toluene (tōl´yōōēn´) or methylbenzene (mĕth´əlbĕn´zēn), C₇H₈, colorless liquid aromatic hydrocarbon that melts at -95°C and boils at 110.8°C. It is insoluble in water but highly soluble in most organic solvents. Toluene is obtained from coal tar and petroleum by distillation. It is used as a solvent and as a starting material for the synthesis of many compounds, including dyes and explosives. When toluene is treated with a mixture of nitric and sulfuric acids (a process known as nitration), trinitrotoluene (TNT) is produced.

toluene (methylbenzene) Aromatic hydrocarbon (C₆H₅CH₃) derived from coal tar and petroleum. It is a colourless, flammable liquid used as an industrial solvent and in aircraft and motor fuels. Toluene is also used to make TNT. Properties: r.d. 0.87; m.p. −94.5°C (−138.1°F); b.p. 110.7°C (231.3°F).

tol·u·ene / ˈtälyoōˌēn/ • n. Chem. a colorless liquid hydrocarbon (C₆H₅CH₃) present in coal tar and petroleum and used as a solvent and in organic synthesis. Also called methylbenzene.

Toluene

Toluene is a potentially hazardous liquid that occurs naturally in crude oil, and can be obtained from the tolu tree. Toluene is used in manufacturing paints, adhesives, and rubber ,as well as in the leather-making and printing industries. People can be exposed to toluene by breathing automobile exhaust, fumes from paints, kerosene, and heating oils, or by drinking well-water that is contaminated with toluene. At high doses, toluene may causes dizziness, unconsciousness, and possibly death. High doses may cause kidney damage, while low doses may cause nausea, hearing and vision problems, disorientation, and fatigue. Toluene in air is very reactive so it increases photochemical smog and leads to the production of peroxybenzoylnitrate, a potent eye irritant. The Environmental Protection Agency (EPA) does not classify toluene as a carcinogen , although the EPA has set a limit of 1 milligram per liter for safe drinking water.

[Marie H. Bundy ]

Toluene

KEY FACTS

OTHER NAMES:

Methylbenzene; phenylmethane; toluol

FORMULA:

C₆H₅CH₃

ELEMENTS:

Carbon, hydrogen

COMPOUND TYPE:

Aromatic hydrocarbon (organic)

STATE:

Liquid

MOLECULAR WEIGHT:

92.14 g/mol

MELTING POINT:

−94.95°C (−138.9°F)

BOILING POINT:

110.63°C (231.13°F)

SOLUBILITY:

Insoluble in water; miscible with ethyl alcohol, ether, acetone, and carbon disulfide

OVERVIEW

Toluene (TOL-yew-een) is a clear, colorless liquid with a benzene-like odor. It is an aromatic hydrocarbon, that is, a compound that contains carbon and hydrogen only, with the carbon atoms arranged in a ring. Aromatic compounds have structures based on that of benzene (C₆H₆). Toluene was discovered in 1838 by French chemist Pierre Joseph Pelletier (1788–1842). Pelletier found the compound in the gas emitted by the bark of the pine tree Pinus maritima. Pelletier named the substance retinnaphte, after pine resin. The compound was re-discovered a number of times in later years and given a variety of names, including heptacarbure quadihydrique, benzoene, and dracyl. Toluene's chemical nature was finally determined by German chemist August Wilhelm von Hofmann (1818–1892) and British chemist James Muspratt (1793–1886) who adopted the name toluol originally proposed by Swedish chemist Jons Jakob Berzelius (1779–1848).

HOW IT IS MADE

Toluene occurs naturally in petroleum and is extracted during the catalytic reforming of the naphtha segment of petroleum. Catalytic reforming is a process by which hydrocarbons in petroleum are heated, often in contact with a catalyst, in order to change their molecular composition. The naptha segment of petroleum consists of a complex mixture of hydrocarbons with boiling points between about 50°C and 200°C (120°F and 400°F). Toluene is separated from other hydrocarbons produced during the reforming process by adding a solvent, such as sulfolane (c-CH₂CH₂CH₂CH₂SO₂; the "c" at the beginning means that this is a ring compound) or tetraethylene glycol (HO(CH₂CH₂O)3CH₂CH₂OH). The toluene dissolves in one or the other of these solvents, while most other hydrocarbons with which it is mixed do not. Toluene is also obtained by a similar process starting with coal tar oil, the liquid that remains when soft coal is heated in the absence of air to make coke.

