History and Discovery of Aniline
Aniline was first isolated from the distillation products of coal tar in 1826 by Otto Unverdorben and Friedlieb Runge. They named it "Kristallin" because of its ability to form crystals. However, it was not until five years later in 1834 that the molecule was first synthesized from benzoyl chloride and ammonia by C. W. Scheele and named aniline, derived from the Latin word "anil", which means indigo. For a long time, aniline was only produced as a byproduct of coal gasification and was not explored for any commercial uses.
Chemical Structure and Properties
Aniline is an aromatic organic compound with the chemical formula C6H5NH2. It is composed of a phenyl group attached to an amino group. Aniline is a colorless, oily liquid that turns into yellowish crystals upon solidification. It has a weak basic property due to the presence of an amino group and can react with acids to form salts. Aniline has a distinctive, strong odor resembling that of rotten fish and evaporates easily at room temperature to form pungent-smelling vapors. It is soluble in water, alcohol, ether, benzene, and other organic solvents.
Uses and Applications
Due to its versatile reactivity, aniline finds numerous industrial applications and serves as an important chemical intermediate. One of the earliest commercial uses was in the manufacturing of dyes and pigments like aniline blue, aniline red, and magenta. Even today, over half of the aniline produced globally goes into the preparation of dyes and pigments. Besides this, other major uses include:
Production of polyurethane foams: Aniline reacts with phosgene to form methylene diphenyl diisocyanate (MDI), which is a key building block in producing polyurethane foams used in mattresses, furniture, insulation, etc.
Manufacture of explosives: Nitroaromatics like TNT and picric acid used in explosives are synthesized by nitrating aniline.
Rubber processing : Aniline is used as a reagent to accelerate vulcanization or hardening of natural and synthetic rubbers.
Pharmaceuticals: Some drugs like acetaminophen are manufactured via reactions involving aniline as an intermediate.
Resins and pesticides: Aniline finds applications in epoxy and polyester resins and synthesis of pesticides like chlordane.
Other miscellaneous uses: As a precursor to dyes, agrochemicals, corrosion inhibitors, polyaniline (a conducting polymer), and research/lab chemicals.
Toxicity and Safety Precautions
Prolonged or repeated exposure to aniline may cause harmful effects on human health. The primary routes of exposure are inhalation of vapors, ingestion, and skin or eye contact. Aniline is readily absorbed into the body through inhalation or ingestion and distributed to internal organs like liver and kidneys. The major target organs are blood and liver. Symptoms of acute overexposure may include methemoglobinemia (blue baby syndrome), headache, dizziness, nausea, weakness, and cyanosis (bluish discoloration of skin). Aniline is also known to damage red blood cells and cause hemolytic anemia. Prolonged exposure may potentially lead to blood, liver or kidney disorders. Due to its carcinogenic effects, aniline has been classified as a suspected human carcinogen by various regulatory agencies. Necessary safety precautions must be taken while handling aniline through adequate ventilation, protective equipment, and limiting exposure. Spills should be cleaned promptly with proper materials. Aniline waste requires careful disposal according to environmental regulations.
Substituted Anilines: 4-Chloroaniline and 2-nitroaniline
The versatility of aniline has led to the synthesis and study of numerous substituted aniline derivatives with different functional groups in the benzene ring. Some industrially important examples are:
4-Chloroaniline: This is a direct chlorination product of aniline prepared on an industrial scale. It is primarily utilized to manufacture dyes, pigments, agricultural chemicals, and polymerization initiators. Interestingly, 4-chloroaniline played a crucial role in the development of the textile during the Industrial Revolution.
2-Nitroaniline: Unlike aniline, 2-nitroaniline is an oil that melts into an orange solid. It undergoes electrophilic aromatic substitution more readily due to the withdrawing nitro group. 2-Nitroaniline serves as a precursor in synthesis of azo dyes, chemical intermediates, pharmaceuticals, explosives etc. One of its derivatives, 2,4,6-trinitroaniline (TNA) is an important marker for taggants in explosives.
Aniline occupies a unique position in organic chemistry as the simplest aromatic amine. Though initially an unwanted byproduct, aniline today forms the backbone for a wide range of industrial chemicals, dyes, pesticides, explosives, drugs and polymers due to its versatile reactivity pattern. While invaluable to many sectors, necessary safety precautions must be exercised when handling this toxic intermediate chemical. Continued research on aniline derivatives also holds promise for developing new materials and technologies.
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Tags – Aniline, Aromatic Chemicals, MDI (Methylene Diphenyl Diisocyanate), Agrochemicals Pharmaceutical Intermediates, Coherent Market Insights.