Xylene

The three xylene isomers: o-xylene, m-xylene, and p-xylene

In organic chemistry, xylene or xylol (Template:Ety;<ref>Template:LSJ.</ref><ref>Template:OEtymD</ref> IUPAC name: dimethylbenzene) are any of three organic compounds with the formula Template:Chem2. 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.<ref name="Ullmann"/>

The mixture is referred to as both xylene and, more precisely, xylenes. Mixed xylenes refers to a mixture of the xylenes plus ethylbenzene. The four compounds have identical molecular formulas Template:Chem2. Typically the four compounds are produced together by various catalytic reforming and pyrolysis methods.<ref>Template:Cite book</ref>

Occurrence and productionEdit

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.<ref name="Ullmann">Template:Cite book</ref> In 2011, a global consortium began construction of one of the world's largest xylene plants in Singapore.<ref>Template:Cite journal</ref>

HistoryEdit

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.<ref>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 (6 instead of 12).</ref>

Industrial productionEdit

Xylenes are produced by the methylation of toluene and benzene.<ref name="Ullmann" /><ref>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.</ref> 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Kandyala">Template:Cite journal</ref><ref name="Sweden">Xylene (Template:Webarchive), Swedish Chemicals Agency, apps.kemi.se, 2010. Accessed 2012-4-28.</ref> The ratio of isomers can be shifted to favor the highly valued p-xylene via the patented UOP-Isomar process<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> or by transalkylation of xylene with itself or trimethylbenzene. These conversions are catalyzed by zeolites.<ref name="Ullmann" />

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

PropertiesEdit

The physical properties of the isomers of xylene differ slightly. The melting point ranges from Template:Convert (m-xylene) to Template:Convert (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 Template:Convert. The density of each isomer is around Template:Convert 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.<ref name="Kandyala" />

Template:Chemical datatable header colspan="5" \| Xylene isomers
Template:Chemical datatable header colspan="5" \| General
Common name	Xylenes (mixture)	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]
Template:Chemical datatable header colspan="5" \| 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 Template:Convert	0.62 cP at Template:Convert	0.34 cP at Template:Convert
Template:Chemical datatable header colspan="5" \| Hazards
SDS	Xylenes	o-Xylene	m-Xylene	p-Xylene
EU pictograms	Template:GHS02 Template:GHS07 Template:GHS08
NFPA 704	Template:NFPA 704 diamond
Flash point	Template:Convert	Template:Convert	Template:Convert	Template:Convert
H & P phrases	Template:H-phrases Template:P-phrases
RTECS number		ZE2450000	ZE2275000	ZE2625000
Template:Chemical datatable header colspan="5" \| Related compounds
Related aromatic hydrocarbons	Toluene, mesitylene, benzene, ethylbenzene
Related compounds	Xylenols – types of phenols
Template:Chemical datatable header colspan="5" \| Except where noted otherwise, data are given for materials in their [[standard state\|standard state (at 25Template:Nbsp°C, 100Template:NbspkPa)]] Infobox disclaimer and references

Xylenes form azeotropes with water and a variety of alcohols. The azeotrope with water consists of 60% xylenes and boils at 94.5 °C.<ref name="Ullmann" /> As with many alkylbenzene compounds, xylenes form complexes with various halocarbons.<ref>Template:Cite journal</ref> The complexes of different isomers often have dramatically different properties from each other.<ref>Template:Cite journal</ref>

ApplicationsEdit

Terephthalic acid and related derivativesEdit

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.<ref name="Sweden" /><ref>ICIS, Paraxylene-Orthoxylene Template:Pipe Prices, News & Market Analysis, icis.com, 2012. Accessed 2012-04-28.</ref> 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 purposesEdit

Xylenes are used as a solvent in printing, rubber, and leather industries. It is a common component of ink, rubber, and adhesives.<ref>Bostik, Safety Data Sheet Blu-Tack (Template:Webarchive), No. 13135, Bostik Corp., June 2007. Accessed 2012-04-28.</ref> 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.<ref>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.</ref> 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 useEdit

Xylene is used in the laboratory to make baths with dry ice to cool reaction vessels,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and as a solvent to remove synthetic immersion oil from the microscope objective in light microscopy.<ref>Template:Citation</ref> In histology, xylene is the most widely used clearing agent.<ref>Template:Cite book</ref> 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 compoundsEdit

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.<ref name="Ullmann"/>

Chemical propertiesEdit

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.<ref name="Ullmann"/>

Health and safetyEdit

Xylene is flammable but of modest acute toxicity, with Template:LD50 ranges from 200 to 5000 mg/kg for animals. Oral Template:LD50 for rats is 4300 mg/kg. The principal mechanism of detoxification is oxidation to methylbenzoic acid and hydroxylation to hydroxylene.<ref name="Ullmann" />

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.<ref name=":0">Template:Cite journal</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

The side effects of exposure to low concentrations of xylene (Template:Nowrap) 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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>Template:Clarify 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.<ref name=":0" />

Hearing disorders have been also linked to xylene exposure, both from studies with experimental animals,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> as well as clinical studies.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

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.<ref name=":0" />

Xylenes are metabolized to methylhippuric acids.<ref name=NIOSH>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=Inoue>Template:Cite journal</ref> The presence of methylhippuric acid can be used as a biomarker to determine exposure to xylene.<ref name=Inoue/><ref>Template:Cite journal</ref>