- , m-Xylene, and p-Xylene]]
In organic chemistry, xylene or xylol (;[.] IUPAC name: dimethylbenzene) is any of three with the formula . They are derived from the substitution of two hydrogen atoms with 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.
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 . Typically the four compounds are produced together by various catalytic reforming and pyrolysis methods.
Occurrence and production
Xylenes are an important
petrochemical produced by catalytic reforming and also by
carbonization 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
jet fuel.
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. In 2011, a global consortium began construction of one of the world's largest xylene plants in Singapore.
History
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) "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).]
Industrial production
Xylenes are produced by the
methylation of
toluene and
benzene.
[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.] Commercial or
chemical purity xylene produced usually contains about 40–65% of
m-Xylene and up to 20% each of
o-Xylene,
p-Xylene and
ethylbenzene.
[ Xylene (), 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
or by
transalkylation of xylene with itself or trimethylbenzene. These conversions are catalyzed by
.
ZSM-5 is used to facilitate some isomerization reactions leading to mass production of modern plastics.
Properties
The physical properties of the isomers of xylene differ slightly. The melting point ranges from (
m-xylene) to (
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 . The density of each isomer is around 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.
! colspan="5" | Xylene isomers |
|
|
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 |
Chemical 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 |
|
Density and phase | 0.864 g/mL, liquid | 0.88 g/mL, liquid | 0.86 g/mL, liquid | 0.86 g/mL, liquid |
Soluble 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 | 0.62 cP at | 0.34 cP at |
|
SDS | Xylenes | o-Xylene | m-Xylene | p-Xylene |
EU pictograms | |
NFPA 704 | |
Flash point | | | | |
H & P phrases |
|
RTECS number | | ZE2450000 | ZE2275000 | ZE2625000 |
|
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 Infobox disclaimer and references |
Xylenes form 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 . The complexes of different isomers often have dramatically different properties from each other.
Applications
Terephthalic acid and related derivatives
p-Xylene is the principal precursor to terephthalic acid and dimethyl terephthalate, both
used in the production of polyethylene terephthalate (PET)
and
polyester clothing. 98% of
p-xylene production, and half of all xylenes produced is consumed in this manner.
[ICIS, Paraxylene-Orthoxylene 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).
Solvent applications and industrial purposes
Xylenes are used as a solvent in
printing,
rubber, and
leather industries. It is a common component of ink,
rubber, and
.
[Bostik, Safety Data Sheet Blu-Tack (), No. 13135, Bostik Corp., June 2007. Accessed 2012-04-28.] In thinning
and
, 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.), 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,
, 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 use
Xylene 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
In one large-scale application, para-xylene is converted to terephthalic acid. The major application of ortho-xylene is as a precursor to
, used as
plasticizer. Meta-xylene is converted to
isophthalic acid derivatives, which are components of
.
Chemical properties
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.
Health and safety
Xylene is flammable but of modest acute toxicity, with ranges from 200 to 5000 mg/kg for animals. Oral for rats is 4300 mg/kg. The principal mechanism of detoxification is oxidation to
toluic acid and hydroxylation to hydroxylene.
The main effect of inhaling xylene vapor is CNS depression (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.
The side effects of exposure to low concentrations of xylene () 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. 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, as well as clinical studies.
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. The presence of methylhippuric acid can be used as a biomarker to determine exposure to xylene.
See also
External links