polycyclic aromatic hydrocarbon 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 combustion of organic matter—for example, in engines and incinerators or when biomass burns in forest fires. three representations of hexabenzocoronene, a polycyclic aromatic hydrocarbon. top: standard line-angle schematic, where carbon atoms are represented by the vertices of the hexagons and hydrogen atoms are inferred. middle: ball-and-stick model showing all carbon and hydrogen atoms. bottom: atomic force microscopy image. polycyclic aromatic hydrocarbons are discussed as possible starting materials for abiotic syntheses of materials required by the earliest forms of life. nomenclature and structure 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: examples of polycyclic aromatic hydrocarbons naphthalene biphenyl fluorene anthracene phenanthrene phenalene tetracene chrysene triphenylene pyrene pentacene perylene benzopyrene corannulene benzoperylene coronene ovalene benzofluorene geometry most pahs, like naphthalene, anthracene, and coronene, are planar. this geometry is a consequence of the fact that the σ-bonds that result from the merger of sp2 hybrid orbitals of adjacent carbons lie on the same plane as the carbon atom. those compounds are achiral, since the plane of the molecule is a symmetry plane. in rare cases, pahs are not planar. in some cases, the non-planarity may be forced by the topology of the molecule and the stiffness (in length and angle) of the carbon-carbon bonds. for example, unlike coronene, corannulene adopts a bowl shape in order to reduce the bond stress. the two possible configurations, concave and convex, are separated by a relatively lowenergy barrier (about 11 kcal/mol) in theory, there are 51 structural isomers of coronene that have six fused benzene rings in a cyclic sequence, with two edge carbons shared between successive rings. all of them must be non-planar and have considerable higher bonding energy (computed to be at least 130 kcal/mol) than coronene; and, as of 2002, none of them had been synthesized. other pahs that might seem to be planar, considering only the carbon skeleton, may be distorted by repulsion or steric hindrance between the hydrogen atoms in their periphery. benzophenantrene, with four rings fused in a "c" shape, has a slight helical distortion due to repulsion between the closest pair of hydrogen atoms in the two extremal rings. this effect also causes distortion of picene. adding another benzene ring to form dibenzophenantrene creates steric hindrance between the two extreme hydrogen atoms. adding two more rings on the same sense yields heptahelicene in which the two extreme rings overlap. these non-planar forms are chiral, and their enantiomers can be isolated. benzenoid hydrocarbons the benzenoid hydrocarbons have been defined as condensed polycyclic unsaturated fully-conjugated hydrocarbons whose molecules are essentially planar with all rings six-membered. full conjugation means that all carbon atoms and carbon-carbon bonds must have the sp2 structure of benzene. this class is largely a subset of the alternant pahs, but is considered to include unstable or hypothetical compounds like triangulene or heptacene. as of 2012, over 300 benzenoid hydrocarbons had been isolated and characterized. bonding and aromaticity 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. benzene-substructure resonance analysis for clar's rule phenanthrene anthracene chrysene for example, phenanthrene has two clar structures: one with just one aromatic sextet (the middle ring), and the other with two (the first and third rings). the latter case is therefore the more characteristic electronic nature of the two. therefore, in this molecule the outer rings have greater aromatic character whereas the central ring is less aromatic and therefore more reactive. in contrast, in anthracene the resonance structures have one sextet each, which can be at any of the three rings, and the aromaticity spreads out more evenly across the whole molecule. this difference in number of sextets is reflected in the differing ultraviolet–visible spectra of these two isomers, as higher clar pi-sextets are associated with larger homo-lumo gaps; the highest-wavelength absorbance of phenanthrene is at 293 nm, while anthracene is at 374 nm. three clar structures with two sextets each are present in the four-ring chrysene structure: one having sextets in the first and third rings, one in the second and fourth rings, and one in the first and fourth rings. superposition of these structures reveals that the aromaticity in the outer rings is greater (each has a sextet in two of the three clar structures) compared to the inner rings (each has a sextet in only one of the three). properties physicochemical pahs are nonpolar and lipophilic. larger pahs are generally insoluble in water, although some smaller pahs are soluble. the larger members are also poorly soluble in organic solvents and in lipids. the larger members, e.g. perylene, are strongly colored. redox polycyclic aromatic compounds characteristically yield radical anions upon treatment with alkali metals. the large pah form dianions as well. the redox potential correlates with the size of the pah. half-cell potential of aromatic compounds against the sce (fc+/0) compound potential (v) benzene −3.42 biphenyl −2.60 (-3.18) naphthalene −2.51 (-3.1) anthracene −1.96 (-2.5) phenanthrene −2.46 perylene −1.67 (-2.2) pentacene −1.35 sources fossil carbon polycyclic aromatic hydrocarbons are primarily found in natural sources such as bitumen. pahs can also be produced geologically when organic sediments are chemically transformed into fossil fuels such as oil and coal. the rare minerals idrialite, curtisite, and carpathite consist almost entirely of pahs that originated from such sediments, that were extracted, processed, separated, and deposited by very hot fluids. natural fires pahs may result from the incomplete combustion of organic matter in natural wildfires. substantially higher outdoor air, soil, and water concentrations of pahs have been measured in asia, africa, and latin america than in europe, australia, the u.s., and canada. high