Aromatic HCs (arenas) are hydrocarbons whose molecules contain one or more benzene rings.
Examples of aromatic hydrocarbons:
Benzene row arenas (monocyclic arenas)
General formula: C n H 2n-6 , n≥6
The simplest representative of aromatic hydrocarbons is benzene, its empirical formula is C 6 H 6 .
The electronic structure of the benzene molecule
The general formula of C n H 2 n -6 monocyclic arenes shows that they are unsaturated compounds.
In 1856, the German chemist A.F. Kekule proposed a cyclic formula for benzene with conjugated bonds (single and double bonds alternate) - cyclohexatriene-1,3,5: 
This structure of the benzene molecule did not explain many of the properties of benzene:
- for benzene, substitution reactions are characteristic, and not addition reactions characteristic of unsaturated compounds. Addition reactions are possible, but they are more difficult than for;
- benzene does not enter into reactions that are qualitative reactions to unsaturated hydrocarbons (with bromine water and KMnO 4 solution).
Later electron diffraction studies showed that all bonds between carbon atoms in a benzene molecule have the same length of 0.140 nm (the average value between the length of a simple C-C connections 0.154 nm and C=C double bond 0.134 nm). The angle between the bonds at each carbon atom is 120°. The molecule is a regular flat hexagon.
Modern theory to explain the structure of the C 6 H 6 molecule uses the concept of hybridization of atomic orbitals.
The carbon atoms in benzene are in a state of sp 2 hybridization. Each "C" atom forms three σ-bonds (two with carbon atoms and one with a hydrogen atom). All σ-bonds are in the same plane:
Each carbon atom has one p-electron, which does not participate in hybridization. Unhybridized p-orbitals of carbon atoms are in the plane, perpendicular to the planeσ-bonds. Each p-cloud overlaps with two neighboring p-clouds, and as a result, a single conjugated π-system is formed (remember the effect of conjugation of p-electrons in the 1,3-butadiene molecule, discussed in the topic “Diene hydrocarbons”):
The combination of six σ-bonds with a single π-system is called aromatic bond.
A ring of six carbon atoms linked by an aromatic bond is called benzene ring, or benzene nucleus.
In accordance with modern ideas about the electronic structure of benzene, the C 6 H 6 molecule is depicted as follows:
Physical properties of benzene
Benzene under normal conditions is a colorless liquid; t o pl = 5.5 o C; t o kip. = 80 about C; has a characteristic smell; immiscible with water, good solvent, highly toxic.
Chemical properties of benzene
The aromatic bond determines the chemical properties of benzene and other aromatic hydrocarbons.
The 6π-electron system is more stable than conventional two-electron π-bonds. Therefore, addition reactions are less typical for aromatic hydrocarbons than for unsaturated hydrocarbons. The most typical for arenes are substitution reactions.
I. Substitution reactions
1.Halogenation
2. Nitration
The reaction is carried out with a mixture of and acids (nitrating mixture): 
3. Sulfonation
4. Alkylation (replacement of the "H" atom by an alkyl group) - Friedel-Crafts reactions, homologues of benzene are formed:
Instead of haloalkanes, alkenes can be used (in the presence of a catalyst - AlCl 3 or inorganic acid):
II. Addition reactions
1. Hydrogenation
2. Addition of chlorine
III.Oxidation reactions
1. Combustion
2C 6 H 6 + 15O 2 → 12CO 2 + 6H 2 O
2. Not complete oxidation (KMnO 4 or K 2 Cr 2 O 7 in an acidic environment). The benzene ring is resistant to oxidizing agents. The reaction does not occur.
Getting benzene
In industry:
1) oil and coal processing;
2) dehydrogenation of cyclohexane:
3) dehydrocyclization (aromatization) of hexane:
In the laboratory:
Fusion of salts of benzoic acid with:
Isomerism and nomenclature of benzene homologues
Any benzene homologue has a side chain, i.e. alkyl radicals attached to the benzene ring. The first homologue of benzene is a benzene nucleus linked to a methyl radical: 
Toluene has no isomers, since all positions in the benzene ring are equivalent.
