Benzene. Chemical properties
Among the various reactions into which aromatic compounds enter with the participation of the benzene ring, the substitution reactions discussed above primarily attract attention. This happens because they proceed contrary to expectations. With the degree of unsaturation that is inherent, for example, in benzene, addition reactions should have been more characteristic of this hydrocarbon. Under certain conditions, this happens; benzene and other arenes add hydrogen atoms, halogens, ozone and other reagents capable of addition.
11.5.5. Hydrogenation. In the presence of hydrogenation catalysts (platinum, palladium, nickel), benzene and its homologues add hydrogen and are converted into the corresponding cyclohexanes. Thus, benzene is hydrogenated over a nickel catalyst at 100-200 0 C and 105 atm:
The hydrogenation of arenes has two features compared to alkenes. Firstly, arenes are significantly inferior to alkenes in reactivity. For comparison with the conditions for the hydrogenation of benzene, we point out that cyclohexene is hydrogenated into cyclohexane already at 25 0 C and a pressure of 1.4 atm. Secondly, benzene either does not add, or attaches three hydrogen molecules at once. It is not possible to obtain partial hydrogenation products such as cyclohexene or cyclohexadiene by hydrogenating benzene.
These features during hydrogenation, a special case of addition reactions to the benzene ring, are due to the structure of benzene. When converted to cyclohexane, benzene ceases to be an aromatic system. Cyclohexane contains 150.73 kJ more energy (resonance energy) and is less stable than benzene. It is clear that benzene is not inclined to go into this thermodynamically less stable state. This explains the lower reactivity of benzene with respect to hydrogen compared to alkenes. Attachment to the aromatic system is possible only with the participation R-electrons of a single electron cloud of the benzene ring. Once the addition process begins, the system ceases to be aromatic and the result is an energy-rich and highly reactive particle that is much more willing to undergo addition than the parent arene.
11.5.6. Halogenation. The result of the interaction of a halogen with benzene depends on the experimental conditions. Catalytic halogenation leads to the formation of substitution products. It turned out that ultraviolet radiation initiates the addition of halogen atoms to the benzene ring of arenes. Benzene itself in the light adds 6 chlorine atoms and turns into hesachlorocyclohexane, which is a mixture of 9 spatial isomers
One of these isomers, in which 3 chlorines occupy axial bonds, and another 3 - equatorial bonds (γ-isomer, hexachlorane), turned out to be an effective insecticide, a means of combating harmful insects. Hexachlorane turned out to be too stable in the biosphere and capable of accumulating in the adipose tissue of warm-blooded animals and therefore is not currently used.
In terms of its reactivity towards halogens in addition reactions, benzene is significantly inferior to alkenes. For example, chlorine and bromine in carbon tetrachloride, even in the dark at room temperature, add to cyclohexene. Under these conditions, benzene does not react. This only happens under ultraviolet light.
11.5.7. Ozonation. Ozonation is another example showing that benzene, as an unsaturated compound, can undergo an addition reaction. The ozonation of benzene and the study of triozonide hydrolysis products were carried out back in 1904 ( Harries)
Interesting results were obtained with ozonation O-xylene (1941, Vibo). The fact is that the composition of ozonation products depends on the position of double bonds in the benzene ring. Structure 1 with double bonds between carbons of the benzene ring bearing methyl substituents, upon ozonation and hydrolysis of the ozonide, will give 2 molecules of methylglyoxal and a molecule of glyoxal
Alternative structure II For O-xylene would form 2 molecules of glyoxal and a molecule of diacetyl
The cyclic structure of benzene was first proposed by F.A. Kekule in 1865
Friedrich August Kekule von Stradonitz - an outstanding German chemist of the 19th century. In 1854, he discovered the first organic compound containing sulfur - thioacetic acid (thioethanoic acid). In addition, he established the structure of diazo compounds. However, his most famous contribution to the development of chemistry is the establishment of the structure of benzene (1866). Kekule showed that the double bonds of benzene alternated around the ring (this idea first occurred to him in a dream). He later showed that the two possible double bond arrangements are identical and that the benzene ring is a hybrid between these two structures. Thus, he anticipated the idea of resonance (mesomerism), which appeared in the theory of chemical bonding in the early 1930s.
