Aniline

Aniline is an organic chemical known as an aromatic amine. It includes a ring of the chemical benzene connected by an amino group, and it can be derived from coal tar. It is a clear to pale yellowish-brown oily liquid that can mix with water and many other organic solvents (substances that can dissolve other substances). Aniline is used in the manufacture of products such as polyurethane foam, agricultural chemicals and herbicides used for weed control, explosives, stabilizers used in the manufacture of rubber, and varnishes. It is particularly associated with the manufacture of synthetic dyes and colorants; aniline dyes are so important that the products that use them often include aniline in their name, such as aniline leathers.

Background

Coal was a major source of fuel in nineteenth-century Europe and America. It was used in several different forms but most commonly in the form of coal gas and coke. These were made by burning bituminous, or soft coal, in special containers that restricted the availability of oxygen for the combustion process. The combination of heat and very low oxygen converted the coal into several components. About 60 percent of it became coke and the remaining 40 percent became coal gas. This coal gas was further refined to separate the gas from some liquid and solid components. The solids were mostly coal tar, a foul-smelling and sticky substance that resembled tar but did not have tar's useful ability to waterproof.

The demand for coal gas and coke meant that there was an abundance of coal tar, but it was useless and toxic to the environment. Chemists began looking for uses for coal tar in the hope of finding a purpose for the nuisance product. In the process, they broke the coal tar down into several component chemicals, including benzene and aniline.

Russian chemist C.J. Fritzsche first named aniline. It had previously been isolated by German chemists Otto Unverdorben in 1826 and Friedlieb Runge in 1834; however, neither of them had given it a name. Fritzsche named it for the plant Indigofera anil, which was a source of indigo blue dyes.

One of the chemists working on the coal tar problem was August Wilhelm von Hofmann, a professor at the Royal College of Chemistry in London, England. By the 1840s, Hofmann had identified a number of components of coal tar and found uses for some of them. His most notable discovery at that point was a substance called creosote, which could be used to preserve wooden items such as railroad ties.

Hofmann had noticed that some of these components were amines and that they had a similar chemical composition to quinine, an important antimalarial drug that was always in short supply. Hofmann assigned one of his best students, William Henry Perkin, to try to synthesize quinine from the amines in coal tar. Perkins, who was eighteen at the time, worked on the problem in a makeshift laboratory in the attic of his parents' home during Easter break. He failed many times. However, when he rinsed out the test tubes from one of the failures, one where he had used aniline as part of the experiment, he noticed a solid black substance that left a bluish-purple residue. When he wiped up the substance, it stained the white cloth a perfect shade of purple.

Prior to this, all dyes were made from natural substances, and dying anything required gathering and preparing large amounts of plant or animal material that would result in the desired color. Perkin's failed attempt at quinine became the first synthetic dye; ultimately, it was discovered that the aniline in the coal tar made the dye work. Additional experimentation resulted in many other aniline dyes and, eventually, into uses in the manufacture of other products.

Overview

Aniline is a colorless to nearly colorless oily substance that is liquid at room temperature and boils at about 362 degrees Fahrenheit (184 degrees Celsius). It is flammable and gives off a great deal of smoke when burned. Aniline has an unpleasant smell that has been compared to that of spoiled fish.

A number of compounds can be made by combining aniline with acids. For example, combining it with acyl halides, which are derived from carboxylic acid, results in the formation of amides. Anilines also react in electrophilic substitution reactions, where a hydrogen atom is replaced with an electrophile, a subatomic particle that is positively or neutrally charged and allows for the formation of a new chemical bond. One such reaction with sulfuric acid produces sulfanilic acid, which can be turned into sulfanilamide, which was one of the first antibacterial agents created in the twentieth century.

Aniline is used in the manufacture of polyurethane foams, such as those found in cars and construction supplies. It is also used to make substances that give rubber certain characteristics, such as the ability to resist exposure to the elements. Aniline is a component in making petroleum, some plastics and explosives, and a number of agricultural products to control pests and weeds. It also continues to be an important part of the manufacture of synthetic dyes, especially those used in the leather industry. Aniline leather is noted for the natural look the leather maintains despite being colored; this is a direct result of the use of aniline dyes as opposed to other coloring agents.

While aniline is very useful, it is also a toxin. The greatest risk is to those who work in the manufacture of products in which aniline is used. It is present in tobacco smoke, and people can be exposed to it through secondhand smoke. Exposure can also occur at toxic waste sites where aniline waste has been disposed.

Contact with aniline can cause damage to hemoglobin, the substance in the blood that transports oxygen. When aniline is inhaled, ingested, or absorbed through the skin, a person can develop a condition known as methemoglobinemia. It can result in the skin turning blue from lack of oxygen, irregular heart rate, dizziness, and headaches. In severe cases, convulsions, coma, and death may result. This can happen whether the exposure is acute and occurs all at once or chronic and occurs over time.

Aniline continues to be used in a number of manufacturing applications. This sometimes leads to it getting into waterways and into soil. As a result, it remains a hazardous substance, much like the coal tar that led to its discovery.

Bibliography

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