For subsequent homologues of benzene, one type of isomerism is possible - side chain isomerism, which can be of two types:
1) isomerism of the number and structure of substituents;
2) isomerism of the position of substituents.
Physical properties of toluene
Toluene- a colorless liquid with a characteristic odor, insoluble in water, soluble in organic solvents. Toluene is less toxic than benzene.
Chemical properties of toluene
I. Substitution reactions
1. Reactions involving the benzene ring
Methylbenzene enters into all substitution reactions in which benzene is involved, and at the same time exhibits a higher reactivity, the reactions proceed at a faster rate.
The methyl radical contained in the toluene molecule is a substituent of the genus, therefore, as a result of substitution reactions in the benzene nucleus, ortho- and para-derivatives of toluene are obtained or, with an excess of the reagent, tri-derivatives of the general formula:
a) halogenation
With further chlorination, dichloromethylbenzene and trichloromethylbenzene can be obtained:
II. Addition reactions
hydrogenation
III.Oxidation reactions
1. Combustion
C 6 H 5 CH 3 + 9O 2 → 7CO 2 + 4H 2 O
2. Incomplete oxidation
Unlike benzene, its homologues are oxidized by some oxidizing agents; in this case, the side chain undergoes oxidation, in the case of toluene, the methyl group. Mild oxidizing agents like MnO 2 oxidize it to an aldehyde group, stronger oxidizing agents (KMnO 4) cause further oxidation to an acid: 
Any homologue of benzene with one side chain is oxidized by a strong oxidizing agent such as KMnO4 to benzoic acid, i.e. there is a break in the side chain with the oxidation of its cleaved off part to CO 2; for example: 
In the presence of several side chains, each of them is oxidized to a carboxyl group and as a result polybasic acids are formed, for example:
Getting toluene:
In industry:
1) oil and coal processing;
2) dehydrogenation of methylcyclohexane:
3) dehydrocyclization of heptane:
In the laboratory:
1) Friedel-Crafts alkylation;
2) Wurtz-Fittig reaction(reaction of sodium with a mixture of halobenzene and haloalkane).
We present to your attention a video tutorial on the topic " Chemical properties benzene". Using this video, you can get an idea of the chemical properties of benzene, as well as the harsh conditions that are required for benzene to react with other substances.
Topic:aromatic hydrocarbons
Lesson:Chemical properties of benzene
Rice. 1. Benzene molecule
Breaking the p-electron cloud in the benzene molecule is difficult. Therefore, benzene enters into chemical reactions much less actively than unsaturated compounds.
In order for benzene to enter into chemical reactions, rather harsh conditions are necessary: an elevated temperature, and in many cases a catalyst. In most reactions, the stable benzene ring is retained.
1. Bromination.
A catalyst (iron(III) or aluminum bromide) is required and even small amounts of water must not be allowed to enter. The role of the catalyst is that the bromine molecule is attracted by one of the bromine atoms to the iron atom. As a result, it polarizes - a pair of bond electrons passes to the bromine atom associated with iron:
Br+…. Br - FeBr 3 .
Br+ is a strong electrophile. It is attracted to the six-electron cloud of the benzene ring and breaks it, forming a covalent bond with the carbon atom:
The bromine anion could join the resulting cation. But the reduction of the aromatic system of the benzene ring is energetically more favorable than the addition of the bromine anion. Therefore, the molecule goes into a stable state by throwing out a hydrogen ion:
All electrophilic substitution reactions in the benzene ring proceed according to a similar mechanism.
2. Nitration
Benzene and its homologues react with a mixture of concentrated sulfuric and nitric acids (nitrating mixture). In the nitrating mixture, in equilibrium, there is a nitronium ion NO 2 +, which is an electrophile:
3. Sulfonation.
Benzene and other arenes, when heated, react with concentrated sulfuric acid or oleum - a solution of SO 3 in sulfuric acid:
4 . Friedel-Crafts Alkylation
5. Alkylation with alkenes
These reactions are energetically unfavorable, therefore, they proceed only when heated or irradiated.
