Benzene

Structure

Properties

Benzene derivatives

Uses

Health issues

Resources

Benzene is an aromatic organic compound with the molecular formula C₆ H₆. Credit for its discovery and identification in 1825 is usually given to the English chemist and physicist Michael Faraday (1791–1867).

Benzene is a clear, colorless, highly flammable liquid with a pronounced characteristic odor. It has a freezing point of 41.9°F (5.5°C), a boiling point of 176.2°F (80.1°C), and a density of 0.8787 g/mL. It is only slightly soluble in water (0.18 g/100 mL at 77°F [25°C]), but is completely miscible with alcohol, chloroform, ether, carbon disulfide, carbon tetrachloride, and other organic solvents. Benzene is not to be confused with benzine, which is not a pure chemical compound but a mixture of petroleum hydrocarbons used as a solvent and a fuel.

Structure

The structure of the benzene molecule proved to be a challenge for chemists for more than 40 years after the compound’s discovery by Faraday. Its formula suggests the existence of multiple double and/or triple carbon-carbon bonds, because there are too few hydrogenatoms for six single-bonded carbon atoms. However, benzene exhibits none of the chemical properties associated with such a structure, the property of addition, for example. That problem was largely solved in 1865 by the German chemist Friedrich August Kekul´. Kekul´’s own story is that he fell asleep in front of his fireplace and dreamed of a snake with its tail in its mouth. He awoke to the realization that the benzene molecule might be a ring consisting of six carbon atoms, with one hydrogen atom attached to each carbon atom. That general structure is still accepted today, although the concept of resonance has replaced that of simple single and double bonds between adjacent carbon atoms in the benzene ring.

Properties

The most common chemical property of benzene is that it undergoes substitution reactions. Substitution is a reaction in which an atom or group of atoms replaces a hydrogen atom in an organic molecule. The halogens, nitric acid, sulfuric acid, and alkyl halides all react with benzene to form substituted derivatives. Two, three, or more substitutions can occur on the same benzene molecule, although the ease and location on the benzene ring of these substitutions varies depending on the earlier substitutions.

Benzene derivatives

A number of the substituted benzene derivatives are well known and commercially important compounds. For example, the substitution of a single methyl, hydroxyl, or amino group in benzene results in the formation, respectively, of toluene (C₆ H₅ CH₃), phenol (C₆ H₅ OH), or aniline (C₆ H₅ NH₂). Probably the best known disubstituted products are the xylenes, C₆ H₄ (CH₃)₂. Three different xylene molecules are possible depending on whether the methyl groups are adjacent to each other on the benzene ring (ortho-xylene), separated by one carbon atom (meta-xylene), or opposite each other on the ring (para-xylene). The removal of one hydrogen atom from the benzene molecule results in a radical known as the phenyl group.

Benzene occurs so abundantly in and is obtained so easily from coal tar and petroleum that there is virtually no reason to make it synthetically. Although benzene had been recognized as a component of petroleum for many years, it was not produced commercially from that source until the beginning of World War II.

Uses

Benzene is used as a solvent in many commercial, industrial, and research operations. It has long been of interest as a fuel because of its high octane number. Some manufacturers, particularly in Europe, have used it as a gasoline additive to increase engine efficiency and to improve starting qualities.

By far the most important use of benzene, however, is in the production of other aromatic compounds. The word aromatic was originally applied to benzene because of its distinctive odor, but it later took on a broader meaning, referring to any compound whose molecular structure includes one or more benzene rings. The largest volume of compounds made from benzene goes toward the production of commercially valuable polymers, such as polystyrene, nylon, and synthetic rubber.

The benzene derivative produced in largest quantity is ethylbenzene (C₆ H₅C₂ H₅). Ethylbenzene is converted to styrene (C₆ H₅CH=CH₂) which, in turn, is polymerized to form polystyrene. Nearly half of all benzene used in chemical synthesis is used for this process.

In another example, benzene is treated with propylene to form cumene (C₆ H₅CH[CH₃]₂). The cumene thus formed is then oxidized to produce phenol. Phenol is the starting point for a large number of polymers known as phenolic resins.

Synthetic fibers are produced by yet a third kind of benzene substitution sequence. The addition of hydrogen to benzene converts it to cyclohexane (C₆ H₁₂), which is then oxidized to adipic acid (COOH[CH₂]₄-COOH) The acid can then be treated with hexamethylene diamine to form nylon.

