Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon. It is a colorless,

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 ...

-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted

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 ato ...

derivative, consisting of a

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 ...

(CH₃) attached to a phenyl group. As such, its systematic IUPAC name is methylbenzene. Toluene is predominantly used as an industrial

feedstock A raw material, also known as a feedstock, unprocessed material, or primary commodity, is a basic material that is used to produce goods, finished goods, energy, or intermediate materials that are feedstock for future finished products. As feedst ...

and 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 ...

. As the solvent in some types of paint thinner, permanent markers,

contact cement Impact glue, contact glue or neoprene glue, is a type of glue, used mainly to glue plastic foams, laminates, metal panels, etc. . It also allows the mounting of supports of different types (metal on wood, cork on plaster, wood on cement, etc) ...

and certain types of glue, toluene is sometimes used as a recreational

inhalant Inhalants are a broad range of household and industrial chemicals whose volatile vapors or pressurized gases can be concentrated and breathed in via the nose or mouth to produce intoxication, in a manner not intended by the manufacturer. They ...

and has the potential of causing severe neurological harm.

History

The compound was first isolated in 1837 through a distillation of

pine A pine is any conifer tree or shrub in the genus ''Pinus'' () of the family (biology), family Pinaceae. ''Pinus'' is the sole genus in the subfamily Pinoideae. The World Flora Online created by the Royal Botanic Gardens, Kew and Missouri Botanic ...

oil by the Polish chemist Filip Walter, who named it ''rétinnaphte''. In 1841, French chemist Henri Étienne Sainte-Claire Deville isolated a hydrocarbon from

balsam of Tolu Tolu balsam or balsam of Tolu is a balsam that originates from South America (Colombia, Peru, Venezuela). It is similar to (and frequently confounded with) the balsam of Peru. It is tapped from the living trunks of '' Myroxylon balsamum var. ...

(an aromatic extract from the tropical Colombian tree '' Myroxylon balsamum''), which Deville recognized as similar to Walter's ''rétinnaphte'' and to benzene; hence he called the new hydrocarbon ''benzoène''. In 1843,

Jöns Jacob Berzelius Baron Jöns Jacob Berzelius (; by himself and his contemporaries named only Jacob Berzelius, 20 August 1779 – 7 August 1848) was a Swedish chemist. Berzelius is considered, along with Robert Boyle, John Dalton, and Antoine Lavoisier, to be ...

recommended the name ''toluin''. In 1850, French chemist Auguste Cahours isolated from a distillate of wood a hydrocarbon which he recognized as similar to Deville's ''benzoène'' and which Cahours named ''toluène''.

Chemical properties

Toluene reacts as a normal aromatic hydrocarbon in

electrophilic aromatic substitution Electrophilic aromatic substitution is an organic reaction in which an atom that is attached to an aromatic system (usually hydrogen) is replaced by an electrophile. Some of the most important electrophilic aromatic substitutions are aromatic ni ...

. Because the methyl group has greater electron-releasing properties than a

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 ...

atom in the same position, toluene is more reactive than

toward electrophiles. It undergoes sulfonation to give ''p''-toluenesulfonic acid, and chlorination by Cl₂ in the presence of FeCl₃ to give ortho and para

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formulae – that is, same number of atoms of each element – but distinct arrangements of atoms in space. Isomerism is existence or possibility of isomers. ...

s of chlorotoluene. Importantly, the methyl side chain in toluene is susceptible to oxidation. Toluene reacts with

potassium permanganate Potassium permanganate is an inorganic compound with the chemical formula KMnO4. It is a purplish-black crystalline salt, that dissolves in water as K+ and , an intensely pink to purple solution. Potassium permanganate is widely used in the c ...

to yield benzoic acid, and with chromyl chloride to yield benzaldehyde (

Étard reaction The Étard reaction is a chemical reaction that involves the direct oxidation of an aromatic or heterocyclic bound methyl group to an aldehyde using chromyl chloride. For example, toluene can be oxidized to benzaldehyde. It is named for the French ...