1. Hydrogenation.
When heated, at elevated pressure, and in the presence of a Ni, Pt, or Pd catalyst, benzene and other arenes add hydrogen to form cyclohexane:
2. Chlorination of benzene.
Under the action of ultraviolet radiation, benzene adds chlorine. If a quartz glass flask with a solution of chlorine in benzene is exposed to sunlight, the solution will quickly discolor, chlorine will combine with benzene to form 1,2,3,4,5,6-hexachlorocyclohexane, which is known as hexachlorane(previously used as an insecticide):
3. burning benzene.
Unlike alkanes, the flame of benzene and other aromatic hydrocarbons is bright and smoky.
Summing up the lesson
In this lesson, you studied the topic "Chemical Properties of Benzene". By using this material, you were able to get an idea of the chemical properties of benzene, as well as the harsh conditions that are required for benzene to react with other substances.
Bibliography
1. Rudzitis G.E. Chemistry. Basics general chemistry. Grade 10: textbook for educational institutions: a basic level of/ G. E. Rudzitis, F.G. Feldman. - 14th edition. - M.: Education, 2012.
2. Chemistry. Grade 10. Profile level: studies. for general education institutions / V.V. Eremin, N.E. Kuzmenko, V.V. Lunin and others - M.: Drofa, 2008. - 463 p.
3. Chemistry. Grade 11. Profile level: textbook. for general education institutions / V.V. Eremin, N.E. Kuzmenko, V.V. Lunin and others - M.: Drofa, 2010. - 462 p.
4. Khomchenko G.P., Khomchenko I.G. Collection of problems in chemistry for those entering the universities. - 4th ed. - M.: RIA "New Wave": Publisher Umerenkov, 2012. - 278 p.
Homework
1. No. 13, 14 (p. 62) Rudzitis G.E., Feldman F.G. Chemistry: Organic Chemistry. Grade 10: textbook for educational institutions: basic level / G. E. Rudzitis, F.G. Feldman. - 14th edition. - M.: Education, 2012.
2. Why do aromatic compounds differ in chemical properties from both saturated and unsaturated hydrocarbons?
3. Write the equations for the combustion reactions of ethylbenzene and xylene.
   |
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| Systematic Name | benzene |
| Abbreviations | PhH |
| Traditional names | hairdryer (Laurent, 1837), phenyl hydrogen, benzene |
| Chem. formula | C₆H₆ |
| State | liquid |
| Molar mass | 78.11 g/mol |
| Density | 0.8786 g/cm³ |
| Dynamic viscosity | 0.0652 Pa s |
| Ionization energy | 9.24 ± 0.01 eV |
| T. melt. | 5.5° |
| T. kip. | 80.1° |
| T. rev. | −11° |
| T. svsp. | 562° |
| Etc. blast | 1.2 ± 0.1 vol% |
| Steam pressure | 75 ± 1 mmHg |
| Solubility in water | 0.073 g/100 ml |
| GOST | GOST 5955-75 |
| Reg. CAS number | 71-43-2 |
| PubChem | 241 |
| Reg. EINECS number | 200-753-7 |
| SMILES | C1=CC=CC=C1 |
| InChI | |
| RTECS | CY1400000 |
| CHEBI | 16716 |
| ChemSpider | 236 |
| Toxicity | toxic, has carcinogenic and narcotic properties |
| signal word | DANGEROUS! |
| Data are given for standard conditions (25°, 100 kPa) unless otherwise noted. | |
Chemical properties
Substitution reactions are characteristic of benzene - benzene reacts with alkenes, chloroalkanes, halogens, nitric and sulfuric acids. Benzene ring cleavage reactions take place under harsh conditions (temperature, pressure).
