Toluene

Toluene
General
Systematic name	toluene
Other names	methylbenzene phenylmethane toluol
Molecular formula	C₇H₈
SMILES	Cc1ccccc1
Molar mass	92.14 g/mol
Appearance	Clear, colourless liquid
CAS number	^*
Properties
Density and phase	0.8669 g/cm³, liquid
Solubility in water	0.053 g/100 mL (20-25°C)
In ethanol, acetone, hexane, dichloromethane	Fully miscible
Melting point	-93 °C (180 K)/(-135.4°F)
Boiling point	110.6 °C (383.8 K)/ 231.08°F
Critical Temperature	320 °C (593 K)/ 608°F
Viscosity	0.590 cP at 20°C/ 68°F
Dipole moment	0.36 D
Hazards
MSDS	External MSDS
Main hazards	highly flammable
NFPA 704
Flash point	4 °C/ 39.2 °F
R-phrases	,
S-phrases	, , ,
RTECS number	XS5250000
Supplementary data page
Structure & properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Related aromatic hydrocarbons	benzene xylene naphthalene
Related compounds	methylcyclohexane
Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa) Chemical infobox

Toluene, also known as methylbenzene or phenylmethane is a clear, water-insoluble liquid with the typical smell of paint thinners, reminiscent of the sweet smell of the related compound benzene. It is an aromatic hydrocarbon that is widely used as an industrial feedstock and as a solvent.

History

The name toluene was derived from the older name toluol that refers to tolu balsam, an aromatic extract from the tropical American 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 chromyl chloride leads to benzaldehyde. Halogenation can be performed under free radical conditions. For example, NBS heated with toluene in the presence of AIBN leads to 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.

Preparation

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, 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), phenol and TNT. Toluene can be used as an octane booster in gasoline fuels used in internal combustion engines. Industrial uses of toluene include dealkylation to benzene and disproportionation to a mixture of benzene and xylene.

Toxicology and metabolism

Inhalation of toluene fumes can be intoxicating, but in larger doses nausea-inducing. Chronic or frequent inhalation of toluene over long time periods leads to irreversible brain damage. 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 ring oxidized to epoxides. Most of the epoxides become glutathione conjugated. However, the remainder will severely damage cells.

Toluene is mainly excreted as benzoic acid and hippuric acid, both formed by further metabolic oxidation of benzyl alcohol.

Behavioural Effects of Long Term Exposure

Serious adverse behavioural effects are often associated with toluene abuse related to the deliberate inhalation of solvents.⁴ Long term toluene exposure is often associated with effects such as: psychoorganic syndrome;⁵ visual evoked potential (VEP) abnormality;⁵ toxic polyneuropathy, cerebellar, cognitive, and pyramidal dysfunctions;⁵ optic atrophy; and brain lesions.⁴

Toluene causes postural tremors by increasing extracellular concentrations of γ-aminobutyric acid (GABA) within the cerebellar cortex.⁴ Treatment with GABA agonists such as, benzodiazepines provide some relief from toluene induced tremor and ataxia.⁴ An alternative to drug treatment is vim thalamotomy.⁴ The tremors associated with toluene misuse do not seem to be a transient symptom, but an irreversible and progressive symptom which continues after solvent abuse has been discontinued.⁴

There is some evidence that low level toluene exposure may cause disruption in the differentiation of astrocyte precursor cells.⁶ This does not appear to be a major hazard to adults; however, exposure of pregnant women to toluene during critical stages of fetal development could cause serious disruption to neuronal development.⁶

References

B. S. Furnell et al., Vogel's Textbook of Practical Organic Chemistry, 5th edition, Longman/Wiley, New York, 1989.
(a) L. G. Wade, Organic Chemistry, 5th ed., p. 871, Prentice Hall, Upper Saddle RIver, New Jersey, 2003.
J. March, Advanced Organic Chemistry, 4th ed., p. 723, Wiley, New York, 1992.
Miyagi, Y., Shima, F., Ishido, K. et al. Tremor induced by toluene misuse successfully treated by a Vim thalamotomy. Journal of Neurology Neurosurgery and Psychiatry 66(6):794-6, (1999).
Urban, P., Lukáš, E., Pelclová, D. et al. Neurological and Neurophysiological Follow-up on Workers With Severe Chronic Exposure to Toluene. Neurotoxicity P25:s130, (2003).
Yamaguchi, H., Kidachi, Y. and Ryoyama, K. Toluene at Environmentally Relevant Low Levels Disrupts Differentiation of Astrocyte Precursor Cells. Archives of Environmental Health 57(3):232-8, (2002).

Aromatic hydrocarbons | Schedule VI controlled substances | Soil contamination | Solvents

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