Toluene //, formerly known as toluol //, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from a group of six atoms from the benzene molecule has been replaced by a univalent group, in this case CH3. As such, its IUPAC systematic name is methylbenzene.
It is an aromatic hydrocarbon that is widely used as an industrial feedstock and as a solvent. Like other solvents, toluene is sometimes also used as an inhalant drug for its intoxicating properties; however, inhaling toluene has potential to cause severe neurological harm. Toluene is an important organic solvent, but is also capable of dissolving a number of notable inorganic chemicals such as sulfur, iodine, bromine, phosphorus, and other non-polar covalent substances.
The compound was first isolated in 1837 by a Polish chemist Filip Walter through a distillation of pine oil who named it retinaptha. This name was replaced shortly after by the word toluene derived from the older name toluol, which refers to tolu balsam, an aromatic extract from the tropical Colombian tree Myroxylon balsamum, from which it was also isolated later. It was originally named by Jöns Jakob Berzelius.
The surface tension of toluene is 27.73 dyn/cm.
Toluene reacts as a normal aromatic hydrocarbon towards electrophilic aromatic substitution. The methyl group makes it around 25 times more reactive than benzene in such reactions. It undergoes smooth sulfonation to give p-toluenesulfonic acid, and chlorination by Cl2 in the presence of FeCl3 to give ortho and para isomers of chlorotoluene.
With other reagents the methyl side chain in toluene may react, undergoing oxidation. Reaction with potassium permanganate and diluted acid (e.g., sulfuric acid) or potassium permanganate with concentrated sulfuric acid, leads to benzoic acid, whereas reaction with chromyl chloride leads to benzaldehyde (Étard reaction). Halogenation can be performed under free radical conditions. For example, N-bromosuccinimide (NBS) heated with toluene in the presence of AIBN leads to benzyl bromide. Toluene can also be treated with elemental bromine in the presence of UV light (direct sunlight) to yield benzyl bromide. Toluene may also be brominated by treating it with HBr and H2O2 in the presence of light.
Toluene occurs naturally at low levels in crude oil and is usually produced in the processes of gasoline via a catalytic reformer, in an ethylene cracker or making coke from coal. Final separation, either via distillation or solvent extraction, takes place in one of the many available processes for extraction of the BTX aromatics (benzene, toluene and xylene isomers).
Toluene can be prepared industrially or in the laboratory by a variety of methods.
Although only of pedagogical interest, benzene reacts with methyl chloride in presence of anhydrous aluminium chloride to form toluene. The formation follows an electrophilic substitution reaction according to this stoichiometry:
- C6H5H + CH3+ → C6H5CH3 + HCl
Many catalysts can be used in place of AlCl3. Their order of reactivity is :AlCl3 > SbCl3 > SnCl4 > BF3 > ZnCl2 > HgCl2. The reaction is not very useful because toluene formed readily undergoes further alkylation at a still-greater speed to produce polysubstituted products.
The Wurtz-Fittig reaction is the reaction of an aryl halide and alkyl halide in presence of sodium metal to give substituted aromatic compounds. When bromobenzene and methyl bromide react with sodium metal in dry ether solution, toluene is obtained.
- C6H5Br + CH3Br + 2Na → C6H5CH3 + 2NaBr
- From toluic acid (decarboxylation)
- C6H4CH3COONa (sodium toluate) + NaOH → C6H5CH3 (toluene) + Na2CO3
- From cresol
When cresol (o-, m-, p-) is distilled from zinc dust, toluene is obtained.
- C6H4CH3OH (cresol) + Zn → C6H5CH3 (toluene) + ZnO
- From toluenesulfonic acid
When toluenesulfonic acid is treated with superheated steam or boiled with HCl, toluene is obtained.
- CH3C6H4SO3H (toluenesulfonic acid) + HOH (steam) → C6H5CH3 (toluene) + H2SO4 (sulfuric acid)
- From toluidine
Toluidine is first diazotized with sodium nitrite (NaNO2) and HCl at low temperature. The diazonium compound thus obtained is heated with alkaline stannous chloride (SnCl2). This reaction gives toluene.
