A) also 2-aminopropanoic acid is a non-essential α-amino acid. ... Glycine (Gly, G) is a nonpolar amino acid. ...

The D/L labeling is unrelated to (+)/(−); it does not indicate which enantiomer is dextrorotatory and which is levorotatory. Rather, it says that the compound's stereochemistry is related to that of the dextrorotatory or levorotatory enantiomer of glyceraldehyde. Nine of the nineteen L-amino acids commonly found in proteins are dextrorotatory (at a wavelength of 589 nm), and D-fructose is also referred to as levulose because it is levorotatory. Dextrorotation is the property of rotating plane polarized light to the right. ... Levorotation (also spelled laevorotation) is the counterclockwise rotation of plane polarized light. ... The general structure of an amino acid molecule, with the amine group on the left and the carboxyl group on the right. ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... Fructose (or levulose) is a simple sugar (monosaccharide) found in many foods and is one of the three most important blood sugars along with glucose and galactose. ...

The dextrorotatory isomer of glyceraldehyde is in fact the D isomer, but this was a lucky guess. At the time this system was established, there was no way to tell which configuration was dextrorotatory. (If the guess had turned out wrong, the labeling situation would now be even more confusing.)

A rule of thumb for determining the D/L isomeric form of an amino acid is the "CORN" rule. The groups: The general structure of an amino acid molecule, with the amine group on the left and the carboxyl group on the right. ...

COOH, R, NH₂ and H (where R is an unnamed carbon chain)

are arranged around the chiral center carbon atom. Sighting from the hydrogen atom, if these groups are arranged counter-clockwise around the carbon atom, then it is the D-form. If clockwise, it is the L-form.

By configuration: R- and S-

The R/S system is another nomenclature system for enantiomers which does not involve a reference molecule such as glyceraldehyde. It labels each chiral center R or S according to a system by which its substituents are each assigned a priority, according to the Cahn Ingold Prelog priority rules, based on atomic number. If the center is oriented so that the lowest-priority of the four is pointed away from a viewer, the viewer will then see two possibilities: if the priority of the remaining three substitutents decreases in clockwise direction, it is labeled R, if it decreases in counterclockwise direction, it is S. In organic chemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon. ... For compounds found in organic chemistry, the Cahn Ingold Prelog priority rules are used to determine the orientation of a molecule for purposes of assigning stereochemistry at a stereocenter and for assigning the name of isomers of molecules possessing double bonds such as alkenes. ...

This system labels each chiral center in a molecule (and also has an extension to chiral molecules not involving chiral centers). It thus has greater generality than the D/L system, and can label, for example, an (R,R) isomer versus an (R,S) — diastereomers. Diastereomers are stereoisomers that are not enantiomers, or mirror images of each other. ...

The R/S system has no fixed relation to the (+)/(−) system. An R isomer can be either dextrorotatory or levorotatory, depending on its exact substituents.

The R/S system also has no fixed relation to the D/L system. For example, the side-chain one of serine contains a hydroxy group, -OH. If a thiol group, -SH, were swapped in for it, the D/L labeling would, by its definition, not be affected by the substitution. But this substitution would invert the molecule's R/S labeling, due to the fact that the CIP priority of CH₂OH is lower than that for CO₂H but the CIP priority of CH₂SH is higher than that for CO₂H. Serine is one of the 20 natural amino acids. ...

For this reason, the D/L system remains in common use in certain areas of biochemistry, such as amino acid and carbohydrate chemistry, because it is convenient to have the same chiral label for all of the commonly-occurring structures of a given type of structure in higher organisms. In the D/L system, they are all L; in the R/S system, they are mostly S but there are some common exceptions.

Properties of enantiomers

Enantiomers are identical with respect to ordinary chemical reactions, but differences arise when they are in the presence of other chiral molecules. Different enantiomers of chiral compounds often taste and smell different. For example, D-form amino acids tend to taste sweet, whereas L-forms are usually tasteless. Spearmint leaves and caraway seeds respectively contain L-carvone and D-carvone - enantiomers of carvone. These smell different to most people because our olfactory receptors also contain chiral molecules which behave differently in the presence of different enantiomers. The general structure of an amino acid molecule, with the amine group on the left and the carboxyl group on the right. ... Binomial name Mentha spicata Crantz Spearmint (Mentha spicata, syn ) is a species of mint native to central and southern Europe, where it grows in wet soils. ... Binomial name Carum carvi L. Caraway or Persian cumin (Carum carvi) is a biennial plant that belongs to the Apiaceae, or parsley, family. ... // Carvone Carvone is a member of a family of chemicals called terpenoids. ... In a sensory system, a sensory receptor is a structure that recognizes a stimulus in the internal or external environment of an organism. ...

