xylene

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.{{Cite book | doi=10.1002/14356007.a28_433|chapter = Xylenes|title = Ullmann's Encyclopedia of Industrial Chemistry|year = 2000|last1 = Fabri|first1 = Jörg| last2=Graeser| first2=Ulrich| last3=Simo| first3=Thomas A.| isbn=978-3527306732}} In 2011, a global consortium began construction of one of the world's largest xylene plants in Singapore.{{cite journal | doi = 10.1021/cen-v089n038.p018 | title = Making Aromatics in Singapore | journal = Chemical & Engineering News Archive | volume = 89 | issue = 38 | pages = 18–19 | year = 2011 | last1 = Tremblay | first1 = Jean-François }}

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.Cahours, Auguste (1850) [http://gallica.bnf.fr/ark:/12148/bpt6k2987x/f319.image.langEN "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 (6 instead of 12).

Xylenes are produced by the methylation of toluene and benzene.Martindale, David C. and Kuchar, Paul J., [https://web.archive.org/web/20180717100321/http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=/netahtml/PTO/search-bool.html&r=1&f=G&l=50&co1=AND&d=PTXT&s1=5043502.PN.&OS=PN/5043502&RS=PN/5043502 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. 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.{{cite web |title=Xylene (Mixed Isomers), Air Toxic Hazard Summary |url=http://www.epa.gov/airtoxics/hlthef/xylenes.html |publisher=United States Environmental Protection Agency |access-date=8 February 2015}}{{cite journal | last1 = Kandyala | first1 = Reena | last2 = Raghavendra | first2 = Sumanth Phani C. | last3 = Rajasekharan | first3 = Saraswathi T. | year = 2010 | title = Xylene: An overview of its health hazards and preventive measures | journal = J Oral Maxillofac Pathol | volume = 14 | issue = 1| pages = 1–5 | doi = 10.4103/0973-029X.64299 | pmid = 21180450 | pmc=2996004 | doi-access = free }}[http://apps.kemi.se/flodessok/floden/kemamne_eng/xylen_eng.htm Xylene] ({{webarchive |url=https://web.archive.org/web/20110811152832/http://apps.kemi.se/flodessok/floden/kemamne_eng/xylen_eng.htm |date=August 11, 2011 }}), Swedish Chemicals Agency, apps.kemi.se, 2010. Accessed 2012-4-28. The ratio of isomers can be shifted to favor the highly valued p-xylene via the patented UOP-Isomar process{{cite web |title=Capturing Opportunities for Para-xylene Production |url=http://www.uop.com/?document=uop-capturing-opportunities-for-para-xylene-production-tech-paper&download=1 |publisher=UOP, A Honeywell Company |access-date=8 February 2015}} or by transalkylation of xylene with itself or trimethylbenzene. These conversions are catalyzed by zeolites.

ZSM-5 is used to facilitate some isomerization reactions leading to mass production of modern plastics.

The physical properties of the isomers of xylene differ slightly. The melting point ranges from {{convert|−47.87|°C|°F|2}} (m-xylene) to {{convert|13.26|°C|°F|2}} (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 {{convert|140|°C|°F|0}}. The density of each isomer is around {{convert|0.87|g/mL|lb/gal lb/impgal|abbr=on}} and thus is less dense than water. 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.

Xylenes form azeotropes with water and a variety of alcohols. The azeotrope with water consists of 60% xylenes and boils at 94.5 °C. As with many alkylbenzene compounds, xylenes form complexes with various halocarbons.{{cite journal |title= Separation of Xylenes |journal= Ind. Eng. Chem. |year= 1955 |volume= 47 |issue= 2 |pages= 250–253 |doi= 10.1021/ie50542a028 |author1=Clark J. E. |author2=Luthy, R. V. }} The complexes of different isomers often have dramatically different properties from each other.{{cite journal |doi = 10.1021/ac980221b |title = Separation of Closely Boiling Isomers and Identically Boiling Isotopomers via Electron-Transfer-Assisted Extraction |journal = Analytical Chemistry |volume = 70 |issue = 18 |pages = 3880 |year = 1998 |last1 = Stevenson |first1 = Cheryl D. |last2 = McElheny |first2 = Daniel J. |last3 = Kage |first3 = David E. |last4 = Ciszewski |first4 = James T. |last5 = Reiter |first5 = Richard C.}}

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.ICIS, [http://www.icis.com/chemicals/paraxylene-orthoxylene/ Paraxylene-Orthoxylene {{pipe}} Prices, News & Market Analysis], icis.com, 2012. Accessed 2012-04-28. 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).

