• 05
  • 02
  • 03
  • 04
  • 05
  • 05
  • 05
  • 05
  • 05

A xylene (from Greek ξυλος, xylos, "wood") is an aromatic hydrocarbon consisting of a benzene ring with two methyl substituents. The three isomeric xylenes each have a molecular formula of C8H10, though the more informative semi-structural formula C6H4(CH3)2 is also used commonly. The xylenes are major petrochemicals, produced by catalytic reforming and also by coal carbonisation in the manufacture of coke fuel. Representing about 0.5–1% of crude oil (depending on the source), xylenes are found in small quantities in gasoline and airplane fuels. Xylenes are mainly produced as part of the BTX aromatics (benzene, toluene and xylenes) extracted from the product of catalytic reforming known as "reformate". The mixture is a slightly greasy, colourless liquid commonly encountered as a solvent. It was named in 1851, having been discovered as a constituent of wood tar. Several million tons are produced annually. In 2011, a global consortium began construction of one of the world’s largest xylene plants in Singapore.

erephthalic acid and related derivatives

p-Xylene is the principal precursor to terephthalic acid and dimethyl terephthalate, both monomers used in the production of polyethylene terephthalate (PET) plastic bottles and polyester clothing. 98% of p-xylene production, and half of all xylene, is consumed in this way.[7][8] o-Xylene is an important precursor to phthalic anhydride. The demand for isophthalic acid is relatively modest so m-xylene is rarely sought (and hence the utility of its conversion to the o- and p-isomers).

Solvent applications

Xylene is used as a solvent. In this application, the mixture of isomers is often referred to as xylenes or xylol. Solvent xylene often contains a small percentage of ethylbenzene. Like the individual isomers, the mixture is colorless, sweet-smelling, and highly flammable. Areas of application include printing, rubber, and leather industries. It is a common component of ink, rubber, adhesive,[9] and leather industries. In thinning paints and varnishes, it can be substituted for toluene where slower drying is desired, and thus is used by conservators of art objects in solubility testing.[10] Similarly it is a cleaning agent, e.g., for steel, silicon wafers, and integrated circuits. In dentistry, xylene can be used to dissolve gutta percha, a material used for endodontics (root canal treatments). In the petroleum industry, xylene is also a frequent component of paraffin solvents; used when the tubing becomes clogged with paraffin wax.

Laboratory uses

It is used in the laboratory to make baths with dry ice to cool reaction vessels, and as a solvent to remove synthetic immersion oil from the microscope objective in light microscopy.[11] In histology, xylene is the most widely used clearing agent.[12] Xylene is used to remove paraffin from dried microscope slides prior to staining. After staining, microscope slides are put in xylene prior to mounting with a coverslip.

Precursor to other compounds

Although conversion to terephthalic acid is the dominant chemical conversion, xylenes are precursors to other chemical compounds. For instance chlorination of both methyl groups gives the corresponding xylene dichlorides (bis(chloromethyl)benzenes) whilst monobromination yields xylyl bromide, a tear gas-agent used in World War I.