levels of such pahs have been detected in the cretaceous-tertiary (k-t) boundary, more than 100 times the level in adjacent layers. the spike was attributed to massive fires that consumed about 20% of the terrestrial above-ground biomass in a very short time. extraterrestrial pahs are prevalent in the interstellar medium (ism) of galaxies in both the nearby and distant universe and make up a dominant emission mechanism in the mid-infrared wavelength range, containing as much as 10% of the total integrated infrared luminosity of galaxies. pahs generally trace regions of cold molecular gas, which are optimum environments for the formation of stars. nasa's spitzer space telescope and james webb space telescope include instruments for obtaining both images and spectra of light emitted by pahs associated with star formation. these images can trace the surface of star-forming clouds in our own galaxy or identify star forming galaxies in the distant universe. in june 2013, pahs were detected in the upper atmosphere of titan, the largest moon of the planet saturn. minor sources volcanic eruptions may emit pahs. certain pahs such as perylene can also be generated in anaerobic sediments from existing organic material, although it remains undetermined whether abiotic or microbial processes drive their production. artificial the dominant sources of pahs in the environment are thus from human activity: wood-burning and combustion of other biofuels such as dung or crop residues contribute more than half of annual global pah emissions, particularly due to biofuel use in india and china. as of 2004, industrial processes and the extraction and use of fossil fuels made up slightly more than one quarter of global pah emissions, dominating outputs in industrial countries such as the united states. a year-long sampling campaign in athens, greece found a third (31%) of pah urban air pollution to be caused by wood-burning, like diesel and oil (33%) and gasoline (29%). it also found that wood-burning is responsible for nearly half (43%) of annual pah cancer-risk (carcinogenic potential) compared to the other sources and that wintertime pah levels were 7 times higher than in other seasons, especially if atmospheric dispersion is low. lower-temperature combustion, such as tobacco smoking or wood-burning, tends to generate low molecular weight pahs, whereas high-temperature industrial processes typically generate pahs with higher molecular weights. pahs are typically found as complex mixtures. distribution in the environment aquatic environments most pahs are insoluble in water, which limits their mobility in the environment, although pahs sorb to fine-grained organic-rich sediments. aqueous solubility of pahs decreases approximately logarithmically as molecular mass increases. two-ringed pahs, and to a lesser extent three-ringed pahs, dissolve in water, making them more available for biological uptake and degradation. further, two- to four-ringed pahs volatilize sufficiently to appear in the atmosphere predominantly in gaseous form, although the physical state of four-ring pahs can depend on temperature. in contrast, compounds with five or more rings have low solubility in water and low volatility; they are therefore predominantly in solid state, bound to particulate air pollution, soils, or sediments. in solid state, these compounds are less accessible for biological uptake or degradation, increasing their persistence in the environment. human exposure human exposure varies across the globe and depends on factors such as smoking rates, fuel types in cooking, and pollution controls on power plants, industrial processes, and vehicles. developed countries with stricter air and water pollution controls, cleaner sources of cooking (i.e., gas and electricity vs. coal or biofuels), and prohibitions of public smoking tend to have lower levels of pah exposure, while developing and undeveloped countries tend to have higher levels. surgical smoke plumes have been proven to contain pahs in several independent research studies. a wood-burning open-air cooking stove. smoke from solid fuels like wood is a large source of pahs globally. burning solid fuels such as coal and biofuels in the home for cooking and heating is a dominant global source of pah emissions that in developing countries leads to high levels of exposure to indoor particulate air pollution containing pahs, particularly for women and children who spend more time in the home or cooking. in industrial countries, people who smoke tobacco products, or who are exposed to second-hand smoke, are among the most highly exposed groups; tobacco smoke contributes to 90% of indoor pah levels in the homes of smokers. for the general population in developed countries, the diet is otherwise the dominant source of pah exposure, particularly from smoking or grilling meat or consuming pahs deposited on plant foods, especially broad-leafed vegetables, during growth. pahs are typically at low concentrations in drinking water. smog in cairo. particulate air pollution, including smog, is a substantial cause of human exposure to pahs. emissions from vehicles such as cars and trucks can be a substantial outdoor source of pahs in particulate air pollution. geographically, major roadways are thus sources of pahs, which may distribute in the atmosphere or deposit nearby. catalytic converters are estimated to reduce pah emissions from gasoline-fired vehicles by 25-fold. people can also be occupationally exposed during work that involves fossil fuels or their derivatives, wood-burning, carbon electrodes, or exposure to diesel exhaust. industrial activity that can produce and distribute pahs includes aluminum, iron, and steel manufacturing; coal gasification, tar distillation, shale oil extraction; production of coke, creosote, carbon black, and calcium carbide; road paving and asphalt manufacturing; rubber tire production; manufacturing or use of metal working fluids; and activity of coal or natural gas power stations. environmental pollution and degradation crude oil on a beach after a 2007 oil spill in korea. pahs typically disperse from urban and suburban non-point sources through road runoff,