Health issues

The health risks associated with exposure to benzene have been known for many years. The compound

KEY TERMS

Acute— A medical condition that arises over a relatively brief period of time, reaches some crisis, and then may be resolved.

Aromatic— In organic chemistry, a compound whose molecular structure includes some variation of the benzene ring.

Chronic— A disease or condition that devlops slowly and exists over a long period of time.

Hydrogenation— A chemical reaction in which hydrogen is added to a compound.

Polymer— A molecule that consists of a few small units repeated over and over again many times.

Substitution— In organic chemistry, a chemical reaction in which an atom or group of atoms substitutes for a hydrogen atom in a molecule.

Synthesis— A chemical process by which some new substance is produced by reacting other substances with each other.

has both chronic and acute effects whether ingested by mouth, taken in through the respiratory system, or absorbed through the skin. Acute effects resulting from inhalation include irritation of the mucous membranes, headache, instability, euphoria, convulsions, excitement or depression, and unconsciousness.

The ingestion of benzene has been associated with the development of bronchitis, pneumonia, while exposure through the skin can cause drying, blistering, and erythema (redness). Death can result from exposure to high concentrations of benzene. Chronic effects resulting from benzene exposure include reduced white and red bloodcell counts, aplasia, and more rarely, leukemia.

See also Hydrocarbon.

Resources

BOOKS

Carey, Francis A. Organic Chemistry (6th ed. ) New York: McGraw-Hill, 2004.

Flick, Ernest W. Industrial Solvents Handbook. Delhi, India: Jaico Book Publishers, 2004.

Icon Health Publications. Benzene: A Medical Dictionary, Bibliography, And Annotated Research Guide To Internet References. San Diego, CA: Icon Health Publications, 2004.

Purcell, William P. “Benzene.” Kirk-Othmer Encyclopedia of Chemical Technology. New York: John Wiley & Sons, 2004.

OTHER

CBS News. “FDA: Too Much Benzene in Some Drinks.” May 19, 2006. <http://www.cbsnews.com/stories/2006/05/19/health/main1638170.shtml> (accessed October 19, 2006).

David E. Newton

Benzene

Benzene is an aromatic organic compound with the molecular formula C6H6. Credit for its discovery and identification in 1825 is usually given to the English chemist and physicist Michael Faraday.

Benzene is a clear, colorless, highly flammable liquid with a pronounced characteristic odor. It has a freezing point of 41.9°F (5.5°C), a boiling point of 176.2°F (80.1°C), and a density of 0.8787 g/mL. It is only slightly soluble in water (0.18 g/100 mL at 77°F [25°C]), but is completely miscible with alcohol , chloroform , ether , carbon disulfide, carbon tetrachloride , and other organic solvents. Benzene is not to be confused with benzine, which is not a pure chemical compound but a mixture of petroleum hydrocarbons used as a solvent and a fuel.

Structure

The structure of the benzene molecule proved to be a challenge for chemists for more than 40 years after the compound's discovery by Faraday. Its formula suggests the existence of multiple double and/or triple carbon-carbon bonds, because there are too few hydrogen atoms for six single-bonded carbon atoms. However, benzene exhibits none of the chemical properties associated with such a structure, the property of addition, for example. That problem was largely solved in 1865 by the German chemist Friedrich August Kekulé. Kekulé's own story is that he fell asleep in front of his fireplace and dreamed of a snake with its tail in its mouth. He awoke to the realization that the benzene molecule might be a ring consisting of six carbon atoms, with one hydrogen atom attached to each carbon atom. That general structure is still accepted today, although the concept of resonance has replaced that of simple single and double bonds between adjacent carbon atoms in the benzene ring.

Properties

The most common chemical property of benzene is that it undergoes substitution reactions. Substitution is a reaction in which an atom or group of atoms replaces a hydrogen atom in an organic molecule. The halogens , nitric acid , sulfuric acid , and alkyl halides all react with benzene to form substituted derivatives. Two, three, or more substitutions can occur on the same benzene molecule, although the ease and location on the benzene ring of these substitutions varies depending on the earlier substitutions.

