Benzene
Overview
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).
Key Facts
Other Names:
Benzol; cyclohexatriene
Formula:
C6H6
Elements:
Carbon, hydrogen
Compound Type:
Aromatic hydrocarbon (organic)
State:
Liquid
Molecular Weight:
78.11 g/mol
Melting Point:
5.49°C (41.9°F)
Boiling Point:
80.09°C (176.2°F)
Solubility:
Slightly soluble in water; soluble in alcohol, ether, and acetone
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 Kekulé (1829–1896). Kekulé 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.
Benzene is a very popular raw material for a variety of industrial chemical reactions. In 2004, U.S. manufacturers produced 8.8 million metric tons (9.7 million short tons) of benzene, placing it in twelfth place among all chemicals made in the United States that year.
How It Is Made
At one time, benzene was obtained from coal tar, the thick gooey liquid left over after soft coal is converted to coke. This method has now been largely replaced by a variety of methods that use crude oil or refined petroleum as a raw material. In the most popular of these methods, toluene (C6H5CH3) from petroleum is heated over a catalyst of platinum metal and aluminum oxide (Al2O3). The toluene loses its methyl group (-CH3), leaving benzene as the primary product. Other methods are available for changing the molecular structure of hydrocarbons found in petroleum and converting them to benzene.
Common Uses and Potential Hazards
By far the most important use of benzene is as a raw material in the synthesis of other organic compounds. More than 90 percent of the benzene produced in the United States is used to make ethylbenzene (55 percent), cumene (24 percent), and cyclohexane (12 percent). The first two compounds rank fifteenth and twentieth, respectively, among all chemicals produced in the United States each year. Another five percent of benzene production goes to the synthesis of a large variety of other organic compounds, including nitrobenzene, chlorobenzene, and maleic anhydride, a raw material for the manufacture of plastics. Smaller amounts of benzene are used as a solvent for cleaning purposes, in chemical reactions, and as a gasoline additive.
As with most chemicals, benzene can enter the body in one of three ways: through the skin, the nose, or the throat. People who handle or work with benzene in their workplaces are at greatest risk of exposure to benzene and should take precautions in working with the material. Because of its serious health hazards, benzene is no longer included in most materials with which the average person comes into contact. On those occasions when a person does come into contact with benzene, first aid and medical attention should be sought for treatment of the exposure.
Interesting Facts
- Kekulé's discovery of the formula for benzene is one of the most interesting in the history of chemistry. The story is told that he worked so hard on the problem that he often dreamed about the compound at night. One evening, he dreamed of a snake with a tail in its mouth. Kekulé immediately awoke, went to his work table, and drew a structure for the benzene molecule inspired by the snake: a molecule in the shape of a ring made of carbon atoms.
The health effects of exposure to liquid benzene or benzene fumes depends on the amount of benzene taken into the body. The most common symptoms of benzene exposure include irritation of the mucous membranes, convulsions, depression, and restlessness. At greater doses, a person may experience respiratory failure, followed by death. Even at low concentrations, benzene can cause long-term effects for people who are regularly in contact with the compound. The most important of these effects are carcinogenic. Benzene is known to cause damage to bone marrow, resulting in a form of cancer of the blood known as leukemia.
Words to Know
A chemical that causes cancer in humans or other animals. A material that increases the rate of a chemical reaction without undergoing any change in its own chemical structure. Tissues that line the moist inner lining of the digestive, respiratory, urinary, and reproductive systems. A chemical reaction in which some desired chemical product is made from simple beginning chemicals, or reactants.For Further Information
"Chronic Toxicity Summary: Benzene." California Office of Health Hazard Assessment. http://www.oehha.org/air/chronic_rels/pdf/71432.pdf (accessed on September 21, 2005).
Newton, David E. "Benzene." In Gale Encyclopedia of Science. Edited by K. Lee Lerner and Brenda Wilmoth Lerner. 3rd ed., vol. 1. Detroit: Gale, 2004.
"Spectrum Chemical Fact Sheet: Benzene." Spectrum Laboratories. http://www.speclab.com/compound/c71432.htm (accessed on September 21, 2005).
"Toxicity Summary for Benzene." The Risk Assessment Information System. http://risk.lsd.ornl.gov/tox/profiles/benzene.shtml (accessed on September 21, 2005).
"Toxicological Profile for Benzene." Agency for Toxic Substances and Disease Registry. http://www.atsdr.cdc.gov/toxprofiles/tp3.html (accessed on September 21, 2005).
See Also
Benzoic Acid; Styrene
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