Xylene

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The xylene isomers

Xylene (from Greek ξύλο, xylo, "wood"), xylol or dimethylbenzene is an aromatic hydrocarbon mixture consisting of a benzene ring with two methyl groups at various substituted positions. The three isomers of xylene have the molecular formula C8H10, also represented by the semi-structural formula C6H4(CH3)2. Xylene is a major petrochemical produced by catalytic reforming and also by coal carbonisation in the manufacture of coke fuel. It represents about 0.5–1% of crude oil (depending on the source), and is found in small quantities in gasoline and aircraft 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, colorless liquid commonly encountered as a solvent. 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.[1] Several million tons are produced annually.[2] In 2011, a global consortium began construction of one of the world’s largest xylene plants in Singapore.[3]

Isomers[edit]

Xylene exists in three isomeric forms. The isomers can be 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. By 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. Of the three isomers, the p-isomer is the most industrially sought after since it can be oxidized to terephthalic acid.[2]

Industrial production[edit]

Xylenes are produced by the methylation of toluene and benzene.[2][4] 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.[5][6][7] The ratio of isomers can be shifted to favor the highly valued p-xylene via the patented UOP-Isomar process[8] or by transalkylation of xylene with itself or trimethylbenzene. These conversions are catalyzed by zeolites.[2]

Properties[edit]

The chemical and physical properties of xylene differ according to the respective isomers. The melting point ranges from −47.87 °C (−54.17 °F) (m-xylene) to 13.26 °C (55.87 °F) (p-xylene). The boiling point for each isomer is around 140 °C (284 °F). The density of each isomer 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 be tasted in water at 0.53 to 1.8 ppm.[6]

Xylene Isomers
General
Common name Xylene o-Xylene m-Xylene p-Xylene
Systematic name Dimethylbenzene 1,2-Dimethylbenzene 1,3-Dimethylbenzene 1,4-Dimethylbenzene
Other names Xylol o-Xylol;
Orthoxylene
m-Xylol;
Metaxylene
p-Xylol;
Paraxylene
Molecular formula C8H10
SMILES Cc1c(C)cccc1 Cc1cc(C)ccc1 Cc1ccc(C)cc1
Molar mass 106.16 g/mol
Appearance clear, colorless liquid
CAS number [1330-20-7] [95-47-6] [108-38-3] [106-42-3]
Properties
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 (68 °F) 0.62 cP at 20 °C (68 °F) 0.34 cP at 30 °C (86 °F)
Hazards
SDS Xylenes[9] o-Xylene m-Xylene p-Xylene
EU Classification Harmful (Xn)
NFPA 704
Flammability code 3: Liquids and solids that can be ignited under almost all ambient temperature conditions. Flash point between 23 and 38 °C (73 and 100 °F). E.g., gasoline) Health code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g., chloroform Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
Flash point 30 °C (86 °F) 17 °C (63 °F) 25 °C (77 °F) 25 °C (77 °F)
R/S statement R10, R20/21, R38: (S2), S25
RTECS number ZE2450000 ZE2275000 ZE2625000
Supplementary data page
Structure & properties n, εr, etc.
Thermodynamic data Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
Related compounds
Related aromatic
hydrocarbons
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.[10] The complexes of different isomers often have dramatically different properties from each other.[11]

Applications[edit]

Terephthalic acid and related derivatives[edit]

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.[7][12] 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[edit]

Xylene is used as a solvent. In this application, 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 the printing, rubber, and leather industries. It is a common component of ink, rubber, and adhesives.[13] 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.[14] 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. For similar reasons, it is often the active ingredient in commercial products for ear wax (cerumen) removal.

Laboratory use[edit]

Xylene is used in the laboratory to make baths with dry ice to cool reaction vessels,[15] and as a solvent to remove synthetic immersion oil from the microscope objective in light microscopy.[16] In histology, xylene is the most widely used clearing agent.[17] 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[edit]

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 mono-bromination yields xylyl bromide, a tear gas agent used in World War I.

