Xylene

The term "xylene" refers to any of three benzene derivative isomers that share the same chemical formula, C₆H₄(CH₃)₂, but differ in their molecular structure. They are useful as solvents, as additives to improve the octane rating of aviation fuels, and as raw materials in the manufacture of fibers, films, dyes, and other synthetic products. Because of their high volatility they are classified as aromatic hydrocarbons (volatile compounds containing only carbon and hydrogen atoms). Xylene fractions were first isolated from coal tar in the mid-nineteenth century in Germany. Coal tar is the thick liquid product of the carbonization, or destructive distillation, of coal, a process in which coal is heated without air to temperatures above 1,600°F (862°C). Benzene and toluene , two aromatic hydrocarbons similar to xylene, are formed in the same process. Coal tar remained the source for xylene until industrial demand outgrew the supply. Later, techniques were developed to obtain xylene from petroleum refining, and today petroleum is a major source.

The three isomeric xylenes are classified by the arrangement of the methyl (CH₃) groups substituting for hydrogen atoms on the benzene ring. In each case, the methyl groups replace two of the hydrogens attached to the six-carbon benzene ring, but the structure and chemical properties of each isomer depend on which hydrogens are replaced. Ortho-xylene (o-xylene) has methyl groups on two adjacent carbons in the benzene ring structure, while in meta-xylene (m-xylene) the methyl-containing carbons are separated by a hydrogen-containing carbon. In para-xylene (p-xylene), two hydrogen-bearing carbons separate the methyl-containing carbons, placing the methyl groups at opposite ends of the molecule. Ortho-xylene, the least volatile of the three forms, serves as raw material for the manufacture of coatings and plastics . Para-xylene is used to manufacture polyesters. Meta-xylene, less useful than the other forms, is used to make coatings, plastics, and dyes. A commercial xylene mixture in the form of a colorless, flammable, non-viscous, toxic liquid is used as a solvent for lacquers and rubber cements. Emulsified xylene has been used as an economical and effective way of controlling aquatic weeds in irrigation systems, but concentrations must be carefully controlled to avoid harming plants and fish exposed to the residues.

Xylene's volatility leads to its easy escape into the atmosphere . Unless care is taken, its fumes soon permeate the air in laboratories where it is used. Although its solubility in water is quite limited, xylene (along with benzene and toluene) is classified as one of the primary components of the "water-soluble" fraction of petroleum. The widespread use of xylene in manufacturing has resulted in significant releases of the chemical into the air and water of the environment . Xylene's toxicity in water is influenced by salinity , temperature, and the presence of other toxic materials. It affects cell permeability and acts as a neurotoxin for fish and other animals. At high concentrations of xylene, fish exhibit a series of behavioral changes including restlessness (rapid and erratic swimming), loss of equilibrium, paralysis, and death. Although each of the three forms of xylene can be detoxified by microorganisms , para-xylene is more difficult to detoxify, and therefore a more persistent threat.

[Douglas C. Pratt ]

RESOURCES

BOOKS

Buikema, L., and A. C. Hendricks. Benzene, Xylene, and Toluene in Aquatic Systems: A Review. Washington, DC: American Petroleum Institute, 1980.

Hancock, E. G., ed. Toluene, the Xylenes, and Their Industrial Derivatives. Amsterdam; New York: Elsevier, 1982.

Parmeggiani, L., ed. Encyclopaedia of Occupational Health and Safety. 3rd ed. Geneva: International Labour Office, 1983.

Walsh, D. F., ed. Residues of Emulsified Xylene in Aquatic Weed Control and Their Impact on Rainbow Trout, Salmo gairdneri. Denver, CO: U.S. Dept. of the Interior [Bureau of Reclamation, Engineering and Research Center, Division of General Research, Applied Sciences Branch], 1977.

xylene (C₆H₄(CH₃)₂) Organic chemical compound obtained from the distillates of coal tar and petroleum, and important as a solvent. Chemically it is dimethyl benzene which exists in three isomeric forms: ortho-, meta-, and para-xylene. The isomers have different physical properties.

xy·lene / ˈzīˌlēn/ • n. Chem. a volatile liquid hydrocarbon, C₆H₄(CH₃)₂, obtained by distilling wood, coal tar, or petroleum, and used in fuels and solvents, and in chemical synthesis.

xylene (dimethylbenzene) (zy-leen) n. a liquid used for increasing the transparency of tissues prepared for microscopic examination after they have been dehydrated.

xylenes See dimethylbenzenes.