Xylenes
Xylenes
(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 | ||||
|---|---|---|---|---|
| Properties | o-xylene | m-xylene | p-xylene | |
| *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.