xylene [key], C6H4(CH3)2, colorless, oily, liquid aromatic hydrocarbon, used extensively as a solvent, obtained from coal tar, wood tar, and sometimes from petroleum. It is a mixture of three isomers that differ structurally from one another in the location of the two methyl groups that have replaced hydrogen atoms in the benzene molecule. Ortho-xylene is 1,2-dimethylbenzene; it melts at −25℃ and boils at 144℃. Meta-xylene is 1,3-dimethylbenzene; it melts at −48℃ and boils at 139℃. Para-xylene is 1,4-dimethylbenzene; it melts at 13℃ and boils at 138℃. The separation of these three isomers from one another by fractional distillation is difficult because their boiling points are so close together. The ortho and para isomers are converted to meta-xylene by treatment with aluminum trichloride and hydrochloric acid at about 80℃. The xylenes are often used in the synthesis of other compounds, e.g., the xylidenes that are amino derivatives used in the synthesis of azo dyes and other compounds.
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