Arena examples. Organic Chemistry: Arenas. Chemical properties of benzene

General review.

Aromatic hydrocarbons (arenes) are substances whose molecules contain one or more benzene rings - cyclic groups of carbon atoms with a special nature of bonds.

The concept of "benzene ring" immediately requires deciphering. To do this, it is necessary to at least briefly consider the structure of the benzene molecule. The first structure of benzene was proposed in 1865 by the German scientist A. Kekule:

This formula correctly reflects the equivalence of six carbon atoms, but does not explain a number of special properties of benzene. For example, despite the unsaturation, benzene does not show a tendency to addition reactions: it does not decolorize bromine water and potassium permanganate solution, i.e. does not give qualitative reactions typical for unsaturated compounds.

The features of the structure and properties of benzene were fully explained only after the development of the modern quantum mechanical theory of chemical bonds. According to modern concepts, all six carbon atoms in the benzene molecule are in the -hybrid state. Each carbon atom forms -bonds with two other carbon atoms and one hydrogen atom lying in the same plane. The bond angles between the three -bonds are 120°. Thus, all six carbon atoms lie in the same plane, forming a regular hexagon (-skeleton of the benzene molecule).

Each carbon atom has one unhybridized p orbital.

Six such orbitals are located perpendicular to the flat -skeleton and parallel to each other (Fig. 21.1, a). All six p-electrons interact with each other, forming -bonds, not localized in pairs, as in the formation of ordinary double bonds, but combined into a single -electron cloud. Thus, circular conjugation occurs in the benzene molecule (see § 19). The highest -electron density in this conjugated system is located above and below the -skeleton plane (Fig. 21.1, b).

Rice. 21.1. The structure of the benzene molecule

As a result, all bonds between carbon atoms in benzene are aligned and have a length of 0.139 nm. This value is intermediate between the single bond length in alkanes (0.154 nm) and the double bond length in alkenes (0.133 nm). The equivalence of connections is usually depicted as a circle inside the cycle (Fig. 21.1, c). Circular conjugation gives an energy gain of 150 kJ/mol. This value is the conjugation energy - the amount of energy that needs to be expended to break the aromatic system of benzene (compare - the conjugation energy in butadiene is only 12 kJ / mol).

This electronic structure explains all the features of benzene. In particular, it is clear why benzene is difficult to enter into addition reactions - this would lead to a violation of conjugation. Such reactions are possible only under very harsh conditions.

Nomenclature and isomerism.

Conventionally, the arenas can be divided into two rows. The first includes benzene derivatives (for example, toluene or diphenyl), the second - condensed (polynuclear) arenes (the simplest of them is naphthalene):

We will consider only the homologous series of benzene with the general formula .

Structural isomerism in the homologous series of benzene is due to the mutual arrangement of substituents in the nucleus. Monosubstituted benzene derivatives do not have position isomers, since all atoms in the benzene nucleus are equivalent. Disubstituted derivatives exist in the form of three isomers that differ in the mutual arrangement of substituents. The position of the substituents is indicated by numbers or prefixes:

Aromatic hydrocarbon radicals are called aryl radicals. The radical is called phenyl.

physical properties.

The first members of the homologous series of benzene (for example, toluene, ethylbenzene, etc.) are colorless liquids with a specific odor. They are lighter than water and insoluble in water. They dissolve well in organic solvents. Benzene and its homologues are themselves good solvents for many organic substances. All arenas burn with a smoky flame due to the high content of carbon in their molecules.

Ways to get.

1. Obtaining from aliphatic hydrocarbons. When straight-chain alkanes having at least 6 carbon atoms in a molecule are passed over heated platinum or chromium oxide, dehydrocyclization occurs - the formation of an arene with the release of hydrogen:

2. Dehydrogenation of cycloalkanes. The reaction occurs when passing vapors of cyclohexane and its homologues over heated platinum:

3. Preparation of benzene by trimerization of acetylene - see § 20.

4. Obtaining benzene homologues by the Friedel-Crafts reaction - see below.

5. Fusion of salts of aromatic acids with alkali:

Chemical properties.