If benzene really had such a structure, then its 1,2-disubstituted derivatives should have two isomers. For example,
However, none of the 1,2-disubstituted benzenes can be isolated into two isomers.
Therefore, Kekule subsequently suggested that the benzene molecule exists as two structures that quickly transform into each other:
Note that such schematic representations of benzene molecules and their derivatives usually do not indicate the hydrogen atoms attached to the carbon atoms of the benzene ring.
In modern chemistry, the benzene molecule is considered as a resonant hybrid of these two limiting resonant forms (see Section 2.1). Another description of the benzene molecule is based on a consideration of its molecular orbitals. In Sect. 3.1 it was indicated that -electrons located in -bonding orbitals are delocalized between all carbon atoms of the benzene ring and form an -electron cloud. In accordance with this representation, the benzene molecule can be conventionally depicted as follows:
Experimental data confirm the presence of just such a structure in benzene. If benzene had the structure that Kekulé originally proposed, with three conjugated double bonds, then benzene should undergo addition reactions like alkenes. However, as mentioned above, benzene does not undergo addition reactions. In addition, benzene is more stable than if it had three isolated double bonds. In Sect. 5.3 it was indicated that the enthalpy of benzene hydrogenation to form cyclohexane has a greater negative
Table 18.3. Length of various carbon-carbon bonds
Rice. 18.6. Geometric structure of the benzene molecule.
value than triple the enthalpy of hydrogenation of cyclohexene. The difference between these quantities is usually called the enthalpy of delocalization, resonance energy or stabilization energy of benzene.
All carbon-carbon bonds in the benzene ring have the same length, which is shorter than the length of the C-C bonds in alkanes, but longer than the length of the C=C bonds in alkenes (Table 18.3). This confirms that the carbon-carbon bonds in benzene are a hybrid between single and double bonds.
The benzene molecule has a flat structure, which is shown in Fig. 18.6.
Physical properties
Benzene under normal conditions is a colorless liquid that freezes at 5.5 °C and boils at 80 °C. It has a characteristic pleasant odor, but, as mentioned above, is highly toxic. Benzene does not mix with water and in a benzene system, water forms the upper of the two layers. However, it is soluble in non-polar organic solvents and is itself a good solvent for other organic compounds.
Chemical properties
Although benzene undergoes certain addition reactions (see below), it does not exhibit the reactivity typical of alkenes. For example, it does not discolor bromine water or -ion solution. Moreover, benzene is not
enters into addition reactions with strong acids, such as hydrochloric or sulfuric acid.
At the same time, benzene takes part in a number of electrophilic substitution reactions. The products of this type of reaction are aromatic compounds, since in these reactions the delocalized -electronic system of benzene is retained. The general mechanism for replacing a hydrogen atom on the benzene ring with an electrophile is described in Section. 17.3. Examples of electrophilic substitution of benzene are its nitration, halogenation, sulfonation and Friedel-Crafts reactions.
Nitration. Benzene can be nitrated (a group added to it) by treating it with a mixture of concentrated nitric and sulfuric acids:
Nitrobenzene
The conditions for this reaction and its mechanism are described in section. 17.3.
Nitrobenzene is a pale yellow liquid with a characteristic almond odor. When benzene is nitrated, in addition to nitrobenzene, crystals of 1,3-dinitrobenzene are also formed, which is the product of the following reaction:
Halogenation. If you mix benzene with chlorine or bromine in the dark, no reaction will occur. However, in the presence of catalysts possessing the properties of Lewis acids, electrophilic substitution reactions occur in such mixtures. Typical catalysts for these reactions are iron(III) bromide and aluminum chloride. The action of these catalysts is that they create polarization in the halogen molecules, which then form a complex with the catalyst:
although there is no direct evidence that free ions are formed in this case. The mechanism of benzene bromination using iron (III) bromide as an ion carrier can be represented as follows:
Sulfonation. Benzene can be sulfonated (replace a hydrogen atom with a sulfo group) by refluxing its mixture with concentrated sulfuric acid for several hours. Instead, benzene can be carefully heated in a mixture with fuming sulfuric acid. Fuming sulfuric acid contains sulfur trioxide. The mechanism of this reaction can be represented by the diagram
Friedel-Crafts reactions. Friedel-Crafts reactions were originally called condensation reactions between aromatic compounds and alkyl halides in the presence of an anhydrous aluminum chloride catalyst.
