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.
Xylenes are produced by dehydrocyclodimerization and by methylating of toluene and benzene. Via the Isomar process, the ratio of isomers can be shifted to favor p-xylene, which is most valued. This conversion is catalyzed by zeolites.
Xylenes encompass three isomers of dimethylbenzene. The isomers are distinguished by the designations ortho- (o-), meta- (m-), and para- (p-), which specify to which carbon atoms (of the benzene ring) the two methyl groups are attached. Counting the carbon atoms around the ring starting from one of the ring carbons bonded to a methyl group, and counting towards the second methyl group, the o-isomer has the IUPAC name of 1,2-dimethylbenzene, the m-isomer is 1,3-dimethylbenzene, and the p-isomer is 1,4-dimethylbenzene.
Some chemical and physical properties differ from isomer to isomer. The melting point ranges from −47.87°C (m-xylene) to 13.26°C (p-xylene). The boiling point for each isomer is around 140°C. The density of each is around 0.87 g/mL (7.26 lb/U.S. gallon or 8.72 lb/imp gallon) and thus is less dense than water. Xylene in air can be smelled at concentrations as low as 0.08 to 3.7 ppm (parts of xylene per million parts of air) and can begin to be tasted in water at 0.53 to 1.8 ppm.
|Other names||Xylols|| o-Xylol;
|Molar mass||106.16 g/mol|
|Appearance||clear, colorless liquid|
|Density and phase||0.864 g/mL, liquid||0.88 g/mL, liquid||0.86 g/mL, liquid||0.86 g/mL, liquid|
|Solubility in water||practically insoluble|
|Soluble in non-polar solvents such as aromatic hydrocarbons|
|Melting point||−47.4 °C (−53.3 °F; 226 K)||−25 °C (−13 °F; 248 K)||−48 °C (−54 °F; 225 K)||13 °C (55 °F; 286 K)|
|Boiling point||138.5 °C (281.3 °F; 412 K)||144 °C (291 °F; 417 K)||139 °C (282 °F; 412 K)||138 °C (280 °F; 411 K)|
|Viscosity||0.812 cP at 20°C||0.62 cP at 20°C||0.34 cP at 30°C|
|EU Classification||Harmful (Xn)|
|R/S statement||R10, R20/21, R38: (S2), S25|
|Supplementary data page|
|Structure & properties||n, εr, etc.|
|Thermodynamic data|| Phase behaviour|
Solid, liquid, gas
|Spectral data||UV, IR, NMR, MS|
| Related aromatic
|toluene, mesitylene, benzene, ethylbenzene|
|Related compounds||xylenols - types of phenols|
| Except where noted otherwise, data are given for|
materials in their standard state (at 25°C, 100 kPa)
Infobox disclaimer and references
Xylenes form azeotropes with water and a variety of alcohols. With water the azeotrope consists of 60% xylenes and boils at 90 °C. As with many alkylbenzene compounds, xylenes form complexes with various halocarbons. The complexes of different isomers often have dramatically different properties from each other.
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. 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).
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, 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. 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 asphaltene and paraffin solvents; used to remove organic deposits in the production or processing systems.
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. 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
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.
Xylenes are not highly toxic as indicated by the high values of the LD50, which range from 200 to 4000 mg/kg for animals. The principal mechanism of detoxification is oxidation to methylbenzoic acid and hydroxylation to hydroxylene. However, the material safety data sheet (MSDS) rating assigned is for a moderate hazard, indicating safe handling precautions should include goggles, protective clothing, ventilation, and proper gloves, with storage of the material in a cool, dry, well-ventilated location since it is highly flammable.
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