Aromatic hydrocarbons make up important part cyclic series of organic compounds. The simplest representative of such hydrocarbons is benzene. The formula of this substance not only distinguished it from a number of other hydrocarbons, but also gave impetus to the development of a new direction in organic chemistry.
Discovery of aromatic hydrocarbons
Aromatic hydrocarbons were discovered in the early 19th century. In those days, the most common fuel for street lighting was lamp gas. From his condensate great English physicist Michael Faraday isolated three grams of an oily substance in 1825, described its properties in detail and named it: carburated hydrogen. In 1834, the German scientist, chemist Mitscherlich, heated benzoic acid with lime and obtained benzene. The formula for this reaction is presented below:
C6 H5 COOH + CaO fusion of C6 H6 + CaCO3.
At that time, the rare benzoic acid was obtained from the resin of benzoic acid, which can be secreted by some tropical plants. In 1845, a new compound was discovered in coal tar, which was a completely accessible raw material for producing a new substance on an industrial scale. Another source of benzene is petroleum obtained from some fields. To provide the need industrial enterprises in benzene, it is also obtained by aromatization of certain groups of acyclic petroleum hydrocarbons.
The modern version of the name was proposed by the German scientist Liebig. The root of the word "benzene" should be found in Arabic languages- there it is translated as “incense”.
Physical properties of benzene
Benzene is a colorless liquid with a specific odor. This substance boils at a temperature of 80.1 o C, hardens at 5.5 o C and turns into a white crystalline powder. Benzene practically does not conduct heat and electricity, is poorly soluble in water and well soluble in various oils. Aromatic properties benzene reflect the essence of its structure internal structure: relatively stable benzene ring and uncertain composition.
Chemical classification of benzene
Benzene and its homologues - toluene and ethylbenzene - are an aromatic series of cyclic hydrocarbons. The structure of each of these substances contains a common structure called a benzene ring. The structure of each of the above substances contains a special cyclic group created by six carbon atoms. It is called the benzene aromatic ring.
History of discovery
The establishment of the internal structure of benzene took several decades. The basic principles of the structure (ring model) were proposed in 1865 by the chemist A. Kekule. As the legend tells, a German scientist saw the formula of this element in a dream. Later, a simplified spelling of the structure of a substance called benzene was proposed. The formula of this substance is a hexagon. The symbols for carbon and hydrogen, which should be located at the corners of the hexagon, are omitted. This produces a simple regular hexagon with alternating single and double lines on the sides. The general formula of benzene is shown in the figure below.
Aromatic hydrocarbons and benzene
The chemical formula of this element suggests that addition reactions are not typical for benzene. For it, as for other elements of the aromatic series, substitution reactions of hydrogen atoms in the benzene ring are typical.
Sulfonation reaction
By ensuring the interaction of concentrated sulfuric acid and benzene, increasing the reaction temperature, benzosulfonic acid and water can be obtained. The structural formula of benzene in this reaction is as follows:
Halogenation reaction
Bromine or chromium reacts with benzene in the presence of a catalyst. This produces halogen derivatives. But the nitration reaction takes place using concentrated nitric acid. The final result of the reaction is a nitrogenous compound:
Using nitriding, a well-known explosive is produced - TNT, or trinitotoluene. Few people know that tol is based on benzene. Many other benzene ring-based nitro compounds can also be used as explosives
Electronic formula of benzene
The standard formula of the benzene ring does not accurately reflect the internal structure of benzene. According to it, benzene must have three localized p-bonds, each of which must interact with two carbon atoms. But, as experience shows, benzene does not have ordinary double bonds. The molecular formula of benzene allows you to see that all the bonds in the benzene ring are equivalent. Each of them has a length of about 0.140 nm, which is intermediate between the length of a standard single bond (0.154 nm) and an ethylene double bond (0.134 nm). The structural formula of benzene, depicted with alternating bonds, is imperfect. A more plausible three-dimensional model of benzene looks like the image below.
Each of the atoms of the benzene ring is in a state of sp 2 hybridization. It spends three valence electrons on the formation of sigma bonds. These electrons cover two neighboring carbohydrate atoms and one hydrogen atom. At the same time, both electrons and S-S connections, H-H are in the same plane.
The fourth valence electron forms a cloud in the shape of a three-dimensional figure eight, located perpendicular to the plane of the benzene ring. Each such electron cloud overlaps above the plane of the benzene ring and directly below it with the clouds of two neighboring carbon atoms.
The density of the n-electron clouds of this substance is evenly distributed between all carbon bonds. In this way, a single ring electron cloud is formed. IN general chemistry This structure is called an aromatic electron sextet.