In condensation reactions, two molecules of reagents (or one reagent) combine with each other, forming a molecule of a new compound, while a molecule of some simple compound, such as water or hydrogen chloride, is split off (eliminates) from them.
Currently, the Friedel-Crafts reaction is called any electrophilic substitution of an aromatic compound in which the role of an electrophile is played by a carbocation or a highly polarized complex with a positively charged carbon atom. The electrophilic agent, as a rule, is an alkyl halide or chloride of some carboxylic acid, although it can also be, for example, an alkene or an alcohol. Anhydrous aluminum chloride is usually used as a catalyst for these reactions. Friedel-Crafts reactions are usually divided into two types: alkylation and acylation.
Alkylation. In this type of Friedel-Crafts reaction, one or more hydrogen atoms on the benzene ring are replaced by alkyl groups. For example, when a mixture of benzene and chloromethane is gently heated in the presence of anhydrous aluminum chloride, methylbenzene is formed. Chloromethane plays the role of an electrophilic agent in this reaction. It is polarized by aluminum chloride in the same way as halogen molecules:
The mechanism of the reaction under consideration can be presented as follows:
It should be noted that in this condensation reaction between benzene and chloromethane, a hydrogen chloride molecule is eliminated. Note also that the real existence of the metal carbocation in the form of a free ion is doubtful.
Alkylation of benzene with chloromethane in the presence of a catalyst - anhydrous aluminum chloride does not result in the formation of methylbenzene. In this reaction, further alkylation of the benzene ring occurs, leading to the formation of 1,2-dimethylbenzene:
Acylation. In this type of Friedel-Crafts reaction, a hydrogen atom on the benzene ring is replaced by an acyl group, resulting in the formation of an aromatic ketone.
The acyl group has the general formula
The systematic name of an acyl compound is formed by replacing the suffix and ending -ova in the name of the corresponding carboxylic acid, of which this acyl compound is a derivative, with the suffix -(o) yl. For example
The acylation of benzene is carried out using the chloride or anhydride of any carboxylic acid in the presence of a catalyst, anhydrous aluminum chloride. For example
This reaction is a condensation in which a hydrogen chloride molecule is eliminated. Note also that the name "phenyl" is often used to refer to the benzene ring in compounds where benzene is not the main group:
Addition reactions. Although benzene is most characterized by electrophilic substitution reactions, it also undergoes some addition reactions. We have already met one of them. We are talking about the hydrogenation of benzene (see section 5.3). When a mixture of benzene and hydrogen is passed over the surface of a finely ground nickel catalyst at a temperature of 150-160 °C, a whole sequence of reactions occurs, which ends with the formation of cyclohexane. The overall stoichiometric equation for this reaction can be represented as follows:
When exposed to ultraviolet radiation or direct sunlight, benzene also reacts with chlorine. This reaction occurs via a complex radical mechanism. Its final product is 1,2,3,4,5,6-hexachlorocyclohexane:
A similar reaction occurs between benzene and bromine under the influence of ultraviolet radiation or sunlight.
Oxidation. Benzene and the benzene ring in other aromatic compounds are, generally speaking, resistant to oxidation even by such strong oxidizing agents as an acidic or alkaline solution of potassium permanganate. However, benzene and other aromatic compounds burn in air or oxygen to produce a very smoky flame, which is typical of hydrocarbons with a high relative carbon content.
The benzene ring is quite stable. It is more prone to substitution reactions for hydrogen atoms of the benzene ring than to addition reactions at the site of double bond cleavage. In this expression its "aromatic character".
Substitution reactions
The most typical reactions are electrophilic substitution: nitration, sulfonation, alkylation (acylation), halogenation (halogenation of benzene homologues can also occur by a radical mechanism.