Safety[edit]

Xylene is flammable but of modest acute toxicity, with LD50 ranges from 200 to 5000 mg/kg for animals. Oral LD50 for rats is 4300 mg/kg.[18] The principal mechanism of detoxification is oxidation to methylbenzoic acid and hydroxylation to hydroxylene.[2]

See also[edit]

References[edit]

  1. ^ 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 nomme xylène." (I will designate it by the name of xylene.) Note: Cahours' empirical formula for xylene is incorrect because chemists at that time used the wrong atomic mass for carbon (8 instead of 16).
  2. ^ a b c d e Fabri, Jörg; Graeser, Ulrich, and Simo, Thomas A., Xylenes, Ullmann's Encyclopedia of Industrial Chemistry, 2000, Wiley-VCH, Weinheim. Accessed 2015-2-8 doi:10.1002/14356007.a28_433.
  3. ^ Tremblay, Jean-François, Making Aromatics In Singapore, Chem. Eng. News, 89 (38), 2011-9-19, pp 18–19. Accessed 2012-4-28.
  4. ^ Martindale, David C. and Kuchar, Paul J., Production of xylenes from light aliphatic hydrocarbons via dehydrocyclodimerization and methylation, United States Patent No. 5,043,502, 1991-8-27. Accessed 2012-4-28.
  5. ^ "Xylene (Mixed Isomers), Air Toxic Hazard Summary)". United States Environmental Protection Agency. Retrieved 8 February 2015. 
  6. ^ a b Kandyala, Reena; Raghavendra, Sumanth Phani C.; Rajasekharan, Saraswathi T. (2010). "Xylene: An overview of its health hazards and preventive measures". J Oral Maxillofac Pathol 14 (1): 1–5. doi:10.4103/0973-029X.64299. PMC 2996004. PMID 21180450. 
  7. ^ a b Xylene, Swedish Chemicals Agency, apps.kemi.se, 2010. Accessed 2012-4-28.
  8. ^ "Capturing Opportunities for Para-xylene Production". UOP, A Honeywell Company. Retrieved 8 February 2015. 
  9. ^ SIRI, Xylenes Materials Safety Data Sheet, MSDS No. X2000, Vermont Safety Information Resources, Inc., 1997-9-8. Accessed 2012-4-27.
  10. ^ Clark J. E.; Luthy, R. V. (1955). "Separation of Xylenes". Ind. Eng. Chem. 47 (2): 250–253. doi:10.1021/ie50542a028. 
  11. ^ Stevenson, Cheryl D., McElheny, Daniel J., Kage, David E., Ciszewski, James T., and Reiter, Richard C., "Separation of Closely Boiling Isomers and Identically Boiling Isotopomers via Electron-Transfer-Assisted Extraction", Analytical Chemistry, 70 (18), 1998-8-8, 3880-3885. doi:10.1021/ac980221b
  12. ^ ICIS, Paraxylene-Orthoxylene | Prices, News & Market Analysis, icis.com, 2012. Accessed 2012-4-28.
  13. ^ Bostik, Safety Data Sheet Blu-Tack, No. 13135, Bostik Corp., 2007-6. Accessed 2012-4-28.
  14. ^ Samet, Wendy, (comp.), Appendix I, Painting Conservation Catalog, American Institute for Conservation of Historic and Artistic Works, conservation-wiki.com, 1997-9. Accessed 2012-4-28.
  15. ^ "Cooling baths". UC Davis Chem Wiki. Retrieved 8 February 2015. 
  16. ^ Cargille, John (1985) [1964], "Immersion Oil and the Microscope", New York Microscopical Society Yearbook 
  17. ^ Carson, Freida; Hladik, Christa (2009). Histotechnology: A Self-Instructional Text (3 ed.). American Society for Clinical Pathology Press. p. 35. ISBN 9780891895817. 
  18. ^ "Xylene: Material Safety Data Sheet" (PDF). West Liberty University. Retrieved 16 December 2013. 

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