Equivalence of internal bonds of benzene
It is the equivalence of all the faces of the hexagon that explains the equalization of aromatic bonds, which determine the characteristic chemical and physical properties, which benzene has. Formula uniform distribution n-electron cloud and the equivalence of all of it internal connections shown below.
As you can see, instead of alternating single and double lines, the internal structure is depicted as a circle.
Essence internal structure benzene provides the key to understanding the internal structure of cyclic hydrocarbons and expands the possibilities of practical application of these substances.
Benzene(Also benzene)- the first representative of the homologous series of aromatic hydrocarbons, molecular formula C 6 H 6. Colorless volatile liquid with a characteristic odor. First obtained by Michael Faraday from whale oil pyrolysis condensate in 1825.
Industrially, benzene was extracted from coal tar fractions, but since the mid-20th century, almost the entire industrial volume of benzene is produced by the dehydrogenation of petroleum feedstocks. Benzene has valuable properties as a solvent, but due to its high toxicity and carcinogenicity, such use is still very limited. This compound is a raw material for industrial organic synthesis, more than two-thirds of benzene goes into the production of cyclohexane, cumene and ethylbenzene.
History of the study
Benzene is the first open by man arenov. IN pure form It was isolated by Michael Faraday by distillation crystallization from a luminous gas, is a product of the high-temperature decomposition of whale oil, and was used in street lamps. At the same time, the relative density of its vapors and the quantitative relationship between the atoms of the elements included in its composition were established; based on these data, Faraday calculated the empirical formula - C 2 H 2. An error in the formula was made due to the fact that at that time it was believed that atomic mass carbon is 6 a.o.m.. 1834 Mitscherlich isolated benzene by dry distillation of benzoic acid with lime, he established the correct empirical formula (C 6 H 6) and called this compound “gasoline” from benzoic acid. However, Liebig suggested using the name benzene, the ending of which is taken from the word dumb. Öl- oil. Modern name"benzene" is recommended for use by IUPAC due to the fact that the suffix -ol corresponds to alcohols. 1860 Kekule named benzene and other compounds with similar properties aromatic, because most of them had a pleasant smell.
On establishing the correct empirical formula of benzene, writing structural formulas organic compounds has not yet been accepted in chemistry. However, even after structural formulas were proposed for many aliphatic hydrocarbons, it was more difficult to do this for benzene: the formula C 6 H 6 indicated that this compound was not saturated hydrocarbons, however, benzene, unlike alkenes and alkynes, undergoes substitution reactions better than addition. In 1865, Kekule proposed a structural formula for benzene in the form of a six-membered ring with three double bonds alternating with single ones. It is widely known that the idea of the cyclic structure of benzene came to Kekula when he dreamed of a snake biting its own tail. Later descriptions of the dream speak of six monkeys holding each other's hind legs. In fact, the cyclic structure of beneznu was first published in his book by the Austrian chemist Joseph Loschmidt in 1861, and Kekule saw this publication.
Kekule's formulas could not explain some of the features of benzene, for example the fact that there were not two different isomers of 1,2-dimethylbenzene. In 1872, the scientist published an article in which he noted that although the existence of two different valence isomers for benzene can be assumed, the actual compound is intermediate between these two due to oscillation (transition) double bonds. However, even this addition could not explain the difference between benzene and known unsaturated hydrocarbons, so other scientists continued to propose alternative versions of the structure of this substance. Among them are the Dewar formulas of 1867 and the prismatic structure of Ladenburg (1869). It is now known that such compounds can indeed be synthesized; they are valence isomers of benzene.
From explanations of the properties of benzene proposed for the discovery of nature covalent bond, the closest to the modern one is the theory of “partial valences” (from lat. Partialis- partial) was proposed by Thiele in 1899. According to it, carbon atoms in unsaturated compounds have partial free valences, which in the benzene molecule “close” with each other, as a result of which the difference between single and double bonds disappears. The creation of the theory of covalent bonds made it possible to better understand the structure of Benezin; in 1926, Ingold suggested that in the molecule of this compound the electrons of π bonds are shifted to simple σ bonds, as a result of which they do not exist in an isolated state, but are aligned between single ones. Later, Linus Pauling, based on quantum mechanical concepts, proposed that there are no individual π bonds in the benzene molecule, and all their electrons are combined into a continuous π cloud.
IN scientific literature To designate benzene, both the Pauling formula and the Kekule formula are used, although the latter do not correctly reflect the structure of this molecule.
