Polycyclic aromatic hydrocarbon
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2008-07-16T01:12:25Z
Freestyle-69
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[[Image:Polycyclic Aromatic Hydrocarbons.png|thumb|250px|An illustration of typical polycyclic aromatic hydrocarbons. Clockwise from top left: benz[e]acephenanthrylene, [[pyrene]] and dibenz[a,h]anthracene ]]
[[Image:Hexa-peri-hexabenzocoronene ChemEurJ 2000 1834.jpg|thumbnail|250px|Crystal structure of a hexa-t-butyl derivatized hexa-peri-hexabenzo[bc,ef,hi,kl,no,qr]coronene, reported by Müllen and coworkers in Chem. Eur. J., 2000, 1834-1839. The t-butyl groups make this compound soluble in common solvents such as hexane, in which the unsubstituted PAH is insoluble.]]
'''Polycyclic aromatic hydrocarbons''' ('''PAH'''s) are [[chemical compound]]s that consist of fused [[aromatic]] [[simple aromatic ring|rings]] and do not contain [[heteroatom]]s or carry [[substituent]]s <ref>{{cite book | last = Fetzer | first = J. C. | title = The Chemistry and Analysis of the Large Polycyclic Aromatic Hydrocarbons | location = New York | publisher = Wiley | year = 2000 }}</ref>. PAHs occur in [[oil]], [[coal]], and [[tar]] deposits, and are produced as byproducts of fuel burning (whether fossil fuel or biomass). As a pollutant, they are of concern because some compounds have been identified as [[carcinogen]]ic, [[mutagen]]ic, and [[teratogen]]ic.
They are also found in the [[interstellar medium]], in [[comet]]s, and in [[meteorite]]s and are a [[PAH world hypothesis|candidate molecule to act as a basis for the earliest forms of life]]. In [[graphene]] the PAH motif is extended to large 2D sheets.
==Occurrence and pollution==
Polycyclic aromatic hydrocarbons are [[lipophilic]], meaning they mix more easily with oil than water. The larger compounds are less water-soluble and less [[Volatility (chemistry)|volatile]] (prone to evaporate). Because of these properties, PAHs in the environment are found primarily in [[soil]], sediment and oily substances, as opposed to in water or air. However, they are also a component of concern in [[particulate matter]] suspended in air.
Natural crude oil and coal deposits contain significant amounts of PAHs, arising from chemical conversion of natural product molecules, such as steroids, to aromatic hydrocarbons. They are also found in processed fossil fuels, [[tar]] and various edible oils.{{Facts|date=April 2008}}
PAHs are one of the most widespread organic pollutants. In addition to their presence in fossil fuels (and thus fuel spills) they are also formed by incomplete [[combustion]] of [[carbon]]-containing fuels such as [[wood]], [[coal]], [[diesel]], [[fat]], [[tobacco]], or [[incense]].[http://news.bbc.co.uk/2/hi/health/1467409.stm] Different types of combustion yield different distributions of PAHs in both relative amounts of individual PAHs and in which [[isomer]]s are produced. Thus coal burning produces a different mixture than motor-fuel combustion or a forest fire, making the compounds potentially useful as markers.
Hydrocarbon emissions from fossil fuel-burning engines are regulated in developed countries.<ref>For example, EPA regulations for small engines are at 40 CFR §90.103; see [[emission standard]] for more information.</ref>
==Human health==
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, [[benzopyrene|benzo[a]pyrene]], is notable for being the first chemical carcinogen to be discovered (and is one of many carcinogens found in [[Chemicals in a cigarette|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.
PAHs known for their [[carcinogen]]ic, [[mutagen]]ic and [[teratogen]]ic properties are benz[a][[anthracene]] and [[chrysene]] , benzo[b][[fluoranthene]], benzo[j]fluoranthene, benzo[k]fluoranthene,[[benzopyrene|benzo[a]pyrene]], benzo[ghi][[perylene]], [[coronene]], dibenz[a,h]anthracene (C<sub>20</sub>H<sub>14</sub>), indeno[1,2,3-cd][[pyrene]] (C<sub>22</sub>H<sub>12</sub>) and [[ovalene]] <ref>Luch, A. (2005). ''The Carcinogenic Effects of Polycyclic Aromatic Hydrocarbons.'' London: Imperial College Press, ISBN 1-86094-417-5</ref>.
