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Aromatic hydrocarbon



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 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.

Table of contents
1 Models of benzene ring electron configurations
2 Properties of aromatic hydrocarbons
3 Members of this group of substances
4 See also
5 External links
6 References

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. But at any time, there's no way to know which bonds are single and which are double. 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

  1. They have closed conjugation.
  2. The Carbon Atoms are sp2 Hybrid,and have a planar structure.
  3. The Carbon-Hydrogen ratio is very small.
  4. They burn with a yellow sooty flame because of the low carbon-hydrogen ratio.
  5. 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 strenth 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 PAH (Polycyclic Aromatic Hydrocarbons); they are also called Polynuclear Aromatic Hydrocarbons. They are composed of more than one aromatic ring. The simplest polycyclic aromatic hydrocarbon is pentalene.

Naphthalene, consisting of two coplanar six membered rings sharing an edge, C10H8 is another aromatic hydrocarbon. Its smell is familiar to those who have encountered mothballs.

Phenol is a common polycyclic arene too.

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 2004 (at the 203rd Meeting of the American Astronomical Society in January 2004) (American Astronomical Society, n.d.), 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.

See also

External links