Physical properties
Benzene is a colorless liquid with a peculiar odor. Density - 0.88 g / cm³. At a temperature of 80.1 ° C it boils, and at 5.5 ° C it freezes into a white crystalline mass.
Benzene, due to its symmetry, is non-polar substance, therefore does not dissolve in water, but forms with it azeotropic mixture(91.17 wt%) with a boiling point of 69.25 ° C. Miscible with most non-polar solvents in all respects, it is itself a good solvent for many organic matter.
In the ultraviolet region of the absorption spectrum it appears as a number of bands fine structure with a distance between them of 5-6 nm (it is most intensely observed in the range of 170-120 nm and less in the range of 270-240 nm).
Structure
The molecular formula is C 6 H 6. X-ray methods have established that the benzene molecule has the shape of a flat hexagon with carbon atoms at the vertices. All C-C connections have same length, which is 0.140 nm. This is more than that of a double (0.134 nm) bond and less than that of a single (0.154 nm) bond. Benzene is a non-polar compound with zero dipole moment (μ).
All carbon atoms in a benzene molecule are in the state sp 2 hybridization. The three hybrid orbitals are located at an angle of 120°, forming C-C and C-H σ bonds. Non-hybridni p-orbitals located perpendicular to the plane of the molecule, forming a continuous electron ring. From a theoretical point of view valence bonds this ring can be viewed as a superposition of two resonance structures of an imaginary 1,3,5-cyclohexatriene with isolated C=C double bonds. From the point of view of molecular orbital theory, it can be considered as the result of delocalization along six carbon atoms of three π orbitals of double C = C bonds. The consequence of delocalization is the lower free energy (greater stability) of benzene compared to 1,3,5-cyclohexatriene. This difference in energy is called conjugation, delocalization, or resonance energy. It can be calculated based on the heats of hydrogenation of cyclohexene and benzene:
- the heat of hydrogenation of cyclohexene is 120 kJ/mol;
- then the expected heat of hydrogenation for 1,3,5-cyclohexatriene should be about 3 × 120 kJ/mol = 360 kJ/mol;
- in fact, the heat of hydrogenation of benzene is 208 kJ/mol;
- then the conjugation energy is 360 kJ/mol - 208 kJ/mol = 152 kJ/mol.
The formation of a continuous π cloud containing six electrons gives the benzene molecule its so-called aromatic character. The carbon skeleton of a benzene molecule with this type of bond is called a benzene ring, or benzene core.
Chemical properties
Due to the significant stability of the π-cloud for benzene, in contrast to non-aromatic unsaturated hydrocarbons, characteristic reactions substitution rather than addition, since they should lead to loss of aromaticity, but addition reactions can also occur under fairly stringent conditions. Substitution occurs by an electrophilic mechanism. Benzene also undergoes oxidation reactions.
Electrophilic substitution reactions
Benzene enters into electrophilic substitution reactions that occur according to the following mechanism: at the first stage, the formation of a π-complex occurs between the electrophile (in the form of a cation or a highly polarized molecule E σ + -Nu σ-) and the benzene molecule, as a result of the overlap of the LUMO Electrophile with the HOMO ( π-cloud) of benzene. After this the couple p-electrons leaves the conjugated benzene ring and participates in the formation of a σ bond with an electrophile, thus the π complex is converted into a σ complex or Welland intermediate. This intermediate connection has positive charge and lacks an aromatic character, which makes it less stable compared to the aromatic ring, which it usually quickly turns into as a result of proton abstraction (this step occurs through another intermediate π-complex).
Friedel-Crafts alkylation and acylation
Alkylation of benzene is carried out by alkyl halides, alkenes and alcohols, acylation - carboxylic acids, acid halides and anhydrides, both types of reactions catalyzed by Lewis acids. These reactions are named after their discoverers Charles Friedel and James Crafts.
The role of the catalyst in this type of reaction is that it interacts with the alkylating or acylating reagent and ensures the formation of a carbocation or polarized complex. For example, when chloromethane interacts with aluminum chloride, a complex is formed with increased electrophilicity of the carbon atom:
An example of an alkylation reaction would be the ethylation of benzene with chloroethane.