==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 [[polyhex (mathematics)|polyhex]]es, which are planar figures composed by conjoining regular [[hexagon]]s 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 [[ultraviolet-visible spectroscopy|UV absorbance spectra]]. These often possess many absorbance bands and are unique for each ring structure. Thus, for a set of [[isomer]]s, 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 [[semiconductor|semi-conducting]] and other behaviors.
[[Naphthalene]] (C<sub>10</sub>H<sub>8</sub> constituent of [[mothball]]s), 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.
Aqueous solubility decreases approximately one order of magnitude for each additional ring.
== PAH compounds ==
{| class="wikitable" style="background: #ffffff; text-align: center;"
|-
! Chemical compound
!
! Chemical compound
!
|-
| [[Anthracene]] || [[Image:Anthracene.svg|85px]] || [[Benzopyrene|Benzo[a]pyrene]] || [[Image:Benzo-a-pyrene.svg|100px]]
|-
| [[Chrysene]] || [[Image:Chrysene.svg|100px]] || [[Coronene]] || [[Image:Coronene.svg|85px]]
|-
| [[Corannulene]] || [[Image:Corannulene.svg|80px]]
|| [[Naphthacene]] || [[Image:Naftacene.svg|100px]]
|-
| [[Naphthalene]] || [[Image:Naphthalene.svg|55px]]
|| [[Pentacene]] || [[Image:Pentacene.svg|100px]]
|-
| [[Phenanthrene]] || [[Image:Phenanthrene.svg|80px]] || [[Pyrene]] || [[Image:Pyrene.svg|80px]]
|-
| [[Triphenylene]] || [[Image:Triphenylene chemical structure.png|100px]]
|| [[Ovalene]] || [[Image:Ovalene.svg|100px]]
|-
|-
|}
==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 PAHs 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 <ref>''Assessment of Clar's aromatic -sextet rule by means of PDI, NICS and HOMA indicators of local aromaticity'' Guillem Portella , Jordi Poater, Miquel Solà J. Phys. Org. Chem. '''2005'''; 18: 785–791 {{DOI|10.1002/poc.938}}</ref>.
:[[Image:TheClarRule.png|400px|The Clar rule]]
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|PAH world hypothesis}}
In January 2004 (at the 203rd Meeting of the [[American Astronomical Society]] <ref>American Astronomical Society. (n.d.). Meeting program contents. Retrieved [[January 11]] [[2004]] from http://www.aas.org/meetings/aas203/</ref>), it was reported <ref>Battersby, S. (2004). Space molecules point to organic origins. Retrieved [[January 11]] [[2004]] from http://www.newscientist.com/news/news.jsp?id=ns99994552</ref> that a team led by A. Witt of the [[University of Toledo|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, by necessity they must originate in nebulae.
==References==
{{reflist}}
==External links==
* [http://www.chem.qmul.ac.uk/iupac/fusedring/ Fused Ring and Bridged Fused Ring Nomenclature]
* [http://ois.nist.gov/pah/ Database of PAH structures]
* [http://www.npi.gov.au/database/substance-info/profiles/74.html National Pollutant Inventory: Polycyclic Aromatic Hydrocarbon Fact Sheet]
* [http://www.spitzer.caltech.edu/features/articles/20050627.shtml Understanding Polycyclic Aromatic Hydrocarbons] NASA Spitzer Space Telescope
*[http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=1992&mode=thread&order=0&thold=0 Astrobiology magazine] ''Aromatic World'' An interview with Professor Pascale Ehrenfreund on PAH origin of life. Accessed June 2006
[[Category:Polycyclic aromatic hydrocarbons| ]]
[[Category:Carcinogens]]
[[Category:Origin of life]]
[[Category:Astrochemistry]]
[[ar:هيدروكربون أروماتي متعدد الحلقات]]
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