General review. Possessing a mobile six -electrons, the aromatic nucleus is a convenient object for attack by electrophilic reagents. This is also facilitated by the spatial arrangement of the -electron cloud on both sides of the flat -skeleton of the molecule (Fig. 21.1, b)

For arenes, reactions proceeding according to the mechanism of electrophilic substitution, denoted by the symbol (from the English substitution electrophilic), are most characteristic.

The mechanism of electrophilic substitution can be represented as follows. The electrophilic reagent XY (X is an electrophile) attacks the electron cloud, and an unstable -complex is formed due to the weak electrostatic interaction. The aromatic system is not yet disturbed. This stage is fast. At the second, slower stage, a covalent bond is formed between the electrophile X and one of the carbon atoms of the ring due to two α-electrons of the ring. This carbon atom changes from to the -hybrid state. The aromaticity of the system is thus disturbed. The four remaining -electrons are distributed among five other carbon atoms, and the benzene molecule forms a carbocation, or -complex.

Violation of aromaticity is energetically unfavorable, therefore the structure of the -complex is less stable than the aromatic structure. To restore aromaticity, a proton is split off from the carbon atom associated with the electrophile (third stage). In this case, two electrons return to the -system, and thereby aromaticity is restored:

Electrophilic substitution reactions are widely used for the synthesis of many benzene derivatives.

Chemical properties of benzene.

1. Halogenation. Benzene does not interact with chlorine or bromine under normal conditions. The reaction can proceed only in the presence of anhydrous catalysts. As a result of the reaction, halogen-substituted arenes are formed:

The role of the catalyst is to polarize the neutral halogen molecule with the formation of an electrophilic particle from it:

2. Nitration. Benzene reacts very slowly with concentrated nitric acid, even when heated strongly. However, under the action of the so-called nitrating mixture (a mixture of concentrated nitric and sulfuric acids), the nitration reaction proceeds quite easily:

3. Sulfonation. The reaction easily takes place under the action of "fuming" sulfuric acid (oleum):

4. Alkylation according to Friedel-Crafts. As a result of the reaction, an alkyl group is introduced into the benzene core to obtain benzene homologues. The reaction proceeds under the action of haloalkanes on benzene in the presence of catalysts - aluminum halides. The role of the catalyst is reduced to the polarization of the molecule with the formation of an electrophilic particle:

Depending on the structure of the radical in the haloalkane, different homologues of benzene can be obtained:

5. Alkylation with alkenes. These reactions are widely used in industry to produce ethylbenzene and isopropylbenzene (cumene). Alkylation is carried out in the presence of a catalyst. The reaction mechanism is similar to that of the previous reaction:

All the reactions discussed above proceed by the mechanism of electrophilic substitution.

Addition reactions to arenes lead to the destruction of the aromatic system and require large amounts of energy, so they proceed only under harsh conditions.

6. Hydrogenation. The reaction of hydrogen addition to arenes proceeds under heating and high pressure in the presence of metal catalysts (Ni, Pt, Pd). Benzene is converted to cyclohexane, and benzene homologues are converted to cyclohexane derivatives:

7. Radical halogenation. The interaction of benzene vapor with chlorine proceeds by a radical mechanism only under the influence of hard ultraviolet radiation. In this case, benzene adds three molecules of chlorine and forms a solid product - hexachlorocyclohexane:

8. Oxidation by atmospheric oxygen. In terms of resistance to oxidizing agents, benzene resembles alkanes. Only with strong heating (400 ° C) of benzene vapor with atmospheric oxygen in the presence of a catalyst, a mixture of maleic acid and its anhydride is obtained:

Chemical properties of benzene homologues.

Benzene homologues have a number of special chemical properties associated with the mutual influence of the alkyl radical on the benzene ring, and vice versa.

