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Benzene

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The simplest organic compounds are hydrocarbons, which contain only carbon and hydrogen. Aromatic hydrocarbons are hydrocarbons that possess one or more benzene rings. The most important one is benzene a clear colorless liquid with a distinct odor. It is carcinogen a cancer causing agent.

Benzene is obtained mainly from petroleum. Other aromatic hydrocarbons are commonly obtained from coal from coal tar, a by-product of soft coal. Benzene is used as a chemical intermediate in the manufacture of styrene, cyclohexane, detergents and pesticides. Benzene is excreted rapidly.

Definition

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"Benzene is also known as benzol, is a colorless liquid with a sweet odor, and can be described as a volatile organic compound. The chemical symbol for benzene is C6H6."

Benzene is formed from both natural processes and human activities. It is produced from volcanoes ad forest fires, and is a natural processes and human activities. It is produced from volcanoes and forest fires, and is a natural part of crude oil, gasoline and cigarette smoke. Benzene is one of the 20 most widely used chemicals in the united states. It is used to make other chemicals that are then used to make plastics, resins, nylons and other synthetic fibers.

Structure

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In 1865, Kekule proposed a six membered ring structure for benzene where each carbon is attached to one hydrogen atom. To satisfy the tetra valency of carbon, he further proposed the presence of three alternate double bonds in benzene ring, which change their position rapidly. The two forms of benzene exist in equilibrium and cannot be isolated.

Benzene Structure

Although Kekule structure satisfied the structural features of benzene and also explained the equivalent nature of hydrogen, it was not able to account for the unusual behavior of benzene.

Properties

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Benzene and many other aromatic compounds have a characteristic aroma and burn with sooty or smoky flame. Being hydrophobic benzene is very sparingly soluble in water but is soluble in organic solvents.

Benzene is used as a solvent for fats and resins. Benzene is commonly used as a solvent and a synthetic intermediate in the chemical and pharmaceutical industries.
Benzene can prevent engine knocking and is added to gasoline as a replacement for alkylated compounds.
Combustion of fossil fuels accounts for approximately 82% of benzene released into the atmosphere as automotive exhaust 14% from industrial activities, 3% from human individual activities and 0.1% from cigarette smoke. Benzene has been produced commercially from coal since 1849, although petroleum has been used since 1951 and is currently the major source of its production. Benzene is chemically stable in soil and water, yet oxidative degradation of benzene occurs within a few hours to days after volatilisation into the air.

Benzene Rings

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Some points about benzene rings
  • The formula for benzene is C6H6.
  • Its molecules are flat, with the 6 carbon atoms arranged in a hexagon.
  • The strength of the bond between each pair of carbon atoms is the same.
  • A ring of the delocalised electrons lies above and below the plane of the molecule.
  • This pi bonding makes the benzene ring stable.
  • However, it is attacked by electrophiles (attracted by the electrons in the ring system). It then undergoes substitution reactions.
Benzene Rings
Kekule in proposed the cyclic structure of benzene in which the carbon atoms were joined by alternating single and double bonds. Certain reactions of benzene such as the catalytic hydrogenation to cyclohexane, which involves the addition of six hydrogen atoms, confirmed that benzene was a ring compound and that it contained three double bonds.

Kekule proposed structure looks more in keeping with our current knowledge of benzene although it does not explain how the double bonds differ from the aliphatic type. Kekule proposed that the equivalent structures oscillated between each other, averaging out the single and double bonds so that the compounds were indistinguishable.
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Production

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Benzene has been produced commercially from coal since 1849, although petroleum has been used since 1951 and is currently the major source of its production. Benzene is consistently one of the top ten organic chemicals. Benzene is chemically stable in soil and water, yet oxidative degradation of benzene occurs within a few hours to days after volatilization in the air.

New processes began to take over in the 1950s which were used for 50% of the benzene in 1859, 94% in 1972, 96% in 1980 and near 100% in the 1990s. These new processes consist of catalytic reforming of naphtha and hydrodealkylation of toluene in a 70:30 capacity ratio.

Benzene Production

The manufacture of benzene, toluene and xylenes by catalytic reforming is shown below.

Production of Benzene

Derivatives

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There are nine chemicals in the top 50 that are manufactured from benzene. They are
  • Ethyl benzene
  • Styrene
  • Cumene
  • Phenol
  • Acetone
  • Bisphenol A
  • Cyclohaxane
  • Adipic acid
  • Nitrobenzene

Exposure to Benzene

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Benzene was commonly used as an industrial and commercial solvent in the past. High exposures suppress the bone marrow, sometimes leading to anemia or pancytopenia. benzene is also a potent carcinogen, leading to leukemia and other hematopoirtic malignancies. Because of this toxicity, benzene is now used much less frequently, although exposures continue to occur in the petrochemical industry and in some other industries.

Benzene exposure still occurs in a variety of industries, including perfume manufacture, chemical research and development and shoe manufacture. Benzene is present in a concentration up to 10% in unleaded gasoline resulting in some benzene exposure to gas station attendants. It is also a constituent of cigarette smoke.