1. Nitration- replacement of the hydrogen of the benzene ring with a nitro group is carried out with a so-called nitrating mixture - a mixture of concentrated nitric and sulfuric acids. The active agent is the nitronium cation N0 2 +:
HO - N0 2 + 2H 2 S0 4 →N0 2 + + 2HS0 4 - + H 3 0
Nitronium cation hydronium cation
The mechanism of nitration (as well as all substitution reactions) is as follows:
The presence of water in the reaction mixture interferes with the progress of the reaction, because water participates in the process reverse to the formation of the nitronium cation. Therefore, to bind the water released in the reaction, an excess of concentrated sulfuric acid is taken.
Rules for replacing hydrogens in the benzene ring. If there is any hydrogen substituent in the benzene ring, then in electrophilic substitution reactions it plays the role of an orientator - the reaction occurs predominantly in the ortho- and para-positions relative to the substituent (first-order orientant) or meta-positions (second-kind orientant) .
Substitutes of the first kind direct the attacking electrophile to ortho- and para-positions relative to themselves. We present them in descending order of the orienting force (electrodonor effect):
Type II substituents direct the attacking electrophile to meta positions relative to themselves. We also present them in descending order of orienting force:
For example, -OH - group - orientant of the first kind:
59. Write the equation and mechanism of the nitration reactions of the following compounds: a) benzene; b) toluene; c) chlorobenzene; d) nitrobenzene; e) sulfobenzene; f) phenyl cyanide; g) methoxybenzene; h) aminobenzene.
Substituents of the first kind are electron-donating, they increase the density of the electron cloud of the benzene ring, especially in the ortho- and para-positions and thereby (facilitate) activate the benzene ring to attack the electrophile. However, the σ-complex (III) is stabilized not by the addition of an anion, but by the elimination of a hydrogen cation (the energy released during the formation of a single π-electron cloud of the benzene ring is 36.6 kcal/mol):
Substituents of the second kind are electron-withdrawing; they seem to draw part of the electron cloud towards themselves, thereby reducing the density of the electron cloud of the benzene ring, especially in the ortho- and para-positions relative to themselves. Substituents of the second type generally hinder electrophilic substitution reactions. But in meta positions relative to the second type substituent, the density of the cloud is slightly higher than in others. Therefore, electrophilic substitution reactions in the case of substituents of the second kind go to meta positions:
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The rules described above are not laws. We are almost always talking only about the main direction of the reaction. For example, the nitration of toluene produces 62% ortho-, 33.5% para- and 4.5% meta-nitrotoluenes.
The reaction conditions (temperature, presence of catalysts, etc.) have a fairly strong influence on the direction of reactions.
In the presence of two orientants in the benzene ring, coordinated and inconsistent orientation of these two substituents is possible. In the case of inconsistent orientation of substituents of the same kind, the direction of the reaction is determined by the stronger one (see rows of substituents of the first and second kind):
In the case of inconsistent orientation of substituents of different types, the direction of the reaction is determined by the substituent of the first kind, since it activates the benzene ring to an electrophilic attack, for example,
60. According to the rules of substitution, write the nitration of the following disubstituted benzenes: a) m-nitrotoluene; b) p-nitrotoluene; c) o-hydroxytoluene; d) p-chlorotoluene; e) m-nitrobenzoic acid; f) p-oxychlorobenzene; g) m-chlorotoluene; h) p-methoxytoluene.
2. Sulfonation reaction occurs when arenes are heated with concentrated sulfuric acid or oleum. The attacking agent is the SO 3 molecule, which plays the role of an electrophile:
The first stage of sulfonation is slow, the reaction is generally reversible:
Sulfonic acids are comparable in strength to mineral acids, therefore in aqueous solutions they are in an ionized state (III).
61. Give equations and mechanisms for sulfonation reactions of the following substances, following the rules of substitution:
a) toluene; b) o-xylene; c) nitrobenzene; d) o-nitrotoluene; e) p-chloronitrobenzene; f) m-nitrotoluene; g) p-aminotoluene; h) o methoxytoluene.