- Interaction with alkenes (alkylation), as a result of the reaction, benzene homologues are formed, for example, ethylbenzene and cumene:
- Interaction with chlorine and bromine in the presence of a catalyst to form chlorobenzene (electrophilic substitution reaction):
- In the absence of a catalyst, when heated or illuminated, a radical addition reaction occurs with the formation of a mixture of hexachlorocyclohexane isomers
- When benzene reacts with bromine in an oleum solution, hexabromobenzene is formed:
- Interaction with halogen derivatives of alkanes (benzene alkylation, Friedel-Crafts reaction) to form alkylbenzenes:
- The Friedel-Crafts acylation reaction of benzene anhydrides, carboxylic acid halides leads to the formation of aromatic and fatty aromatic ketones:
6 6 + 6 5 COCl → AlCl 3 6 5 COC 6 5 + HCl
In the first and second reactions, acetophenone (methylphenyl ketone) is formed, replacing aluminum chloride with antimony chloride allows the reaction temperature to be reduced to 25 ° C. In the third reaction, benzophenone (diphenyl ketone) is formed.
- The formylation reaction - the interaction of benzene with a mixture of CO and HCl, proceeds at high pressure and under the action of a catalyst, the reaction product is benzaldehyde:
- Sulfonation and nitration reactions (electrophilic substitution):
- Reduction of benzene with hydrogen (catalytic hydrogenation):
Oxidation reactions
Benzene, due to its structure, is very resistant to oxidation, it is not affected, for example, by a solution of potassium permanganate. However, oxidation to maleic anhydride can be carried out using a vanadium oxide catalyst:
- ozonolysis reaction. Also, benzene undergoes ozonolysis, but the process is slower than with unsaturated hydrocarbons:
The result of the reaction is the formation of dialdehyde - glyoxal (1,2-ethandial).
- combustion reaction. The combustion of benzene is the limiting case of oxidation. Benzene is highly flammable and burns in air with a very smoky flame:
Structure
By composition, benzene belongs to unsaturated hydrocarbons (homologous series n 2n−6), but unlike hydrocarbons of the ethylene series, 2 4 , it exhibits properties inherent in unsaturated hydrocarbons (they are characterized by addition reactions), only under harsh conditions, but benzene is more prone to substitution reactions. This "behavior" of benzene is explained by its special structure: the presence of atoms in the same plane and the presence of a conjugated 6π-electron cloud in the structure. The modern idea of the electronic nature of bonds in benzene is based on the hypothesis of Linus Pauling, who proposed to depict the benzene molecule as a hexagon with an inscribed circle, thereby emphasizing the absence of fixed double bonds and the presence of a single electron cloud covering all six carbon atoms of the cycle.
In specialized and popular literature, the term benzene ring, referring, as a rule, to the carbon structure of benzene without taking into account other atoms and groups associated with carbon atoms. The benzene ring is part of many different compounds.
Production
To date, there are several fundamentally different methods for the production of benzene.
Application
Transportation of benzene by rail is carried out in specialized tank cars
A significant part of the resulting benzene is used for the synthesis of other products:
- 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% of benzene is used for the synthesis of chlorobenzene.
In much smaller quantities, benzene is used for the synthesis of some 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. In the 1920s and 1930s, benzene was added ru de to straight-run gasoline to increase its octane rating, but by the 1940s such blends could not compete with high-octane gasolines. Due to the high toxicity, the content of benzene in fuel is limited by modern standards to the introduction of up to 1%.
Biological action and toxicology
Benzene is one of the most common anthropogenic xenobiotics.
Benzene is highly toxic. The minimum lethal dose for oral administration is 15 ml, the average is 50-70 ml. With a short inhalation of benzene vapor, no immediate poisoning occurs, therefore, until recently, the procedure for working with benzene was not particularly regulated. In large doses, benzene causes nausea and dizziness, and in some severe cases, poisoning can be fatal. The first sign of benzene poisoning is often euphoria. Benzene vapor can penetrate intact skin. Liquid benzene is quite irritating to the skin. If the human body is exposed to long-term exposure to benzene in small quantities, the consequences can also be very serious.
Benzene is a strong carcinogen. Studies show the association of benzene with diseases such as aplastic anemia, acute leukemia (myeloid, lymphoblastic), chronic myeloid leukemia, myelodysplastic syndrome and bone marrow diseases.
