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(dimethylbenzenes). Three isomers (ortho-xylene, meta-xylene, and para-xylene) are known. All are colorless liquids that are readily soluble in organic solvents and poorly soluble in water. (See Table 1 for some physical properties of xylenes.) Their structural formulas are
Xylenes form highly explosive vapor-air mixtures. They cause acute and chronic damage to the blood-forming organs, as well as dermatitis upon contact with skin. Oxidation of xylenes yields toluic and phthalic acids. Xylenes are obtained from the products formed as a result of the coking of coal or during petroleum aromatization. The isomers are separated by fractional
|Table 1. Some properties of xylenes|
|*1 cp (centipoise) = 10−3 newtons • sec/m2|
|Melting point, °C ...........||−25.18||B47.87||13.26|
|Boiling point, °C............||144.41||139.10||138.35|
|Density at 20°C, g/cm3.........||0.8802||0.8642||0.8611|
|Refractive index np20 .....||1.5054||1.4972||1.4958|
|Viscosity at 20°C, cp* ........||0.809||0.617||0.644|
crystallization and rectification. Xylenes are used as solvents, high-octane additives to aviation gasoline, and the initial material in the preparation of xylidines. Of greatest importance is ρ-xylene, which is used in the manufacture of terephthalic acid.