Benzene derivatives

A number of the substituted benzene derivatives are well known and commercially important compounds. For example, the substitution of a single methyl, hydroxyl, or amino group in benzene results in the formation, respectively, of toluene (C6H5CH3), phenol (C6H5OH), or aniline (C6H5NH2). Probably the best known disubstituted products are the xylenes, C6H4(CH3)2. Three different xylene molecules are possible depending on whether the methyl groups are adjacent to each other on the benzene ring (ortho-xylene), separated by one carbon atom (meta-xylene), or opposite each other on the ring (paraxylene). The removal of one hydrogen atom from the benzene molecule results in a radical known as the phenyl group .

Benzene occurs so abundantly in and is obtained so easily from coal tar and petroleum that there is virtually no reason to make it synthetically. Although benzene had been recognized as a component of petroleum for many years, it was not produced commercially from that source until the beginning of World War II.

Uses

Benzene is used as a solvent in many commercial, industrial, and research operations. It has long been of interest as a fuel because of its high octane number. Some manufacturers, particularly in Europe , have used it as a gasoline additive to increase engine efficiency and to improve starting qualities.

By far the most important use of benzene, however, is in the production of other aromatic compounds. The word aromatic was originally applied to benzene because of its distinctive odor, but it later took on a broader meaning, referring to any compound whose molecular structure includes one or more benzene rings. The largest volume of compounds made from benzene goes toward the production of commercially valuable polymers, such as polystyrene, nylon, and synthetic rubber.

The benzene derivative produced in largest quantity is ethylbenzene (C6H5C2H5). Ethylbenzene is converted to styrene (C6H5CH=CH2) which, in turn, is polymerized to form polystyrene. Nearly half of all benzene used in chemical synthesis is used for this process.

In another example, benzene is treated with propylene to form cumene (C6H5CH[CH3]2). The cumene thus formed is then oxidized to produce phenol. Phenol is the starting point for a large number of polymers known as phenolic resins .

Synthetic fibers are produced by yet a third kind of benzene substitution sequence. The addition of hydrogen to benzene converts it to cyclohexane (C6H12), which is then oxidized to adipic acid (COOH[CH2]4-COOH) The acid can then be treated with hexamethylene diamine to form nylon.

Health issues

The health risks associated with exposure to benzene have been known for many years. The compound has both chronic and acute effects whether ingested by mouth, taken in through the respiratory system , or absorbed through the skin. Acute effects resulting from inhalation include irritation of the mucous membranes, headache, instability, euphoria, convulsions, excitement or depression , and unconsciousness.

The ingestion of benzene has been associated with the development of bronchitis and pneumonia , while exposure through the skin can cause drying, blistering, and erythema (redness). Death can result from exposure to high concentrations of benzene. Chronic effects resulting from benzene exposure include reduced white and red blood cell counts, aplasia, and more rarely, leukemia .

See also Hydrocarbon.

Resources

books

Browning, E. Toxicity and Metabolism of Industrial Solvents. New York: Elsevier, 1965, pp. 3-65.

Carey, Francis A. Organic Chemistry. New York: McGraw-Hill, 2002.

Graham, John D., Laura C. Green, and Marc J. Roberts. InSearch of Safety: Chemicals and Cancer Risk. Cambridge, MA: Harvard University Press, 1988.

Purcell, William P. "Benzene." Kirk-Othmer Encyclopedia ofChemical Technology. 4th ed. Suppl. New York: John Wiley & Sons, 1998.

Solomons, T. W. Graham. Organic Chemistry. 2nd edition. New York: John Wiley, 1980, Chapter 11.

David E. Newton

KEY TERMS

Acute

—A medical condition that arises over a relatively brief period of time, reaches some crisis, and then may be resolved.

Aromatic

—In organic chemistry, a compound whose molecular structure includes some variation of the benzene ring.

Chronic

—A disease or condition that devlops slowly and exists over a long period of time.

Hydrogenation

—A chemical reaction in which hydrogen is added to a compound.

Polymer

—A molecule that consists of a few small units repeated over and over again many times.

Substitution

—In organic chemistry, a chemical reaction in which an atom or group of atoms substitutes for a hydrogen atom in a molecule.

Synthesis

—A chemical process by which some new substance is produced by reacting other substances with each other.

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

CARCINOGEN

A chemical that causes cancer in humans or other animals.

CATALYST

A material that increases the rate of a chemical reaction without undergoing any change in its own chemical structure.

MUCOUS MEMBRANES

Tissues that line the moist inner lining of the digestive, respiratory, urinary, and reproductive systems.