COMMON USES AND POTENTIAL HAZARDS

Toluene generally ranks among the top thirty compounds produced in the United States annually. In 2002, American chemical plants produced 330 million liters (87 million gallons) of toluene for domestic use and export. About half that amount was used in the production of benzene and about 20 percent for the production of xylene (C₆H₄(CH₃)₂). Another 18 percent went to the manufacture of other organic chemicals, one of the most important of which is toluene diisocyanate (CH₃C₆H₃(NCO)₂) used primarily for the production of polyurethane foams and artificial rubber products. The other major use for toluene is as a solvent for paints and other coatings, gums, oils, natural and artificial rubber products, adhesives, and plastics. Some toluene is also used as a blending agent in aviation fuels and other gasoline mixtures with high-octane ratings.

Interesting Facts

• The name toluol (and toluene) originally comes from an aromatic substance obtained from the bark of the South American tree Myroxylon balsamum.
• Toluene was first synthesized artificially by German chemists Rudolf Fittig (1835–1910) and B. C. G. Tollens (1841–1918).
• Toluene is the recommended solvent for the removal of stains made by airplane cement.

One application of toluene that consumes relatively small amounts of the compound but that is very important is in the production of the explosive trinitrotoluene (TNT). TNT is a very desirable explosive because it detonates (explodes) only when shocked or heated to temperatures above 230°C (450°F), but then releases very large amounts of energy. For many decades, TNT was one of the world's most widely used explosives. Even today, the power of other explosives is measured by comparison with TNT. For example, the power of nuclear weapons is said to be 10 kilotons or 10 megatons, meaning that they have an explosive power comparable to 10,000 tons or 10 million tons, respectively, of TNT.

Toluene is a very hazardous chemical compound. It catches fire easily and, under proper conditions, is explosive. It is also an irritant to the skin, eyes, and respiratory tract. It may produce symptoms such as headache, dizziness, drowsiness, confusion, fatigue, and peculiar sensations of the skin, such as a "pins and needles" effect. It large quantities, toluene may cause unconsciousness, coma, and death. In contact with the skin or eyes, it may produce redness and pain. Prolonged exposure to toluene may cause serious damage to the liver and kidneys resulting in anemia, decreased blood cell count, and bone marrow hypoplasia (diminished ability to produce blood cells). Toluene is thought to be teratogenic, capable of damaging a fetus while still in the womb.

Words to Know

AROMATIC COMPOUND

A compound whose chemical structure is based on that of benzene (C₆H₆).

CATALYTIC REFORMING

A process by which hydrocarbons in petroleum are heated, often in contact with a catalyst, in order to change their molecular composition.

MISCIBLE

Able to be mixed; especially applies to the mixing of one liquid with another.

SOLVENT

A substance that is able to dissolve one or more other substances.

TERATOGENIC

Capable of causing an organism to develop incorrectly.

FOR FURTHER INFORMATION

Harte, John, et al. Toxics A to Z. Berkeley: University of California Press, 1991, 415-417.

Martin, Kevin A. "Toxicity, Toluene." eMedicine. http://www.emedicine.com/emerg/topic594.htm (accessed on December 29, 2005).

"Toluene." International Chemical Safety Cards. http://www.cdc.gov/niosh/ipcsneng/neng0078.html (accessed on December 29, 2005).

"Toluene." J. T. Baker. http://www.jtbaker.com/msds/englishhtml/t3913.htm (accessed on December 29, 2005).

"ToxFAQs™ for Toluene." Agency for Toxic Substances and Disease Registry. http://www.atsdr.cdc.gov/tfacts56.html (accessed on December 29, 2005).

See AlsoBenzene