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Xylene (isomer)  (updated in 2010)

Name CAS no
(ortho-xylene) 95-47-6 
(meta-xylene) 108-38-3
(para-xylene) 106-42-3

CAS no 1330-20-7

Synonyms
Dimethylbenzene (IUPAC)
Xylol

Physical data
Melting point: ca -50░C
Boiling point: 135-145░C
Vapour pressure: 0.6-1.1 kPa =
5-8 mm Hg
Water solubility: <0.1%

Xylene is a colourless liquid whose smell becomes noticeable at 20-40 ppm (the current occupational exposure limit value is 50 ppm). It consists of three substances, the three isomers 1,2-dimethylbenzene or ortho-xylene, 1,3-dimethylbenzene or meta-xylene and 1,4-dimenthylbenzene or para-xylene.

The isomer mixture is produced by repeated extractions or distillations of light petroleum fractions. It consists of 4-20% ortho-xylene, 44-60% meta-xylene, 12-20% para-xylene and 17-23% ethylbenzene.

Just as with toluene, the overwhelming quantity of these aromatic hydrocarbons is used as motor fuel, due to gasoline containing about 10% xylenes.

In 2010, the capacity for production of pure xylene was 35,000,000 tonnes in Asia, the USA and the EU. The main part is used for further separation in the three isomers through distillation fractionated crystallisation. 82 % ends up in  p-xylene for the production of dimethylterephthalate and terephthalic acid, which is used for polyester, e.g. PET. 9 % ends up as ortho-xylene used for synthesis of phthalic acid anhydride to polymers and phthalates. 1 % becomes meta-xylene for isophthalic acid. All three isomers are raw materials for synthesis of special chemicals such as vitamins, pharmaceuticals, flavouring agents etc.

About 10 % of world production, 3,500,000 tonnes,  is estimated to be used as the isomer mixture xylene. It is used as a solvent in a number of products where good solvent capacity and relatively fast evaporation are needed, such as paints, degreasing agents, glues, sealing agents and carcare products.

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