An illustration of typical polycyclic aromatic hydrocarbons. Source: NASA

Crystal structure of a hexa-peri-hexabenzocoronene, reported by Müllen and coworkers in Chem. Eur. J., 2000, 1834-1839.

Polycyclic aromatic hydrocarbons (PAHs) are chemical compounds that consist of fused aromatic rings and do not contain heteroatoms or carry substituents ^[1]. These compounds can be point source pollutants (e.g. oil spill) or non-point source (e.g. atmospheric deposition) and are one of the most widespread organic pollutants. Some of them are known or suspected carcinogens, and are linked to other health problems. They are primarily formed by incomplete combustion of carbon-containing fuels such as wood, coal, diesel, fat, or tobacco. Tar also contains PAHs. Since human civilization relies so heavily on combustion, PAHs are inevitably linked to our energy production. In this sense, PAH can be thought of as marker molecules as their abundance can be directly proportional to combustion processes in the region and therefore directly related to air quality. Different types of combustion yield different distributions of PAHs in both relative amounts of individual PAHs and in which isomers are produced. Thus, those produced from coal burning are different than those produced by motor-fuel combustion, which differ from those produced by forest fires. Some PAHs occur within crude oil, arising from chemical conversion of natural product molecules, such as steroids, to aromatic hydrocarbons. Image File history File links NASA20050627a_PAH_molecules. ... Image File history File links NASA20050627a_PAH_molecules. ... Image File history File linksMetadata Download high-resolution version (664x687, 157 KB) This is a picture generated from crystal structure data reported by Peter T. Herwig, Volker Enkelmann, Oliver Schmelz, Klaus Müllen in Chemistry – A European Journal, Year 2000, Pages 1834-1839. ... Image File history File linksMetadata Download high-resolution version (664x687, 157 KB) This is a picture generated from crystal structure data reported by Peter T. Herwig, Volker Enkelmann, Oliver Schmelz, Klaus Müllen in Chemistry – A European Journal, Year 2000, Pages 1834-1839. ... A chemical compound is a chemical substance of two or more different chemically bonded chemical elements, with a fixed ratio determining the composition. ... In chemistry, an aromatic molecule is one in which electrons are free to cycle around circular arrangements of atoms, which are alternately singly and doubly bonded to one another. ... Simple aromatic rings are aromatic organic compounds (also known as arenes or aromatics) that consist only of conjugated planar ring systems with delocalized pi electron clouds instead of discrete alternating single and double bonds. ... In the nomenclature of organic chemistry, a heteroatom (from Ancient Greek heteros, different, + atomos) is any atom that is not carbon or hydrogen, typically, but not exclusively, nitrogen, oxygen, sulfur, phosphorus or boron. ... 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. ... The hazard symbol for carcinogenic chemicals in the Globally Harmonized System. ... A combustion reaction taking place in a igniting match Combustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Standard atomic weight 12. ... Trunks A tree trunk as found at the Veluwe, The Netherlands Wood is a solid material derived from woody plants, notably trees but also shrubs. ... Coal Coal (IPA: ) is a fossil fuel formed in swamp ecosystems where plant remains were saved by water and mud from oxidization and biodegradation. ... Diesel or diesel fuel is a specific fractional distillate of fuel oil (mostly petroleum) that is used as fuel in a diesel engine invented by German engineer Rudolf Diesel. ... Fats consist of a wide group of compounds that are generally soluble in organic solvents and largely insoluble in water. ... Shredded tobacco leaf for pipe smoking Tobacco can also be pressed into plugs and sliced into flakes Tobacco is an agricultural product processed from the fresh leaves of plants in genus Nicotiana. ... Tar can be produced from corn stalks by heating in a microwave. ...

They are also found in the interstellar medium, in comets, and in meteorites and are a candidate molecule to act as a basis for the earliest forms of life. In graphene the PAH motive is extended to large 2D sheets. The interstellar medium (or ISM) is the name astronomers give to the tenuous gas and dust that pervade interstellar space. ... Comet Hale-Bopp Comet West For other uses, see Comet (disambiguation). ... Willamette Meteorite A meteorite is a natural object originating in outer space that survives an impact with the Earths surface without being destroyed. ... The PAH world hypothesis proposes that the use of polycyclic aromatic hydrocarbon (PAH) was a means for a pre-RNA World basis for the origin of life. ... It has been suggested that Quasam be merged into this article or section. ...