Toluene is mainly used as a precursor to benzene. The process involves hydrodealkylation:
- C6H5CH3 + H2 → C6H6 + CH4
Toluene is a common solvent, able to dissolve paints, paint thinners, silicone sealants, many chemical reactants, rubber, printing ink, adhesives (glues), lacquers, leather tanners, and disinfectants. It can also be used as a fullerene indicator, and is a raw material for toluene diisocyanate (used in the manufacture of polyurethane foam) and TNT. In addition, it is used as a solvent for carbon nanotubes including fullerenes (the color of the toluene solution of C60 is bright purple). It is also used as a cement for fine polystyrene 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 cells in order to extract hemoglobin in biochemistry experiments.
Toluene can be used as an octane booster in gasoline fuels used in internal combustion engines. Toluene at 86% by volume fueled all the turbo Formula 1 teams in the 1980s, first pioneered by the Honda team. The remaining 14% was a "filler" of n-heptane, to reduce the octane to meet Formula 1 fuel restrictions. Toluene at 100% can be used as a fuel for both two-stroke and four-stroke engines; however, due to the density of the fuel and other factors, the fuel does not vaporize easily unless preheated to 70 degrees Celsius (Honda accomplished this in their Formula 1 cars by routing the fuel lines through the muffler system to heat the fuel). Standard gasoline in the United States currently contain many alcohols in a variety of seasonal blends, including tolulene. Tolulene alone can account for 18 percent of the near 50 to 60 percent overall alcohol levels found in gasoline.
In Australia, toluene has been found to have been illegally combined with petrol in fuel outlets for sale as standard vehicular fuel. Toluene attracts no fuel excise, while other fuels are taxed at over 40%, so fuel suppliers are able to profit from substituting the cheaper toluene for petrol. The extent of toluene substitution has not been determined.
Toluene has also been used as a coolant for its good heat transfer capabilities in sodium cold traps used in nuclear reactor system loops.
Toxicology and metabolism
Inhalation of toluene in low to moderate levels can cause tiredness, confusion, weakness, drunken-type actions, memory loss, nausea, loss of appetite, and hearing and color vision loss. These symptoms usually disappear when exposure is stopped. Inhaling high levels of toluene in a short time may cause light-headedness, nausea, or sleepiness. It can also cause unconsciousness, and even death.
Toluene is, however, much less toxic than benzene, and has, as a consequence, largely replaced it as an aromatic solvent in chemical preparation. For example, benzene is a known carcinogen, whereas toluene has very little carcinogenic potential.
Similar to many other solvents such as 1,1,1-trichloroethane and some alkylbenzenes, toluene has been shown to act as a non-competitive NMDA receptor antagonist and GABAA receptor positive allosteric modulator. It is abused as an inhalant likely on account of the euphoric and dissociative effects these actions produce. Additionally, toluene has been shown to display antidepressant-like effects in rodents in the forced swim test (FST) and the tail suspension test (TST).
Several species of fungi; Cladophialophora, Exophiala, Leptodontium, Pseudeurotium zonatum, and Cladosporium sphaerospermum, and certain species of bacteria, can degrade toluene; using it as a source of carbon and energy. 
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- title=Isolation and characterisation of fungi growing on volatile aromatic hydrocarbons as their sole carbon and energy source|author1=Francesc X. PRENAFETA-BOLDU|author2=Andrea KUHN|author3=Dion M. A. M. LUYKX|author4=Heidrun ANKE|author5=Johan W. van GROENESTIJN|author6=Jan A. M. de BONT|volume=105|issue=4|pages=477-484|date=April 2001|
|Wikimedia Commons has media related to toluene.|
- ATSDR - Case Studies in Environmental Medicine: Toluene Toxicity U.S. Department of Health and Human Services (public domain)
- Toluene CDC - NIOSH Workplace Safety and Health Topic (DHHS)