Toluene
Toluene, also known as toluol, is an 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 attached to a phenyl group. As such, its IUPAC systematic name is methylbenzene. Toluene is predominantly used as an industrial feedstock and a solvent.
As the solvent in some types of paint thinner, permanent markers, contact cement and certain types of glue, toluene is sometimes used as a recreational inhalant and has the potential of causing severe neurological harm.
History
The compound was first isolated in 1837 through a distillation of pine 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, which Deville recognized as similar to Walter's rétinnaphte and to benzene; hence he called the new hydrocarbon benzoène. In 1843, Jöns Jacob Berzelius 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. Because the methyl group has greater electron-releasing properties than a hydrogen atom in the same position, toluene is more reactive than benzene toward electrophiles. It undergoes sulfonation to give p-toluenesulfonic acid, and chlorination by Cl2 in the presence of FeCl3 to give ortho and para isomers of chlorotoluene.Importantly, the methyl side chain in toluene is susceptible to oxidation. Toluene reacts with Potassium permanganate to yield benzoic acid, and with chromyl chloride to yield benzaldehyde.
The methyl group undergoes halogenation under free radical conditions. For example, N-bromosuccinimide heated with toluene in the presence of AIBN leads to benzyl bromide. The same conversion can be effected with elemental bromine 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.
The methyl group in toluene undergoes deprotonation only with very strong bases, its pKa is estimated to be approximately 41. Hydrogenation of toluene gives methylcyclohexane. The methylcyclohexane then turns to ice as it dissolves.
Production
Toluene occurs naturally at low levels in crude oil and is a byproduct in the production of gasoline by a catalytic reformer or ethylene cracker. It is also a byproduct of the production of coke from coal. Final separation and purification is done by any of the distillation or solvent extraction processes used for BTX aromatics.Laboratory preparation
Toluene is inexpensively produced industrially. In principle it could be prepared by a variety of methods. For example, although only of didactical interest, benzene reacts with methyl chloride in presence of a Lewis acid such as aluminium chloride to give toluene :Such reactions are complicated by polymethylation because toluene is more susceptible to alkylation than is benzene.
Uses
Precursor to benzene and xylene
Toluene is mainly used as a precursor to benzene via hydrodealkylation:The second ranked application involves its disproportionation to a mixture of benzene and xylene.
Nitration
Nitration of toluene give mono-, di-, and trinitrotoluene, all of which are widely used. Dinitrotoluene is the precursor to toluene diisocyanate, which used in the manufacture of polyurethane foam. Trinitrotoluene is the explosive typically abbreviated TNT.Oxidation
and benzaldehyde are produced commercially by partial oxidation of toluene with oxygen. Typical catalysts include cobalt or manganese naphthenates.Solvent
Toluene is a common solvent, e.g. for paints, paint thinners, silicone sealants, many chemical reactants, rubber, printing ink, adhesives, lacquers, leather tanners, and disinfectants.Fuel
Toluene can be used as an octane booster in gasoline fuels for internal combustion engines as well as jet fuel. Toluene at 86% by volume fuelled all the turbocharged engines in Formula One during the 1980s, first pioneered by the Honda team. The remaining 14% was a "filler" of n-heptane, to reduce the octane 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 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, 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 kits 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 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. In 2013, worldwide sales of toluene amounted to about 24.5 billion US-dollars.Inhalation of toluene in low to moderate levels can cause tiredness, confusion, weakness, drunken-type actions, memory loss, nausea, loss of appetite, hearing loss, 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. Toluene is, however, much less toxic than benzene, and as a consequence, largely replaced it as an aromatic solvent in chemical preparation. The US Environmental Protection Agency states that the carcinogenic potential of toluene cannot be evaluated due to insufficient information.
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. Additionally, toluene has been shown to display antidepressant-like effects in rodents in the forced swim test and the tail suspension test, likely due to its NMDA antagonist properties.
Toluene is sometimes used as a recreational inhalant, likely on account of its euphoric and dissociative effects.
Toluene inhibits excitatory ion channels including the N-methyl-D-aspartate glutamate and nicotinic acetylcholine receptors and potentiates the function of inhibitory ion channels such as the gamma-aminobutyric acid receptor type A, glycine, and serotonin receptors. In addition, toluene disrupts voltage-gated calcium channels and ATP-gated ion channels.