Xylenes are used as a solvent in printing, rubber, and leather industries. It is a common component of ink, rubber, and adhesives.Bostik, [http://www.bostik-amer.com/upload/download/MSDS%20Blu%20Tack.pdf Safety Data Sheet Blu-Tack] ({{webarchive |url=https://web.archive.org/web/20110911020151/http://www.bostik-amer.com/upload/download/MSDS%20Blu%20Tack.pdf |date=September 11, 2011 }}), No. 13135, Bostik Corp., June 2007. Accessed 2012-04-28. 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.Samet, Wendy, (comp.), [http://www.conservation-wiki.com/index.php?title=Appendix_I Appendix I, Painting Conservation Catalog], American Institute for Conservation of Historic and Artistic Works, conservation-wiki.com, September 1997. Accessed 2012-04-28. 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.

Xylene is used in the laboratory to make baths with dry ice to cool reaction vessels,{{cite web |title=Cooling baths |url=http://chemwiki.ucdavis.edu/Reference/Lab_Techniques/Cooling_baths |publisher=UC Davis Chem Wiki |access-date=8 February 2015 |date=2013-10-02 |archive-date=2015-02-08 |archive-url=https://web.archive.org/web/20150208042031/http://chemwiki.ucdavis.edu/Reference/Lab_Techniques/Cooling_baths |url-status=dead }} and as a solvent to remove synthetic immersion oil from the microscope objective in light microscopy.{{Citation |last = Cargille |first = John |title = Immersion Oil and the Microscope |journal = New York Microscopical Society Yearbook |orig-year = 1964 |year = 1985 |url = http://www.cargille.com/immersionoilmicroscope.shtml |access-date = 2011-03-10 |archive-url = https://web.archive.org/web/20110911190610/http://www.cargille.com/immersionoilmicroscope.shtml |archive-date = 2011-09-11 |url-status = dead }} In histology, xylene is the most widely used clearing agent.{{cite book |title=Histotechnology: A Self-Instructional Text |last1=Carson |first1=Freida |last2=Hladik |first2=Christa |edition=3 |year=2009 |publisher=American Society for Clinical Pathology Press |isbn=9780891895817 |page=35 }} 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.

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.

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 corresponding xylene dichlorides (bis(chloromethyl)benzenes), while mono-bromination yields xylyl bromide, a tear gas agent. Oxidation and ammoxidation also target the methyl groups, affording dicarboxylic acids and the dinitriles. Electrophiles attack the aromatic ring, leading to chloro- and nitroxylenes.

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. The principal mechanism of detoxification is oxidation to methylbenzoic acid and hydroxylation to hydroxylene.

The main effect of inhaling xylene vapor is depression 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.{{Cite journal |title = Xylene: An overview of its health hazards and preventive measures |journal = Journal of Oral and Maxillofacial Pathology |date = 2010-01-01 |issn = 0973-029X |pmc = 2996004 |pmid = 21180450 |pages = 1–5 |volume = 14 |issue = 1 |doi = 10.4103/0973-029X.64299 |first1 = Reena |last1 = Kandyala |first2 = Sumanth Phani C. |last2 = Raghavendra |first3 = Saraswathi T. |last3 = Rajasekharan |doi-access = free }}{{Cite web |url=http://oehha.ca.gov/air/acute_rels/pdf/XylenesA.pdf |title=ACUTE TOXICITY SUMMARY: XYLENES |url-status=dead |archive-url=https://web.archive.org/web/20151022131350/http://www.oehha.ca.gov/air/acute_rels/pdf/xylenesA.pdf |archive-date=October 22, 2015 }}

The side effects of exposure to low concentrations of xylene ({{nowrap|< 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.{{Cite web | url=http://www.inchem.org/documents/ehc/ehc/ehc190.htm | title=Xylenes (EHC 190, 1997)}}{{clarify |reason=short-term memory loss? |date=June 2016}} 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.

