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, ...

, xylene or xylol (;

IUPAC name In chemical nomenclature, a preferred IUPAC name (PIN) is a unique name, assigned to a chemical substance and preferred among the possible names generated by IUPAC nomenclature. The "preferred IUPAC nomenclature" provides a set of rules for choo ...

: 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. T ...

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-toxic ...

atoms with

methyl group In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula . In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in ma ...

s 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 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 intermedia ...

. 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 ''py ...

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 renewable so ...

produced by

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 organi ...

and aircraft fuels. Xylenes are produced mainly as part of the BTX aromatics (

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) a ...

, and xylenes) extracted from the product of

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 Tar is a dark brown or black viscous liquid of hydrocarbons and free carbon, obtained from a wide variety of organic materials through destructive distillation. Tar can be produced from coal, wood, petroleum, or peat. "a dark brown or black bi ...

Industrial production

Xylenes are produced by the methylation of

and

. 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

.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 In organic chemistry, transalkylation is a chemical reaction involving the transfer of an alkyl group from one organic compound to another. The reaction is used for the transfer of methyl and ethyl groups between benzene rings. This is of particul ...

of xylene with itself or trimethylbenzene. These conversions are catalyzed by

zeolite Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ･y where is either a metal ion or H+. These p ...

s. 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 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 compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...

. 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

azeotrope An azeotrope () or a constant heating point mixture is a mixture of two or more liquids whose proportions cannot be altered or changed by simple distillation.Moore, Walter J. ''Physical Chemistry'', 3rd e Prentice-Hall 1962, pp. 140–142 This ...

s 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 Dimethyl terephthalate (DMT) is an organic compound with the formula C6H4(COOCH3)2. It is the diester formed from terephthalic acid and methanol. It is a white solid that melts to give a distillable colourless liquid.Richard J. Sheehan "Terepht ...

, 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 (PET)

plastic bottle A plastic bottle is a bottle constructed from high-density or low density plastic. Plastic bottles are typically used to store liquids such as water, soft drinks, motor oil, cooking oil, medicine, shampoo, milk, and ink. The size ranges from v ...

s and polyester 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 Phthalic anhydride is the organic compound with the formula C6H4(CO)2O. It is the anhydride of phthalic acid. Phthalic anhydride is a principal commercial form of phthalic acid. It was the first anhydride of a dicarboxylic acid to be used commer ...

. The demand for

isophthalic acid Isophthalic acid is an organic compound with the formula C6H4(CO2H)2. This colorless solid is an isomer of phthalic acid and terephthalic acid. The main industrial uses of purified isophthalic acid (PIA) are for the production of polyethylene te ...

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, an ...

, 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,

, and adhesives. In thinning paints and varnishes, it can be substituted for

where slower drying is desired, and thus is used by

conservators In certain areas of England, Conservators are statutory bodies which manage areas of countryside for the use of the public. Establishment, Role and Powers Conservators are bodies corporate generally established, and granted their powers, by a ...

of art objects in solubility testing. Similarly it is a

cleaning agent Cleaning agents or hard-surface cleaners are substances (usually liquids, powders, sprays, or granules) used to remove dirt, including dust, stains, bad smells, and clutter on surfaces. Purposes of cleaning agents include health, beauty, removin ...

, e.g., for steel,

silicon wafer In electronics, a wafer (also called a slice or substrate) is a thin slice of semiconductor, such as a crystalline silicon (c-Si), used for the fabrication of integrated circuits and, in photovoltaics, to manufacture solar cells. The wafer serv ...

s, and integrated circuits. In dentistry, xylene can be used to dissolve

gutta percha Gutta-percha is a tree of the genus '' Palaquium'' in the family Sapotaceae. The name also refers to the rigid, naturally biologically inert, resilient, electrically nonconductive, thermoplastic latex derived from the tree, particularly from ...

, 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 from the microscope

objective Objective may refer to: * Objective (optics), an element in a camera or microscope * ''The Objective'', a 2008 science fiction horror film * Objective pronoun, a personal pronoun that is used as a grammatical object * Objective Productions, a Brit ...

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 vi ...

, 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 ester Phthalates (, ), or phthalate esters, are esters of phthalic acid. They are mainly used as plasticizers, i.e., substances added to plastics to increase their flexibility, transparency, durability, and longevity. They are used primarily to soften ...

s, used as

plasticizer A plasticizer ( UK: plasticiser) is a substance that is added to a material to make it softer and more flexible, to increase its plasticity, to decrease its viscosity, and/or to decrease friction during its handling in manufacture. Plasticiz ...

. Meta-xylene is converted to

derivatives, which are components of

alkyd resin An alkyd is a polyester resin modified by the addition of fatty acids and other components. Alkyds are derived from polyols and organic acids including dicarboxylic acids or carboxylic acid anhydride and triglyceride oils. The term ''alkyd'' is a ...

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 Xylyl bromide, also known as methylbenzyl bromide or T-stoff ('substance-T'), is any member or a mixture of organic chemical compounds with the molecular formula C6 H4(CH3)(CH2 Br). The mixture was formerly used as a tear gas and has an odor r ...

, 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 In organic chemistry, ammoxidation is a process for the production of nitriles () using ammonia () and oxygen (). It is sometimes called the SOHIO process, acknowledging that ammoxidation was developed at Standard Oil of Ohio. The usual substrate ...

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 Chronic solvent-induced encephalopathy (CSE) is a condition induced by long-term exposure to organic solvents, often - but not always - in the workplace, that lead to a wide variety of persisting sensorimotor polyneuropathies and neurobehavioral d ...

, 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 In biomedical contexts, a biomarker, or biological marker, is a measurable indicator of some biological state or condition. Biomarkers are often measured and evaluated using blood, urine, or soft tissues to examine normal biological processes, p ...

to determine exposure to xylene.

References

American Industrial Hygiene Association The American Industrial Hygiene Association (AIHA) is a 501(c)6 non-profit organization, whose mission is "Creating knowledge to protect worker health." The American Industrial Hygiene Association works to provide information and resources to Ind ...

, November, 2018 {{Authority control Alkylbenzenes C2-Benzenes Hazardous air pollutants Hydrocarbon solvents Soil contamination Commodity chemicals Petrochemicals Aromatic solvents Occupational safety and health D