Benzene
Benzene is the simplest of the aromatic hydrocarbons. It is a ring structure with the empirical formula C6H6. Benzene was first discovered by Michael Faraday in 1825 from the liquid condensed by compressing oil gas.
The chemical structure of the molecule was originally worked out by Kekulé in 1865. The structure of benzene is formally that of a regular hexagonal array of the carbon atoms, termed a ring structure, comprising three alternating carbon-carbon double bonds and three carbon-carbon single bonds. The electrons of the double bond are now known not to be static, however, but form a symmetrical "-cloud" molecular orbital above and below the ring where these electrons are delocalized (or resonating) around the ring. This resonance strongly stabilizes benzene and profoundly influences its chemical properties.
Benzene is a clear, highly refractive liquid at standard temperature and pressure with a characteristic, sweet smell. It is highly flammable, and burns with a smoky yellow flame (indicating a high level of carbon in the structure). Benzene is a good solvent for fats and lower molecular weight aromatic compounds. It is freely miscible with ethanol, diethylether, acetone (propanone), and acetic acid (ethanoic acid). Excessive exposure to the vapor is toxic, and chronic exposure to benzene results in damage to the bone marrow, liver, and kidneys. Chronic exposure is also associated with an increased risk for leukemia and possibly other cancers..
Like other hydrocarbons with double bonds, addition reactions can occur to change the double bonds to single bonds. For example, catalytic hydrogenation adds six hydrogen atoms to produce cyclohexane, a colorless liquid with the properties of an alkane. The requirements to affect this reduction of benzene to cyclohexane, in terms of catalyst, reaction temperature, and reaction time, are much more vigorous than for the reduction of simple alkenes. This increased difficulty reflects the resonance stabilization that benzene possesses. Benzene burns in air to produce carbon dioxide, water, and carbon but it is very resistant to chemical oxidation. Thus potassium permanganate (KMnO4) will not decolorize in its presence. This resistance to chemical oxidation is likewise due to the resonance stability of the benzene. Under rigorous conditions benzene will undergo substitution for one of its hydrogen substituents, by a process called electrophilic aromatic substitution. For example, a mixture of concentrated nitric and sulfuric acids will nitrate benzene. A yellow oil separates after this mixture and benzene are added to cold water. This liquid is nitrobenzene, C6H5NO2. Benzene will also form a range of compounds with the transition metals. Halogenation will occur in the presence of strong sunlight or ultraviolet light. Benzene will react with chlorine to give hexachlorobenzene, which once was used as an insecticide. A similar reaction occurs with bromine but not with iodine or fluorine. When atoms or functional groups are added to the benzene ring, their position is indicated by numbering. Each carbon atom is numbered from 1-6 and when the two substitutents are located at the 1,2 position it is ortho-; 1,3 is meta-; and 1,4 is para-.
Benzene is manufactured industrially by dehydrogenation and dealkylation of appropriate fractions of petroleum. In the laboratory benzene can be made by the action of heat on a mixture of sodium benzoate and sodium hydroxide. The Friedel-Crafts reaction, the common name for electrophilic aromatic substitution, can be used to make hydrocarbon derivatives of benzene. For example, benzene and ethyl bromide can be use to make ethylbenzene. In the 1990s, United States production of benzene was in the order of 5 million megatons (1 megaton = 1 million metric tons). Industrially, benzene is used in the manufacture of nylon, phenol, styrene (and by polymerization polystyrene), and cyclohexane. It is a common constituent of automobile gasoline. Benzene is also used in the manufacture of the insecticide DDT.
Benzene is the simplest aromatic hydrocarbon and it has many commercial uses.
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