Reactions in the side chain. In terms of chemical properties, alkyl radicals are similar to alkanes. Hydrogen atoms in them are replaced by halogens by a free radical mechanism. Therefore, in the absence of a catalyst during heating or UV irradiation, a radical substitution reaction occurs in the side chain. The effect of the benzene ring on alkyl substituents always results in the replacement of the hydrogen atom at the carbon atom directly bonded to the benzene ring (a-carbon atom).

Substitution in the benzene ring is possible only by the mechanism in the presence of a catalyst:

Below you will find out which of the three isomers of chlorotoluene are formed in this reaction.

Under the action of potassium permanganate and other strong oxidants on the homologues of benzene, the side chains are oxidized. No matter how complex the substituent chain is, it is destroyed, with the exception of the -carbon atom, which is oxidized into a carboxyl group.

Homologues of benzene with one side chain give benzoic acid:

Orientation (substitution) rules in the benzene ring.

The most important factor determining the chemical properties of a molecule is the distribution of electron density in it. The nature of the distribution depends on the mutual influence of the atoms.

In molecules that have only -bonds, the mutual influence of atoms is carried out through the inductive effect (see § 17). In molecules that are conjugated systems, the action of the mesomeric effect is manifested.

The influence of substituents, transmitted through a conjugated system of -bonds, is called the mesomeric (M) effect.

In a benzene molecule, the -electron cloud is distributed evenly over all carbon atoms due to conjugation.

If, however, some substituent is introduced into the benzene ring, this uniform distribution is disturbed, and the electron density in the ring is redistributed. The place of entry of the second substituent into the benzene ring is determined by the nature of the already existing substituent.

Substituents are divided into two groups depending on the effect they exhibit (mesomeric or inductive): electron support and electron acceptor.

Electron-donor substituents exhibit an effect and increase the electron density in the conjugated system. These include the hydroxyl group -OH and the amino group. The lone pair of electrons in these groups enters into general conjugation with the -electronic system of the benzene ring and increases the length of the conjugated system. As a result, the electron density is concentrated in the ortho and para positions:

Alkyl groups cannot participate in general conjugation, but they exhibit an effect under which a similar redistribution of -electron density occurs.

Electron-withdrawing substituents exhibit the -M effect and reduce the electron density in the conjugated system. These include the nitro group, the sulfo group, the aldehyde group -CHO and the carboxyl group -COOH groups. These substituents form a common conjugated system with the benzene ring, but the overall electron cloud shifts towards these groups. Thus, the total electron density in the ring decreases, and it decreases least of all in the meta positions:

For example, toluene containing a substituent of the first kind is nitrated and brominated in the para and ortho positions:

Nitrobenzene containing a substituent of the second kind is nitrated and brominated in the meta position:

In addition to the orienting effect, substituents also affect the reactivity of the benzene ring: orientants of the 1st kind (except for halogens) facilitate the introduction of the second substituent; orientants of the second kind (and halogens) make it difficult.

BENEFITS-TUTOR IN CHEMISTRY.

Arenas. Benzene .

The article is devoted to aromatic hydrocarbons (arenes) and their simplest representative, benzene. Material contains
the theoretical part in the amount necessary to prepare for the exam, test and tasks. There are also answers and

to some problems, solutions.

I.V.TRIGUBCHAK

aromatichydrocarbons(arena).Benzene

Plan 1. Definition, general formla homologous series, structuremolecules (for example, benzene).2. Physical properties of benzene.3. Chemical properties of benzene:a) substitution reactions (halogennating, nitrating, sulfiation, alkylation);b) addition reactions (gidraining, chlorination);c) oxidation reactions (wonie).4. Obtaining benzene (in prothinking - processingoil and coal, dehydrogenationcyclohexane, aromatizationhexane, acetyl trimerizationon the; in the laboratory - by fusionsalts of benzoic acid withlochs).

Arenas are hydrocarbons,whose molecules contain oneor more benzene rings.Under the benzene ringof course the ring systemcarbon atoms with delocalizedπ-electrons. In 1931E. Hückel formulated the rightfork stating that the connectionshould show aromaticproperties, if in its molecule withheld flat ring with (4n + 2)generalized electrons, where ncan display integer valuesnumbers from 1 onwards (Hyuk's rulecell). According to this rulesystems containing 6, 10, 14 andetc. generalized electrons, isare aromatic. Distinguishthree groups of arenas by numberand relative position of the binash rings.