Penicillin's activity is stereoselective. The antibiotic only works on peptide links of D-alanine which occur in the cell walls of bacteria - but not in humans. The antibiotic can kill only the bacteria, and not us, because we don't have these D-amino acids. Penicillin nucleus Penicillin (sometimes abbreviated PCN) refers to a group of β-lactam antibiotics used in the treatment of bacterial infections caused by susceptible, usually Gram-positive, organisms. ...

Chiral objects have different interactions with the two enantiomers of other chiral objects. Enzymes, which are chiral, often distinguish between the two enantiomers of a chiral substrate. Imagine an enzyme as having a glove-like cavity which binds a substrate. If this glove is right handed, then one enantiomer will fit inside and be bound while the other enantiomer will have a poor fit and is unlikely to bind. Ribbon diagram of the enzyme TIM, surrounded by the space-filling model of the protein. ...

One chiral 'object' that interacts differently with the two enantiomers of a chiral compound is circularly polarised light: An enantiomer will absorb left- and right-circularly polarised light to differing degrees. This is the basis of circular dichroism (CD) spectroscopy. Usually the difference in absorptivity is relatively small (parts per thousand). CD spectroscopy is a powerful analytical technique for investigating the secondary structure of proteins and for determining the absolute configutations of chiral compounds, particularly transition metal complexes. CD spectroscopy is replacing in polarimetry as a method for characterising chiral compounds, although the latter is still popular with sugar chemists. Circular dichroism (CD), is the differential absorption of left and right hand circularly polarized light. ... Polarimetry is the measurement of the polarisation of light; a polarimeter is the scientific instrument used to make these measurements. ...

Even isotopic differences must be considered when examining chirality. If one replaces one the two ¹H atoms at the CH₂ position of benzyl alcohol with a deuterium (²H) makes that carbon a stereocenter. The resulting benzyl-α-d alcohol exists as two distinct enantiomers, which can be assigned by the usual stereochemical naming conventions. The S enantiomer has [α]_D=+0.715°.^[5] Benzyl alcohol, also known as phenylmethanol, is a clear, colorless liquid with a mild pleasant aromatic odor. ... Deuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of one atom in 6400 of hydrogen (see VSMOW; the abundance changes slightly from one kind of natural water to another). ...

Chirality in biology

Many biologically-active molecules are chiral, including the naturally-occurring amino acids (the building blocks of proteins), and sugars. Interestingly, in biological systems most of these compounds are of the same chirality: most amino acids are L and sugars are D. The origin of this homochirality in biology is the subject of much debate. Many chiral drugs must be made with high enantiomeric purity due to potential side-effects of the other enantiomer. (The other enantiomer may also merely be inactive.) Consider a racemic sample of thalidomide. One enantiomer is effective against morning sickness while the other is teratogenic. Unfortunately, in this case administering just one of the enantiomers to a pregnant patient would still be very dangerous as the two enantiomers are readily interconverted in vivo. Thus, if a person is given either enantiomer, both the D and L isomers will eventually be present in the patient's serum. Steroid receptor sites also show stereoisomer specificity. Biology (from Greek βίος λόγος, see below) is the branch of science dealing with the study of living organisms. ... The general structure of an amino acid molecule, with the amine group on the left and the carboxyl group on the right. ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... Magnification of typical sugar In non-scientific use, the term sugar means sucrose, also called table sugar or saccharose, a white crystalline solid disaccharide. ... Homochirality is a term used to refer to a group of molecules that possess the same sense of chirality. ... Biology (from Greek βίος λόγος, see below) is the branch of science dealing with the study of living organisms. ... // Thalidomide is a sedative, hypnotic, and anti-inflammatory medication. ... Morning sickness, also called nausea and vomiting of pregnancy (NVP), or pregnancy sickness, affects between 50 and 95 percent of all pregnant women. ... Teratogenesis is a medical term from the Greek, literally meaning monster-making, which derives from teratology, the study of the frequency, causation, and development of congenital malformations—misleadingly called birth defects. ... Steroid skeleton. ... Main article: stereochemistry Stereoisomerism is the arrangement of atoms in molecules whose connectivity remains the same but their arrangement in space is different in each isomer. ...

