History and Discovery of Aniline
Aniline was first isolated in 1826 by Otto Unverdorben and Friedrich Runge by dry distillation of indigo. They called the new substance "krystallinischer tearsäure" ("crystalline tears acid") due to its sharp smell. The name aniline was introduced in 1840 by Eugène-Melchior Péligot, who reobtained the substance from the oxidation products of indigo. Aniline is named after the Latin name for indigo dye anil, which itself comes from the Sanskrit word nīla meaning "blue". Industrial scale production of aniline started in 1856 when the German chemists William Henry Perkin and his brother began manufacturing aniline for use as a dye. This marked the birth of the synthetic dye and the beginning of the golden age for the organic chemical .
Sources and Manufacturing
Aniline is produced by two main routes. In the most common process, benzene is nitrated to nitrobenzene, followed by hydrogenation to aniline. Another process involves hydrogenating toluene to produce methyl aniline followed by removal of the methyl group to yield aniline. The global production capacity of aniline is over 4 million metric tons per year. Major producers are China, India, Taiwan, South Korea and Western Europe.
Properties and Structure
Aniline is an organic compound with the formula C6H5NH2. It is a colorless, toxic liquid with an amine like odor. It is soluble in water, ethanol, ether and acids but insoluble in non-polar solvents. Aniline has a boiling point of 184°C and freezing point of -6°C. The planar phenyl group and the amino group attached provide aniline properties between those of aromatics and amines. Aniline exists predominantly in the neutral form in nonpolar solvents while in polar solvents it exists as a resonance stabilized.
Uses and Applications
MDI, Rubber Chemicals, Agrochemicals: Aniline is primarily used to produce methylene diphenyl diisocyanate (MDI), which is further processed to produce polyurethane foam for items like upholstery, insulation, car interiors etc. It is also used to produce rubber chemicals like aniline black that are used in making tires, tubes and belts. Aniline is used to synthesize agricultural chemicals like DDT and paraquat.
Dyes and Pigments: An important historic use of aniline was for production of synthetic dyes like fuchsine and aniline black. It was the first dye produced on industrial scale starting the modern synthetic dye . Aniline is still used to make dyes, albeit to a lesser extent now. It is a key intermediate for indigo and other azo dyes.
Pharmaceuticals: Selective reduction or alkylation of aniline derivatives leads to drugs like acetaminophen, sulfanilamide, barbiturates, local anesthetics etc. Aniline itself is a precursor for manufacturing sulfanilamide, a widely used antibiotic. Some anilines also have reported anti-inflammatory and antimicrobial properties.
Others: Small amounts of aniline are used in the production of explosives like TNT and pesticides. It is also used as a chemical intermediate in the synthesis of pigments, agrochemicals, rubber processing chemicals, antioxidants and corrosion inhibitors. Aniline wood is a common model compound used for teaching organic synthesis techniques in academia.
Health and Environmental Effects
Aniline is toxic by inhalation, ingestion and skin absorption. The primary health effects are methemoglobinemia and hemolytic anemia. Chronic exposure can potentially cause cancer in humans. The International Agency for Research on Cancer has classified aniline as possibly carcinogenic to humans. Strong fumes released during production and processing are also dangerous. Aniline is banned or restricted in many consumer products due to safety issues.
Being a moderately soluble organic compound, aniline can leach into groundwater from disposal sites. It does not bioaccumulate in organisms or food chains but spills in water can have adverse ecological impacts. Aniline is combustible in its vapor state forming toxic gases on combustion. Strict precautions are necessary during manufacturing and handling to avoid environmental pollution and occupational exposure. Responsible waste management practices must be followed for its safe disposal.
Aniline is one of the most important aromatic chemicals that marks the beginning of the synthetic chemical. Though hazardous, aniline and its derivatives have numerous industrial applications especially in manufacturing MDI for polyurethane foams and plastics. With proper safety protocols, aniline can continue to serve as a valuable aromatic feedstock and intermediate in specialty chemical synthesis.
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