Monocyclic arenas.

Picture Structural Shapesmules of benzene, toluene, o-xylene,cumene. Name these substancessystematic nomenclature.

Polycyclic arenas withisolated cores.

Picture Structural Shapesmules of diphenyl, diphenylmethane,stilbene.

Polycyclic arenas withcondensed nuclei.

Picture Structural Shapesmules of naphthalene, anthracene.

The general formula of monocyclic arenes of the benzene series is С6Н2n–6, where n ≥ 6. The simplest representative is benzene (С6Н6). Proposed in 1865 by a German chemist
F.A. Kekule the cyclic formula of benzene with conjugated bonds (cyclohexatriene-1,3,5) did not explain many properties of benzene.
Benzene is characterized by substitution reactions, and not addition reactions, as for unsaturated hydrocarbons. Addition reactions are possible, but proceed
they are harder than those of alkenes.
Benzene does not enter into reactions that are qualitative for unsaturated hydrocarbons (with bromine water and potassium permanganate solution).
Later studies showed that all bonds between carbon atoms in a benzene molecule have the same length - 0.140 nm (the average value between the length of a single C–C bond of 0.154 nm and a double C=C bond of 0.134 nm). The angle between bonds at each carbon atom is 120°. The benzene molecule is a regular flat hexagon.
The modern theory of the structure of the benzene molecule is based on the concept of hybridization of the orbitals of the carbon atom. According to this theory, the carbon atoms in benzene are in a state of sp2 hybridization. Each carbon 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 more p-electron that does not participate in hybridization. The unhybridized p-orbitals of carbon atoms are in a plane perpendicular to the plane of σ-bonds. Each p-cloud overlaps with two neighboring p-clouds, resulting in the formation unified conjugate π-system. A single π-electron cloud is located above and below the benzene ring, and the p-electrons are not associated with any carbon atom and can move relative to them in one direction or another. The complete symmetry of the benzene nucleus, due to conjugation, gives it special stability.
Thus, along with the Kekule formula, the benzene formula is used, where the generalized electron cloud is depicted as a closed line inside the ring.
Draw Kekule's formula and the formula showing the conjugate π-system.

The radical formed from benzene has a trivial name phenyl.
Draw its structural formula.

Physical properties

Under normal conditions, benzene is a colorless liquid with a melting point of 5.5 °C and a boiling point of 80 °C; has a characteristic smell; lighter than water and does not mix with it; good organic solvent; toxic.

Chemical properties

The chemical properties of benzene and its homologues are determined by the specifics of the aromatic bond. The most typical arenas are substitution reactions(for benzene they proceed harder than for its homologues).

Halogenation.
Write the reaction for the chlorination of benzene.

Nitration.
Write the reaction of interaction of benzene with nitric acid.

Sulfonation.
Write the reaction between benzene and sulfuric acid.

Alkylation (Free reactiondel-Crafts).

Write reacttions for obtaining ethylbenzene atinteraction of benzene with chlorineethane and ethylene.

A system of 6 π-electrons is more stable than a 2π-electron system, therefore addition reactions for arenes are less typical than for alkenes; they are possible, but under more stringent conditions.

Hydrogenation.

Write the hydrogenation reaction of benzene to cyclohexane.

addition of chlorine.

Write the reaction for the chlorination of benzene to hexachlorane.

Oxidation reactions for benzene, it is possible only in the form of combustion, because the benzene ring is resistant to the action of oxidizing agents.
Write the combustion reaction of benzene. Explain why aromatic hydrocarbons burn with a smoky flame.

Getting arenas

Ways to get. one. Obtaining from aliphatic hydrocarbons. To obtain benzene and its homologues in industry, they use aromatization saturated hydrocarbons that are part of the oil. When alkanes with a straight chain consisting of at least six carbon atoms are passed over heated platinum or chromium oxide, dehydrogenation occurs with simultaneous ring closure ( dehydrocyclization). In this case, benzene is obtained from hexane, and toluene is obtained from heptane.

