Polycyclic aromatic hydrocarbon

A polycyclic aromatic hydrocarbon (PAH) is a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings and the three-ring compounds anthracene and phenanthrene. PAHs are uncharged, non-polar and planar. Many are colorless. Many of them are found in coal and in oil deposits, and are also produced by the incomplete combustion of organic matter—for example, in engines and incinerators or when biomass burns in forest fires. Polycyclic aromatic hydrocarbons are discussed as possible starting materials for abiotic syntheses of materials required by the earliest forms of life. The terms polyaromatic hydrocarbon or polynuclear aromatic hydrocarbon are also used for this concept. By definition, polycyclic aromatic hydrocarbons have multiple rings, precluding benzene from being considered a PAH. Some sources, such as the US EPA and CDC, consider naphthalene to be the simplest PAH. Other authors consider PAHs to start with the tricyclic species phenanthrene and anthracene. Most authors exclude compounds that include heteroatoms in the rings, or carry substituents. A polyaromatic hydrocarbon may have rings of various sizes, including some that are not aromatic. Those that have only six-membered rings are said to be alternant. The following are examples of PAHs that vary in the number and arrangement of their rings: File:Naphthalene-2D-Skeletal.svg | [[Naphthalene]] File:Biphenyl.svg | [[Biphenyl]] File:Fluorene.svg | [[Fluorene]] File:Anthracene.svg | [[Anthracene]] File:Phenanthrene.svg | [[Phenanthrene]] File:Phenalene.svg | [[Phenalene]] File:Naftacene.svg | [[Tetracene]] File:Chrysene.svg | [[Chrysene]] File:Triphenylene.svg | [[Triphenylene]] File:Pyrene.svg | [[Pyrene]] File:Pentacene.svg | [[Pentacene]] File:Perylene.svg | [[Perylene]]

File:Benzo-a-pyrene.svg | [[Benzo(a)pyrene|Benzo[''a'']pyrene]] File:Corannulene.svg | [[Corannulene]] File:Benzo(ghi)perilene.