Xylene

The term xylene refers to a group of three derived from the Benzène, corresponding to the Isomère S ortho-, méta- and para of dimethylbenzene. The technical xylene is a mixture of three isomers, of composition close to méta- (60%), ortho- (10-25%) and para (10-25%).

Structure and isomers

Just like for the Benzene, the structure of xylene is plane. It is a aromatic Composé, and the electron S forming the connections π cycle are delocalized, which involves an important stability of the structure.

The prefix O (xylene), p or m indicate to which Atome S of Carbone of the aromatic nucleus is fixed the groupings methyl S. One numbering the carbon atoms starting from an atom related to a grouping methyl, the compound is O-xylene (1,2-dimethylbenzene) if adjacent the carbon atom, numbered 2, is also related to a grouping methyl. If carbon numbered 3 is related to a grouping methyl, the compound is the m-xylene (1,3-dimethylbenzene). If carbon numbered 4 is related to a grouping methyl, the compound is the p-xylene (1,4-dimethylbenzene).

Physicochemical properties

Xylene is a colorless Liquide, of pleasant and very flammable odor. It is naturally present in the Pétrole and the coal tar, and is formed during the forest fires. The chemical properties differ little from a Isomère with the other. The Melting point lies between (- 47,87 °C (m-Xylene) and 13,26 °C (p-Xylene). The Boiling point is close to 140°C for all isomers. The Densité is of 0,87 (the compound is lighter than the Eau. The odor of xylene becomes detectable for concentration S of about 0,08 to 3,7 ppm, and the taste is apparent in water for concentrations of about 0,53 to 1,8 ppm.

Production and use

Xylene is produced starting from the Pétrole in petrochemical industry . In term of volume, it is one of the 30 chemical compounds most produced with the USA (approximately 450.000 tons per annum). It is used like Solvant, like by industries of the Impression, the rubber and the Cuir. It is employed like starting reagent for the production of Terephtalic acid, used like Monomère S for the production of Polymère S of the terephthalates type. Xylene is also used for cleaning, like Pesticide, thinner for the Peinture like in the varnished Peinture and the . It is present in small quantities in the Carburant S for the Aviation like in the gasoline. In the presence of reagents Oxidant S, like the Permanganate of potassium KMnO4, the grouping methyl can be oxidized until forming a Carboxylic acid . When the two groupings methyls are oxidized, O-xylene forms the Phtalic acid and p-xylene the Terephtalic acid .

Separation of isomers

The boiling points of three isomers being very close, it is not possible to separate them by Distillation. On the other hand, the products of Sulfonation can be separated by distillation. O-xylene can be separate of two other isomers by correction. Lastly, the melting points being sufficiently distinct, it is possible to carry out a separation of three isomers by split Cristallisation.

Effects on health

Xylene has a harmful effect on the Cerveau. Levels of exposures raised for even short periods can involve headaches, a defect of coordination of the Muscle S, giddinesses, the confusion and losses of the direction of balance. Exposures to high rates for short periods of time can also cause an irritation of the skin, eyes, nose and throat, difficulties of breathing, problems pulmonary, an increase in the reaction times, losses of memory, irritations of Estomac and deteriorations of the operation of the Foie and Rein S. Of the very high rates of exposure can involve the loss of conscience even death.

Studies on animals showed that concentration S of xylene high involve an increase in the number of dull animals, as well as delays of growth and development. In much of case, these same concentrations also have negative effects on health of the mothers. The effect of exposures of the mother to weak xylene concentrations on the Fœtus is not known at present.

Sentences of safety

  • R: 10 (Flammable)
  • R: 20/21 (Harmful by inhalation and contact with the skin)
  • R: 38 (Irritating for the skin)
  • S: 2 (To preserve out of the range of the children)
  • S: 25 (To avoid the contact with the eyes)

See too

References

  • Bewertung von Toluol und Xylol-Immissionen. Erich Schmidt Verlag, Berlin (2000), ISBN 3503040714 ( German )

External bonds

  • Card of safety of p-xylene

  • Card of safety of m-xylene
  • Card of safety of O-xylene

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