..... Click the link for more information. , 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 [N/A]. 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 chemical bond, mechanism whereby atoms combine to form molecules. There is a chemical bond between two atoms or groups of atoms when the forces acting between them are strong enough to lead to the formation of an aggregate with sufficient stability to be regarded as
..... Click the link for more information. ).
Benzene is the parent substance of the aromatic compounds aromatic compound, any of a large class of compounds that includes benzene and compounds that resemble benzene in certain of their chemical properties. Originally applied to a small class of pleasant-smelling chemicals derived from vegetables, it now encompasses a
..... Click the link for more information. , 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 toluene or methylbenzene , C7H8, colorless liquid aromatic hydrocarbon that melts at −95°C; and boils at 110.8°C;. It is insoluble in water but highly soluble in most organic solvents.
..... Click the link for more information. , C6H5CH3, from which trinitrotoluene trinitrotoluene or TNT , CH3C6H2(NO2)3, crystalline, aromatic compound that melts at 81°C;. It is prepared by the nitration of toluene.
..... Click the link for more information. is derived, and the third member is xylene xylene or dimethylbenzene , C6H4(CH3)2, colorless, oily, liquid aromatic hydrocarbon, used extensively as a solvent, obtained from coal tar, wood tar, and sometimes from petroleum.
..... Click the link for more information. , 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 naphthalene , colorless, crystalline, solid aromatic hydrocarbon with a pungent odor. It melts at 80°C;, boils at 218°C;, and sublimes upon heating. It is insoluble in water, somewhat soluble in ethanol, soluble in benzene, and very soluble in ether,
..... Click the link for more information. , anthracene anthracene , C14H10, solid organic compound derived from coal tar. It melts at 218°C; and boils at 354°C;. When pure it is colorless and has a violet fluorescence; it darkens when exposed to sunlight.
..... Click the link for more information. , and phenanthrene; or other atoms, such as nitrogen, may be substituted for carbon atoms in the ring, as in pyridine pyridine or azine , C5H5N, colorless, flammable, toxic liquid with a putrid odor. It melts at −42°C; and boils at 115.5°C;. Chemically, it is a heterocyclic aromatic tertiary amine (see under amino group).
..... Click the link for more information. (C5H5N) and pyrimidine pyrimidine , type of organic base found in certain coenzymes and in the nucleic acids of plant and animal tissue. The three major pyrimidines of almost universal distribution in living systems are cytosine, thymine, and uracil.
..... Click the link for more information. (C4H4N2). Among the important derivatives of benzene are phenol phenol , C6H5OH, a colorless, crystalline solid that melts at about 41°C;, boils at 182°C;, and is soluble in ethanol and ether and somewhat soluble in water.
..... Click the link for more information. , aniline aniline , C6H5NH2, colorless, oily, basic liquid organic compound; chemically, a primary aromatic amine whose molecule is formed by replacing one hydrogen atom of a benzene molecule with an amino group.
..... Click the link for more information. , and picric acid picric acid or 2,4,6-trinitrophenol , C6H2(NO2)3OH, a toxic yellow crystalline solid that melts at 122°C; and is soluble in most organic solvents. Picric acid is a derivative of phenol.
..... Click the link for more information. . 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
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.
benzene
benzene [′ben‚zēn]
Benzene
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.
REFERENCE
Nenitescu, 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.
REFERENCES
Omel’ianenko, L. M., and N. A. Senkevich. Klinika i profilaktika otravlenii benzolom. Moscow, 1957.Professional’nye bolezni, 2nd ed. Moscow, 1964.
Want to thank TFD for its existence? Tell a friend about us, add a link to this page, add the site to iGoogle, or visit the webmaster's page for free fun content.
| ?Page tools | ||
|---|---|---|
|