Xylene

The xylenes (from Greek ξύλον ( Xylon ) = " wood " ) (also Xylene or according to IUPAC nomenclature Dimethylbenzene ) are liquid organic chemical compounds with a characteristic aromatic odor and the general empirical formula C8H10. They are among the aromatic hydrocarbons and consist of a benzene ring with two methyl ( - CH3). 1,2- xylene (ortho- xylene), 1,3- xylene ( meta-xylene ) and 1,4- xylene ( para-xylene ) with different arrangement of the three methyl groups are constitutional isomers of the xylene yield. In the art (eg, solvent), they are mostly used as ( unseparated ) isomer mixture and put in the rule of 60% m -xylene, o -xylene 10-25% and 10-25 % p -xylene together. Xylene mixtures used as a solvent frequently also contain ethylbenzene boils in the same range of temperatures and has similar solubility characteristics.

Properties

Xylenes are colorless liquids, which are sparingly soluble in water (0.2 g / l). However, in ethers, alcohols, benzene and acetone exhibit good solubility. Aqueous solutions show already in the concentration range from 0.53 to 1.8 ppm a recognizable taste. Xylenes have typical aromatic odor; the odor threshold is 0.2 and 174 mg/m3. The boiling points of the three isomers are close together, while their melting points differ significantly. The p -xylene has, for example benzene or cyclohexane, due to its high symmetry a relatively high melting point. The density is 0.86 to 0.88 g/cm3; the xylenes 're all lighter than water. The flash point is (depending on the isomer) at about 30 ° C, the ignition temperature of 463-528 ° C. They burn with a bright and sooty flame.

Production and representation

Sources of raw materials for the production of xylenes are carbon ( from coal tar ) and petroleum ( by cracking ), with mixtures of isomers. As the isomers have similar boiling points, a separation by distillation is difficult. The separation of the o- isomer is carried out industrially by rectification, and then p-xylene can be separated by freezing due to its relatively high melting point. A large scale makes the separation of p-xylene with the aid of a molecular sieve adsorption with an important role.

Use

Xylenes are used as solvent use and used for the production of plastics and adhesives. Since the flash point of xylene is above 21 ° C, it is in practice next to one of the main butyl acetate solvent for paints. Further, they are added to increase the fuel octane number.

P- xylene is the starting material for the preparation of terephthalic acid ( > 1 million tons per year ) and o- xylene in similar quantities for the production of phthalic for the plastics industry. By nitration yields the Nitroxylole who serve by subsequent reduction to represent the xylidines.

In addition to the benzene and the toluene, the xylenes are among the industrially most important aromatic compounds, the so-called BTEX aromatics.

Reactions

The methylene groups (- CH 3) may be oxidized to carboxyl groups. Suitable oxidizing agents for the implementation of both methyl groups are, for example, potassium permanganate or chromic acid. o-xylene is converted to phthalic acid, isophthalic acid and m-xylene to p-xylene to terephthalic acid. o-xylene may be catalytically - such as vanadium (V) oxide - can be oxidized in the gas phase to phthalic anhydride. By oxygen (e.g., in the presence of cobalt stearate as catalyst) to give the corresponding monocarboxylic acids, the toluic ( methylbenzoic ). Also by selective oxidation of the Benzoldicarbaldehyde can be obtained. Xylenes are making a typical aromatic substitution reactions.

Hazards

Xylenes are flammable and act harmful if absorbed through the skin and the respiratory tract. They can cause headaches, for example, orientation and memory disturbances, dizziness, and shortness of breath. Xylenes are hazardous ( class 2 ). Between an air volume ratio of 1 to 8%, they form explosive mixtures. Emissions of xylenes are mainly due to motor vehicle traffic. In recent years, a decrease in Xylolemissionen is recorded.