Mechanism of transformation and mutagenic effect of benzene
There are several variants of the mechanism of transformation of benzene in the human body. In the first variant, the benzene molecule is hydroxylated by the microsomal oxidation system with the participation of cytochrome P450. According to the mechanism, benzene is first oxidized to a highly reactive epoxide, which is further converted to phenol. In addition, free radicals (reactive oxygen species) are generated due to the high activation of P450 according to the reaction:
Molecular mechanism of benzene mutagenesis
Benzene is promutagen, it acquires mutagenic properties only after biotransformation, as a result of which highly reactive compounds are formed. One of these is benzene epoxide. Due to the high angular stress of the epoxy cycle, the -C-O-C- bonds break and the molecule becomes an electrophile, it easily reacts with the nucleophilic centers of the nitrogenous bases of nucleic acid molecules, especially DNA.
The mechanism of interaction of the epoxy cycle with nucleophilic centers - amino groups of nitrogenous bases (arylation reaction) proceeds as a nucleophilic substitution reaction 2 . As a result, fairly strong covalently bound DNA adducts are formed; such derivatives are most often observed in guanine (because the guanine molecule has maximum amount nucleophilic centers), for example, N7-phenylguanine. The resulting DNA adducts can lead to a change in the native structure of DNA, thereby disrupting the proper course of transcription and replication. What is the source of genetic mutations. The accumulation of epoxide in hepatocytes (liver cells) leads to irreversible consequences: an increase in DNA arylation, and at the same time an increase in the expression (overexpression) of mutant proteins that are products of a genetic mutation; inhibition of apoptosis; cell transformation and even death. In addition to pronounced genotoxicity and mutagenicity, benzene has strong myelotoxicity and carcinogenic activity, especially this effect is manifested in the cells of myeloid tissue (the cells of this tissue are very sensitive to such effects of xenobiotics).
Benzene and substance abuse
Benzene has a stupefying effect on a person and can lead to drug addiction.
Acute poisoning
At very high concentrations - almost instantaneous loss of consciousness and death within a few minutes. The color of the face is cyanotic, the mucous membranes are often cherry red. At lower concentrations - excitation, similar to alcohol, then drowsiness, general weakness, dizziness, nausea, vomiting, headache, loss of consciousness. Muscle twitches are also observed, which can turn into tonic convulsions. The pupils are often dilated and unresponsive to light. Breathing is first quickened, then slowed down. Body temperature drops sharply. Pulse quickened, small filling. The blood pressure is lowered. Cases of severe cardiac arrhythmias have been reported.
After severe poisoning that does not lead directly to death, long-term health disorders are sometimes observed: pleurisy, catarrhs of the upper respiratory tract, diseases of the cornea and retina, liver damage, heart disorders, etc. A case of vasomotor neurosis with swelling of the face and extremities, disorders sensitivity and convulsions shortly after acute benzene vapor poisoning. Sometimes death occurs some time after poisoning.
chronic poisoning
In severe cases, there are: headaches, extreme fatigue, shortness of breath, dizziness, weakness, nervousness, drowsiness or insomnia, indigestion, nausea, sometimes vomiting, lack of appetite, increased urination, menstruation, persistent bleeding from the oral mucosa, especially gums, often develops. , and nose, lasting for hours and even days. Sometimes persistent bleeding occurs after tooth extraction. Numerous small hemorrhages (hemorrhages) in the skin. Blood in stools, uterine bleeding, retinal hemorrhage. Usually, it is the bleeding, and often the accompanying fever (temperature up to 40 ° and above) that brings the poisoned to the hospital. In such cases, the prognosis is always serious. The cause of death is sometimes secondary infections: there are cases of gangrenous inflammation of the periosteum and necrosis of the jaw, severe ulcerative inflammation of the gums, general sepsis with septic endometritis.
Sometimes, with severe poisoning, symptoms of nervous diseases develop: an increase in tendon reflexes, bilateral clonus, a positive Babinsky symptom, a deep sensitivity disorder, pseudo-tabetic disorders with paresthesia, ataxia, paraplegia and motor disorders (signs of damage to the posterior columns of the spinal cord and pyramidal tract).