Chemistry

The simplest PAHs, as defined by the International Union on Pure and Applied Chemistry (IUPAC) {G.P Moss, IUPAC nomenclature for fused-ring systems), are phenanthrene and anthracene. Smaller molecules, such as benzene and naphthalene, are not formally PAHs, although they are chemically related they are called one-ring (or mono) and two-ring (di) aromatics. Phenanthrene is a polycyclic aromatic hydrocarbon composed of three fused benzene rings--as the above formula shows. ... In chemistry, anthracene is a solid polycyclic aromatic hydrocarbon consisting of three benzene rings derived from coal-tar. ... Benzene is an organic chemical compound with the formula C6H6. ... Naphthalene (not to be confused with naphtha) (also known as naphthalin, naphthaline, tar camphor, white tar, albocarbon, or naphthene), is a crystalline, aromatic, white, solid hydrocarbon, best known as the primary ingredient of mothballs. ...

PAHs may contain four-, five-, six- or seven-member rings, but those with five or six are most common. PAHs composed only of six-membered rings are called alternant PAHs. Certain alternant PAHs are called "benzenoid" PAHs. The name comes from benzene, an aromatic hydrocarbon with a single, six-membered ring. These can be benzene rings interconnected with each other by single carbon-carbon bonds and with no rings remaining that do not contain a complete benzene ring. Benzene is an organic chemical compound with the formula C6H6. ... An aromatic hydrocarbon (abbreviated as AH) or arene [1] is a hydrocarbon, the molecular structure of which incorporates one or more planar sets of six carbon atoms that are connected by delocalised electrons numbering the same as if they consisted of alternating single and double covalent bonds. ...

The set of alternant PAHs is closely related to a set of mathematical entities called polyhexes, which are planar figures composed by conjoining regular hexagons of identical size. In recreational mathematics, a polyhex is a polyform with a regular hexagon (or hex for short) as the base form. ... A regular hexagon. ...

PAHs containing up to six fused aromatic rings are often known as "small" PAHs and those containing more than six aromatic rings are called "large" PAHs. Due to the availability of samples of the various small PAHs, the bulk of research on PAHs has been of those of up to six rings. The biological activity and occurrence of the large PAHs does appear to be a continuation of the small PAHs. They are found as combustion products, but at lower levels than the small PAHs due to the kinetic limitation of their production through addition of successive rings. Additionally, with many more isomers possible for larger PAHs, the occurrence of specific structures is much smaller.

PAHs possess very characteristic UV absorbance spectra. These often possess many absorbance bands and are unique for each ring structure. Thus, for a set of isomers, each isomer has a different UV absorbance spectrum than the others. This is particularly useful in the identification of PAHs. Most PAHs are also fluorescent, emitting characteristic wavelengths of light when they are excited (when the molecules absorb light). The extended pi-electron electronic structures of PAHs lead to these spectra, as well as to certain large PAHs also exhibiting semi-conducting and other behaviors. Ultraviolet-visible spectroscopy or ultraviolet-visible spectrophotometry (UV/ VIS) involves the spectroscopy of photons and spectrophotometry. ... In chemistry, isomers are molecules with the same chemical formula and often with the same kinds of chemical bonds between atoms, but in which the atoms are arranged differently (analogous to a chemical anagram). ... Fluorescence induced by exposure to ultraviolet light in vials containing various sized cadmium selenide (CdSe) quantum dots. ... A semiconductor is a fuckin solid whose electrical conductivity is in between that of a metal and that of an insulator, and can be controlled over a wide range, either permanently or dynamically. ...

PAHs of three rings or more have low solubilities in water and a low vapor pressure. As molecular weight increases, aqueous solubility and vapor pressure decrease. The aqueous solubility decreases approximately one order of magnitude for each additional ring. PAHs with two rings are more soluble in water and more volatile. Because of these properties, PAHs in the environment are found primarily in soil and sediment, as opposed to in water or air. PAHs, however, are also often found in particles suspended in water and air. Natural crude oil and coal deposits contain significant amounts of PAHs, as do combustion products and smoke from naturally occurring forest fires. In chemistry and physics, Vapor pressure is the pressure of a vapor in equilibrium with its non-vapor phases. ... Loess field in Germany Surface-water-gley developed in glacial till, Northern Ireland Technically, soil forms the pedosphere: the interface between the lithosphere (rocky part of the planet) and the biosphere, atmosphere, and hydrosphere. ...

PAHs toxicity is very structurally dependent, with isomers (PAHs with the same formula and number of rings) varying from being nontoxic to being extremely toxic. Thus, highly carcinogenic PAHs may be small or large. One PAH compound, benzo[a]pyrene, is notable for being the first chemical carcinogen to be discovered (and is one of many carcinogens found in cigarette smoke). The EPA has classified seven PAH compounds as probable human carcinogens: benz[a]anthracene, benzo[a]pyrene, benzo[b]fluoranthene, benzo[k]fluoranthene, chrysene, dibenz[a,h]anthracene, and indeno[1,2,3-cd]pyrene. Benzo[a]pyrene, C20H12, is a five-ring polycyclic aromatic hydrocarbon that is mutagenic and highly carcinogenic. ... It has been suggested that this article or section be merged with Cigarette#Contents of a cigarette. ...