However, in industry, ethylbenzene is more often obtained by reaction with ethylene, which also takes place in the presence of aluminum oxide, phosphoric or sulfuric acid:
The products of benzene acylation reactions are aromatic ketones. An example would be a reaction with acetyl chloride, the product of which is methyl aryl ketone:
Halogenation
Unlike unsaturated hydrocarbons, benzene does NOT discolor bromine water. But it is characterized by halogenation reactions that occur by the mechanism of electrophilic substitution in the presence of Lewis acids. For example, when reacting with bromine, bromobenzene is formed:
Nitration
The nitration reaction characteristic of benzene uses a nitrating mixture that consists of concentrated nitric acid and concentrated sulfuric acid as a dewatering agent. This reaction produces nitrobenzene, which is a precursor in the synthesis of aniline
Sulfonation
When benzene is exposed to concentrated sulfuric acid, it sulfonates to form benzosulfonic acid, which can be a precursor in the synthesis of phenol:
Addition reactions
Benzene also undergoes addition reactions, but it is much more difficult than in substitution reactions. At the same time, it exhibits the properties of unsaturated hydrocarbons. Thus, in the presence of a nickel catalyst and upon heating, the hydrogenation reaction of benzene occurs with the formation of cyclohexane:
In this case, hydrogen atoms are attached to the benzene molecule due to the breaking of double bonds. Benzene also reacts by adding one, two or three chlorine molecules. This reaction occurs by a free radical mechanism; ultraviolet light is required for the formation of chlorine radicals (achieved by irradiation with a mercury-quartz lamp). The product of complete addition is hexachlorocyclohexane:
Oxidation reactions
In air, benzene burns with a very sooty flame, since the carbon content in it is very low. A mixture of benzene vapor and air is explosive. Due to its aromatic nature, benzene is resistant to oxidizing agents: it is not oxidized by potassium permanganate solution and nitric acid. In the presence of a catalyst, vanadium(V) oxide reacts with molecular oxygen, resulting in the formation of maleic anhydride:
Benzene is also oxidized by ozone, a reaction historically used to determine its structure.
Receipt and production
Today there are several fundamental different ways benzene production.
- Coking coal. This process was historically the first and served as the main source of benzene until World War II. IN Lately the proportion of benzene obtained by this method is less than 10%. It should be added that benzene obtained from coal tar contains a significant amount of thiophene, which makes such benzene a raw material unsuitable for a number of technological processes.
- Catalytic reforming (aromazing) of gasoline fractions of oil. This process is the main source of benzene in the United States. IN Western Europe, Russia and Japan obtain 40-60% of the total amount of the compound using this method. In addition to benzene, this process produces toluene and xylenes. Considering that toluene is formed in quantities exceeding the demand for it, it is also partially processed into: benzene - by hydrodealkylation; a mixture of benzene and xylenes - by disproportionation method;
- Pyrolysis of gasoline and heavier oil fractions. Up to 50% of benzene is produced by this method. Along with benzene, toluene and xylenes are formed. In some cases, this entire fraction is sent to the dealkylation stage, where both toluene and xylenes are converted to benzene.
- trimerization of acetylene
By passing acetylene at 600 °C over activated carbon with good solution benzene and other aromatic hydrocarbons are formed (reaction of N. D. Zelinsky):
3C 2 H 2 → C 6 H 6
Application
Benzene is important raw material For chemical industry. Large quantities it is used to obtain nitrobenzene, which is reduced to aniline according to the reaction of M. M. Zinin:
In technology, this reaction is carried out by exposure to benzene of hydrochloric acid in the presence of iron filings. Iron, reacting with acid, forms hydrogen, which, at the time of release, reduces nitrobenzene. Most organic dyes and pharmaceuticals are synthesized from aniline. Significant amounts of benzene are used for the synthesis of phenol, which is used in the production of phenol-formaldehyde resins. Hexachlorocyclohexane, obtained from benzene (reaction given above), called hexachlorane, is used in agriculture as one of the most effective means to kill insects. In addition, benzene is used for the synthesis of many other organic compounds and as a solvent.