The most typical changes in the blood. The number of erythrocytes is usually sharply reduced, down to 1-2 million and below. The content of hemoglobin also falls sharply, sometimes up to 10%. The color index in some cases is low, sometimes close to normal, and sometimes high (especially with severe anemia). Anisocytosis and poikilocytosis, basophilic puncture and the appearance of nuclear erythrocytes, an increase in the number of reticulocytes and the volume of erythrocytes are noted. A sharp decrease in the number of leukocytes is more typical. Sometimes initially leukocytosis, quickly replaced by leukopenia, acceleration of ESR. Changes in the blood do not develop simultaneously. Most often, the leukopoietic system is affected earlier, later thrombocytopenia joins. The defeat of erythroblastic function often occurs even later. In the future, a characteristic picture of severe poisoning may develop - aplastic anemia.
The effects of poisoning may persist and even progress months and years after the cessation of work with benzene.
First aid for poisoning and treatment
In case of acute poisoning with benzene (benzene vapor), the victim must first be taken out to fresh air, in case of respiratory arrest, artificial respiration is carried out to normalized, oxygen and lobeline are used as respiratory stimulants. The use of adrenaline as an analeptic is strictly prohibited! If vomiting occurs, intravenously 40% glucose solution, in case of circulatory disorders - injection of caffeine solution. If poisoning occurred orally and benzene got into the stomach, it is necessary to rinse it with vegetable oil (benzene absorbs well), the procedure should be carried out with caution, since aspiration is possible. With mild poisoning, the patient is shown rest. In excited states, sedatives are needed. In the event of anemia, blood transfusions, vitamin B12, folic acid are carried out, in case of leukopenia - vitamin B6, pentoxyl. In case of a decrease in immunity (immunodeficiency state) - immunostimulants.
The action of benzene on biomembranes
Biological membranes are supramolecular structures - a double lipid layer, into which are integrated (embedded) or attached on the surface of the molecules of proteins, polysaccharides. The lipids that make up biomembranes are by their nature amphiphilic (amophilic) compounds, that is, capable of dissolving both in polar and non-polar substances, due to the presence of polar groups in them, the so-called. "head"(carboxylic -COOH, hydroxyl -OH, amino groups -NH 2 and others) and non-polar so-called. "tails"(hydrocarbon radicals - alkyls, aryls, polycyclic structures such as cholestan and others).
Benzene is an effective solubilizer of biological membranes, it quickly dissolves non-polar groups (the so-called hydrocarbon "tails") lipids, mainly cholesterol, which is part of the membranes. The solubilization process is limited by the concentration of benzene, the more it is, the faster this process proceeds. In the process of solubilization, energy is released, literally breaking the double lipid layer (lipid bilayer), which leads to complete destruction (structure destruction) of the membrane and subsequent cell apoptosis (during the destruction of biomembranes, membrane receptors are activated (such as: CD95, TNFR1, DR3, DR4, and others) that activate cell apoptosis).
Action on the skin
With frequent contact of hands with benzene, dry skin, cracks, itching, redness (usually between the fingers), swelling, and millet-like blisters are observed. Sometimes, due to skin lesions, workers are forced to quit their jobs.
The maximum allowable concentration is 5 mg/m 3 .
Safety
Working with benzene carries the risk of poisoning and serious health problems. Benzene is a highly volatile liquid (volatility 320 mg / l at 20 ° C) with a high degree of flammability, therefore, when working with it, it is necessary to observe the safety precautions for working with flammable liquids. Benzene vapors are of great danger, as they can form explosive mixtures with air. Currently, the use of benzene as an organic solvent is severely limited due to the toxicity and carcinogenic effects of its vapors and negative effects on the skin. Working with benzene in laboratories also provides for its limitation (strictly regulated). Benzene is recommended to be used in experiments only in small volumes (no more than 50 ml), work should be carried out exclusively with fluororubber gloves (latex dissolves and swells when exposed to benzene).
