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benzene from coal tar

Product name benzene from coal tar
GOST 8448-78
CAS 71-43-2

Benzene is a clear, colorless, volatile, flammable liquid with a characteristic "aromatic" smell. It is slightly soluble in water (0.073% wt at 25°C). The solubility of water in benzene is 0.05% wt at 26°C. It is fully miscible in alkanes, olefins, aromatic hydrocarbons, ethers, esters. It is boundedly soluble in methanol, insoluble in ethylene glycol and glycerol. Benzene is a good solvent for greases, fats, rubber, sulfur, phosphorus, iodine. It forms an azeotropes with water (b.p. is 69.8°C, 91.0% wt of benzene), isopropyl alcohol (71.9°C, 66.2% wt), n-propanol (77.1°C, 83.1% wt), ethanol (67.9°C, 67.5% wt), methanol (58.0°C, 62.0% wt), formic acid (71.0°C, 67.0% wt).

Benzene was isolated for the first time in 1825 by Michael Faraday as a product of whale oil pyrolysis. First synthetic benzene was obtained bu German chemist Eilhard Mitscherlich in 1833 by decarboxylation of benzoic acid. He also gave the name benzin for a new compound. A cyclic structural formula were suggested by German chemist Friedrich August Kekule in 1865.

Benzene is a parent aromatic hydrocarbon. It's molecule is a regular hexagon. Each angle is 120°, all bond C-C lengths are 0.139nm. This value is typical for aromatic bond and it is intermediate between single (0.154nm) and double (0.134nm) bond lengths.

The compound with brutto formula C6H6 can be presented with a number of structural formulae with localized (non-aromatic) single and double bonds. Some of them are shown below. That are cyclohexatriene (1), benzvalene (2), Dewar benzene (3), prisman (4), fulvene (5).

Benzene structure isomers


Anyway cyclohexatriene is regarded only as resonance form. At the same time the existence of Dewar benzene structure is really proved. Oxidation of cyclobutadiene complexes of transition metals with cerium(IV) salts or with silver salts in the presence of diphenyl acetylene results in cyclobutadiene liberation. Free cyclobutadiene condences with diphenyl acetylene resulting in diphenyl substituted Dewar benzene. It is clear that Dewar benzene relatively unstable and spontaneously gradually converts into simple diphenyl benzene. Being irradiated with UV light benzene and its homologues isomerize to give derivatives with benzvalene and fulvene sceleton.

Benzene is a thermodynamically unstable compound because of ΔG of this hypothetical reaction at room temperature is negative:

C6H6 → 6C(solid) + 3H2(gas) ΔG<0

It means that the decomposition of benzene is allowed by thermodynamics. At the same time one can recognize that benzene is stable at room temperature and do not decompose into elements. This occurs because the energy barrier of decomposition is quite large. So, benzene is an example of kinetically stable compound.

Chemical properties

Chemical properties of benzene were investigated in details and described in many sources. The main characteristic properties of aromatic compounds are stability upon oxidizing agents action and stability in addition to multiple bond reactions. Benzene show the best correlation with both rules because it do not discolour neither potassium permanganate nor bromine water solutions. Benzene reacts with oxygen at high temperature 350-450°C in the presence of catalysts (V, Mo) breaking aromatic ring and to form finally maleic anhydride.

Hydrogenation of benzene occurs also at hard conditions. It hydrogenates at 120-200°C and pressure 2.96-6.94MPa in the presence of nickel catalyst to form cyclohexane. It also hydrogenates in liquid ammonia upon action of alkali metals resulting in 1,4-cyclohexadiene formation.

Electrophilic substitution is a characteristic reaction for aromatic compounds. Benzene can be nitrated, sulfinated, halogenated, acylated and so on via electrophilic substitution mechanism. This reaction proceeds via subsequent formation of π-complex and σ-complex:

the mechanism of electrophilic substitution in benzene


Electrophilic substitution in benzene is accelerated in the presence of aluminum chloride AlCl3, iron(III) chloride FeCl3 or other Lewis acids. The catalyst facilitate formation of intermediate complexes. The Friedel-Crafts reaction is a variety of electrophilic substitution in benzene as applied to acylation and alkylation.

