Benzene

A number of the substituted benzene derivatives are well known and commercially important compounds. For example, the substitution of a single methyl, hydroxyl, or amino group in benzene results in the formation, respectively, of toluene (C₆H₅CH₃), phenol (C₆H₅OH), or aniline (C₆H₅NH₂). Probably the best known disubstituted products are the xylenes, C₆H₄(CH₃)₂. Three different xylene molecules are possible depending on whether the methyl groups are adjacent to each other on the benzene ring (ortho-xylene), separated by one carbon atom (meta-xylene), or opposite each other on the ring (paraxylene). The removal of one hydrogen atom from the benzene molecule results in a radical known as the phenyl group.

Benzene occurs so abundantly in and is obtained so easily from coal tar and petroleum that there is virtually no reason to make it synthetically. Although benzene had been recognized as a component of petroleum for many years, it was not produced commercially from that source until the beginning of World War II.