1911 Encyclopædia Britannica/Xylene

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XYLENE, or Dimethyl Benzene, C6H4(CH3)2. Three isomeric hydrocarbons of this formula exist; they occur in the light oil fraction of the coal tar distillate, but they cannot be separated by fractional distillation owing to the closeness of their boiling points. The mixture can be separated by shaking with sulphuric acid, whereupon the ortho and meta forms are converted into soluble sulphonic acids, the para form being soluble only in concentrated acid; the ortho and meta acids may be separated by crystallization of their salts or sulphonamides. Ortho-xylene is obtained from ortho-bromtoluene, methyl iodide and sodium as a colourless mobile liquid boiling at 142°, melting at -28°, and having a specific gravity of 0.8932 at 0°. Oxidation by potassium permanganate gives phthalic acid; whilst chromic acid gives carbon dioxide and water. Meta- or iso-xylene, the most important isomer, may be prepared by nucleus-synthetic reactions, or by distilling mesitylenic acid, C6H3(CH3)2CO2H, an oxidation product of mesitylene, C6H3(CH3)3, which is produced on the condensation of acetone, with lime; this reaction is very important, for it orientates meta-compounds. It boils at 139°, melts at -54°, and has a specific gravity of 0.8812. Para-xylene is obtained when camphor is distilled with zinc chloride, but it is best prepared from para-brom-toluene or dibrombenzene, methyl iodide and sodium. Dilute nitric acid oxidizes it first to para-toluic acid and then to terephthalic acid. It boils at 138°, melts at 15°, and has a specific gravity of 0.8801 at 0°.