Toluene, also known as
methylbenzene, or
Toluol, is a clear
water-insoluble liquid with the typical smell of
paint thinners, reminiscent of the related compound
benzene. It is an
aromatic hydrocarbon that is widely used as an industrial
feedstock and as a
solvent. Like other solvents, toluene is also used as an
inhalant drug for its intoxicating properties; however this causes severe neurological harm.
History
The name
toluene was 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 first isolated. It was originally named by
Jöns Jakob Berzelius.
Chemical properties
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. It undergoes
nitration to give ortho and para
nitrotoluene isomers, but if heated it can give
dinitrotoluene and ultimately the explosive
trinitrotoluene (TNT).
With other
reagents the methyl
side chain in toluene may react, undergoing oxidation. Reaction with
potassium permanganate 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.
Catalytic
hydrogenation of toluene to
methylcyclohexane requires a high pressure of
hydrogen to go to completion, because of the stability of the aromatic system.
pK
a is approximately 45.
Production
Toluene occurs naturally at low levels in
crude oil and is usually produced in the processes of making
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 a
BTX plant.
Uses
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. 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.
Industrial uses of toluene include dealkylation to benzene, and the
disproportionation to a mixture of benzene and
xylene in the
BTX process. When oxidized it yields benzaldehyde and benzoic acid, two important intermediates in chemistry. It is also used as a carbon source for making Multi-Wall
Carbon Nanotubes. 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). Toluene also poses similar problems as alcohol fuels, as it eats through standard rubber fuel lines and has no lubricating properties as standard gasoline does, which can break down fuel pumps and cause upper cylinder bore wear.
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. This substitution is likely to affect engine performance and result in additional wear and tear. 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.
Toluene can be inhaled for its intoxicating effects. 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 may negatively affect kidney function.
Toluene has also been used in the process of
removing the cocaine from coca leaves in the production of Coca-Cola syrup.
Toxicology and metabolism
Inhalation of toluene fumes can be intoxicating, but in larger doses nausea-inducing. Toluene may enter the human system not only through vapour inhalation from the liquid evaporation, but also following
soil contamination events, where human contact with soil, ingestion of contaminated groundwater or soil vapour off-gassing can occur.
The toxicity of toluene can be explained mostly by its metabolism. As toluene has very low water solubility, it cannot exit the body via the normal routes (urine, feces, or sweat). It must be metabolized in order to be excreted. The methyl group of toluene is more easily oxidized by
cytochrome P450 than the benzene ring. Therefore, in the metabolism of toluene, 95% is oxidized to become
benzyl alcohol. The toxic metabolites are created by the remaining 5% that are oxidized to
benzaldehyde and
cresols. Most of the reactive products are
detoxified by conjugation to
glutathione but the remainder may severely damage cells.
The metabolism of toluene
Toluene is mainly excreted as
benzoic acid and
hippuric acid, both formed by further metabolic oxidation of benzyl alcohol.
The metabolism of benzyl alcohol
See also
Footnotes
