:class=skin-invert-image|thumb|400px|The three xylene isomers: [[o-Xylene|o-xylene, m-xylene, and p-xylene]]

In organic chemistry, xylene or xylol (; IUPAC name: dimethylbenzene) is any of three organic compounds with the formula . They are derived from the substitution of two hydrogen atoms with methyl groups in a benzene ring; which hydrogens are substituted determines which of three structural isomers results. It is a colorless, flammable, slightly greasy liquid of great industrial value.

Occurrence and production

Xylenes are an important petrochemical produced by catalytic reforming and also by coal carbonisation in the manufacture of coke fuel. They also occur in crude oil in concentrations of about 0.5–1%, depending on the source. Small quantities occur in gasoline and aircraft fuels.

Xylenes are produced mainly as part of the BTX aromatics (benzene, toluene, and xylenes) extracted from the product of catalytic reforming known as reformate.

Several million tons are produced annually. In 2011, a global consortium began construction of one of the world's largest xylene plants in Singapore.

History

Xylene was first isolated and named in 1850 by the French chemist Auguste Cahours (1813–1891), having been discovered as a constituent of wood tar.

Industrial production

Xylenes are produced by the methylation of toluene and benzene. Commercial or laboratory-grade xylene produced usually contains about 40–65% of m-xylene and up to 20% each of o-xylene, p-xylene and ethylbenzene. The ratio of isomers can be shifted to favor the highly valued p-xylene via the patented UOP-Isomar process or by transalkylation of xylene with itself or trimethylbenzene. These conversions are catalyzed by zeolites. The complexes of different isomers often have dramatically different properties from each other.

Applications

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 xylenes produced is consumed in this manner. 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 and industrial purposes

Xylenes are used as a solvent in printing, rubber, and leather industries. It is a common component of ink, rubber, and adhesives. 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. 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 use

Xylene 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. In histology, xylene is the most widely used clearing agent. 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

In one large-scale application, para-xylene is converted to terephthalic acid. The major application of ortho-xylene is as a precursor to phthalate esters, used as plasticizer. Meta-xylene is converted to isophthalic acid derivatives, which are components of alkyd resins.

The side effects of exposure to low concentrations of xylene () are reversible and do not cause permanent damage. Long-term exposure may lead to headaches, irritability, depression, insomnia, agitation, extreme tiredness, tremors, hearing loss, impaired concentration and short-term memory loss. A condition called chronic solvent-induced encephalopathy, commonly known as "organic-solvent syndrome" has been associated with xylene exposure. There is very little information available that isolates xylene from other solvent exposures in the examination of these effects. as well as clinical studies.

Xylene is also a skin irritant and strips the skin of its oils, making it more permeable to other chemicals. The use of impervious gloves and masks, along with respirators where appropriate, is recommended to avoid occupational health issues from xylene exposure. The presence of methylhippuric acid can be used as a biomarker to determine exposure to xylene.

See also

References

  • NIOSH Pocket Guide to Chemical Hazards (o-Xylene)
  • NIOSH Pocket Guide to Chemical Hazards (m-Xylene)
  • NIOSH Pocket Guide to Chemical Hazards (p-Xylene)
  • Xylene, Hazard Summary (EPA) (Mixed Isomers)
  • The Ear Poisons , The Synergist, American Industrial Hygiene Association, November, 2018