| Receipt | Substance | Application | |
| + Cl 2 / AlCl 3 → C 6 H 5 Cl | + Cl 2 / AlCl 3 → 1,4-dichlorobenzene | 1,4-dichlorobenzene | Insecticide |
| + NaOH/Cu → Phenol | Phenol | Solvent, organic synthesis reagent, plastics, dyes, drugs, explosives | |
| + H 2 SO 4 → Benzenesulfonic acid (C 6 H 5 -SO 2 OH) + NaOH → Phenol | |||
| + Propene (CH 3 -CH = CH 2) → Cumene (C 6 H 5 -CH (CH 3) 2) + O 2 → Cumene hydroperoxide (C 6 H 5 -C (CH 3) 2 -OOH) → Phenol + acetone | |||
| + HNO 3 → nitrobenzene + 6H → aniline | Aniline (C 6 H 5 -NH 2) | Dyes, medicines | |
| + H 2 / Ni → Cyclohexane → Caprolactam | Caprolactam | Synthetic fibers | |
| + O 2 / V 2 O 5 → Maleic acid → Maleic anhydride | Maleic anhydride | Polyesters | |
| + Ethylene (CH 2 = CH 2) → Ethylbenzene (C 6 H 5 -CH 2 -CH 3) + ZnO → styrene (C 6 H 5 -CH = CH 2) + H 2 | Styrene | Plastics, synthetic rubbers | |
| + HOSO 2 Cl → Benzosulfanyl chloride (C 6 H 5 -SO 2 Cl) → Benzosulfonamide | Benzosulfonamide | Medicines, dyes |
Below are percentage use
- About 50% of benzene is converted to ethylbenzene (alkylation of benzene with ethylene)
- about 25% of benzene is converted to cumene (alkylation of benzene with propylene)
- approximately 10 - 15% of benzene is hydrogenated to cyclohexane;
- about 10% of benzene is spent on the production of nitrobenzene;
- 2 - 3% of benzene is converted into linear alkylbenzenes;
- approximately 1% benzene is used to synthesize chlorobenzene.
Benzene is used in significantly smaller quantities for the synthesis of other compounds. Occasionally and in extreme cases, due to its high toxicity, benzene is used as a solvent. In addition, benzene is part of gasoline. Due to its high toxicity, its content is limited to 5% by new standards.
Benzene homologues
Benzene, like other hydrocarbons, forms its own homologous series, It has general formula CnH2n-6. Benzene homologues can be considered as products of the substitution of one or more hydrogen atoms in the benzene molecule by various hydrocarbon radicals, forming side chains.
The simplest homologue of benzene is methylbenzene - the product of the replacement of the hydrogen atom in the benzene molecule with a methyl group - CH 3
Methylbenzene having technical name toluene is a colorless liquid with a characteristic odor. Boiling point 110.6 ° C. Density 0.867 g / cm In its chemical properties, methylbenzene, or toluene, like other homologues of benzene, is very close to benzene. Thus, under the action of concentrated nitric acid, in the presence of sulfuric acid, it easily lends itself to nitration with the formation of trinitrotoluene, a highly explosive substance.
Methylbenzene (toluene) is extracted from coal tar and coke oven gas along with benzene and then separated by fractional distillation. Methylbenzene, or toluene, is used mainly for the production of explosives - trinitrotoluene, which is also called TNT and toluene. In addition, toluene serves as a raw material for the production of dyes and other organic products.
Video on the topic
The main use of benzene is in the synthesis of many other organic substances. The process during which the product can be obtained is coal coking. If you heat this raw material at high temperatures and at the same time limiting the access of air, many volatile combustion products will be formed, among which benzene is emitted.
Formation of matter
The scientist N.D. Zelinsky once proved that benzene can be obtained not only by coking coal. This substance can also be obtained from a product such as cyclohexane if the catalytic effect of platinum or palladium on this substance is observed (at a temperature of 300 degrees Celsius). In addition, a substance such as hexane can also be converted to benzene if the correct catalytic process and heating procedure are applied.
To date, great practical significance received such operations as the production of benzene from saturated hydrocarbons and cycloparaffins. This is due to the fact that the demand for this substance is growing rapidly.
Use of volatile substance
The scope of benzene is quite extensive. The main direction was the production of other substances based on this reagent. So, for example, if you use the nitration reaction, you can get nitrobenzene, if you carry out the chlorination procedure, you can get chlorobenzene, which in life is most often called a solvent, as well as many other compounds.
The procedure for using benzene as a starting product for creating medicinal and aromatic substances has become widespread. Often used in monomer synthesis processes for high molecular weight compounds, to create dyes.
Derivatives of chlorine and benzene are currently successfully used in agriculture. Here they are used as chemical plant protection products. For example, a product in which hydrogen atoms are replaced by chlorine atoms, hexachlorobenzene, is actively used as a product for dry dressing of wheat and rye seeds.
Chemical industry
If we list the areas where benzene is used, there are a lot of them. However, in some he plays one of key roles, for example in the chemical industry. Here this component is one of the most in demand, since it is the starting element for the production of many others, and is also a solvent in many operations. It is worth noting that benzene can dissolve almost any organic compound. If in the first half of the 20th century the use of benzene was mainly for the creation of compounds such as nitro- and dinitro compounds, today the most common substances are ethylbenzene, cumene and cyclohexane. 60% of all benzene comes from the creation of the first two elements.
Types of composition and their application
Benzene itself is practically never used in its pure form. However, its derivatives are very widely used.
Ethylbenzene, for example, is common as an intermediate in the manufacture of styrene and is also successfully used as a motor fuel additive.
