Benzene (BEN-zeen) is a clear, colorless liquid with an
aromatic (fragrant) odor. It occurs in coal and petroleum,
from which it is extracted for commercial use. Benzene is
very flammable, burning with a smoking flame. The compound
was discovered in 1825 by the English chemist and
physicist Michael Faraday (1791–1867), who gave the compound
the name of bicarburet of hydrogen. It was given its
modern name of benzene (benzin, at the time) by the German
chemist Eilhardt Mitscherlich (1794–1863).
The chemical structure of benzene remained one of the
great mysteries in chemistry for nearly half a century. The
compound’s formula, C6H6, suggests that it contains three
double bonds. A double bond consists of four electrons that
hold two atoms in close proximity to each other in a molecule.
Yet benzene has none of the chemical properties common
to double-bonded substances. The solution to this problem
was suggested in 1865 by the German chemist Friedrich
August Kekule´ (1829–1896). Kekule´ suggested that the six
carbon atoms in the benzene molecule are arranged in a
ring, with one hydrogen atom attached to each carbon. The
ring itself consists of three double bonds and three single
bonds, alternating with each other in the ring. The fact that
the double bonds in benzene do not act like double bonds
in other compounds was explained by the German chemist
Johannes Thiele (1860–1935), who suggested that the bonds
in benzene shift back and forth between single and double
bonds so rapidly that they are not able to behave like typical
double bonds. Chemists now use a variety of chemical formulas
for representing the character of chemical bonds in
Benzene is a very popular raw material for a variety ofindustrial chemical reactions. In 2004, U.S. manufacturersproduced 8.8 million metric tons (9.7 million short tons) ofbenzene, placing it in twelfth place among all chemicalsmade in the United States that year.
At one time, benzene was obtained from coal tar, thethick gooey liquid left over after soft coal is converted tocoke. This method has now been largely replaced by a varietyof methods that use crude oil or refined petroleum as a rawmaterial. In the most popular of these methods, toluene(C6H5CH3) from petroleum is heated over a catalyst of platinummetal and aluminum oxide (Al2O3). The toluene loses itsmethyl group (-CH3), leaving benzene as the primary product.Other methods are available for changing the molecularstructure of hydrocarbons found in petroleum and convertingthem to benzene.
By far the most important use of benzene is as a rawmaterial in the synthesis of other organic compounds.More than 90 percent of the benzene produced in theUnited States is used to make ethylbenzene (55 percent),cumene (24 percent), and cyclohexane (12 percent). Thefirst two compounds rank fifteenth and twentieth, respectively,among all chemicals produced in the United Stateseach year. Another five percent of benzene production goesto the synthesis of a large variety of other organic compounds,including nitrobenzene, chlorobenzene, and maleicanhydride, a raw material for the manufacture of plastics.Smaller amounts of benzene are used as a solvent forcleaning purposes, in chemical reactions, and as a gasolineadditive. As with most chemicals, benzene can enter the bodyin one of three ways: through the skin, the nose, or thethroat. People who handle or work with benzene in theirworkplaces are at greatest risk of exposure to benzeneand should take precautions in working with the material.Because of its serious health hazards, benzene is nolonger included in most materials with which the averageperson comes into contact. On those occasions when aperson does come into contact with benzene, first aidand medical attention should be sought for treatment ofthe exposure. The health effects of exposure to liquid benzene orbenzene fumes depends on the amount of benzene takeninto the body. The most common symptoms of benzeneexposure include irritation of the mucous membranes, convulsions,depression, and restlessness. At greater doses, aperson may experience respiratory failure, followed bydeath. Even at low concentrations, benzene can cause longtermeffects for people who are regularly in contact withthe compound. The most important of these effects arecarcinogenic. Benzene is known to cause damage to bonemarrow, resulting in a form of cancer of the blood known asleukemia.