Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon with the chemical formula C6H5CH3, often abbreviated as PhCH3, where Ph stands for the phenyl group. Toluene is a colorless, water-insoluble liquid with a sweet smell, and is partially responsible for the aroma of gasoline and paint fumes. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) attached to a phenyl group by a single bond. As such, its systematic IUPAC 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 and kept in 1837 through a distillation of pine oil by Pierre Joseph Pelletier and Filip Neriusz Walter, who named it rétinnaphte. In 1841, Henri Étienne Sainte-Claire Deville isolated a hydrocarbon from balsam of Tolu (an aromatic extract from the tropical Colombian tree Myroxylon balsamum), 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 The distance between carbon atoms in the toluene ring is 0.1399 nm. The C-CH3 bond is longer at 0.1524 nm, while the average C-H bond length is 0.111 nm.
Ring reactions 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. Nitration of toluene gives mono-, di-, and trinitrotoluene, all of which are widely used. Dinitrotoluene is the precursor to toluene diisocyanate, a precursor to polyurethane foam. Trinitrotoluene (TNT) is an explosive. Complete hydrogenation of toluene gives methylcyclohexane. The reaction requires a high pressure of hydrogen and a catalyst.
Side chain reactions The C-H bonds of the methyl group in toluene are benzylic, therefore they are weaker than C-H bonds in simpler alkanes. Reflecting this weakness, the methyl group in toluene undergoes a variety of free radical reactions. For example, when heated with N-bromosuccinimide (NBS) in the presence of AIBN, toluene converts 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.
C6H5CH3 + Br2 → C6H5CH2Br + HBr Benzoic acid and benzaldehyde are produced commercially by partial oxidation of toluene with oxygen. Typical catalysts include cobalt or manganese naphthenates. Related but laboratory-scale oxidations involve the use of potassium permanganate to yield benzoic acid and chromyl chloride to yield benzaldehyde (Étard reaction).
The methyl group in toluene undergoes deprotonation only with very strong bases; its pKa is estimated using acidity trends to be approximately 43 in dimethyl sulfoxide (DMSO) and its ion pair acidity is extrapolated to be 41.2 in cyclohexylamine (Cesium Cyclohexylamide) using a Bronsted correlation.
Miscibility Toluene is miscible (soluble in all proportions) with ethanol, benzene, diethyl ether, acetone, chloroform, glacial acetic acid and carbon disulfide, but immiscible with water.
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 (benzene, toluene, and xylene isomers).
Other preparative routes Toluene can be prepared by a variety of methods. For example, benzene reacts with methanol in presence of a solid acid to give toluene and water:
C 6 H 6 + CH 3 OH → t o C 6 H 5 CH 3 + H 2 O {\displaystyle {\ce {C6H6 + CH3OH ->[t^o]C6H5CH3 + H2O}}}
Uses Toluene is one of the most abundantly produced chemicals. Its main uses are (1) as a precursor to benzene and xylenes, (2) as a solvent for thinners, paints, lacquers, adhesives, and (3) as an additive for gasoline. In 2013, worldwide sales of toluene amounted to about 24.5 billion US dollars.
Precursor to benzene and xylenes Toluene is converted to benzene via hydrodealkylation:
C6H5CH3 + H2 → C6H6 + CH4 Its transalkylation gives a mixture of benzene and xylenes.
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