Polycyclic aromatic hydrocarbon
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An illustration of typical polycyclic aromatic hydrocarbons - NASA |
Polycyclic aromatic hydrocarbons (
PAHs) are
chemical compounds that consist of fused
aromatic rings and do not contain
heteroatoms or carry
substituents. Many of them are known or suspected
carcinogens. They are formed by incomplete
combustion of
carbon-containing fuels such as
wood,
coal,
diesel,
fat, or
tobacco.
Tar also contains PAHs.
They are also found in the
interstellar medium, in
comets, and in
meteorites.
They are a candidate molecule to act as a basis for the earliest forms of life. (see below)
The simplest PAH is
benzocyclobutene (
86).
PAHs composed only of six-membered rings are called
benzenoid PAHs. The name comes from
benzene, an
aromatic hydrocarbon with a single, six-membered ring. The set of benzenoid 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 4 fused benzene rings are known as light PAHs and those containing more than four benzene rings are known as heavy PAHs. Heavy PAHs are more stable and more toxic than light ones.
PAHs of three rings or more have low solubilities in water and a low
vapor pressure. As molecular weight increases, solubility and vapor pressure decrease. 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. However, PAHs are often found in particles suspended in water and air.
As molecular weight increases, the
carcinogenicity of PAHs also increases, and acute
toxicity decreases. One PAH compound,
benzo(a)pyrene, is notable for being the first chemical carcinogen to be discovered.
Naphthalene (C
10H
8), consisting of two coplanar six-membered rings sharing an edge, is another aromatic hydrocarbon. By some conventions, it is not a true PAH, but is referred to as a bicyclic aromatic hydrocarbon. Its smell is familiar to those who have encountered
mothballs.
PAHs are
lipophilic in nature. Their presence has been reported in different edible oils from different countries.
*
Anthracene*
Benzo(a)pyrene*
Benzo(ghi)pyrene*
Chrysene*
Coronene*
Fluoranthene*
Naphthacene (
Tetracene)
*
Naphthalene*
Phenanthrene*
Pyrene*
TriphenylenePAHs known for their
carcinogenic,
mutagenic and
teratogenic properties are:
* Benzo(a)
anthracene and
chrysene (C
18H
12);
* Benzo(b)
fluoranthene,
* Benzo(j)fluoranthene,
* Benzo(k)fluoranthene,
*
Benzo(a)pyrene (C
20H
12),
* Benzo(ghi)
perylene*
Coronene,
* Dibenzo(a,h)anthracene (C
20H
14).
* Indeno(1,2,3-cd)
pyrene (C
22H
12),
*
Ovalene,
*
PhenanthreneIn January 2004 (at the 203rd Meeting of the
American Astronomical Society), it was reported (as cited in Battersby, 2004) 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, nebulae, by necessity, are where they originate.
*
Graphene*American Astronomical Society. (n.d.). Meeting program contents. Retrieved January 11, 2004 from http://www.aas.org/meetings/aas203/
*Battersby, S. (2004). Space molecules point to organic origins. Retrieved January 11, 2004 from http://www.newscientist.com/news/news.jsp?id=ns99994552
*
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