- store near sources of heat, open flames, strong oxidizers, food products, and so on,
- leave containers containing benzene open, smoke,
- use benzene containers for food use, washing hands, dishes,
- work in a closed, poorly ventilated room with an air temperature of more than 30 ° C,
- use a large volume of a substance as a solvent,
- work without protective equipment for the skin of hands, eyes and respiratory organs.
Ecology
Benzene is an environmentally unsafe substance, a toxicant of anthropogenic origin. The main sources of benzene entering the environment with wastewater or air emissions are petrochemical and coke industries, fuel production and transport. From reservoirs, benzene easily volatilizes, is capable of transformation from soils into plants, which poses a serious threat to ecosystems.
Benzene has the property of cumulation, due to its lipophilicity, it is able to be deposited in the cells of the adipose tissue of animals, thereby poisoning them.
The first group of reactions is substitution reactions. We said that arenes do not have multiple bonds in the molecular structure, but contain a conjugated system of six electrons, which is very stable and gives additional strength to the benzene ring. Therefore, in chemical reactions the replacement of hydrogen atoms occurs first, and not the destruction of the benzene ring.
We have already encountered substitution reactions when talking about alkanes, but for them these reactions proceeded according to a radical mechanism, while arenes are characterized by an ionic mechanism of substitution reactions.
First chemical property - halogenation. Substitution of a hydrogen atom for a halogen atom - chlorine or bromine.
The reaction proceeds when heated and always with the participation of a catalyst. In the case of chlorine, it can be aluminum chloride or iron chloride three. The catalyst polarizes the halogen molecule, as a result of which heterolytic bond breaking occurs and ions are obtained.
The positively charged chloride ion reacts with benzene.
If the reaction occurs with bromine, then iron tribromide or aluminum bromide acts as a catalyst.
It is important to note that the reaction occurs with molecular bromine and not with bromine water. Benzene does not react with bromine water.
The halogenation of benzene homologues has its own characteristics. In the toluene molecule, the methyl group facilitates substitution in the ring, the reactivity increases, and the reaction proceeds under milder conditions, that is, already at room temperature.
It is important to note that the substitution always occurs in the ortho and para positions, so a mixture of isomers is obtained.
Second property - nitration of benzene, the introduction of a nitro group into the benzene ring.
A heavy yellowish liquid with the smell of bitter almonds is formed - nitrobenzene, so the reaction can be qualitative for benzene. For nitration, a nitrating mixture of concentrated nitric and sulfuric acids is used. The reaction is carried out by heating.
Let me remind you that for the nitration of alkanes in the Konovalov reaction, dilute nitric acid was used without the addition of sulfuric acid.
In the nitration of toluene, as well as in the halogenation, a mixture of ortho- and para-isomers is formed.
Third property - alkylation of benzene with haloalkanes.
This reaction allows the introduction of a hydrocarbon radical into the benzene ring and can be considered a method for obtaining benzene homologues. Aluminum chloride is used as a catalyst, which promotes the decomposition of the haloalkane molecule into ions. It also needs heating.
Fourth property - alkylation of benzene with alkenes.
In this way, for example, cumene or ethylbenzene can be obtained. The catalyst is aluminum chloride.
2. Reactions of addition to benzene
The second group of reactions is addition reactions. We said that these reactions are not characteristic, but they are possible under rather harsh conditions with the destruction of the pi-electron cloud and the formation of six sigma bonds.
Fifth property in the general list - hydrogenation, addition of hydrogen.
Temperature, pressure, catalyst nickel or platinum. Toluene is able to react in the same way.
sixth property - chlorination. Please note that we are talking specifically about the interaction with chlorine, since bromine does not enter into this reaction.
The reaction proceeds under hard ultraviolet irradiation. Hexachlorocyclohexane, another name for hexachlorane, is formed, a solid.
It is important to remember that for benzene not possible addition reactions of hydrogen halides (hydrohalogenation) and addition of water (hydration).
3. Substitution in the side chain of benzene homologues
The third group of reactions concerns only benzene homologues - this is a substitution in the side chain.
seventh a property in the general list is halogenation at the alpha carbon atom in the side chain.
The reaction occurs when heated or irradiated, and always only at the alpha carbon. As the halogenation continues, the second halogen atom will return to the alpha position.