One of the new processes that can be used to obtain styrene directly from benzene is also generating widespread interest. The use of this substance in combination with ethylene and a Pd catalyst during the oxidation process is such a method. It is worth noting that when producing ethylbenzene, a by-product is released, which became known as diethylbenzene. This element itself is not very actively used, but with its help it has become possible to receive divinylbenzene, and this component is already a very valuable monomer for the production
One more an important component is cumene. This product is also a derivative of benzene, and it is used to create a substance - phenol, which has received wide practical application.
It is worth noting that there are very, very many substances that are formed with the help of benzene.
Benzyl chloride is a product of chlormetation. It is most widely used in the manufacture of benzyl alcohol, esters, dyes, etc.
Diphenylmethane is a substance obtained by reacting benzene with components such as benzyl chloride or formaldehyde. This product can be used as a fragrance, as it has a geranium scent, or as a solvent for paint and varnish products.
Sulfo derivatives of benzene are also known. These products are intermediates whose main purpose is to produce more complex intermediates. Based on certain benzenesulfonic acids, final compositions can be obtained that can be used in production polymer materials.
The first attempts to use benzene in medicine were made a very long time ago. The first area where it was applied was oncohematology. The main idea behind benzene was to use it to treat a disease such as leukemia. The speed at which this idea spread was enormous. In 1912, doctors around the world used this substance to treat leukemia in patients. At first, the substance was used only for oral administration. However, fairly soon injection attempts were made. By this time, there was a surge in the use of raw materials in medicinal purposes has already subsided. It turned out that leukemia cannot be cured in this way. In addition, this chemical has many dangerous side effects.
However, while the composition was still in use, doctors isolated some positive points. For example, benzene caused the number of white balls in the blood to decrease significantly by the end of the 2nd and beginning of the 3rd week. Red blood cells initially decreased in number, but then this quickly passed and the number increased again. It was also noted that benzene can improve the leukemic composition of the blood in cases where the X-ray method was not able to cope.
However, as already mentioned, this method was quickly recognized as ineffective and dangerous.
Mrs. Khimiya finally and irrevocably acquired such a compound as benzene only in 1833. Benzene is a compound that has a hot-tempered, one might even say explosive, character. How did you find out?
Story
Johann Glauber in 1649 turned his attention to a compound that was successfully formed when a chemist was processing coal tar. But it wished to remain incognito.
About 170 years later, or to be much more precise, in the mid-twenties of the 19th century, by chance, benzene was extracted from the illuminating gas, namely from the released condensate. Humanity owes such efforts to Michael Faraday, a scientist from England.
The baton for the acquisition of benzene was taken over by the German Eilgard Mitscherlich. This happened during the processing of anhydrous calcium salts of benzoic acid. Perhaps that is why the compound was given such a name - benzene. Alternatively, the scientist called it gasoline. Incense, if translated from Arabic.
Benzene burns beautifully and brightly; in connection with these observations, Auguste Laurent recommended calling it “fen” or “benzene”. Bright, shining - if translated from Greek.
Based on the concept of the nature of electronic communication and the qualities of benzene, the scientist provided the molecule of the compound in the form next image. This is a hexagon. A circle is inscribed in it. The above suggests that benzene has a complete electron cloud, which safely encloses six (without exception) carbon atoms of the cycle. No fastened binary bonds are observed.
Benzene was previously used as a solvent. But basically, as they say, he was not a member, did not participate, was not involved. But this is in the 19th century. Significant changes took place in the 20th century. The properties of benzene are expressed the most valuable qualities, which helped him become more popular. The octane number, which turned out to be high, made it possible to use it as a fuel element for refueling cars. This action served as the impetus for the extensive withdrawal of benzene, its extraction is carried out as a secondary product of coking steel production.
By the forties, benzene began to be used in the chemical field in the manufacture of substances that quickly explode. The 20th century crowned itself with the fact that the oil refining industry produced so much benzene that it began to supply the chemical industry.
Characteristics of benzene
Unsaturated hydrocarbons are very similar to benzene. For example, the ethylene hydrocarbon series characterizes itself as an unsaturated hydrocarbon. It is characterized by an addition reaction. Benzene readily enters into all this thanks to the atoms that are in the same plane. And as a fact - a conjugate electron cloud.
If the formula contains benzene ring, which means we can come to elementary conclusion that it is benzene, structural formula which looks exactly like this.
