Inorganic chemistry,xylene orxylol (from Greek ξύλον (xylon)'wood';[1][2]IUPAC name:dimethylbenzene) are any of threeorganic compounds with the formula(CH3)2C6H4. They are derived from the substitution of twohydrogen atoms withmethyl groups in abenzene ring; which hydrogens are substituted determines which of threestructural isomers results. It is a colorless, flammable, slightly greasy liquid of great industrial value.[3]
The mixture is referred to as both xylene and, more precisely, xylenes.Mixed xylenes refers to a mixture of the xylenes plusethylbenzene. The four compounds have identicalmolecular formulasC8H10. Typically the four compounds are produced together by variouscatalytic reforming andpyrolysis methods.[4]
Several million tons are produced annually.[3] In 2011, a global consortium began construction of one of the world's largest xylene plants inSingapore.[5]
The physical properties of the isomers of xylene differ slightly. The melting point ranges from −47.87 °C (−54.17 °F) (m-xylene) to 13.26 °C (55.87 °F) (p-xylene)—as usual, the para isomer's melting point is much higher because it packs more readily in the crystal structure. The boiling point for each isomer is around 140 °C (284 °F). The density of each isomer is around 0.87 g/mL (7.3 lb/US gal; 8.7 lb/imp gal) and thus is less dense thanwater. The odor of xylene is detectable at concentrations as low as 0.08 to 3.7 ppm (parts of xylene per million parts of air) and can be tasted in water at 0.53 to 1.8 ppm.[9]
Xylenes formazeotropes with water and a variety of alcohols. The azeotrope with water consists of 60% xylenes and boils at 94.5 °C.[3] As with manyalkylbenzene compounds, xylenes form complexes with varioushalocarbons.[12] The complexes of different isomers often have dramatically different properties from each other.[13]
Xylenes are used as a solvent inprinting,rubber, andleather industries. It is a common component of ink,rubber, andadhesives.[15] In thinningpaints andvarnishes, it can be substituted fortoluene where slower drying is desired, and thus is used byconservators of art objects in solubility testing.[16] Similarly it is acleaning agent, e.g., forsteel,silicon wafers, andintegrated circuits. In dentistry, xylene can be used to dissolvegutta 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.
Xylene is used in the laboratory to make baths with dry ice to cool reaction vessels,[17] and as asolvent to remove syntheticimmersion oil from the microscopeobjective inlight microscopy.[18] Inhistology, xylene is the most widely used clearing agent.[19] 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.
Generally, two kinds of reactions occur with xylenes: those involving the methyl groups and those involving the ring C–H bonds. Being benzylic and hence weakened, the C–H bonds of the methyl groups are susceptible to free-radical reactions, including halogenation to the correspondingxylene dichlorides (bis(chloromethyl)benzenes), whilemono-bromination yieldsxylyl bromide, atear gas agent. Oxidation andammoxidation also target the methyl groups, affording dicarboxylic acids and the dinitriles. Electrophiles attack the aromatic ring, leading to chloro- and nitroxylenes.[3]
Xylene is flammable but of modest acute toxicity, withLD50 ranges from 200 to 5000 mg/kg for animals. OralLD50 for rats is 4300 mg/kg. The principal mechanism of detoxification is oxidation tomethylbenzoic acid and hydroxylation to hydroxylene.[3]
The main effect of inhaling xylene vapor isdepression of the central nervous system (CNS), with symptoms such as headache, dizziness, nausea and vomiting. At an exposure of 100 ppm, one may experience nausea or a headache. At an exposure between 200 and 500 ppm, symptoms can include feeling "high", dizziness, weakness, irritability, vomiting, and slowed reaction time.[20][21]
The side effects of exposure to low concentrations of xylene (< 200 ppm) 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.[22][clarification needed] A condition calledchronic 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.[20]
Hearing disorders have been also linked to xylene exposure, both from studies with experimental animals,[23][24] as well as clinical studies.[25][26][27]
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.[20]
^Cahours, Auguste (1850)"Recherches sur les huiles légéres obtenues dans la distillation du bois" (Investigations of light oils obtained by the distillation of wood),Compte rendus,30 : 319-323; see especially p. 321. From p. 321:"Je le désignerai sous le nommexylène." (I will designate it by the name ofxylene.) Note: Cahours' empirical formula for xylene is incorrect because chemists at that time used the wrong atomic mass for carbon (6 instead of 12).
^Clark J. E.; Luthy, R. V. (1955). "Separation of Xylenes".Ind. Eng. Chem.47 (2):250–253.doi:10.1021/ie50542a028.
^Stevenson, Cheryl D.; McElheny, Daniel J.; Kage, David E.; Ciszewski, James T.; Reiter, Richard C. (1998). "Separation of Closely Boiling Isomers and Identically Boiling Isotopomers via Electron-Transfer-Assisted Extraction".Analytical Chemistry.70 (18): 3880.doi:10.1021/ac980221b.
^Samet, Wendy, (comp.),Appendix I, Painting Conservation Catalog, American Institute for Conservation of Historic and Artistic Works, conservation-wiki.com, September 1997. Accessed 2012-04-28.
^"Cooling baths". UC Davis Chem Wiki. 2013-10-02. Archived fromthe original on 2015-02-08. Retrieved8 February 2015.
^Gagnaire, F.; Marignac, B.; Langlais, C.; Bonnet, P. (July 2001). "Ototoxicity in rats exposed to ortho-, meta- and para-xylene vapours for 13 weeks".Pharmacology & Toxicology.89 (1):6–14.doi:10.1034/j.1600-0773.2001.d01-129.x (inactive 15 February 2025).ISSN0901-9928.PMID11484912.{{cite journal}}: CS1 maint: DOI inactive as of February 2025 (link)
^Fuente, Adrian; McPherson, Bradley; Hood, Linda J. (November 2012). "Hearing loss associated with xylene exposure in a laboratory worker".Journal of the American Academy of Audiology.23 (10):824–830.doi:10.3766/jaaa.23.10.7.hdl:10533/137495.ISSN1050-0545.PMID23169198.
^abInoue, O.; Seiji, K.; Kawai, T.; Watanabe, T.; Jin, C.; Cai, S. X.; Chen, Z.; Qu, Q. S.; Zhang, T.; Ikeda, M. (1993). "Excretion of methylhippuric acids in urine of workers exposed to a xylene mixture: Comparison among three xylene isomers and toluene".International Archives of Occupational and Environmental Health.64 (7):533–539.Bibcode:1993IAOEH..64..533I.doi:10.1007/bf00381104.PMID8482596.S2CID21534640.