). The C-H bonds of the methyl group in toluene are benzylic, which means that they are weaker than C-H bonds in simpler alkanes. Reflecting this weakness, the methyl group in toluene undergoes halogenation under

free radical A daughter category of ''Ageing'', this category deals only with the biological aspects of ageing. Ageing Ailments of unknown cause Biogerontology Biological processes Causes of death Cellular processes Gerontology Life extension Metabo ...

conditions. For example, when heated with ''N''-bromosuccinimide (NBS) in the presence of AIBN, toluene converts to benzyl bromide. The same conversion can be effected with elemental

bromine Bromine is a chemical element with the symbol Br and atomic number 35. It is the third-lightest element in group 17 of the periodic table (halogens) and is a volatile red-brown liquid at room temperature that evaporates readily to form a simil ...

in the presence of UV light or even sunlight. Toluene may also be brominated by treating it with HBr and H₂O₂ in the presence of light. :C₆H₅CH₃ + Br₂ → C₆H₅CH₂Br + HBr :C₆H₅CH₂Br + Br₂ → C₆H₅CHBr₂ + HBr The methyl group in toluene undergoes deprotonation only with very strong bases; its p''K''_a is estimated to be approximately 41. Complete

hydrogenation Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate org ...

of toluene gives methylcyclohexane. The reaction requires a high pressure of hydrogen and a

catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

Miscibility

Toluene is miscible (soluble in all proportions) with

ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a h ...

, diethyl ether,

acetone Acetone (2-propanone or dimethyl ketone), is an organic compound with the formula . It is the simplest and smallest ketone (). It is a colorless, highly volatile and flammable liquid with a characteristic pungent odour. Acetone is miscible wi ...

chloroform Chloroform, or trichloromethane, is an organic compound with formula C H Cl3 and a common organic solvent. It is a colorless, strong-smelling, dense liquid produced on a large scale as a precursor to PTFE. It is also a precursor to various re ...

, glacial

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...

and carbon disulfide, but immiscible with water.

Production

Toluene occurs naturally at low levels in crude oil and is a byproduct in the production of

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 ...

by a

catalytic reformer 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-oct ...

ethylene Ethylene ( IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds). Ethylene ...

cracker Cracker, crackers or The Crackers may refer to: Animals * ''Hamadryas'' (butterfly), or crackers, a genus of brush-footed butterflies * '' Sparodon'', a monotypic genus whose species is sometimes known as "Cracker" Arts and entertainment Films ...

. It is also a byproduct of the production of coke from

coal Coal is a combustible black or brownish-black sedimentary rock, formed as stratum, rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen ...

. Final separation and purification is done by any of the

distillation Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distillation is the he ...

or solvent extraction processes used for BTX aromatics (benzene, toluene, and xylene isomers).

Other preparative routes

Toluene can be prepared by a variety of methods. For example,

reacts with methanol in presence of a solid acid to give toluene: :C6H6 + CH3OH -> ^o6H5CH3 + H2O

Uses

Precursor to benzene and xylene

Toluene is mainly used as a precursor to benzene via

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, ...

: :C₆H₅CH₃ + H₂ → C₆H₆ + CH₄ The second ranked application involves its

disproportionation In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term ca ...

to a mixture of benzene and

xylene In organic chemistry, xylene or xylol (; IUPAC name: dimethylbenzene) are any of three organic compounds with the formula . They are derived from the substitution of two hydrogen atoms with methyl groups in a benzene ring; which hydrogens are sub ...

Nitration

Nitration of toluene gives mono-, di-, and trinitrotoluene, all of which are widely used. Dinitrotoluene is the precursor to toluene diisocyanate, which used in the manufacture of

polyurethane Polyurethane (; often abbreviated PUR and PU) refers to a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethan ...

foam.

Trinitrotoluene Trinitrotoluene (), more commonly known as TNT, more specifically 2,4,6-trinitrotoluene, and by its preferred IUPAC name 2-methyl-1,3,5-trinitrobenzene, is a chemical compound with the formula C6H2(NO2)3CH3. TNT is occasionally used as a reag ...

is the explosive typically abbreviated TNT.