3. Halogenation reaction arenes in the cold in the presence of catalysts such as AlCl 3, AlBr 3, FeCl 3 - a typical electrophilic reaction, because catalysts contribute to the polarization of the bond in the halogen molecule (up to its rupture): 
Anhydrous ferric chloride works in the same way:
Under radical reaction conditions (light, heat), halogens (chlorine, bromine) replace the hydrogens of the side chains (similar to the halogenation of alkanes):
Under more severe conditions, radical addition of halogens to the aromatic ring occurs.
62 . Write the equations and reaction mechanisms and name the products:
a) toluene + chlorine (in bright light and heating);
b) toluene + chlorine (in the cold in the presence of a catalyst);
c) nitrobenzene + chlorine (in the cold in the presence of a catalyst);
d) p-nitrotoluene + chlorine (in bright light and heating);
e) p-nitrotoluene + chlorine (in the cold in the presence of a catalyst):
e) ethylbenzene + chlorine (in bright light and heating);
g) ethylbenzene + chlorine (in the cold in the presence of a catalyst);
h) p-hydroxytoluene + chlorine (in the cold in the presence of a catalyst);
i) m-nitrotoluene + chlorine (in the cold in the presence of a catalyst);
j) m-xylene + chlorine (in the cold in the presence of a catalyst).
4. Alkylation of arenes. In the presence of anhydrous AlCl 3 (AlBr3), haloalkanes alkylate benzene, even more easily than its homologues, as well as their halogen derivatives (Gustavson-Friedel-Crafts reactions). The catalyst, forming the A1Cl 3 complex, polarizes the C-Gal bond until it breaks, and therefore the attacking electrophile agent:
Alkylation with alkenes in the presence of A1Cl 3, BF 3 or H 3 PO 4 leads to similar results (the mechanism is also electrophilic):
Alkylation with haloalkanes and alkenes as electrophilic reactions proceeds in accordance with the rules for the substitution of hydrogens on the benzene ring. However, the process is complicated by further alkylation of the reaction products and other undesirable phenomena. To minimize the latter, the reaction is carried out at the lowest possible temperature, the optimal amount of catalyst and a large excess of arene.
63. Give the equations and reaction mechanism under Gustavson-Friedel-Crafts conditions between the following substances:
a) benzene + 2-chloropropane; b) benzene + 2-chloro-2-megylpropane; c) benzene + benzyl chloride; d) bromobenzene + bromoethane; e) toluene + butyl chloride; f) toluene + bromoethane; i) p-bromotoluene + isopropyl bromide; h) m-bromotoluene + bromoethane; i) p-bromotoluene + isopropyl bromide; j) chlorobenzene + benzyl chloride.
64. Write the reaction equations for the alkylation of arenes with alkenes in the presence of phosphoric acid, give the mechanism:
a) benzene + ethylene; b) benzene + propylene; c) toluene + ethylene; d) toluene + propylene; e) benzene + isobutylene; f) toluene + isobutylene; g) m-xylene + ethylene; h) p-xylene + ethylene.
5. Oxidation reaction (determining the number of side chains). The aromatic core is very resistant to oxidizing agents. Thus, benzene and its homologues do not react with potassium permanganate like alkanes. This also expresses their “aromatic character”. But when benzene homologues are heated with oxidizing agents under harsh conditions, the benzene ring is not oxidized, and all side hydrocarbon chains, regardless of their length, are oxidized to carboxyl groups; the oxidation products are aromatic acids. The number of side chains in the original benzene homologue 1 is determined by the number of carboxyl groups in the latter.
65 . Write the equations for the oxidation reactions of the following substances: a) ethylbenzene; b) o-dimethylbenzene; c) propylbenzene; d) ordinary trimethylbenzene; e) p-methylisopropylbenzene; f) o-nitrotoluene; g) 3-nitro-1-methyl-4-ethylbenzene; h) symmetrical trimethylbenzene.
6. Addition reactions. Although the aromatic ring is less prone to addition reactions than substitution reactions, they do occur under certain conditions. A feature of addition reactions is that three moles of halogen, hydrogen, ozone are always added to one mole of benzene (or its homologue), which is explained by the presence of a single π-electron cloud in the aromatic nucleus with a certain single, total energy of three double bonds (or rather, six π electrons).
a) Hydrogenation occurs in the presence of catalysts (Pt, Pd, etc.) at 110°C (N.D. Zelinsky and others).
b) Halogenation occurs when bromine or chlorine vapor is passed through boiling benzene under the influence of direct sunlight or when illuminated with UV rays (quartz lamp):
V) Ozonation. Like alkenes, aromatic hydrocarbons are easily subject to ozonolysis.