4. Oxidation of benzene homologues
The fourth group of reactions is oxidation.
The benzene ring is too strong, so benzene does not oxidize potassium permanganate - does not discolor its solution. This is very important to remember.
On the other hand, benzene homologues are oxidized with an acidified solution of potassium permanganate when heated. And this is the eighth chemical property.
It turns out benzoic acid. Discoloration of the solution is observed. In this case, no matter how long the carbon chain of the substituent is, it always breaks after the first carbon atom and the alpha atom is oxidized to a carboxyl group with the formation of benzoic acid. The rest of the molecule is oxidized to the corresponding acid or, if it is only one carbon atom, to carbon dioxide.
If the benzene homologue has more than one hydrocarbon substituent on the aromatic ring, then the oxidation occurs according to the same rules - the carbon in the alpha position is oxidized.
In this example, a dibasic aromatic acid is obtained, which is called phthalic acid.
In a special way, I note the oxidation of cumene, isopropylbenzene, with atmospheric oxygen in the presence of sulfuric acid.
This is the so-called cumene method for producing phenol. As a rule, one has to deal with this reaction in matters relating to the production of phenol. This is the industrial way.
ninth property - combustion, complete oxidation with oxygen. Benzene and its homologs burn to carbon dioxide and water.
Let us write the equation for the combustion of benzene in a general form.
According to the law of conservation of mass, there should be as many atoms on the left as there are atoms on the right. Because after all, in chemical reactions, atoms do not go anywhere, but the order of bonds between them simply changes. So there will be as many carbon dioxide molecules as there are carbon atoms in an arene molecule, since the molecule contains one carbon atom. That is n CO 2 molecules. There will be two times fewer water molecules than hydrogen atoms, that is, (2n-6) / 2, which means n-3.
There are the same number of oxygen atoms on the left and on the right. On the right, there are 2n from carbon dioxide, because there are two oxygen atoms in each molecule, plus n-3 from water, for a total of 3n-3. On the left, there are the same number of oxygen atoms - 3n-3, which means there are half as many molecules, because the molecule contains two atoms. That is (3n-3)/2 oxygen molecules.
Thus, we have compiled the equation for the combustion of benzene homologues in a general form.
Arenes are aromatic hydrocarbons containing one or more benzene rings. The benzene ring is made up of 6 carbon atoms, between which double and single bonds alternate.
It is important to note that the double bonds in the benzene molecule are not fixed, but constantly move in a circle.
Arenes are also called aromatic hydrocarbons. First Member homologous series- benzene - C 6 H 6 . The general formula for their homologous series is C n H 2n-6.
For a long time, the structural formula of benzene remained a mystery. The formula proposed by Kekule with two triple bonds could not explain the fact that benzene does not enter into addition reactions. As mentioned above, according to modern concepts, double bonds in a molecule are constantly moving, so it is more correct to draw them in the form of a ring.
Double bonds form a conjugation in the benzene molecule. All carbon atoms are in a state of sp 2 hybridization. Valence angle - 120°.
Nomenclature and isomerism of arenes
The names of arenes are formed by adding the names of substituents to the main chain - the benzene ring: benzene, methylbenzene (toluene), ethylbenzene, propylbenzene, etc. Substituents are, as usual, listed in alphabetical order. If there are several substituents on the benzene ring, then the shortest path between them is chosen.
Arenes are characterized by structural isomerism associated with the position of substituents. For example, two substituents on a benzene ring may be in different positions.
The name of the position of the substituents in the benzene ring is formed on the basis of their location relative to each other. It is denoted by the prefixes ortho-, meta- and para. Below you will find mnemonic clues for their successful memorization;)
Getting arenas
Arenas are obtained in several ways:
Chemical properties of arenes
Arenes are aromatic hydrocarbons that contain a benzene ring with conjugated double bonds. This feature makes addition reactions difficult (and yet possible!)
Remember that, unlike other unsaturated compounds, benzene and its homologues do not discolor bromine water and potassium permanganate solution.
© Bellevich Yuri Sergeevich 2018-2020
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