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Toluene (systematic chemical name: methylbenzene; CAS No.: 108-88-3; molecular formula: C7H8) is a volatile aromatic liquid at room temperature. Less toxic than benzene, it is a major basic chemical raw material; it is widely used as an industrial solvent and as a chemical intermediate for further processing, and also serves as a blending component for gasoline.
Electrophilic Substitution Reactions on the Benzene Ring (Meta‑directing Effect)
The methyl group is an ortho‑para director and activates the benzene ring. The main reactions are as follows:
1. Nitration: Reacts with mixed acid (concentrated HNO₃ + concentrated H₂SO₄) to produce o‑nitrotoluene and p‑nitrotoluene (used for dyes and pesticides). Deep nitration can produce TNT explosive (2,4,6‑trinitrotoluene).
2. Sulfonation: Reacts with concentrated sulfuric acid to produce o‑/p‑toluenesulfonic acid (used for synthesizing saccharin and detergents).
3. Halogenation: Reacts with chlorine or bromine under iron catalysis to produce o‑/p‑chlorotoluene (used for organic synthesis).
Reactions on the Side‑chain Methyl Group (α‑H Reactivity)
Affected by the benzene ring, the α‑hydrogen atoms are relatively active and prone to the following reactions:
1. Oxidation: Strong oxidants (e.g. acidified KMnO₄, concentrated HNO₃) can oxidize the methyl group into benzoic acid (a key raw material for food preservatives). Note: Toluene can decolorize acidified potassium permanganate solution (which distinguishes it from benzene).
2. Side‑chain Halogenation: Under light irradiation or heating conditions, it undergoes free‑radical substitution with halogens to produce benzyl chloride. (The product depends on halogen dosage; benzyl chloride is an important intermediate for plasticizers and fragrances.)
Other Properties
1. Addition reaction: Methylcyclohexane (used as a solvent) can be generated under high‑pressure hydrogenation conditions.
2. Auto‑ignition property: When vapor mixes with air at a specific concentration, open flame will cause deflagration (the core of chemical hazards).
Incompatible Substances: Strictly avoid contact with strong oxidants (potassium permanganate, potassium chlorate, peroxides, etc.), strong acids (concentrated nitric acid, fuming sulfuric acid), active halogens (chlorine), and other reactive substances (aluminum chloride, copper chlorate, etc.).
Chemical Intermediates (Core Consumption): Approximately 60%‑70% of toluene is consumed to be converted into other chemicals. The main routes include producing pure benzene (basic raw material) via hydrodealkylation, as well as manufacturing benzoic acid (food preservative), caprolactam (nylon monomer), and toluene diisocyanate (TDI, raw material for polyurethane foams and coating curing agents). TDI is an important raw material for polyurethane foam and PU resin.
High‑performance Solvent (Key Feature): Benefiting from strong dissolving power and fast volatilization rate, it is widely used in paints and coatings (nitro lacquer, polyurethane paint, acrylic paint, industrial spray paint as diluent solvent), inks (printing inks, screen‑printing inks for adjusting viscosity and evaporation rate), adhesives (diluting solvent for rubber adhesives, neoprene adhesives and synthetic adhesives). It is used for cleaning precision components in the electronics industry and acts as a solvent for pesticide emulsifiable concentrates in agriculture.
Gasoline Blending Component: It is used as a high‑octane gasoline additive to improve anti‑knock performance. However, due to its toxicity and environmental restrictions, its blending ratio has been strictly limited in developed countries and some cities in China.
Other Fine‑chemical Fields: It is used for extracting pharmaceutical intermediates (such as antibiotics) and serves as a dehydrating agent in chemical reactions. It acts as a synthetic intermediate for pharmaceuticals, pesticides and dyes, laboratory analytical solvent and chromatographic reagent. It is also used to produce dyes, synthetic fragrances and fixatives for cosmetics.
It must be stored in a cool, well-ventilated, dedicated warehouse, away from heat sources, ignition sources, and oxidizing agents. The storage temperature should not exceed 30°C; explosion-proof lighting and ventilation equipment must be used, and emergency facilities for handling leaks must be provided. Due to its high volatility, containers must be sealed while leaving 5%–7% of the volume as expansion space.
Vehicles used for transport must be qualified for the carriage of hazardous materials and display "Flammable Liquid" placards. During summer, transport should be conducted in the early morning or evening, and the cargo must be protected from direct sunlight en route. Grounding connections must be used during loading and unloading to dissipate static electricity; mixed loading or transport with oxidizing agents, food-grade chemicals, or similar substances is strictly prohibited. Tank trucks must be equipped with flame arresters.
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