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benzene





benzene
Benzene is often drawn as a ring of six carbon atoms, with alternating double bonds and single bonds. This simple picture has some complications, however. Carbon-carbon single bonds are longer than carbon-carbon double bonds, so if there were "real" single and double bonds in the molecule, the shape of the benzene molecule would be a distorted hexagon.
benzene
What is actually found is that all of the bond lengths in the benzene rings are 1.397 angstroms, which is roughly intermediate between the typical lengths of single bonds (~1.5 Å) and double bonds (~1.3 Å). All of the carbon atoms in the benzene rings are sp2-hybridized: the overlap of the sp2 orbitals around the ring produces a framework of six sigma bonds, while the unhybridized p-orbitals which are perpendicular to this plane overlap in a side-to-side fashion to form three pi-bonds. These pi-bonds are delocalized around the ring, leading to an unusual stability for the benzene ring compared to other alkenes.
A common organic chemical compound made up of six carbon atoms arranged in a hexagonal ring, with hydrogen atoms attached to each corner. Benzene is a clear, colorless, sweet-smelling, toxic liquid hydrocarbon with the chemical formula C6H6. It is the archetypical aromatic compound. Although it consists of unsaturated molecules, benzene is stable and not very reactive. However, it does take part in many substitution reactions in which hydrogen atoms are replaced by other atoms or groups. It doesn't readily undergo addition reactions but does form various addition products with halogens – including γ-benzene hexachloride, a powerful insecticide.

Benzene is volatile and highly flammable; it boils at 80.1°C, and melts at 5.5°C.

Benzene is present in small proportion in some crude oils and is made commercially from petroleum by the catalytic reforming of naphthenes in petroleum naphtha. It is also made from coal in the manufacture of coke. Benzene is used as a solvent in the manufacture of detergents, synthetic fibers, petrochemicals, and as a component of high-octane gasoline. It is also used as the starting material for a great variety of aromatic compounds, including phenol, styrene, aniline, and maleic anhydride. It is toxic by ingestion and inhalation, and may cause bone marrow problems or leukemia with prolonged exposure.


History

Benzene was first isolated by Michael Faraday in 1825, from the whale oil used in gaslights; he also determined that it had an empirical formula of CH. Eilhardt Mitscherlich synthesized benzene in 1834, and showed it to have a molecular formula of C6H6.

Many other compounds with similar properties to benzene were discovered in the 1800s, all having a low ratio of hydrogen to carbon. This usually indicates the presence of carbon-carbon double bonds, but these compounds did not undergo the reactions typical of alkenes under ordinary lab conditions. Since many of these molecules had pleasant aromas, they were called "aromatic compounds." The structure of benzene and its derivatives was a puzzle for many years, until Friedrich August Kekulé proposed the cyclic structure in the top diagram in 1866 (supposedly after having a dream about a snake biting its own tail). The term "aromatic" in organic chemistry now means that the molecule contains benzene, or its structural relatives.


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   • ORGANIC CHEMISTRY