benzene

benzene

[ben-zeen, ben-zeen]
benzene, colorless, flammable, toxic liquid with a pleasant aromatic odor. It boils at 80.1°C; and solidifies at 5.5°C;. Benzene is a hydrocarbon, with formula C6H6. The simplest picture of the benzene molecule, proposed by the German chemist Friedrich Kekulé (1865), is a hexagon of six carbon atoms joined by alternating single and double bonds and each bearing one hydrogen atom, symbolized by #x232c;. However, modern studies have shown that the six carbon-carbon bonds are all of equal strength and distance; thus the double-bond electrons do not belong to any particular bonds but rather are delocalized about the ring, with the result that the strength of each bond is between that of a single bond and that of a double bond (see chemical bond).

Benzene is the parent substance of the aromatic compounds, a large and important group of organic compounds. It is the first of a series of hydrocarbons known as the benzene series, formed by the substitution of methyl groups, CH3, for the hydrogen atoms of the benzene molecule. The second member of the series is toluene, C6H5CH3, from which trinitrotoluene is derived, and the third member is xylene, C6H4(CH3)2, a solvent. In xylene and other benzene derivatives in which two of the hydrogens have been replaced, there are three possible arrangements of the substitution groups; in the ortho (o) configuration the groups are on adjacent carbon atoms, in the meta (m) configuration the groups are separated by one carbon atom, and in the para (p) configuration the groups are on opposite sides of the ring. The three forms of xylene (dimethylbenzene) are shown below:

In addition to derivatives formed by the substitution of other groups for one or more of the hydrogen atoms of the benzene ring, two or more rings may be joined together, as in naphthalene, anthracene, and phenanthrene; or other atoms, such as nitrogen, may be substituted for carbon atoms in the ring, as in pyridine (C5H5N) and pyrimidine (C4H4N2). Among the important derivatives of benzene are phenol, aniline, and picric acid. Benzene and the other aromatic hydrocarbons are obtained for industrial purposes from the distillation of coal tar, a byproduct in the manufacture of coke, and from petroleum by special reforming methods. They are used in the manufacture of plastics, synthetic rubber, dyes, and drugs. Benzene is a known carcinogen.

benzene, dimethyl: see xylene.
benzene, 1,2,3-trihydroxy-, IUPAC name for pyrogallol. See gallic acid.

Simplest aromatic hydrocarbon (see aromatic compound), parent substance of a large class of chemical compounds. It was discovered in 1825 by Michael Faraday. The chemical formula is C6H6; August Kekule von Stradonitz in 1865 was the first to propose the correct structure, a six-membered ring of carbon atoms, each with one hydrogen atom bonded to it (see bonding). Although benzene is often represented with alternating single and double bonds between carbon atoms, the electrons in the bonds are shared or delocalized in such a way as to make all carbon-carbon bonds alike. Benzene is a colourless, mobile liquid with a characteristic odour. An excellent solvent, it is also widely used as a starting material for many plastics, dyes, detergents, insecticides, and other industrial chemicals. Benzene is highly toxic, and long exposure may cause leukemia.

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(Bis(trifluoroacetoxy)iodo)benzene, C10H5F6IO4, is a hypervalent iodine compound used as a reagent in organic chemistry. The reagent is used in an acidic modification of the Hofmann rearrangement. One example is the conversion of cyclobutanecarboxamine to cyclobutylamine hydrochloride. It also brings around the conversion of a hydrazone to a diazo compound, for example in the diazo-thioketone coupling. It also converts thioacetals to their parent carbonyl compounds.

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