Oxidation

Benzoic acid and benzaldehyde are produced commercially by partial oxidation of toluene with

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

. Typical catalysts include

cobalt Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, ...

manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy u ...

naphthenates. :

Solvent

Toluene is a common

, e.g. for

paint Paint is any pigmented liquid, liquefiable, or solid mastic composition that, after application to a substrate in a thin layer, converts to a solid film. It is most commonly used to protect, color, or provide texture. Paint can be made in many ...

s, paint thinners, silicone sealants, many chemical reactants,

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, a ...

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 ...

ink, adhesives (glues), lacquers, leather tanners, and disinfectants.

Fuel

Toluene can be used as an octane booster in

fuels for

internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal co ...

s as well as jet fuel. Toluene at 86% by volume fuelled all the

turbocharged In an internal combustion engine, a turbocharger (often called a turbo) is a forced induction device that is powered by the flow of exhaust gases. It uses this energy to compress the intake gas, forcing more air into the engine in order to pro ...

engines in

Formula One Formula One (also known as Formula 1 or F1) is the highest class of international racing for open-wheel single-seater formula racing cars sanctioned by the Fédération Internationale de l'Automobile (FIA). The World Drivers' Championship ...

during the 1980s, first pioneered by the

Honda is a Japanese public multinational conglomerate manufacturer of automobiles, motorcycles, and power equipment, headquartered in Minato, Tokyo, Japan. Honda has been the world's largest motorcycle manufacturer since 1959, reaching a producti ...

team. The remaining 14% was a "filler" of ''n''- heptane, to reduce the octane rating to meet Formula One fuel restrictions. Toluene at 100% can be used as a fuel for both two-stroke and four-stroke engines; however, due to the

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...

of the fuel and other factors, the fuel does not vaporize easily unless preheated to . Honda solved this problem in their Formula One cars by routing the fuel lines through a

heat exchanger A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct conta ...

, drawing energy from the water in the cooling system to heat the fuel. In Australia in 2003, toluene was found to have been illegally combined with petrol in fuel outlets for sale as standard vehicular fuel. Toluene incurs no fuel excise tax, while other fuels are taxed at more than 40%, providing a greater profit margin for fuel suppliers. The extent of toluene substitution has not been determined.

Niche applications

In the laboratory, toluene is used as a solvent for carbon nanomaterials, including nanotubes and fullerenes, and it can also be used as a fullerene indicator. The color of the toluene solution of C₆₀ is bright purple. Toluene is used as a cement for fine

polystyrene Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per unit weight. It is ...

kits (by dissolving and then fusing surfaces) as it can be applied very precisely by brush and contains none of the bulk of an adhesive. Toluene can be used to break open

red blood cell Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...

s in order to extract

hemoglobin Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyte ...

in biochemistry experiments. Toluene has also been used as a coolant for its good heat transfer capabilities in sodium cold traps used in nuclear reactor system loops. Toluene had also been used in the process of removing the cocaine from coca leaves in the production of Coca-Cola syrup.

Toxicology and metabolism

The environmental and toxicological effects of toluene have been extensively studied. Inhalation of toluene in low to moderate levels can cause tiredness, confusion, weakness, drunken-type actions, memory loss, nausea, loss of appetite,

hearing loss Hearing loss is a partial or total inability to hear. Hearing loss may be present at birth or acquired at any time afterwards. Hearing loss may occur in one or both ears. In children, hearing problems can affect the ability to acquire spoken la ...

, and colour vision loss. Some of these symptoms usually disappear when exposure is stopped. Inhaling high levels of toluene in a short time may cause light-headedness, nausea, or sleepiness, unconsciousness, and even

death Death is the irreversible cessation of all biological functions that sustain an organism. For organisms with a brain, death can also be defined as the irreversible cessation of functioning of the whole brain, including brainstem, and brain ...