Physical properties
Benzene is a liquid that has no color, but has a regrettable odor. Benzene melts when the temperature reaches 5.52 degrees Celsius. Boils at 80.1. The density is 0.879 g/cm 3, the molar mass is 78.11 g/mol. When burning it smokes a lot. Forms explosive compounds when air enters. rocks (gasoline, ether and others) combine with the described substance without problems. Creates an azeotropic compound with water. Heating before vaporization begins at 69.25 degrees (91% benzene). At 25 degrees Celsius it can dissolve in water 1.79 g/l.
Chemical properties
Benzene reacts with sulfuric and nitric acid. And also with alkenes, halogens, chloroalkanes. The substitution reaction is what is characteristic of it. The pressure temperature affects the breakthrough of the benzene ring, which occurs under rather harsh conditions.
We can consider each benzene reaction equation in more detail.
1. Electrophilic substitution. Bromine, in the presence of a catalyst, reacts with chlorine. As a result, we obtain chlorobenzene:
С6H6+3Cl2 → C6H5Cl + HCl
2. Friedel-Crafts reaction, or alkylation of benzene. The appearance of alkylbenzenes occurs due to the combination with alkanes, which are halogen derivatives:
C6H6 + C2H5Br → C6H5C2H5 + HBr
3. Electrophilic substitution. Here the reaction of nitration and sulfonation takes place. The equation for benzene will look like this:
C6H6 + H2SO4 → C6H5SO3H + H2O
C6H6 + HNO3 → C6H5NO2 + H2O
4. Benzene when burning:
2C6H6 + 15O2 → 12CO2 + 6H2O
Under certain conditions, it exhibits a character characteristic of saturated hydrocarbons. The P-electron cloud, which is located in the structure of the substance in question, explains these reactions.
Depends on special technology different kinds benzene This is where petroleum benzene is labeled. For example, purified and highly purified, for synthesis. I would like to separately note the homologues of benzene, and more specifically, their Chemical properties. These are alkylbenzenes.
Benzene homologues react much more readily. But the above reactions of benzene, namely homologues, take place with some differences.
Halogenation of alkylbenzenes
The form of the equation is as follows:
C6H5-CH3 + Br = C6H5-CH2Br + HBr.
The tendency of bromine into the benzene ring is not observed. It comes out into the chain from the side. But thanks to the Al(+3) salt catalyst, bromine easily enters the ring.
Nitration of alkylbenzenes
Thanks to sulfur and nitric acids Benzenes and alkylbenzenes are nitrated. Reactive alkylbenzenes. Two of the presented three products are obtained - these are para- and ortho-isomers. You can write one of the formulas:
C6H5 - CH3 + 3HNO3 → C6H2CH3 (NO2)3.
Oxidation
This is unacceptable for benzene. But alkylbenzenes react readily. For example, benzoic acid. The formula is given below:
C6H5CH3 + [O] → C6H5COOH.
Alkylbenzene and benzene, their hydrogenation
In the presence of an amplifier, hydrogen begins to react with benzene, resulting in the formation of cyclohexane, as discussed above. In a similar way alkybenzenes are easily converted to alkylcyclohexanes. To obtain alkylcyclohexane, it is necessary to hydrogenate the desired alkylbenzene. This is basically a necessary procedure to produce a pure product. And this is not all the reactions of benzene and alkylbenzene.
Benzene production. Industry
The foundation of such production is based on the processing of components: toluene, naphtha, tar, which is released during cracking of coal, and others. Therefore, benzene is produced at petrochemical and metallurgical enterprises. It is important to know how to get benzene varying degrees cleaning, because the brand of a given substance directly depends on the principle of manufacture and purpose.
The lion's share is produced by thermocatalytic reforming of the caustobiolite part, boiling at 65 degrees, having an extract effect, distillation with dimethylformamide.
When producing ethylene and propylene, liquid products are obtained that are formed during the decomposition of inorganic and organic compounds under the influence of heat. Benzene is isolated from them. But, unfortunately, there is not so much source material for this option for benzene extraction. Therefore, the substance we are interested in is extracted by reforming. By this method the volume of benzene is increased.
By dealkylation at a temperature of 610-830 degrees with a plus sign, in the presence of steam formed by the boiling of water and hydrogen, benzene is obtained from toluene. There is another option - catalytic. When the presence of zeolites, or, alternatively, oxide catalysts, is observed, subject to temperature regime 227-627 degrees.
There is another, older, method for developing benzene. By absorption by absorbents of organic origin, it is isolated from final result coking of coal. The product is a vapor-gas product and has been cooled in advance. For example, oil is used, the source of which is petroleum or coal. When distillation is carried out with steam, the absorbent is separated. Hydrotreating helps remove excess substances from crude benzene.
