It was first isolated in 1826 by Otto Unverdorben from the beta-naphthylamine present in coal tar. He was able to isolate a volatile crystalline substance from the tar which was given the name aniline. Its structure and chemical nature remained unknown for many years. In 1834, it was shown to be a flammable oily liquid. Further chemical analysis established it contained carbon, hydrogen and nitrogen but its exact structure was still undetermined. It was not until 1860 when German chemists August Wilhelm von Hofmann and Emil Erlenmeyer established its structure as a derivative of benzene with an amino substituent. This finally proved that it was the first aromatic amine to be isolated.
Properties and Structure
It is an organic compound with the molecular formula C6H5NH2. It is a colorless, transparent oily liquid which becomes pale yellow on exposure to air and light. It has a strong characteristic aromatic smell. It has a molecular weight of 93.13 g/mol and melts at 6°C and boils at 184°C. It is weakly basic in nature and soluble in water, ethanol, ether and benzene.
Its structure consists of a benzene ring with an amino group (-NH2) directly attached to one of the carbon atoms of the ring. This amino substituent makes it an aromatic amine. It can exist as either neutral molecules or as anilinium cations (C6H5NH3+) in solution depending on the pH. The amino group is an electron donating substituent which increases the electron density of the benzene ring.
Industrial Uses and Commercial Production
One of the most important industrial uses of it is in the production of precursors for polyurethane polymers. Approximately 45% of commercially produced it is used to manufacture methylenediphenyl diisocyanate (MDI) and toluene diisocyanate (TDI) which are important intermediates in polyurethane production.
Another major use is in the manufacture of dyes and organic compounds. It reacts with benzaldehyde to produce azobenzene dye which is orange-brown in color. Azo dyes are a largest class of synthetic dyes and find widespread application in coloring goods and textiles. It is also used to prepare phenol for manufacturing phenol-formaldehyde resins.
Commercially, it is prepared by two different processes - reduction of nitrobenzene and catalytic ammoxidation of benzene. Nitrobenzene provides about 90% of world's supply. It is reduced to aniline by hydrogenation over metal catalysts such as Raney nickel or iron. The most common industrial process involves ammoxidation of benzene to produce cyclohexylamine which is then dehydrogenated to aniline.
Toxicology and Safety Precautions
Due to its chemical nature, it has some toxicity associated with it. The major routes of exposure are through inhalation of vapors, ingestion and skin contact. Symptoms of overexposure may include headache, dizziness, skin and eye irritation, unconsciousness and even death in severe cases. It is classified as a carcinogen as it can cause cancer with long term exposure.
It is readily absorbed through the skin and easily metabolized in the liver. The major toxic effects are on the blood due to its methemoglobin forming potential. Aniline and its metabolites bind to hemoglobin and prevent it from carrying oxygen leading to hypoxia or suffocation. It can also affect the central nervous system, kidney and gastrointestinal tract on overexposure.
Proper protective equipment should always be worn while handling it. It is advisable to work in a well ventilated area. Skin contact should be avoided and hands should be thoroughly washed after contact. It is moderately toxic to aquatic organisms as well so care should be taken during transportation and storage near water sources. Overall, it requires responsible handling and disposal as per safety regulations due to its toxic nature.
Applications in Pharmaceuticals
Despite its toxicity issues, itcontinues to find application in pharmaceutical synthesis. Its derivatives are an important class of pharmaceutical compounds used as antibacterial, antifungals and anti-inflammatories. Sulphanilamide, one of the earliest antibacterial sulfonamides, is synthesized from it. It was one of the first antibacterials to treat streptococcal infections in the 1930s.
Other aniline derived drugs include analgesic phenacetin, antihistamine chlorpheniramine, antimalarial primaquine and muscle relaxant carisoprodol. It serves as a precursor in synthesizing indole alkaloids like reserpine which lowers blood pressure and mescaline which has psychoactive properties. Thus its versatility in chemical reactions helps produce important drugs even today through introduction of different substituents on the aromatic ring. Researchers continue developing newer aniline analogs for potentials in therapeutics.
Health and Environmental Hazards
While it finds many commercial uses, it also poses hazards due to its toxicity profile if not handled properly. Long term health effects were observed in workers exposed to it fumes in their occupational setting. They exhibited methemoglobinemia and symptoms of hypoxia even at low exposure levels. Studies found association of it exposure with increased risk of cancers like bladder cancer on chronic contact.
Being a pollutant, it can contaminate air, water and soil if spilled during storage, transport or industrial use. It is moderately toxic to aquatic life and can bioaccumulate in fish tissues. Biodegradation of it is a slow process and it may leach into groundwater from hazardous waste sites polluting drinking sources. Spills need to be contained and cleaned promptly as per standard protocols.
Stringent regulations are in place for its manufacture, use and disposal globally. Producers are obligated to adopt all safety precautions to prevent release into the environment. Monitoring of air and water at facilities near habitations helps check compliance. While its availability is necessary for industries, human and environmental protection should not be compromised at any cost. With responsible production and engineered risk management, its risks can be contained.
aniline is an important chemical building block discovered in coal tar. Its versatile reactivity helps generate dyes, polymers, pharmaceuticals and other consumer products essential for modern life. However, as an aromatic amine it also displays toxic effects primarily through methemoglobin formation. Safe handling practices and adherence to safety guidelines during industrial synthesis and applications are necessary considering potential health and environmental impacts. Continued research in its chemistry will possibly offer more application opportunities. Overall, with appropriate risk control measures, it can be utilized sustainably for various end uses.
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