An aromatic hydrocarbon (abbreviated as AH), or arene is a hydrocarbon, the molecular structure of which incorporates one or more planar sets of six carbon atoms that are connected by delocalised electrons numbering the same as if they consisted of alternating single and double covalent bonds. After the simplest possible aromatic hydrocarbon, benzene, such a configuration of six carbon atoms is known as a benzene ring. Aromatic hydrocarbons were so named due to their generally intense smell, long before their molecular structure was understood.
Models of benzene ring electron configurations
Each carbon atom in the hexagonal cycle has four electrons to share. One goes to the hydrogen atom, and one each to the two neighboring carbons. This leaves one to share with one of its two neighboring carbon atoms, which is why the benzene molecule is drawn with alternating single and double bonds around the hexagon.
Many chemists just draw a circle around the inside of the ring to show that there are six electrons floating around. The electrons float above and below the ring, and the electromagnetic fields they generate keep the ring flat.
In modern terminology, benzene rings can be described as compounds in which a continuous, closed system of rings contains separate sets of sigma and pi electrons. The atomic orbitals forming the sigma system are sp2 hybridized, and those forming the pi system are pure p orbitals.
Erich Hückel's "4n+2" rule can be used to predict aromaticity by the count of delocalized (pi) electrons—if it equals (4n+2), where n is a non-negative integer, then the molecule is likely aromatic.
Properties of aromatic hydrocarbons
- They have close conjugation.
- The Carbon atoms are sp2 hybridized, and therefore have a trigonal planar structure.
- The Carbon-Hydrogen ratio is very large.
- They burn with a sooty yellow flame because of the high carbon-hydrogen ratio.
- They undergo electrophilic substitution reactions, unlike aliphatic compounds which undergo nucleophilic substitution reactions.
Members of this group of substances
Aromatic hydrocarbons can be monocyclic or polycyclic.
Benzene, C6H6, is the simplest AH and was recognized as the first aromatic hydrocarbon, with the nature of its bonding first being recognized by Friedrich August Kekulé von Stradonitz in the 19th century. He envisioned the delocalised electons rapidly shifting configuration between one of two forms, or resonance structures, in which double bonds rapidly move about the hexagonal ring. However, the total strength of the aromatic bonds involved in aromaticity is stronger than the total strength of the bonds when formulated as a combination of single and double bonds. Aromatic bonding, therefore, must be recognized as a type of bonding distinct from other types of multiple bonding, such as double or triple bonds. This can be described more quantitatively with molecular orbital theory.
Some important arenes are the polycyclic aromatic hydrocarbons (PAH); they are also called Polynuclear Aromatic Hydrocarbons. They are composed of more than one aromatic ring. The simplest polycyclic aromatic hydrocarbon is pentalene.
PAHs of three rings or more have low solubilities in water and a low vapor pressure. As molecular weight increases, solubility decreases and vapor pressure decreases. 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.
Other PAHs are anthracene, fluoranthene, chrysene, pyrene, coronene and ovalene.
Famous PAHs known for their carcinogenic and teratogenic properties are benz(a)anthracene and Chrysene (C18H12); Benzo(b)fluoranthene, Benzo(j)fluoranthene, Benzo(k)fluoranthene and Benzo(a)pyrene (C20H12); Indeno(1,2,3-cd)pyrene (C22H12); and Dibenz(a,h)anthracene (C20H14).
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.