Toluene, named after a Colombian tree balsam, flavours petrol fumes and makes TNT
Part of that sweet smell at the petrol pump and in fresh paint is toluene, a benzene ring carrying one extra methyl group. Its name traces to balsam of Tolu, a resin from a tropical tree, and the same molecule ends up in foam cushions, nail polish, Formula One fuel and TNT.
Pierre Joseph Pelletier and Filip Neriusz Walter first isolated the compound in 1837 by distilling pine oil, and gave it the name rétinnaphte. Four years on, in 1841, the French chemist Henri Étienne Sainte-Claire Deville pulled a similar hydrocarbon from balsam of Tolu, an aromatic extract of the Colombian tree Myroxylon balsamum, and recognised its kinship with benzene. The eventual name toluene reflects that balsam. Chemically it is methylbenzene: a colourless liquid that will not mix with water but blends freely with ethanol, acetone, ether and many other solvents.
That single methyl group changes a lot. It pushes electrons into the ring, so toluene reacts more readily than benzene with electrophiles; chlorinating it gives ortho and para chlorotoluenes. The methyl group's own hydrogens are relatively weakly held, so light or radical initiators can swap one for bromine to make benzyl bromide. Nitration produces mono-, di- and trinitrotoluene. Dinitrotoluene leads on to toluene diisocyanate and hence polyurethane foam, while trinitrotoluene is the explosive TNT. Partial oxidation with oxygen yields benzoic acid and benzaldehyde commercially.
Toluene is one of the most heavily produced chemicals, worth about 24.5 billion dollars in sales in 2013. It occurs naturally in small amounts in crude oil and arrives as a byproduct of petrol refining and of turning coal into coke. Its main uses are as a feedstock for benzene and xylenes, as a solvent in paints, lacquers, glues, inks and rubber, and as an octane booster in petrol, jet fuel and turbocharged Formula One engines.
Its solvent role has a darker side: found in thinners, markers and contact cement, it is sometimes inhaled recreationally and can cause severe neurological damage. It has also been used for fraud. In Australia in 2003 it was found illegally blended into petrol sold as standard fuel, because toluene escaped fuel excise while other fuels were taxed at more than 40 per cent.
Source: Toluene