2. Dehydrogenation of cycloalkanes also leads to aromatic hydrocarbons; for this, a pair of cyclohexane and its homologues is passed over heated platinum.

3. Benzene can be obtained from acetylene trimerization, why acetylene is passed over activated carbon at 600 °C.

4. Benzene homologues are obtained from benzene by its interaction with alkyl halides in the presence of aluminum halides (alkylation reaction, or Friedel-Crafts reaction).

5. When fusion salts of aromatic acids with alkali, arenes are released in gaseous form.

Chemical properties. The aromatic nucleus, which has a mobile system of n-electrons, is a convenient object for attack by electrophilic reagents. This is also facilitated by the spatial arrangement of the n-electron cloud on both sides of the flat a-skeleton of the molecule (see Fig. 23.1, b).

For arenes, the most typical reactions proceed according to the mechanism electrophilic substitution, denoted by the symbol S E(from English, substitution, electrophilic).

Mechanism S E can be represented as follows:

At the first stage, the electrophilic particle X is attracted to the n-electron cloud and forms an n-complex with it. Then two of the six n-electrons of the ring form an a-bond between X and one of the carbon atoms. In this case, the aromaticity of the system is violated, since only four n-electrons remain in the ring, distributed among five carbon atoms (a-complex). To preserve aromaticity, the a-complex ejects a proton, and two C-H bond electrons pass into the n-electron system.

The following reactions of aromatic hydrocarbons proceed according to the mechanism of electrophilic substitution.

1. Halogenation. Benzene and its homologues react with chlorine or bromine in the presence of anhydrous A1C1 3 , FeCl 3 , A1Br 3 catalysts.

This reaction produces a mixture from toluene. ortho- and para-isomers (see below). The role of the catalyst is to polarize the neutral halogen molecule with the formation of an electrophilic particle from it.

2. Nitration. Benzene reacts very slowly with concentrated nitric acid, even when heated strongly. However, when acting nitrating mixture(mixtures of concentrated nitric and sulfuric acids), the nitration reaction proceeds quite easily.

3. Sulfonation. The reaction easily passes with "fuming" sulfuric acid (oleum).

• 4. Friedel-Crafts Alkylation- see above methods for obtaining benzene homologues.
• 5. Alkylation with alkenes. These reactions are widely used in industry to produce ethylbenzene and isopropylbenzene (cumene). Alkylation is carried out in the presence of a catalyst A1C1 3 . The reaction mechanism is similar to that of the previous reaction.

All the above reactions proceed according to the mechanism electrophilic substitution S E .

Along with substitution reactions, aromatic hydrocarbons can enter into addition reactions, however, these reactions lead to the destruction of the aromatic system and therefore require large amounts of energy and proceed only under harsh conditions.

6. hydrogenation benzene goes under heating and high pressure in the presence of metal catalysts (Ni, Pt, Pd). Benzene is converted to cyclohexane.

Hydrogenation of benzene homologues gives cyclohexane derivatives.

7. Radical halogenation benzene occurs when its vapor interacts with chlorine only under the influence of hard ultraviolet radiation. At the same time, benzene joins three chlorine molecules and forms solid product hexachlorocyclohexane (hexachloran) C 6 H 6 C1 6 (hydrogen atoms are not indicated in the structural formulas).

8. Oxidation by atmospheric oxygen. In terms of resistance to the action of oxidizing agents, benzene resembles alkanes - the reaction requires harsh conditions. For example, the oxidation of benzene with atmospheric oxygen occurs only when its vapor is strongly heated (400 °C) in air in the presence of a V 2 0 5 catalyst; the products are a mixture of maleic acid and its anhydride.

Benzene homologues. The chemical properties of benzene homologues are different from those of benzene, which is due to the mutual influence of the alkyl radical and the benzene ring.

