Everything about Polycyclic Aromatic Hydrocarbon totally explained
Polycyclic aromatic hydrocarbons (
PAHs) are
chemical compounds that consist of fused
aromatic rings and don't contain
heteroatoms or carry
substituents . 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're of concern because some compounds have been identified as
carcinogenic,
mutagenic,and
teratogenic.
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 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
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're 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.
PAHs are one of the most widespread organic pollutants. In addition to their presence in fossil fuels (and thus fuel spills) they're also formed by incomplete
combustion of
carbon-containing fuels such as
wood,
coal,
diesel,
fat,
tobacco, or
incense.
(External Link
) Different types of combustion yield different distributions of PAHs in both relative amounts of individual PAHs and in which
isomers 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.
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,, 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.
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[ghi]
perylene,
coronene, dibenz[a,h]anthracene (C
20H
14), indeno[1,2,3-cd]
pyrene (C
22H
12) and
ovalene .
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're chemically related they're 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 don't 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're 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.
Naphthalene (C
10H
8 constituent of
mothballs), consisting of two coplanar six-membered rings sharing an edge, is another aromatic hydrocarbon. By formal convention, it isn't 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
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—for example benzene-like moieties—is the most important for the characterization of the properties .
»
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
In January 2004 (at the 203rd Meeting of the
American Astronomical Society ), it was reported 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, by necessity they must originate in nebulae.
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