SYNTHESIS

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 AlsoBenzoic Acid; Styrene

BENZENE

Benzene is a ubiquitous component of the petrochemical era. Present in crude petroleum, benzene is produced from the combustion of fossil fuels. It has been known to cause toxicity to human bone marrow since the late nineteenth century, at high levels destroying the bone marrow machinery responsible for the production of mature red blood cells, white blood cells, and platelets. When severe, this is a frequently fatal condition known as aplastic anemia. Lesser levels of benzene exposure result in sufficient bone marrow destruction to cause partial decrements in the levels of circulating blood cells, a condition known as pancytopenia.

Benzene is also a known cause of acute myelogenous leukemia, the adult form of acute leukemia, and a more than probable cause of other forms of blood and bone marrow cancers, including non-Hodgkin's lymphoma, multiple myeloma, and acute lymphatic leukemia, the childhood form of acute leukemia. There are recent indications that subpopulations vary in their susceptibility to benzene toxicity based upon their metabolic capabilities.

In the latter half of the twentieth century there was a dramatic decline in the allowable levels of benzene at the workplace. To protect workers from this known human carcinogen, the United States permissible standard progressively decreased from 100 parts per million (ppm) to 1 ppm on an eight-hour time-weighted average. There was a corresponding fall in shorter term exposure limits, and an increase in requirements for industrial hygiene monitoring and in the use of respirators and other personal protective equipment. Even more stringent standards have been proposed.

At high concentrations, well above 100 ppm, benzene is also a central nervous system anesthetic-like agent. This effect is due to its solubility in lipids and its other physicochemical characteristics, and it is predictable based upon what is known about analogous compounds such as toluene and xylenes. In contrast, the bone marrow toxicity of benzene is a result of its metabolism and this toxicity does not occur with toluene, xylenes, and other related compounds that are metabolized differently. In fact, at high concentrations toluene is known to protect against the bone marrow toxicity of benzene because it occupies the metabolic machinery that otherwise would produce toxic benzene metabolites. However, concentrations in the general environment are too low to produce this result. For benzene, outdoor environmental exposure is usually in the 1 to 5 parts per billion (ppb) range in the United States. Benzene levels from natural sources are negligible in comparison.

For most nonsmoking individuals in the general population, it is indoor exposure that is the most dominant source of benzene, often reflecting the storage of gasoline or of benzene-containing consumer products within the home. Gasoline in the United States contains about 1 to 2 percent benzene, and higher levels are present elsewhere. Cigarette smokers inhale benzene directly in tobacco smoke, causing contamination of indoor air with benzene that is then inhaled by nonsmokers. Drinking water supplies are sometimes contaminated with benzene, most frequently from leaking underground petroleum storage tanks. This can also lead to inhalation of benzene through offgassing from contaminated water during cooking or showering. Skin absorption can occur in those working with products that contain benzene, as well as during the refueling of automobiles with gasoline.

As with other cancer-causing agents, it is unclear what level of exposure, if any, can be considered completely safe, or what level might be certain to cause cancer. As benzene is a component of gasoline, a useful solvent, and an organic building block in many chemical reactions, it cannot simply be banned. However, there have been many actions taken to decrease the extent to which the general population is exposed to benzene from gasoline and from industrial effluents.

Bernard D. Goldstein

(see also: Cancer; Carcinogen; Carcinogenesis; Environmental Tobacco Smoke; Fuel Additives; Groundwater Contamination; Hazardous Air Pollutants; One-Hit Model )

Bibliography

Goldstein, B. D., and Witz, G. (1999). "Benzene." In Environmental Toxicants: Human Exposures and Their Health Effects, 2nd edition, ed. M. Lippman. New York: John Wiley.

Krewski, D., and Snyder, R., eds. (2000). "Assessing the Health Risks of Benzene: A Report on the Benzene State-of-the-Science Workshop." Journal of Toxicology and Environmental Health 61:307–338.

Benzene

A hydrocarbon with chemical formula C₆H₆, benzene contains six carbon atoms in a ring structure. A clear volatile liquid with a strong odor, it is one of the most extensively used hydrocarbons . Because it is an excellent solvent and a necessary component of many industrial chemicals , including gasoline , benzene is classified by United States federal agencies as a known human carcinogen based on studies that show an increased incidence of nonlymphocytic leukemia from occupational exposure and increased incidence of neoplasia in rats and mice exposed by inhalation and gavage. Because of these cancer-causing properties, benzene has been listed as a hazardous air pollutant under Section 112 of the Clean Air Act .