Hearing disorders have been also linked to xylene exposure, both from studies with experimental animals,{{Cite journal |last1=Gagnaire |first1=F. |last2=Marignac |first2=B. |last3=Langlais |first3=C. |last4=Bonnet |first4=P. |date=July 2001 |title=Ototoxicity in rats exposed to ortho-, meta- and para-xylene vapours for 13 weeks |journal=Pharmacology & Toxicology |volume=89 |issue=1 |pages=6–14 |doi=10.1034/j.1600-0773.2001.d01-129.x |doi-broken-date=15 February 2025 |issn=0901-9928 |pmid=11484912}}{{Cite journal |last1=Gagnaire |first1=F. |last2=Marignac |first2=B. |last3=Blachère |first3=V. |last4=Grossmann |first4=S. |last5=Langlais |first5=C. |date=2007-03-07 |title=The role of toxicokinetics in xylene-induced ototoxicity in the rat and guinea pig |journal=Toxicology |volume=231 |issue=2–3 |pages=147–158 |doi=10.1016/j.tox.2006.11.075 |issn=0300-483X |pmid=17210216|bibcode=2007Toxgy.231..147G }} as well as clinical studies.{{Cite journal |last1=Fuente |first1=Adrian |last2=McPherson |first2=Bradley |last3=Cardemil |first3=Felipe |date=September 2013 |title=Xylene-induced auditory dysfunction in humans |journal=Ear and Hearing |volume=34 |issue=5 |pages=651–660 |doi=10.1097/AUD.0b013e31828d27d7 |issn=1538-4667 |pmid=23598724|s2cid=45206975 |hdl=10533/134303 |hdl-access=free }}{{Cite journal |last1=Draper |first1=T. H. J. |last2=Bamiou |first2=D.-E. |date=April 2009 |title=Auditory neuropathy in a patient exposed to xylene: case report |journal=The Journal of Laryngology & Otology |volume=123 |issue=4 |pages=462–465 |doi=10.1017/S0022215108002399 |pmid=18439334 |issn=1748-5460|url=http://discovery.ucl.ac.uk/70543/1/S0022215108002399.pdf |archive-url=https://web.archive.org/web/20170809163223/http://discovery.ucl.ac.uk/70543/1/S0022215108002399.pdf |archive-date=2017-08-09 |url-status=live }}{{Cite journal |last1=Fuente |first1=Adrian |last2=McPherson |first2=Bradley |last3=Hood |first3=Linda J. |date=November 2012 |title=Hearing loss associated with xylene exposure in a laboratory worker |journal=Journal of the American Academy of Audiology |volume=23 |issue=10 |pages=824–830 |doi=10.3766/jaaa.23.10.7 |issn=1050-0545 |pmid=23169198|hdl=10533/137495 |hdl-access=free }}

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.

Xylenes are metabolized to methylhippuric acids.{{cite web | url = https://www.cdc.gov/niosh/docs/2003-154/pdfs/8301.pdf | work = NIOSH Manual of Analytical Methods (NMAM) | edition = Fourth | title = HIPPURIC and METHYL HIPPURIC ACIDS in urine }}{{Cite journal |pmid = 8482596 |year = 1993 |last1 = Inoue |first1 = O. |title = Excretion of methylhippuric acids in urine of workers exposed to a xylene mixture: Comparison among three xylene isomers and toluene |journal = International Archives of Occupational and Environmental Health |volume = 64 |issue = 7 |pages = 533–539 |last2 = Seiji |first2 = K. |last3 = Kawai |first3 = T. |last4 = Watanabe |first4 = T. |last5 = Jin |first5 = C. |last6 = Cai |first6 = S. X. |last7 = Chen |first7 = Z. |last8 = Qu |first8 = Q. S. |last9 = Zhang |first9 = T. |last10 = Ikeda |first10 = M. |doi=10.1007/bf00381104|bibcode = 1993IAOEH..64..533I |s2cid = 21534640 }} The presence of methylhippuric acid can be used as a biomarker to determine exposure to xylene.{{Cite journal | author = Kira S. | title = Measurement by gas chromatography of urinary hippuric acid and methylhippuric acid as indices of toluene and xylene exposure | journal = Occupational and Environmental Medicine | date = 1977 | volume = 34 | issue = 305–309 | pages = 305–309 | doi = 10.1136/oem.34.4.305 | pmid = 588486 | pmc = 1008281 }}

• Alkylbenzene
• C2-Benzenes
• Hydrodealkylation
• Transalkylation
• Xylene cyanol

{{Reflist|30em}}

{{commons category|Xylenes}}

• {{Cite EB1911|wstitle=Xylene}}
• [https://www.cdc.gov/niosh/npg/npgd0668.html NIOSH Pocket Guide to Chemical Hazards] (o-Xylene)
• [https://www.cdc.gov/niosh/npg/npgd0669.html NIOSH Pocket Guide to Chemical Hazards] (m-Xylene)
• [https://www.cdc.gov/niosh/npg/npgd0670.html NIOSH Pocket Guide to Chemical Hazards] (p-Xylene)
• [https://www.epa.gov/haps/health-effects-notebook-hazardous-air-pollutants Xylene, Hazard Summary (EPA)] (Mixed Isomers)
• [https://synergist.aiha.org/201811-the-ear-poisons The Ear Poisons], The Synergist, American Industrial Hygiene Association, November, 2018

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Category:Alkylbenzenes

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Category:Hydrocarbon solvents

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