Polycyclic aromatic hydrocarbon

Polycyclic aromatic hydrocarbons (PAHs, also polyaromatic hydrocarbons or polynuclear aromatic hydrocarbons) are hydrocarbons—organic compounds containing only carbon and hydrogen—that are composed of multiple aromatic rings (organic rings in which the electrons are delocalized). The simplest such chemicals are naphthalene, having two aromatic rings, and the three-ring compounds anthracene and phenanthrene.AnthracenePhenanthrenePhenaleneTetraceneChryseneTriphenylenePyrenePentaceneBenzopyreneCorannuleneBenzoperyleneCoroneneOvaleneBenzofluorenePhenanthreneAnthraceneChrysene Polycyclic aromatic hydrocarbons (PAHs, also polyaromatic hydrocarbons or polynuclear aromatic hydrocarbons) are hydrocarbons—organic compounds containing only carbon and hydrogen—that are composed of multiple aromatic rings (organic rings in which the electrons are delocalized). The simplest such chemicals are naphthalene, having two aromatic rings, and the three-ring compounds anthracene and phenanthrene. PAHs are uncharged, non-polar molecules found in coal and in tar deposits. They are also produced by the thermal decomposition of organic matter (for example, in engines and incinerators or when biomass burns in forest fires). PAHs are abundant in the universe, and have recently been found to have formed possibly as early as the first couple of billion years after the Big Bang, in association with formation of new stars and exoplanets. Some studies suggest that PAHs account for a significant percentage of all carbon in the universe. Polycyclic aromatic hydrocarbons are discussed as possible starting materials for abiotic syntheses of materials required by the earliest forms of life. By definition, polycyclic aromatic hydrocarbons have multiple cycles, precluding benzene from being considered a PAH. Naphthalene is considered the simplest polycyclic aromatic hydrocarbon by the US EPA and US CDC for policy contexts. Other authors consider PAHs to start with the tricyclic species phenanthrene and anthracene. PAHs are not generally considered to contain heteroatoms or carry substituents. PAHs with five or six-membered rings are most common. Those composed only of six-membered rings are called alternant PAHs, which include benzenoid PAHs. The following are examples of PAHs that vary in the number and arrangement of their rings: PAHs are nonpolar and lipophilic. Larger PAHs are generally insoluble in water, although some smaller PAHs are soluble and known contaminants in drinking water. The larger members are also poorly soluble in organic solvents and in lipids. They are usually colorless. The aromaticity varies for PAHs. According to Clar's rule, the resonance structure of a PAH that has the largest number of disjoint aromatic pi sextets—i.e. benzene-like moieties—is the most important for the characterization of the properties of that PAH.