Naphthalene (C₁₀H₈ constituent of Mothballs), consisting of two coplanar six-membered rings sharing an edge, is another aromatic hydrocarbon. By formal convention, it is not a true PAH, though is referred to as a bicyclic aromatic hydrocarbon. Naphthalene (not to be confused with naphtha) (also known as naphthalin, naphthaline, tar camphor, white tar, albocarbon, or naphthene), is a crystalline, aromatic, white, solid hydrocarbon, best known as the primary ingredient of mothballs. ... Mothballs are small balls of chemical pesticide and deodorant used when storing clothing and other articles susceptible to damage from mold or moth larvae. ...

PAHs are lipophilic. Their presence has been reported in different edible oils from different parts of the world. This article or section does not cite its references or sources. ...

PAH compounds

PAHs known for their carcinogenic, mutagenic and teratogenic properties are: Benz[a]anthracene and chrysene , Benzo[b]fluoranthene, Benzo[j]fluoranthene, Benzo[k]fluoranthene,Benzo[a]pyrene, Benzo[ghi]perylene, Coronene, Dibenz[a,h]anthracene (C₂₀H₁₄), Indeno[1,2,3-cd]pyrene (C₂₂H₁₂) and Ovalene ^[2]. In chemistry, anthracene is a solid polycyclic aromatic hydrocarbon consisting of three benzene rings derived from coal-tar. ... Image File history File links This is a lossless scalable vector image. ... Benzo[a]pyrene, C20H12, is a five-ring polycyclic aromatic hydrocarbon that is mutagenic and highly carcinogenic. ... Chemical structure of benzopyrene Selfmade by cacycle File links The following pages link to this file: Benzopyrene Categories: GFDL images ... Chrysene is a polycyclic aromatic hydrocarbon (PAH) with the molecular formula C18H12 that consists of four fused benzene rings. ... Image File history File links Chrysene. ... Coronene is an aromatic molecule consisting of 7 benzene rings. ... Image File history File links Coronene_chemical_structure. ... Corannulene is a polycyclic aromatic hydrocarbon with chemical formula C20H10 . ... Image File history File links No higher resolution available. ... Tetracene, also called naphthacene and 2,3-benzanthracene, is a polycyclic aromatic hydrocarbon. ... Image File history File links Tetracene. ... Naphthalene (not to be confused with naphtha) (also known as naphthalin, naphthaline, tar camphor, white tar, albocarbon, or naphthene), is a crystalline, aromatic, white, solid hydrocarbon, best known as the primary ingredient of mothballs. ... Image File history File links Naphthalene. ... Pentacene is an aromatic molecule consisting of 5 fused benzene rings. ... Image File history File links Pentacene. ... Phenanthrene is a polycyclic aromatic hydrocarbon composed of three fused benzene rings--as the above formula shows. ... Image File history File links Phenanthrenec1. ... Flash point non-flammable R/S statement R: 36/37/38-45-53 S: 24/25-26-36 RTECS number UR2450000 Supplementary data page Structure and properties n, εr, etc. ... Image File history File links Pyrene. ... In chemistry, triphenylene is the name of an organic compound. ... Image File history File links Download high-resolution version (1010x919, 6 KB) Description: Chemical structure of Triphenylene. ... Ovalene is a polycyclic aromatic hydrocarbon with a formula of C32H14, which consists of ten peri-fused six-member rings. ... Image File history File links No higher resolution available. ... The hazard symbol for carcinogenic chemicals in the Globally Harmonized System. ... In biology, a mutagen (Latin, literally origin of change) is a physical or chemical agent that changes the genetic information (usually DNA) of an organism and thus increases the number of mutations above the natural background level. ... Teratogenesis is a medical term from the Greek, literally meaning monster making. ... In chemistry, anthracene is a solid polycyclic aromatic hydrocarbon consisting of three benzene rings derived from coal-tar. ... Chrysene is a polycyclic aromatic hydrocarbon (PAH) with the molecular formula C18H12 that consists of four fused benzene rings. ... Fluoranthene is a polycyclic aromatic hydrocarbon consisting of a naphtalene and a benzene unit connected by a five-membered ring. ... Benzo[a]pyrene, C20H12, is a five-ring polycyclic aromatic hydrocarbon that is mutagenic and highly carcinogenic. ... Perylene or perilene is a polycyclic aromatic hydrocarbon with chemical formula C20H12 and CAS number 198-55-0, occurring as a brown solid. ... Coronene is an aromatic molecule consisting of 7 benzene rings. ... Flash point non-flammable R/S statement R: 36/37/38-45-53 S: 24/25-26-36 RTECS number UR2450000 Supplementary data page Structure and properties n, εr, etc. ... Ovalene is a polycyclic aromatic hydrocarbon with a formula of C32H14, which consists of ten peri-fused six-member rings. ...

