In
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J ...
, xylene or xylol (;
IUPAC name: dimethylbenzene) are any of three
organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...
s with the formula . They are derived from the substitution of two
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
atoms with
methyl groups in a
benzene
Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...
ring; which hydrogens are substituted determines which of three
structural isomers
In chemistry, a structural isomer (or constitutional isomer in the IUPAC nomenclature) of a compound is another compound whose molecule has the same number of atoms of each element, but with logically distinct bonds between them. The term meta ...
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
Ethylbenzene is an organic compound with the formula . It is a highly flammable, colorless liquid with an odor similar to that of gasoline. This monocyclic aromatic hydrocarbon is important in the petrochemical industry as an reaction interme ...
. The four compounds have identical
empirical formulas . Typically the four compounds are produced together by various
catalytic reforming
Catalytic reforming is a chemical process used to convert petroleum refinery naphthas distilled from crude oil (typically having low octane ratings) into high-octane liquid products called reformates, which are premium blending stocks for high-oc ...
and
pyrolysis
The pyrolysis (or devolatilization) process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere. It involves a change of chemical composition. The word is coined from the Greek-derived elements '' ...
methods.
Occurrence and production
Xylenes are an important
petrochemical
Petrochemicals (sometimes abbreviated as petchems) are the chemical products obtained from petroleum by refining. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewabl ...
produced by
catalytic reforming
Catalytic reforming is a chemical process used to convert petroleum refinery naphthas distilled from crude oil (typically having low octane ratings) into high-octane liquid products called reformates, which are premium blending stocks for high-oc ...
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
Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organic c ...
and
aircraft fuels.
Xylenes are produced mainly as part of the
BTX aromatics (
benzene
Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...
,
toluene
Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon. It is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) ...
, and xylenes) extracted from the product of
catalytic reforming
Catalytic reforming is a chemical process used to convert petroleum refinery naphthas distilled from crude oil (typically having low octane ratings) into high-octane liquid products called reformates, which are premium blending stocks for high-oc ...
known as
reformate
Catalytic reforming is a chemical process used to convert petroleum refinery naphthas distilled from crude oil (typically having low octane ratings) into high-octane liquid products called reformates, which are premium blending stocks for high-o ...
.
Several million tons are produced annually.
In 2011, a global consortium began construction of one of the world's largest xylene plants in
Singapore
Singapore (), officially the Republic of Singapore, is a sovereign island country and city-state in maritime Southeast Asia. It lies about one degree of latitude () north of the equator, off the southern tip of the Malay Peninsula, bor ...
.
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.
Industrial production
Xylenes are produced by the
methylation of
toluene
Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon. It is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) ...
and
benzene
Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms ...
.
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
Ethylbenzene is an organic compound with the formula . It is a highly flammable, colorless liquid with an odor similar to that of gasoline. This monocyclic aromatic hydrocarbon is important in the petrochemical industry as an reaction interme ...
.
[Xylene](_blank)
(), 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
zeolites.
ZSM-5
ZSM-5, Zeolite Socony Mobil–5 (framework type MFI from ZSM-5 (five)), is an aluminosilicate zeolite belonging to the pentasil family of zeolites. Its chemical formula is NanAlnSi96–nO192·16H2O (0 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 0.87 g/mL (7.26
lb/U.S. gallon or 8.72 lb/
imp gallon) and thus is less dense than
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
. 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
The alkylbenzenes are derivatives of benzene, in which one or more hydrogen atoms are replaced by alkyl groups of different sizes. They are a subset of the aromatic hydrocarbons. The simplest member is toluene, in which a hydrogen atom of the benze ...
compounds, xylenes form complexes with various
halocarbons. 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
monomer
In chemistry, a monomer ( ; '' mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or three-dimensional network in a process called polymerization.
Classification
...
s used in the production of
polyethylene terephthalate
Polyethylene terephthalate (or poly(ethylene terephthalate), PET, PETE, or the obsolete PETP or PET-P), is the most common thermoplastic polymer resin of the polyester family and is used in fibres for clothing, containers for liquids and food ...
(PET)
plastic bottles and
polyester
Polyester is a category of polymers that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include natura ...
clothing. 98% of ''p''-xylene production, and half of all xylenes produced is consumed in this manner.
''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
Printing is a process for mass reproducing text and images using a master form or template. The earliest non-paper products involving printing include cylinder seals and objects such as the Cyrus Cylinder and the Cylinders of Nabonidus. The ...
,
rubber
Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Thailand, Malaysia, and ...
, and
leather
Leather is a strong, flexible and durable material obtained from the tanning, or chemical treatment, of animal skins and hides to prevent decay. The most common leathers come from cattle, sheep, goats, equine animals, buffalo, pigs and hog ...
industries. It is a common component of ink,
rubber
Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Thailand, Malaysia, and ...
, and
adhesive
Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation.
The use of adhesives offers certain advant ...
s. In thinning
paints and
varnishes, it can be substituted for
toluene
Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon. It is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) ...
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
Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistan ...
,
silicon wafers, and
integrated circuit
An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
s. 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
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
to remove synthetic
immersion oil
In light microscopy, oil immersion is a technique used to increase the resolving power of a microscope. This is achieved by immersing both the objective lens and the specimen in a transparent oil of high refractive index, thereby increasing the ...
from the microscope
objective in
light microscopy. In
histology
Histology,
also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures v ...
, 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
phthalate esters, used as
plasticizer. Meta-xylene is converted to
isophthalic acid derivatives, which are components of
alkyd resins.
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
Tear gas, also known as a lachrymator agent or lachrymator (), sometimes colloquially known as "mace" after the early commercial aerosol, is a chemical weapon that stimulates the nerves of the lacrimal gland in the eye to produce tears. In ...
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
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.
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
*C2-Benzenes
The C2benzenes are a class of organic aromatic compounds which contain a benzene ring and two other carbon atoms. For the hydrocarbons with no further unsaturation, there are four isomers. There are three xylenes and one ethylbenzene. The substance ...
*Xylene cyanol
Xylene cyanol can be used as an electrophoretic color marker, or tracking dye, to monitor the process of agarose gel electrophoresis and polyacrylamide gel electrophoresis. Bromophenol blue
Bromophenol blue (3′,3″,5′,5″-tetrabromophen ...
*Hydrodealkylation
Hydrodealkylation is a chemical reaction that often involves reacting an aromatic hydrocarbon, such as toluene, in the presence of hydrogen gas to form a simpler aromatic hydrocarbon devoid of functional groups. An example is the conversion of 1,2, ...
* Transalkylation
*Alkylbenzene
The alkylbenzenes are derivatives of benzene, in which one or more hydrogen atoms are replaced by alkyl groups of different sizes. They are a subset of the aromatic hydrocarbons. The simplest member is toluene, in which a hydrogen atom of the benze ...
References
External links
*
NIOSH Pocket Guide to Chemical Hazards
(''o''-Xylene)
(''m''-Xylene)
(''p''-Xylene)
Xylene, Hazard Summary (EPA)
(Mixed Isomers)
The Ear Poisons
''The Synergist'', American Industrial Hygiene Association, November, 2018
{{Authority control
Alkylbenzenes
C2-Benzenes
Hazardous air pollutants
Hydrocarbon solvents
Soil contamination
Commodity chemicals
Petrochemicals
Aromatic solvents
Occupational safety and health
D