Reactions in the side chain. The chemical properties of alkyl substituents in the benzene ring are similar to alkanes. Hydrogen atoms in them are replaced by halogens by a radical mechanism (S R). That's why in the absence of a catalyst, when heated or UV irradiated, a radical substitution reaction occurs in the side chain. However, the influence of the benzene ring on alkyl substituents leads to the fact that, first of all, the hydrogen at the carbon atom directly bonded to the benzene ring is replaced (and -atom carbon).

Substitution on the benzene ring by mechanism S E maybe only in the presence of a catalyst(A1C1 3 or FeCl 3). Substitution in the ring occurs in ortho- and para positions to the alkyl radical.

Under the action of potassium permanganate and other strong oxidizing agents on benzene homologues, the side chains are oxidized. No matter how complex the chain of the substituent is, it is destroyed, with the exception of the a-carbon atom, which is oxidized into a carboxyl group.

Homologues of benzene with one side chain give benzoic acid.

Aromatic chemical compounds, or arenes, are a large group of carbocyclic compounds whose molecules contain a stable cycle of six carbon atoms. It is called the "benzene ring" and determines the special physical and chemical properties of arenes.

Aromatic hydrocarbons primarily include benzene and its various homologues and derivatives.

Arene molecules can contain several benzene rings. Such compounds are called polynuclear aromatic compounds. For example, naphthalene is a well-known drug for protecting woolen products from moths.

Benzene

This simplest representative of arenes consists only of a benzene ring. Its molecular formula is C6Η6. The structural formula of the benzene molecule is most often represented by the cyclic form proposed by A. Kekule in 1865.

The advantage of this formula is the correct reflection of the composition and equivalence of all C and H atoms in the ring. However, she could not explain many of the chemical properties of arenes, so the statement about the presence of three conjugated C=C double bonds is erroneous. This became known only with the advent of modern connection theory.

Meanwhile, even today it is common to write the formula of benzene in the manner proposed by Kekule. Firstly, with its help it is convenient to write down the equations of chemical reactions. Secondly, modern chemists see in it only a symbol, not a real structure. The structure of the benzene molecule is today conveyed by various types of structural formulas.

The structure of the benzene ring

The main feature of the benzene nucleus can be called the absence of single and double bonds in it in the traditional sense. In accordance with modern concepts, the benzene molecule is represented by a flat hexagon with side lengths equal to 0.140 nm. It turns out that the length of the C-C bond in benzene is an intermediate value between single (its length is 0.154 nm) and double (0.134 nm). The C–H bonds lie in the same plane, forming an angle of 120° with the edges of the hexagon.

Each C atom in the benzene structure is in the sp2 hybrid state. It is connected through its three hybrid orbitals with two neighboring C atoms and one H atom. That is, it forms three s-bonds. Another, but already unhybridized, its 2p orbital overlaps with the same orbitals of neighboring C atoms (right and left). Its axis is perpendicular to the plane of the ring, which means that the orbitals overlap above and below it. In this case, a common closed π-electron system is formed. Due to the equivalent overlapping of the 2p orbitals of six C atoms, a kind of "equalization" of the C-C and C=C bonds occurs.

The result of this process is the similarity of such "one-and-a-half" bonds with both double and single bonds. This explains the fact that arenes exhibit chemical properties that are characteristic of both alkanes and alkenes.

The energy of the carbon-carbon bond in the benzene ring is 490 kJ/mol. Which is also the average between the energies of a single and a multiple double bond.

Arena nomenclature

The basis for the names of aromatic hydrocarbons is benzene. Atoms in the ring are numbered from the highest substituent. If the substituents are equivalent, then the numbering is carried out along the shortest path.

For many homologues of benzene, trivial names are often used: styrene, toluene, xylene, etc. To reflect the mutual arrangement of substituents, it is customary to use the prefixes οptο-, meta-, para-.

If the molecule contains functional groups, for example, carbonyl or carboxyl, then the arene molecule is considered as an aromatic radical connected to it. For example, -C6H5 is phenyl, -C6H4 is phenylene, C6H5-CH2- is benzyl.

