Polycyclic aromatic hydrocarbon

    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, tobacco, or incense.[1] 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 from 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.

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.

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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.

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.

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.

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.

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.

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.

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.

PAHs are lipophilic. Their presence has been reported in different edible oils from different parts of the world.

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].

Aromaticity

Although PAHs 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 most disjoint aromatic п-sextets—i.e. benzene-like moieties—is the most important for the characterization of the properties ^[3].

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 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.

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.

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