Toluene: properties, production and applications
Toluene (methylbenzene, phenylmethane) is the simplest homologue of benzene, one of the most important aromatic hydrocarbons and large-tonnage petrochemical products. It is a colourless, transparent liquid with a characteristic odour, lighter than water and insoluble in it, but miscible in all proportions with most organic solvents. Toluene occupies a key place in the production of solvents, high-octane fuel components and as a chemical feedstock for the synthesis of benzene, xylenes and many other compounds.
Physicochemical Properties
The molecular formula of toluene is C₇H₈, with a relative molecular mass of 92.14. Density at 20 °C is 0.865–0.867 g/cm³, refractive index — 1.4969. Boiling point — 110.6 °C, melting point — minus 95 °C. Toluene is highly flammable: flash point — about 4 °C, autoignition temperature — 536 °C, explosive limits in air — 1.25–6.5% by volume. Toluene vapours are heavier than air and can accumulate in low areas of premises.
Toluene belongs to hazard class 3 substances, has a narcotic and irritating effect, and affects the nervous system and mucous membranes. The maximum allowable concentration of vapours in the working area air is 50 mg/m³. When working with toluene, effective ventilation and personal protective equipment are required.
Production Technology
The main industrial sources of toluene are catalytic reforming of gasoline fractions, pyrolysis of petroleum feedstock and toluene hydrodealkylation in benzene production. Significant quantities of toluene are also recovered from coal tar in coke-chemical production. Modern technologies allow toluene of high purity suitable for reactive and polymer synthesis to be obtained.
The quality of petroleum toluene in Russia and CIS countries is regulated by GOST 31906-2013, which establishes requirements for the product intended for use as a feedstock in the petrochemical and chemical industry. For coal toluene, GOST 9880-76 applies, and for reagent-grade toluene — GOST 5789-78. Key quality indicators are the mass fraction of the main substance, density, distillation limits, sulfur, thiophene and non-volatile residue content.
Applications
Solvent. Toluene is widely used as a solvent in the paint and coatings industry for diluting alkyd, epoxy and other resins, in the production of adhesives, printing inks and sealants. It effectively dissolves many polymers, rubbers and resins, providing the required consistency and processability of compositions.
Feedstock for organic synthesis. The largest consumption area for toluene is its conversion to benzene by hydrodealkylation, as well as to xylenes by disproportionation. Significant volumes of toluene are used to produce toluene diisocyanate (TDI) — a key polyurethane component — as well as for the synthesis of benzoic acid, dyes, pharmaceuticals and explosives (TNT). Toluene serves as a feedstock for chlorobenzene, nitrotoluenes and other intermediates.
High-octane additive. Toluene has a high octane number and is used as a motor fuel component that increases the anti-knock resistance of gasolines. However, its content in commercial gasolines is limited by environmental regulations.
Other applications. Toluene is used in the production of polymeric materials, as an extractant in petroleum refining, in laboratory practice and as a component of special solvents for surface degreasing.
Safety and Storage
Toluene is stored in hermetically sealed containers made of steel or polymeric materials, in well-ventilated areas, away from ignition sources and oxidisers. Transportation is carried out in specialised tank cars, barrels and cans in compliance with the rules for the transport of flammable liquids. When working with toluene, personal protective equipment must be used and fire safety measures observed.