66. Write equations for addition reactions (hydrogenation, halogenation under UV irradiation, ozonation) with the following arenes: a) toluene; b) o-xylene; c) m-xylene; d) p-xylene; e) ethylbenzene; f) o-ethyltoluene; g) m-ethyltoluene; h) p-isopropyltoluene. Name the products obtained.
what benzene reacts with and their reaction equations
- The most characteristic reactions for them are the substitution of hydrogen atoms of the benzene ring. They proceed more easily than with saturated hydrocarbons. Many organic compounds are obtained in this way. Thus, when benzene reacts with bromine (in the presence of the FeBr2 catalyst), the hydrogen atom is replaced by a bromine atom:
With another catalyst, all the hydrogen atoms in benzene can be replaced with halogen. This happens, for example, when chlorine is passed into benzene in the presence of aluminum chloride:
Hexachlorobenzene is a colorless crystalline substance used for treating seeds and preserving wood.
If benzene is treated with a mixture of concentrated nitric and sulfuric acids (nitrating mixture), then the hydrogen atom is replaced by the nitro group NO2:
In a benzene molecule, the hydrogen atom can be replaced by an alkyl radical by the action of halogenated hydrocarbons in the presence of aluminum chloride:
Addition reactions to benzene occur with great difficulty. For their occurrence, special conditions are required: increased temperature and pressure, selection of a catalyst, light irradiation, etc. Thus, in the presence of a catalyst - nickel or platinum - benzene is hydrogenated, i.e., it adds hydrogen, forming cyclohexane:
Under ultraviolet irradiation, benzene adds chlorine:
Hexachlorocyclohexane, or hexachlorane, is a crystalline substance used as a powerful insect killer.
Benzene does not add hydrogen halides and water. It is very resistant to oxidizing agents. Unlike unsaturated hydrocarbons, it does not discolor bromine water and KMnO4 solution. Under normal conditions, the benzene ring is not destroyed by the action of many other oxidizing agents. However, benzene homologues undergo oxidation more easily than saturated hydrocarbons. In this case, only radicals associated with the benzene ring undergo oxidation:
Thus, aromatic hydrocarbons can enter into both substitution and addition reactions, but the conditions for these transformations differ significantly from similar transformations of saturated and unsaturated hydrocarbons.
Receipt. Benzene and its homologues are obtained in large quantities from petroleum and coal tar formed during the dry distillation of coal (coking). Dry distillation is carried out at coke and gas plants.
The reaction of converting cyclohexane into benzene (dehydrogenation or dehydrogenation) occurs when it is passed over a catalyst (platinum black) at 300C. Saturated hydrocarbons can also be converted into aromatic hydrocarbons by dehydrogenation reactions. For example:
Dehydrogenation reactions make it possible to use petroleum hydrocarbons to produce hydrocarbons of the benzene series. They indicate the connection between different groups of hydrocarbons and their mutual transformation into each other.
According to the method of N.D. Zelinsky and B.A. Kazansky, benzene can be obtained by passing acetylene through a tube with activated carbon heated to 600 C. The entire process of polymerization of three acetylene molecules can be represented by a diagram
- 1) substitution reaction
a) in the presence of a catalyst—iron (III) salts—benzene undergoes a substitution reaction:
C6H6+Br2=C6H5Br+Rick
benzene reacts similarly with chlorine
b) substitution reactions also include the interaction of benzene with nitric acid:
C6H6+HONO2=C6H5NO2+H2O
2)ADDITION REACTION
A) when exposed to sunlight or ultraviolet rays, benzene undergoes an addition reaction. For example, benzene adds chromium in the light and forms hexachlorocyclohexane:
C6H6+3Cl2=C6H6Cl6
b) benzene can also be hydrogenated:
C6HC+3H2=C6H12
3) OXIDATION REACTIONS
a) under the action of energetic oxidizing agents (KMnO4) on benzene homologues, only the side chains undergo oxidation.
