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benzene (bĕnˈzēn, bĕnzēnˈ), colorless, flammable, toxic liquid with a pleasant aromatic odor. It boils at 80.1℃ and solidifies at 5.5℃. 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 . 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.
an organic compound, C6H6; the simplest aromatic hydrocarbon. It is a mobile, colorless, volatile liquid with a distinct, mild odor: Tm = 5.5° C; Tb = 80. 1° C. Density, 879.1 kg/m3 (0.8791 g/cm3) at 20°C; nD20, 1.5011. Benzene forms an explosive mixture in a volume concentration of 1.5 to 8 percent in air. Benzene is completely miscible with ether, gasoline, and other organic solvents. At 26° C, 0.054 g of water dissolve in 100 g of benzene; with water it forms an azeotropic (constantly boiling) mixture (91.2 percent benzene by weight) with Tb= 69.25° C.
Benzene was discovered by M. Faraday (1825), who extracted it from a liquid condensate of illuminating gas; it was obtained in its pure form by E. Mitscherlich in 1833 by the dry distillation of the calcium salt of benzoic acid (hence the name).
In 1865, F. A. Kekulé proposed for benzene the structural formula (I), corresponding to cyclohexatriene: a closed chain made up of six carbon atoms with alternating single and double bonds. Kekulé’s formula is rather widely used, although many facts have been accumulated which indicate that benzene does not have the structure of cyclohexatriene. It has long been established that ortho-disubstituted benzenes (for example, 1,2 and 1,6) exist in only one form, whereas Kekulé’s formula permits isomerism of such compounds (substituents at the carbon atoms linked with single or double bonds). In 1872, Kekulé provided the additional hypothesis that the bonds in benzene are constantly and rapidly shifting or oscillating. Other formulas were postulated for the structure of benzene; however, they were not accepted.
The chemical properties of benzene formally correspond to formula (I) to some degree. Thus, under certain conditions, three chlorine molecules or three hydrogen molecules will attach to the benzene molecule; benzene is formed during condensation of three acetylene molecules. However, electrophilic substitution reactions—rather than the addition reactions typical for unsaturated compounds—are generally characteristic for benzene. In addition, the benzene nucleus is very resistant to the action of oxidizers—for example, potassium permanganate—which also contradicts the theory that localized double bonds are present in benzene. The special, so-called aromatic, properties of benzene are explained by the fact that all the bonds in the molecule are equat—that is, the distance between neighboring carbon atoms is identical and equals 0.14 nanometers (1.40 angstroms); the length of a single C—C bond is 0.154 nm (1.54 Å), and the length of the double C=C bond is 0.132 nanometers (1.32 Å). A benzene molecule has an axis of symmetry of the sixth order; benzene, as an aromatic compound, is characterized by the presence of a sextet of π-electrons, which forms a single closed, stable electron system. However, there is still no generally accepted formula for its structure; formula (II) is often used.
Benzene is contained in the products of dry distillation of coal (coke oven gas) and in a small quantity in coke oven tar. A significant quantity of benzene is obtained from the catalytic cyclization of the aliphatic petroleum hydrocarbons. Benzene is a very important raw material for the chemical industry. Under the action of nitric acid, benzene forms nitrobenzene, C6H5NO2, which can be reduced to aniline, C6H5NH2—the initial product in the production of many dyes. When benzene reacts with sulfuric acid, the product is ben-zenesulfonic acid, C6H5SO2OH, the alkaline fusion of whose salts is one of the basic methods of producing phenol. When benzene is alkylated with ethylene in the presence of aluminum chloride, ethylbenzene is produced; the catalytic dehydration of this substance is the basic means of producing styrene, C6H5CH=CH2. Analogously, isopropylbenzene, C6H5CH(CH3)2—the initial product for obtaining phenol and acetone on an industrial scale—is produced from benzene and propylene. Halogen-derivatives of benzene are also widely used. Thus, chlorobenzene is saponified in phenol; by means of Grignard’s reaction, phenyl magnesium chloride, C6H5MgCl, is obtained from chlorobenzene and magnesium. The reaction of phenyl magnesium chloride with ethylene oxide forms phenylethyl alcohol, C6H5CH2CH2OH, which is used in the perfume industry as artificial rose oil. During catalytic hydrogenation, benzene is converted into cyclohex-ane, the initial product in one of the means of producing caprolactam, by the polymerization of which kapron synthetic fiber is produced. Upon irradiation of benzene, the attachment of 3 moles of chlorine and the formation of a mixture of stereoisomeric hexachlorcyclohexanes—one of which (hexachlorane) has strong insecticidal properties—occurs. Benzene is used in the production of explosives and as a solvent and extraction agent in producing varnishes and paints.
REFERENCENenitescu, C. D. Organicheskaia khimiia, vol. 1. Moscow, 1962. Pages 304, 323. (Translated from Rumanian.)
Acute poisoning usually occurs during accidents; its most characteristic symptoms are headache, dizziness, nausea, vomiting, agitation alternating with depression, quickened pulse, lowered blood pressure, and—in serious cases—convulsions and loss of consciousness. Chronic benzene poisoning is manifested by a change in the blood (destruction of bone marrow function), dizziness, general weakness, disturbance of sleep, and quick exhaustion; in women, the menstrual function is damaged. A reliable method of protection against poisoning by benzene vapors is good ventilation of manufacturing installations.
Treatment. Treatment of acute poisoning includes rest, heat, bromide preparations, and cardiovascular remedies; for chronic poisoning with pronounced anemia, treatment includes transfusion of the erythrocyte portion of the blood, vitamin B12, and iron preparations.
REFERENCESOmel’ianenko, L. M., and N. A. Senkevich. Klinika i profilaktika otravlenii benzolom. Moscow, 1957.
Professional’nye bolezni, 2nd ed. Moscow, 1964.