Aromaticity

Although PAH's clearly are aromatic compounds, the degree of aromaticity can be different for each ring segment. According to Clar's rule (formulated by Eric Clar in 1964) for PAH's the resonance structure with the largest number of disjoint aromatic п-sextets, i.e. benzene-like moieties, is the most important for the characterization of the properties ^[3]. Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. ... Resonance structures are diagrammatic tools in organic chemistry to symbolize resonant bonds between atoms in molecules. ...

For example in phenanthrene the Clar structure 1A has two sextets at the extremities while resonance structure 1B has just one central sextet. Therefore in this molecule the outer rings are firmly aromatic while its central ring is less aromatic and therefore more reactive. In contrast, in anthracene 2 the number of sextets is just one and aromaticity spreads out. This difference in number of sextets is reflected the the UV absorbance spectra of these two isomers. Phenanthrene has a highest wavelength absorbance around 290 nm, while anthracene has highest wavelength bands around 380 nm. Three Clar structures with two sextets are present in chrysene (4) and by superposition the aromaticity in the outer ring is larger than in the inner rings. Image File history File links Size of this preview: 800 × 567 pixelsFull resolution (1047 × 742 pixel, file size: 22 KB, MIME type: image/png) The Clar Rule File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Phenanthrene is a polycyclic aromatic hydrocarbon composed of three fused benzene rings--as the above formula shows. ... In chemistry, anthracene is a solid polycyclic aromatic hydrocarbon consisting of three benzene rings derived from coal-tar. ... Phenanthrene is a polycyclic aromatic hydrocarbon composed of three fused benzene rings--as the above formula shows. ... In chemistry, anthracene is a solid polycyclic aromatic hydrocarbon consisting of three benzene rings derived from coal-tar. ... Chrysene is a polycyclic aromatic hydrocarbon (PAH) with the molecular formula C18H12 that consists of four fused benzene rings. ...

Origins of life

Main article: PAH world hypothesis

In January 2004 (at the 203rd Meeting of the American Astronomical Society ^[4]), it was reported ^[5] that a team led by A. Witt of the University of Toledo, Ohio studied ultraviolet light emitted by the Red Rectangle nebula and found the spectral signatures of anthracene and pyrene. (No other such complex molecules had ever before been found in space.) This discovery was considered confirmation of a hypothesis that as nebulae of the same type as the Red Rectangle approach the ends of their lives, convection currents cause carbon and hydrogen in the nebulae's core to get caught in stellar winds, and radiate outward. As they cool, the atoms supposedly bond to each other in various ways and eventually form particles of a million or more atoms. Witt and his team inferred (as cited in Battersby, 2004) that since they discovered PAHs, which may have been vital in the formation of early life on Earth—in a nebula, then nebulae, by necessity, are where they originate. The PAH world hypothesis proposes that the use of polycyclic aromatic hydrocarbon (PAH) was a means for a pre-RNA World basis for the origin of life. ... The American Astronomical Society (AAS) is a US society of professional astronomers and other interested individuals, headquartered in Washington, DC. The main aim of the AAS is provide a political voice for its members and organise their lobbying. ... The University of Toledo is a public university situated in Toledo, Ohio. ... The Red Rectangle nebula, so called because of its unique shape and color, is a nebula near the Monoceros constellation. ... In chemistry, anthracene is a solid polycyclic aromatic hydrocarbon consisting of three benzene rings derived from coal-tar. ... Flash point non-flammable R/S statement R: 36/37/38-45-53 S: 24/25-26-36 RTECS number UR2450000 Supplementary data page Structure and properties n, εr, etc. ...

References

A digital object identifier (or DOI) is a standard for persistently identifying a piece of intellectual property on a digital network and associating it with related data, the metadata, in a structured extensible way. ...

External links

• Fused Ring and Bridged Fused Ring Nomenclature
• Database of PAH structures
• National Pollutant Inventory - Polycyclic Aromatic Hydrocarbon Fact Sheet
• Understanding Polycyclic Aromatic Hydrocarbons NASA Spitzer Space Telescope
• Astrobiology magazine Aromatic World An interview with Professor Pascale Ehrenfreund on PAH origin of life. - Accessed June 2006