C6H5-CH3+3O=C7H6O2+H2O
b) benzene and its homologues burn with flame in air:
2C6H6+15O2=12CO2+6H2O
The chemical properties of benzene and other aromatic hydrocarbons differ from saturated and unsaturated hydrocarbons. The most characteristic reactions for them are the substitution of hydrogen atoms of the benzene ring. They proceed more easily than with saturated hydrocarbons. Many organic compounds are obtained in this way. Thus, when benzene reacts with bromine (in the presence of the FeBr 2 catalyst), the hydrogen atom is replaced by a bromine atom:
With another catalyst, all the hydrogen atoms in benzene can be replaced with halogen. This happens, for example, when chlorine is passed into benzene in the presence of aluminum chloride:
If benzene is treated with a mixture of concentrated nitric and sulfuric acids (nitrating mixture), then the hydrogen atom is replaced by a nitro group - NO 2:
This is the nitration reaction of benzene. Nitrobenzene is a pale yellow oily liquid with the smell of bitter almonds, insoluble in water, used as a solvent and also for the production of aniline.
In a benzene molecule, the hydrogen atom can be replaced by an alkyl radical by the action of halogenated hydrocarbons in the presence of aluminum chloride:
Addition reactions to benzene occur with great difficulty. For their occurrence, special conditions are required: increased temperature and pressure, selection of a catalyst, light irradiation, etc. Thus, in the presence of a catalyst - nickel or platinum - benzene is hydrogenated, i.e. adds hydrogen to form cyclohexane:
Cyclohexane is a colorless, volatile liquid with a gasoline odor and is insoluble in water.
Under ultraviolet irradiation, benzene adds chlorine:
Hexachlorocyclohexane, or hexachlorane, is a crystalline substance used as a powerful insect killer.
Benzene does not add hydrogen halides and water. It is very resistant to oxidizing agents. Unlike unsaturated hydrocarbons, it does not discolor bromine water and KMnO 4 solution. Under normal conditions, the benzene ring is not destroyed by the action of many other oxidizing agents. However, benzene homologues undergo oxidation more easily than saturated hydrocarbons. In this case, only radicals associated with the benzene ring undergo oxidation:
Thus, aromatic hydrocarbons can enter into both substitution and addition reactions, but the conditions for these transformations differ significantly from similar transformations of saturated and unsaturated hydrocarbons.
Receipt. Benzene and its homologues are obtained in large quantities from petroleum and coal tar formed during the dry distillation of coal (coking). Dry distillation is carried out at coke and gas plants.
The reaction of converting cyclohexane into benzene (dehydrogenation or dehydrogenation) occurs when it is passed over a catalyst (platinum black) at 300°C. Saturated hydrocarbons can also be converted into aromatic hydrocarbons by dehydrogenation reactions. For example:
Dehydrogenation reactions make it possible to use petroleum hydrocarbons to produce hydrocarbons of the benzene series. They indicate the connection between different groups of hydrocarbons and their mutual transformation into each other.
According to the method of N.D. Zelinsky and B.A. Kazan benzene can be obtained by passing acetylene through a tube with activated carbon heated to 600 ° C. The entire process of polymerization of three acetylene molecules can be represented by a diagram
Electrophilic substitution reactions- replacement reactions in which the attack is carried out electrophile- a particle that is positively charged or has a deficiency of electrons. When a new bond is formed, the outgoing particle is electrofuge splits off without its electron pair. The most popular leaving group is the proton H+.
All electrophiles are Lewis acids.
General view of electrophilic substitution reactions:
(cationic electrophile)
(neutral electrophile)
Reaction mechanism S E Ar or aromatic electrophilic substitution reactions is the most common and most important among the substitution reactions of aromatic compounds and consists of two stages. At the first stage, the electrophile is added, and at the second stage, the electrofuge is separated:
During the reaction, a positively charged intermediate is formed (in Figure 2b). It's called Ueland intermediate, aronium ion or σ-complex. This complex is generally very reactive and is easily stabilized, quickly eliminating the cation.