. Toluene is, however, much less toxic than

, and as a consequence, largely replaced it as an aromatic solvent in chemical preparation. The US Environmental Protection Agency (EPA) states that the

carcinogen A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive subst ...

ic potential of toluene cannot be evaluated due to insufficient information. In 2013, worldwide sales of toluene amounted to about 24.5 billion US-dollars. Similar to many other solvents such as

1,1,1-trichloroethane The organic compound 1,1,1-trichloroethane, also known as methyl chloroform, is a chloroalkane. This colorless, sweet-smelling liquid was once produced industrially in large quantities for use as a solvent. It is regulated by the Montreal Prot ...

and some

alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloal ...

s, toluene has been shown to act as a non-competitive NMDA receptor antagonist and GABA_A receptor positive allosteric modulator. Additionally, toluene has been shown to display

antidepressant Antidepressants are a class of medication used to treat major depressive disorder, anxiety disorders, chronic pain conditions, and to help manage addictions. Common side-effects of antidepressants include dry mouth, weight gain, dizziness ...

-like effects in rodents in the

forced swim test The behavioural despair test (or Porsolt forced swimming test) is a test, centered on a rodent's response to the threat of drowning, whose result has been interpreted as measuring susceptibility to negative mood. It is commonly used to measure the ...

(FST) and the tail suspension test (TST), likely due to its NMDA antagonist properties. Toluene is sometimes used as a recreational

("glue sniffing"), likely on account of its euphoric and dissociative effects. Toluene inhibits excitatory ion channels such as the NMDA receptor, nicotinic acetylcholine receptor, and the serotonin 5-HT₃ receptor. It also potentiates the function of inhibitory ion channels, such as the GABA_A and glycine receptors. In addition, toluene disrupts voltage-gated calcium channels and ATP-gated ion channels.

Recreational use

Toluene is used as an intoxicative inhalant in a manner unintended by manufacturers. People inhale toluene-containing products (e.g., paint thinner,

, model glue, etc.) for its intoxicating effect. The possession and use of toluene and products containing it are regulated in many jurisdictions, for the supposed reason of preventing minors from obtaining these products for recreational drug purposes. As of 2007, 24 U.S. states had laws penalizing use, possession with intent to use, and/or distribution of such inhalants. In 2005 the

European Union The European Union (EU) is a supranational political and economic union of member states that are located primarily in Europe. The union has a total area of and an estimated total population of about 447million. The EU has often been ...

banned the general sale of products consisting of greater than 0.5% toluene.

Bioremediation

Several types of fungi including ''

Cladophialophora ''Cladophialophora'' is a genus of fungi in the family Herpotrichiellaceae. It has 35 species. The genus contains black yeast-like fungi, some of which are species of important medical significance. '' Cladophialophora bantiana'' causes the rare ...

'', '' Exophiala'', ''

Leptodontidium ''Leptodontidium'' is a genus of fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are ...

'' (

syn. The Botanical and Zoological Codes of nomenclature treat the concept of synonymy differently. * In botanical nomenclature, a synonym is a scientific name that applies to a taxon that (now) goes by a different scientific name. For example, Linnae ...

''Leptodontium''), '' Pseudeurotium zonatum'', and '' Cladosporium sphaerospermum'', and certain species of bacteria can degrade toluene using it as a source of carbon and energy.

References

External links

ATSDR – Case Studies in Environmental Medicine: Toluene Toxicity
U.S. Department of Health and Human Services (public domain) * American Industrial Hygiene Association
The Ear Poisons
The Synergist, November 2018.
Toluene
CDC – NIOSH Workplace Safety and Health Topic (DHHS) * OSHA-NIOSH 2018. Preventing Hearing Loss Caused by Chemical (Ototoxicity) and Noise Exposure Safety and Health Information Bulletin (SHIB), Occupational Safety and Health Administration and the National Institute for Occupational Safety and Health. SHIB 03-08-2018. DHHS (NIOSH) Publication No. 2018-124. {{Authority control Hazardous air pollutants Hydrocarbon solvents Antiknock agents Alkylbenzenes Commodity chemicals Petrochemicals GABAA receptor positive allosteric modulators Glycine receptor agonists Euphoriants Inhalants Aromatic solvents Phenyl compounds Occupational safety and health NMDA receptor antagonists