Physical properties

The first representatives in the homologous series of benzene are colorless liquids with a specific odor. Their weight is lighter than water, in which they practically do not dissolve, but are readily soluble in most organic solvents.

All aromatic hydrocarbons burn with the appearance of a smoky flame, which is explained by the high content of C in the molecules. Their melting and boiling points increase with increasing values ​​of molecular weights in the homologous series of benzene.

Chemical properties of benzene

Of the various chemical properties of arenes, substitution reactions should be mentioned separately. Also very significant are some addition reactions carried out under special conditions, and oxidation processes.

Substitution reactions

Quite mobile π-electrons of the benzene ring are able to react very actively with attacking electrophiles. This electrophilic substitution involves the benzene ring itself in benzene and the hydrocarbon chain associated with it in its homologues. The mechanism of this process has been studied in detail by organic chemistry. The chemical properties of arenes associated with the attack of electrophiles are manifested through three stages.

• First stage. The appearance of the π-complex due to the binding of the π-electron system of the benzene nucleus to the X+ particle, which binds to six π-electrons.

• Second stage. The transition of the π-complex to s, due to the release of a pair of six π-electrons to form a covalent C-X bond. And the remaining four are redistributed between the five C atoms in the benzene ring.
• Third stage. Accompanied by rapid elimination of a proton from the s-complex.

Bromination of benzene in the presence of iron or aluminum bromides without heating leads to the production of bromobenzene:

C6H6+ Br2 -> C6H5-Br + ΗBr.

Nitration with a mixture of nitric and sulfuric acids leads to compounds with a nitro group in the ring:

C6H6+ ΗONO2 -> C6H5-NO2+ Η2O.

Sulfonation is carried out by the bisulfonium ion formed as a result of the reaction:

3Η2SO4 ⇄ SO3Η++ Η3O++ 2ΗSO4-,

or sulfur trioxide.

The reaction corresponds to this chemical property of arenes:

C6H6+ SO3H+ -> C6H5-SO3H + H+.

Alkyl and acyl substitution reactions, or Friedel-Crafts reactions, are carried out in the presence of anhydrous AlCl3.

These reactions are unlikely for benzene and proceed with difficulty. The addition of hydrogen halides and water to benzene does not occur. However, at very high temperatures in the presence of platinum, a hydrogenation reaction is possible:

C6H6 + 3H2 -> C6H12.

When irradiated with ultraviolet, chlorine molecules can join the benzene molecule:

C6H6 + 3Cl2 -> C6H6Cl6.

Oxidation reactions

Benzene is very resistant to oxidizing agents. So, it does not decolorize the pink solution of potassium permanganate. However, in the presence of vanadium oxide, it can be oxidized by atmospheric oxygen to maleic acid:

C6H6 + 4O -> COOH-CH = CH-COOH.

In air, benzene burns with the appearance of soot:

2C6H6 + 3O2 → 12C + 6H2O.

Chemical properties of arenes

1. Substitution.

• Halogenation can proceed in different ways depending on the reaction conditions. In the presence of an appropriate iron or aluminum halide, substitution will occur in the ring by the mechanism detailed above. To introduce a halogen atom into the side chain, the interaction is carried out by heating without catalysts or in the light.
• Nitration of aromatic hydrocarbons with a nitronium ion, which is formed by mixing sulfuric and nitric acid, leads to the connection of the nitro group with the benzene ring. The connection of the nitro group with the side chain is possible during the Konovalov reaction. 2. Oxidation. This chemical property of arenes can be considered from two points of view. On the one hand, they are quite easily oxidized, and the side chain is exposed to the formation of a carboxyl group. If two substituents are connected to the ring in an aromatic hydrocarbon molecule, then a dibasic acid is formed. On the other hand, they, like benzene, burn to form soot and water.

