Introduction to Aromatic Hydrocarbons

James Richard Fromm

When a double bond replaces one of the single carbon-carbon bonds in cyclohexane, the double bond behaves just as a single bond in hexene or any other alkene. Introduction of a second double bond on a non-adjacent carbon gives cyclohexadiene; the two double bonds behave much like double bonds when only one is present in the molecule. However, introduction of a third double bond in the ring structure to give cyclohexatriene produces a radical change in the electron distribution in the ring. Halogens and hydrogen no longer are capable of adding to it easily, nor is it easily oxidized. All carbon-carbon bonds are found to be identical in length and reactivity. All of the carbon-hydrogen bonds are identical as well. The carbon atoms now lie in a plane, where in cyclohexane and cyclohexene they do not. Thus the structural representations appropriate for cyclohexatriene do not accurately describe the nature of the cyclic C6H6 molecule.

As a consequence, the ring structure is called benzene rather than cyclohexatriene and is often represented by a circle rather than alternating bonds.

Benzene is the simplest of the aromatic hydrocarbons, which are those hydrocarbons in which one or more ring has aromatic character due to the rearrangement of the pi bonding electrons in what would otherwise be alternating single and double bonds. Aromatic character can arise whenever two equivalent canonical structures involving alternate single and double bonds can be drawn for an organic molecule, even if it is not a ring. The actual structure is sometimes called a resonance hybrid of the two equivalent canonical structures. Rings with aromatic character are remarkably stable to chemical attack.

Substituted benzenes can be formed by reagents such as chlorine. Chlorine adds directly across the double bond of cyclohexene to give 1,2-dichlorohexane, but chlorine reacts only under more extreme conditions with benzene and when it does so the products are chlorobenzene and hydrogen chloride.

While substituted benzenes can be named by the usual IUPAC substitution method, many of them are commonly known by trivial names. Methylbenzene, for example, is known as toluene, and the three possible dimethylbenzenes are known as the xylenes.

The denoting of adjacent groups on a ring as ortho, those of one carbon apart as meta, and those on opposite sides of the ring as para is a common, though non-systematic, method of designating positions of substituent groups on aromatic rings in more complex cyclic compounds. Other examples of trivial names for substituted benzenes are phenol (hydroxybenzene), aniline (aminobenzene or, more commonly, benzylamine), anisole (methoxybenzene), and phenetole (ethoxybenzene).

More complex cyclic hydrocarbons may involve many rings, some aromatic and some non-aromatic or aliphatic. In most cases the entire ring structure is given a particular non-systematic name. Two of the simplest are the two-ring and three-ring aromatic structures, which are called napthalene and anthracene. Multiple-ring structures are often called polynuclear aromatic hydrocarbons.

Substitution upon polynuclear hydrocarbons whether aromatic or aliphatic is described in the usual way, but the numbering of the carbons is not obvious in complex structures. Reference works giving the ring structure and indicating carbon number have been compiled and should be consulted in order to name the structures.

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Copyright 1997 James R. Fromm