An aromatic ring  is a hydrocarbon containing benzene or some other related ring structure. Benzene (C 6 H 6) is usually drawn as a ring of six carbon atoms with alternating double and single bonds.


However, there is some complexity to this simple picture. The carbon-carbon single bond is longer than the carbon-carbon double bond, so if there were "real" single and double bonds in the molecule, the shape of the benzene molecule would be a twisted hexagon.

What is actually found is that all the bonds in the benzene ring are 1.397 ?, which is roughly between the typical length of a single bond (~1.5 ?) and a double bond (~1.3 ?). All carbon atoms in the benzene ring are sp 2 hybridized: the overlap of sp 2 orbitals around the ring creates a framework of six σ bonds, while unhybridized p orbitals perpendicular to this plane overlap on one side to-side fashion to form three pi bonds. These pi bonds are delocalized around the ring, resulting in unusual stability of the benzene ring compared to other olefins.

In place of the three "real" single bonds and three "real" double bonds in the ring, benzene can be thought of as consisting of "six one-and-a-half" bonds, represented by the resonance structure shown below.




Thus, all positions on the benzene ring are equivalent; benzene is sometimes represented as a hexagon with a circle inside to emphasize this equivalence.

Since the atoms on the ring are all sp 2 hybridized, they are all triangular planes with bond angles of 120 ?, and benzene rings are flat molecules, shaped like hexagons.

Aromatic hydrocarbons are non-polar and insoluble in water. However, when substituted with other atoms on the benzene ring, they may be very soluble in water. For example, phenol with an -OH group on the benzene ring is very soluble in water.

Because of the stability conferred by the leaving domain pi electrons in the ring, aromatic molecules do not undergo many reactions typical of olefins, but they do undergo many interesting and useful substitution reactions in which hydrogen atoms are substituted in the ring by other functional groups.

Molecules that do not contain benzene rings are called aliphatic ("fatty-like") compounds.

Aromatic molecules with simple alkyl substituents are called benzene derivatives. For example, benzene with an ethyl group on one carbon in the ring is simply called "ethylbenzene.

Many aromatic molecules are known by common names. For example, benzene with a -CH 3 group is called "toluene"; benzene with a -NH 2 group is called "aniline"; benzene with a -CO 2 H group is called "benzoic acid", etc.

For more complex substituents, the benzene ring is referred to as a substituent, in this case, "phenyl". For example, an eight-carbon chain with a benzene ring at the third carbon is called "3-phenylacetate".

When there are two substituents on the benzene ring, the position of the substituents can be identified numerically, but usually, an older naming system is used in which the prefixes o-, m- and p- (often abbreviated as o-, m- and p-) are used to indicate the relative positions of the substituents.