All compounds that contain a benzene ring are aromatic, and substituted derivatives of benzene make up the largest class of aromatic compounds. Many such compounds are named by attaching the name of the substituent as a prefix to benzene.
Many simple monosubstituted derivatives of benzene have common names of long stand- ing that have been retained in the IUPAC system. Dimethyl derivatives of benzene are called xylenes. There are three xylene isomers, the ortho (o)-, meta (m)-, and para (p)- substituted derivatives.
The prefix ortho signifies a 1,2-disubstituted benzene ring, meta signifies 1,3-disubstitu- tion, and para signifies 1,4-disubstitution. The prefixes o, m, and p can be used when a substance is named as a benzene derivative or when a specific base name (such as ace- tophenone) is used. For example,
The o, m, and p prefixes are not used when three or more substituents are present on benzene; numerical locants must be used instead.
In these examples the base name of the benzene derivative determines the carbon at which numbering begins: anisole has its methoxy group at C-l, toluene its methyl group at C-l, and aniline its amino group at C-1. The direction of numbering is chosen to give the next substituted position the lowest number irrespective of what substituent it bears. The order of appearance of substituents in the name is alphabetical. When no simple base name other than benzene is appropriate, positions are numbered so as to give the lowest locant at the first point of difference. Thus, each of the following examples is named as a 1,2,4-trisubstituted derivative of benzene rather than as a 1,3,4-derivative:
When the benzene ring is named as a substituent, the word phenyl stands for C6H5–. Similarly, an arene named as a substituent is called an aryl group. A benzyl group is C6H5CH2-.
Biphenyl is the accepted IUPAC name for the compound in which two benzene rings are connected by a single bond.
Members of a class of arenes called polycyclic aromatic hydrocarbons possess substantial resonance energies because each is a collection of benzene rings fused together. Naphthalene, anthracene, and phenanthrene are the three simplest members of this class. They are all present in coal tar, a mixture of organic substances formed when coal is converted to coke by heating at high temperatures (about 1000 C) in the absence of air. Naphthalene is bicyclic (has two rings), and its two benzene rings share a common side. Anthracene and phenanthrene are both tricyclic aromatic hydrocarbons. Anthracene has three rings fused in a “linear” fashion; an “angular” fusion characterizes phenan- threne. The structural formulas of naphthalene, anthracene, and phenanthrene are shown along with the numbering system used to name their substituted derivatives:
In general, the most stable resonance structure for a polycyclic aromatic hydro- carbon is the one with the greatest number of rings that correspond to Kekule formulations of benzene. Naphthalene provides a fairly typical example:
Notice that anthracene cannot be represented by any single Lewis structure in which all three rings correspond to Kekule formulations of benzene, but phenanthrene can.
A large number of polycyclic aromatic hydrocarbons are known. Many have been synthesized in the laboratory, and several of the others are products of com- bustion. Benzo[a]pyrene, for example, is present in tobacco smoke, contaminates food cooked on barbecue grills, and collects in the soot of chimneys. Benzo[a]pyrene is a carcinogen (a cancer-causing substance). It is converted in the liver to an epoxy diol that can induce mutations leading to the uncontrolled growth of certain cells.