A polycyclic aromatic hydrocarbon (PAH) is any member of a class of organic compounds that is composed of multiple fused aromatic rings. Most are produced by the incomplete combustion of organic matter—by engine exhaust fumes, tobacco, incinerators, in roasted meats and cereals, or when biomass burns at lower temperatures as in forest fires. 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 also found in fossil fuel deposits such as coal and in petroleum. Exposure to PAHs can lead to different types of cancer, fetal development complications, and cardiovascular issues. 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 (abbreviated as PNA) are also used for this concept. By definition, polycyclic aromatic hydrocarbons have multiple aromatic rings, precluding benzene from being considered a PAH. Sources such as the US EPA and CDC consider naphthalene to be the simplest PAH. 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 sets of examples illustrate several of the types of variations that are possible.
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 low energy 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; 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. Benzo[c]phenanthrene, 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 dibenzo[c,g]phenanthrene 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.
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, such as perylene, are strongly colored.
Redox Polycyclic aromatic compounds characteristically yield radicals and anions upon treatment with alkali metals. The large PAH form dianions as well. The redox potential correlates with the size of the PAH.
Sources
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