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any of three isomeric dimethylbenzenes [which have the same chemical formula, C6H4(CH3)2, but different molecular structure], used as solvents, as components of aviation fuel, and as raw materials for the manufacture of dyes, fibres, and films. The three isomers, designated ortho (o), meta (m), and para (p), differ structurally only in the location of the methyl groups. All three are obtained from coal-tar distillate and petroleum as a mixture containing 50-60 percent by volume of m-xylene and 20-25 percent of each of the other isomers. Fractional distillation of the mixture removes the meta and para isomers, which have very similar boiling points, from the less volatile ortho isomer. Upon cooling the mixture of meta and para isomers, much of the p-xylene crystallizes in nearly pure form. The meta isomer, the principal component of the remaining liquid, then can be purified by taking advantage of its solubility in a mixture of hydrofluoric acid and boron trifluoride. Meta- and para-xylene undergo nitration and reduction to give xylidines, used in making dyes. The meta isomer also is converted to trinitro-t-butyl-m-xylene, or xylene musk, a component of perfumes. Oxidation of the xylenes gives monocarboxylic acids (toluic acids), and then dicarboxylic acids (phthalic acids).