Types

Most commonly, chiral molecules have point chirality, centering around a single atom, usually carbon, which has four different substituents. The two enantiomers of such compounds are said to have different absolute configurations at this center. This center is thus stereogenic (i.e., a grouping within a molecular entity that may be considered a focus of stereoisomerism), and is exemplified by the α-carbon of amino acids. A molecule can have multiple chiral centers without being chiral overall then called a meso compound if there is a symmetry element (a mirror plane or inversion center) which relates the two (or more) chiral centers. It is also possible for a molecule to be chiral without having actual point chirality. Commonly encountered examples include 1,1'-bi-2-naphthol (BINOL) and 1,3-dichloro-allene which have axial chirality, and (E)-cyclooctene which has planar chirality. Meso compound is a chemical compound whose molecules contain asymmetric centers but which is optically inactive due to overall symmetry. ... 1,1-bi-2-naphthol or 1,1-binaphthol or 1,1-binaphthalene-2,2-diol or BINOL is an organic compound that is often used as a ligand for transition-metal catalysed asymmetric synthesis. ... Term used to refer to stereoisomerism resulting from the non-planar arrangement of four groups in pairs about a chirality axis. ... Planar chirality is a special case of chirality in which a molecule does not posess a asymmetric chiral carbon atom but perpendicular disymmetric planes due to restricted rotation around a chemical bond in the molecule. ...

It is important to keep in mind that molecules which are dissolved in solution or are in the gas phase usually have considerable flexibility and thus may adopt a variety of different conformations. These various conformations are themselves almost always chiral. However, when assessing chirality, one must use a structural picture of the molecule which corresponds to just one chemical conformation - the most symmetric conformation possible. In chemistry, a chemical conformation is the spatial arrangement of atoms in a molecule. ...

When the optical rotation for an enantiomer is too low for practical measurement it is said to exhibit cryptochirality. Cryptochirality in stereochemistry is a special case of chirality where due to the electronic properties of the chiral molecule its specific rotation is non-measurable. ...

Chirality in inorganic chemistry

Many coordination compounds are chiral; for example the well-known [Ru(2,2'-bipyridine)₃]²⁺ complex in which the three bipyridine ligands adopt a chiral propeller-like arrangement ^[6]. In this case, the Ru atom may be regarded as a stereogenic centre, with the complex having point chirality. The two enantiomers of complexes such as [Ru(2,2'-bipyridine)₃]²⁺ may be designated as Λ (left-handed twist of the propeller described by the ligands) and Δ (right-handed twist). Hexol is a chiral cobalt complex which was first investigated by Alfred Werner. Resolved hexol is significant as being the first compound devoid of carbon to display optical activity. Synthesis of copper(II)-tetraphenylporphine, a metal complex, from tetraphenylporphine and copper(II) acetate monohydrate. ... Hexol is a cobalt compound that was first prepared by Alfred Werner in 1914 and represented the first non-carbon-containing chiral compound. ...

See also

• Stereochemistry for overview of stereochemistry in general
• Chiral synthesis for preparation of enantiomerically pure compounds
• Chirality for description in other fields of science
• Enantiomer for more details on enantiopure compounds

The different types of isomers. ... The two optical isomers of bromochlorofluoromethane Chiral synthesis (also called asymmetric synthesis) is organic synthesis which preserves or introduces a desired chirality. ... Chirality is a manga by Satoshi Urushihara Chirality (Greek handedness, derived from the word stem χειρ~, ch[e]ir~ - hand~) is an asymmetry property important in several branches of science. ... In chemistry, enantiomers are stereoisomers that are mirror images of each other. ...

References & notes

1. ↑ Lakhtakia, A. (ed.) (1990). Selected Papers on Natural Optical Activity (SPIE Milestone Volume 15). SPIE.
2. ↑ Pasteur, L. (1848). "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".
3. ↑ Bose, J. C. (1898). "On the rotation of plane of polarisation of electric waves by a twisted structure, Proc. R. Soc. Lond. (Vol. 63, pp. 146-152), facsimile reproduction by Wiley in a 2000 book".
4. ↑  Ernest L. Eliel and Samuel H. Wilen (1994). The Sterochemistry of Organic Compounds. Wiley-Interscience.
5. ↑ Streitwieser, A., Jr.; Wolfe, J. R., Jr.; Schaeffer, W. D. (1959). "Stereochemistry of the Primary Carbon. X. Stereochemical Configurations of Some Optically Active Deuterium Compounds". Tetrahedron 6: 338–344.
6. ↑  Alex von Zelewsky (1996). Stereochemistry of Coordination Compounds, Wiley.

External links

• http://www.chemguide.co.uk/basicorg/isomerism/optical.html#top
• http://www.nature.com/horizon/chemicalspace/highlights/s5_nonspec1.html
• IUPAC nomenclature for amino acid configurations.