Benzene can act as a six-electron ligand in the complexes with transition metals. These adducts are called arene complexes. They may be both homoligand (Cr(C6H6)2) or heteroligand (Cr(C6H6)(CO)3). Most stable of arene complexes bisbenzene chromium Cr(C6H6)2 is used in industry for preparation of chromium-carbide coatings. Bisbenzene chromium can be obtained in apparatus at low pressure. The electric current heated chromium wire is placed into the center of apparatus, the cooled sides are coated with solid benzene. The chromium vapours interact with benzene to give bisbenzene chromium. The chemical industry use another process for the preparation of bisbenzene chromium derivatives. This method were found in 1955 by German chemist Ernst Otto Fischer. It is suitable for preparation of arene complexes of chromium, vanadium, molybdenum, tungsten, technetium, rhenium, iron, ruthenium, cobalt, rhodium, nickel, osmium, iridium and platinum. The reaction proceeds at 130-140°C. The metal aluminum is used as reducing agent and can be replaced with zinc powder. Aluminum chloride acts as a catalyst.

3CrCl3 + 2Al + AlCl3 + 6C6H6 → 3[(C6H6)2Cr]+AlCl4-

Then bisbenzene chromium cation is reduced with sodium hydrosulfite Na2S2O4 to produce neutral bisbenzene chromium.

Toxicity.

Benzene exposure has serious health effects. Its action have a cumulative effect. The inhalation may cause drowsiness, dizziness, rapid heart rate, headaches, tremors, confusion, unconsciousness or death. Foods or beverages containing the high concentrations of benzene may cause vomiting, irritation of the stomach, dizziness, sleepiness, convulsions, rapid heart rate. The benzene is poisonous for immune system and hematosis and causes anemia. The US Department of Health and Human Services (DHHS) classifies benzene as a human carcinogen.

Production.

Up until 1940s, most benzene was produced as a byproduct of coke production in the steel industry. However, then, growing plastics industry, necessitated the production of benzene from petroleum. Today, most benzene comes from the petrochemical industry, with only a small fraction being produced from coal.

Three chemical processes contribute equally to industrial benzene production: catalytic reforming, toluene hydrodealkylation, and steam cracking.

  • Catalytic reforming. That is most wide-spread industrial method. Oil fraction which boils between 60-200°C is mixed with dihydrogen and then exposed at 500-525°C and pressure 0.8-5MPa in the presence of platinum chloride or rhenium chloride catalyst. Chain aliphatic hydrocarbons undergo isomerization and partial dehydrogenation under this conditions to form ring structures. Final reaction mixture then separated into aromatics and non-aromatics by extraction with any one or a mixture of solvents such as diethylene glycol or sulfolane. The aromatics mixture then separated by distillation.
  • Toluene hydrodealkylation. Some petrochemical processes produce toluene. If any excess amount of toluene exist, it can be converted into benzene. There are two main methods used for this conversion. In thermal method toluene is dealkylated at 600-820°C in the presence of dihydrogen and steam. In catalytical process toluene is dealkylated at 227-627°C and 4-5MPa in the presence of ceolites or chromium, molybdenum, or platinum oxide catalysts. The yield of this reaction exceed 95%.
  • Steam cracking. That is most economically effective method. Steam cracking is used for industrial preparation of ethylene, propylene and other olefins. The byproduct of this process is liquid pyrolysis gasoline which contains a large amount of benzene. Pyrolysis gasoline then is distilled to separate into components.

Uses.

  • Large amounts of benzene are alkylated with ethylene to obtain ethylbenzene and diethylbenzenes. Ethylbenzene is an intermediate in styrene production.
  • Hydrogenation of benzene results in cyclohexane which is used in caprolactam (a source for Nylon) manufacturing.
  • Being alkylated with propylene benzene gives cumene which is used on phenol and acetone industrial preparation.
  • Previously benzene was frequently used as an additive to motor fuels for increasing of octane number. But now due to high carcinogenic effect and resinification upon burning it is not applied for this purpose. European and US petrol specifications now contain 1% limit on benzene content.
  • As a parent compound in industrial synthesis of maleic anhydride.
  • As a solvent and extracting agent in a laboratory and industrial chemical processes.
  • As a solvent in a laboratory for compound molecular mass determination by cryoscopy method.

Manufacturer(s) JSC KOKS
OJSC Nizhniy Tagil Iron and Steel Works, COKE OVEN AND BY-PRODUCT PLANT
JSC SEVERSTAL, COKE OVEN AND BY-PRODUCT PLANT
JSC Uralorgsintez
JSC West-Siberian Metallurgical Plant, COKE OVEN AND BY-PRODUCT PLANT
Chemical structure of benzene

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