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)

Chemistry

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 (C₁₀H₈), 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.

Examples

• Anthracene
• Benzo(a)pyrene
• Benzo(ghi)pyrene
• Chrysene
• Coronene
• Fluoranthene
• Naphthacene (Tetracene)
• Naphthalene
• Phenanthrene
• Pyrene
• Triphenylene

• Benzo(a)anthracene and chrysene (C₁₈H₁₂);
• Benzo(b)fluoranthene,
• Benzo(j)fluoranthene,
• Benzo(k)fluoranthene,
• Benzo(a)pyrene (C₂₀H₁₂),
• Benzo(ghi)perylene
• Coronene,
• Dibenzo(a,h)anthracene (C₂₀H₁₄).
• Indeno(1,2,3-cd)pyrene (C₂₂H₁₂),
• Ovalene,
• Phenanthrene

PAHs and the origins of life

In 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.

External Links

• [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

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