Hydrocarbons are compounds of carbon and hydrogen only and may be divided into two types: aliphatic hydrocarbons and aromatic hydrocarbons. The latter possess multiple bonding plus a special kind of stabilization called aromaticity, which modifies the way they chemically interact. Toluene and xylene are aromatic hydrocarbons, derivatives of benzene, with its chemical formula C6H6. Skeletal carbon atoms in benzene form what resembles a Ferris wheel, with the six hydrogen atoms corresponding to the cars. Benzene changes into toluene and xylene through substitution of hydrogen atoms by methyl groups (CH3-).
Toluene and xylene differ in their degree of methyl-substitution. Only one hydrogen atom is replaced by a methyl in toluene, which has the chemical formula C6H5CH3. Although there are six carbon atoms in a benzene ring, they are all equal chemically, as symmetry suggests. For that reason, it doesn’t matter to which carbon the methyl group attaches, as the result is the same in each instance. There is no need to number the ring carbon atoms of toluene, since every molecule of toluene is identical, which simplifies processing, since there are no isomers to separate.
Xylene is di-substituted benzene, having two methyl groups replacing two hydrogen atoms. There are actually three different types or isomers of xylene. Atoms in the benzene ring portion of the structure are labeled as one to six. Beginning with the top ring carbon and going clockwise, the three varieties are ortho-xylene, or 1, 2-dimethylbenzene; meta-xylene, or 1, 3-dimethylbenzene; and para-xylene, or 1,4-dimethyl benzene. The benzene ring portion of each structure is written C6H4.
Benzene possesses outstanding solvent properties for many applications. Unfortunately, benzene damages the body’s ability to fight off infection and has been closely linked to the development of leukemia in workers. Toluene and xylene are especially important as replacement solvents for benzene. These substances are not only used as solvents; as the well-known letters "TNT" indicate, toluene is used in the manufacture of the powerful explosive, trinitrotoluene. Another useful toluene derivative is produced by simple oxidation of its methyl group, resulting in benzoic acid, a commercially important preservative.
The xylenes may be used together, as “xylol” or “mixed xylenes,” or they may be used individually. As in the case of benzene and toluene, xylenes are used as solvents. In synthesis, the ortho- and para-isomers of xylene may have their methyl side-chains oxidized to form the important dicarboxylic acids, terephthalic acid and orthophthalic acid. Phthalic acids are used in the manufacture of polyesters, alkyd resins and plasticizers. They are also used in manufacturing specialty chemicals such as anthraquinone and phenolphthalein.