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hydrogen atom from a group of six atoms from the benzene molecule has been replaced by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.
History The compound was first isolated in 1837 through
a distillation of pine oil by a Polish chemist named Filip Walter, 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 toluene was also isolated later. It was originally named by Jöns Jakob
Berzelius. Physical properties
The surface tension of toluene is 27.73 dyn/cm. 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.
With other reagents the methyl side chain in toluene may react, undergoing oxidation.
Reaction with potassium permanganate and diluted acid or potassium permanganate with concentrated
sulfuric acid, leads to benzoic acid, whereas reaction with chromyl chloride leads to benzaldehyde.
Halogenation can be performed under free radical conditions. For example, N-bromosuccinimide
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 to yield benzyl bromide. Toluene
may also be brominated by treating it with HBr and H2O2 in the presence of light.
Catalytic hydrogenation of toluene to methylcyclohexane requires a high pressure of hydrogen to go
to completion, because of the stability of the aromatic system. pKa is approximately
45. Production
Toluene occurs naturally at low levels in crude oil and is usually produced in the process
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 one
of the many available processes for extraction of the BTX aromatics.
Laboratory preparation Toluene can be prepared industrially or in
the laboratory by a variety of methods. From benzene
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 + CH3Cl+ → 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. From bromobenzene
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
When sodium salt of toluic acid is heated with soda lime, toluene is obtained.
C6H4CH3COONa + NaOH → C6H5CH3 + Na2CO3 From cresol
When cresol is distilled from zinc dust, toluene is obtained.
C6H4CH3OH + Zn → C6H5CH3 + ZnO From toluenesulfonic acid
When toluenesulfonic acid is treated with superheated steam or boiled with HCl, toluene
is obtained. CH3C6H4SO3H + HOH → C6H5CH3 + H2SO4
From toluidine Toluidine is first diazotized with sodium
nitrite and HCl at low temperature. The diazonium compound thus obtained is heated with alkaline
stannous chloride. This reaction gives toluene. Uses
Toluene is mainly used as a precursor to benzene. The process involves hydrodealkylation:
C6H5CH3 + H2 → C6H6 + CH4 The second ranked application involves its
disproportionation to a mixture of benzene and xylene. When oxidized it yields benzaldehyde
and benzoic acid, two important intermediates in chemistry.
Solvent Toluene is a common solvent, able to dissolve
paints, paint thinners, silicone sealants, many chemical reactants, rubber, printing
ink, adhesives, lacquers, leather tanners, and disinfectants. It is also a raw material
for toluene diisocyanate and TNT. In addition, it 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. It is also 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. Fuel
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.
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 is another in a group of fuels that have recently been used as components for
jet fuel surrogate blends. Toluene is used as a jet fuel surrogate for its content of
aromatic compounds. Niche applications
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
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
and the tail suspension test. Bioremediation
Several types of fungi including 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. References
External links ATSDR – Case Studies in Environmental Medicine:
Toluene Toxicity U.S. Department of Health and Human Services
Toluene CDC – NIOSH Workplace Safety and Health Topic