Coal raw materials
In metallurgy, when using coal, or, to be more precise, dry distilling it, coke is obtained. During this procedure, the air supply is limited. Do not forget that coal is heated to a temperature of 1200-1500 Celsius.
Coal chemical benzene needs thorough purification. It is imperative to get rid of methyl cyclohexane and its friend n-heptane. should also be confiscated. Benzene faces a process of separation and purification, which will be carried out more than once.
The method described above is the oldest, but over time it loses its high position.
Oil fractions
0.3-1.2% - these are the composition indicators of our hero in crude oil. Meager indicators to invest money and effort. It is best to use an industrial procedure for processing petroleum fractions. That is, catalytic reforming. In the presence of an aluminum-platinum-rhenium amplifier, the percentage of aromatic carbohydrates increases, and the indicator that determines the ability of the fuel not to spontaneously ignite during its compression increases.
Pyrolysis resins
If we extract our petroleum product from non-solid raw materials, namely by pyrolysis of propylene and ethylene arising during the production, then this approach will be the most acceptable. To be precise, benzene is released from the pyrocondensate. The decomposition of certain proportions requires hydrotreating. During cleaning, sulfur and unsaturated mixtures are removed. The initial result contained xylene, toluene, and benzene. Using distillation, which is extractive, the BTK group is separated to produce benzene.
Hydrodealkylation of toluene
The main characters of the process, a cocktail of hydrogen flow and toluene, are fed heated into the reactor. Toluene passes through the catalyst bed. During this process, the methyl group is separated to form benzene. Appropriate here a certain way cleansing. The result is a highly pure substance (for nitration).
Disproportionation of toluene
As a result of the rejection of the methyl class, creation occurs to benzene, and xylene is oxidized. IN this process transalkylation was observed. The catalytic effect occurs thanks to palladium, platinum and neodymium, which are located on aluminum oxide.
Taluene and hydrogen are supplied to the reactor with a stable catalyst bed. Its purpose is to keep hydrocarbons from settling onto the catalyst plane. The stream that leaves the reactor is cooled, and hydrogen is safely recovered for recycling. What's left is distilled three times. On initial stage compounds that are non-aromatic are removed. Benzene is extracted second, and the last step is the separation of xylenes.
Acetylene trimerization
Thanks to the work of the French physical chemist Marcelin Berthelot, benzene began to be produced from acetylene. But what stood out was a heavy cocktail of many other elements. The question was how to lower the reaction temperature. The answer was received only in the late forties of the 20th century. V. Reppe found the appropriate catalyst, it turned out to be nickel. Trimerization is the only option to obtain benzene from acetylene.
Benzene is formed by activated carbon. At high heat levels, acetylene passes over the coal. Benzene is released if the temperature is at least 410 degrees. At the same time, various aromatic hydrocarbons are also born. Therefore, you need good equipment that can efficiently clean acetylene. With such a labor-intensive method as trimerization, a lot of acetylene is consumed. To obtain 15 ml of benzene, take 20 liters of acetylene. You can see how it looks and the reaction will not take long.
3C2H2 → C6H6 (Zelinsky equation).
3CH → CH = (t, kat) = C6H6.
Where is benzene used?
Benzene is a fairly popular brainchild of chemistry. It was especially often noticed how benzene was used in the production of cumene, cyclohexane, and ethylbenzene. To create styrene, you cannot do without ethylbenzene. The starting material for the production of caprolactam is cyclohexane. When making thermoplastic resin, caprolactam is used. The described substance is indispensable in the manufacture of various paints and varnishes.
How dangerous is benzene?
Benzene is toxic substance. The manifestation of a feeling of malaise, which is accompanied by nausea and severe dizziness, is a sign of poisoning. It's not even possible death. A feeling of indescribable delight is no less alarming bells for benzene poisoning.
Benzene in liquid state causes skin irritation. Benzene vapors easily penetrate even intact skin. With very short-term contacts with the substance in a small dose, but on a regular basis, unpleasant consequences will not be long in coming. This may be bone marrow damage and leukemia acute nature different types.
In addition, the substance causes addiction in humans. Benzene acts like dope. Tobacco smoke produces a tar-like product. When they studied it, they came to the conclusion that its contents are unsafe for humans. In addition to the presence of nicotine, the presence of aromatic carbohydrates such as benzpyrene was also discovered. Distinctive feature benzopyrene are carcinogenic substances. They have a very harmful effect. For example, they cause cancer.
Despite the above, benzene is a starting raw material for the production of various medicines, plastics, synthetic rubber and, of course, dyes. This is the most common brainchild of chemistry and an aromatic compound.