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Hydrocarbons are divided into aliphatic and aromatic types. Toluene and xylene are aromatic hydrocarbons derived from benzene. Toluene has one methyl group, while xylene has two, with three isomers. They are used as solvents and in the production of explosives, preservatives, and specialty chemicals.
Hydrocarbons are composed only of carbon and hydrogen and can be divided into two types: aliphatic hydrocarbons and aromatic hydrocarbons. The latter possess multiple bonds plus a special type of stabilization called aromaticity, which changes the way they interact chemically. Toluene and xylene are aromatic hydrocarbons, derivatives of benzene, with its chemical formula C6H6. The skeletal carbon atoms in benzene form what looks like a Ferris wheel, with the six hydrogen atoms corresponding to automobiles. Benzene is transformed into toluene and xylene by substitution of the hydrogen atoms with methyl groups (CH3-).
Toluene and xylene differ in their degree of methyl substitution. Only one hydrogen atom is replaced by one methyl in toluene, which has the chemical formula C6H5CH3. Although there are six carbon atoms in a benzene ring, they are all chemically the same, as the symmetry suggests. For this reason, it doesn’t matter which carbon the methyl group attaches to, as the result is the same in each case. There is no need to number the carbon atoms of the toluene ring, as each toluene molecule is identical, which simplifies processing, as 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 one through six. Starting at 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 has outstanding solvent properties for many applications. Unfortunately, benzene impairs the body’s ability to fight infection and has been closely linked to the development of leukemia in workers. Toluene and xylene are particularly important as substitute solvents for benzene. These substances are not only used as solvents; as the familiar letters “TNT” indicate, toluene is used in the manufacture of the powerful explosive, trinitrotoluene. Another useful derivative of toluene is produced by simple oxidation of its methyl group, resulting in benzoic acid, a commercially important preservative.
The xylenes can be used together, as “xylene” or “mixed xylene”, or they can be used individually. As with benzene and toluene, xylenes are used as solvents. In summary, the ortho- and para-isomers of xylene can have their methyl side chains oxidized to form the important dicarboxylic acids, terephthalic acid and orthophthalic acid. Phthalic acids are used in the production of polyesters, alkyd resins and plasticizers. They are also used in the production of specialty chemicals such as anthraquinone and phenolphthalein.
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