Orientation rules

What position (o-, m- or p-) will the substituent take during the interaction of the electrophilic agent with the benzene ring is determined by the rules:

• if there is already a substituent in the benzene nucleus, then it is he who directs the incoming group to a certain position;
• all orienting substituents are divided into two groups: orientants of the first kind direct the incoming group of atoms to the ortho and para positions (-NH2, -OH, -CH3, -C2H5, halogens); orientants of the second kind direct the entering substituents to the meta-position (-NO2, -SO3Η, -СΗО, -СООΗ).

The orients are listed here in order of decreasing guiding force.

It should be noted that such a division of the substituents of the group is conditional, due to the fact that in most reactions the formation of all three isomers is observed. Orientants only affect which of the isomers will be obtained in greater quantities.

Getting arenas

The main sources of arenes are dry distillation of hard coal and oil refining. Coal tar contains a huge amount of various aromatic hydrocarbons. Some types of oil contain up to 60% arenes, which are easy to isolate by simple distillation, pyrolysis or cracking.

The methods of synthetic preparation and the chemical properties of arenes are often interrelated. Benzene, like its homologues, is obtained by one of the following methods.

1. Reforming of petroleum products. The dehydrogenation of alkanes is the most important industrial method for the synthesis of benzene and many of its homologues. The reaction is carried out by passing gases over a heated catalyst (Pt, Cr2O3, Mo and V oxides) at t = 350–450 °C:

C6H14 -> C6H6 + 4H2.

2. Wurtz-Fittig reaction. It is carried out through the stage of obtaining organometallic compounds. As a result of the reaction, several products can be obtained.

3. Trimerization of acetylene. Acetylene itself, as well as its homologues, are capable of forming arenes when heated with a catalyst:

3C2H2 -> C6H6.

4. Friedel-Crafts reaction. Above, the method for obtaining and converting benzene homologues has already been considered in the chemical properties of arenes.

5. Obtaining from the corresponding salts. Benzene can be isolated by distillation of salts of benzoic acid with alkali:

C6H5-COONa + NaOH -> C6H6 + Na2CO3.

6. Recovery of ketones:

C6H5–CO–CH3 + Zn + 2HCl -> C6H5–CH2–CH3 + Η2O + ZnCl2;

CΗ3–C6Η5–CO–CΗ3+ NΗ2–NΗ2 -> CΗ3–C6Η5–CΗ2–CΗ3+ Η2O.

Application of arenes

The chemical properties and applications of arenes are directly related, since the main part of aromatic compounds is used for further synthesis in chemical production, and is not used in finished form. The exception is substances used as solvents.

Benzene C6H6 is mainly used in the synthesis of ethylbenzene, cumene and cyclohexane. On its basis, semi-products are obtained for the manufacture of various polymers: rubbers, plastics, fibers, dyes, surfactants, insecticides, drugs.

Toluene C6H5-CH3 is used in the manufacture of dyes, drugs and explosives.

Xylenes C6H4(CH3)2 in mixed form (technical xylene) are used as a solvent or initial preparation for the synthesis of organic substances.

Isopropylbenzene (or cumene) C6H4-CH(CH3)2 is the initial reagent for the synthesis of phenol and acetone.

Vinylbenzene (styrene) C6Η5-CΗ=СΗ2 is a raw material for obtaining the most important polymeric material - polystyrene.

Physical properties

Benzene and its closest homologues are colorless liquids with a specific odor. Aromatic hydrocarbons are lighter than water and do not dissolve in it, however, they easily dissolve in organic solvents - alcohol, ether, acetone.

Benzene and its homologues are themselves good solvents for many organic substances. All arenas burn with a smoky flame due to the high carbon content of their molecules.

The physical properties of some arenes are presented in the table.

Table. Physical properties of some arenas

Name	Formula	t°.pl., °C	t°.bp., °C
Benzene	C 6 H 6	5,5	80,1
Toluene (methylbenzene)	C 6 H 5 CH 3	95,0	110,6
Ethylbenzene	C 6 H 5 C 2 H 5	95,0	136,2
Xylene (dimethylbenzene)	C 6 H 4 (CH 3) 2
ortho-		25,18	144,41
meta-		47,87	139,10
pair-		13,26	138,35
Propylbenzene	C 6 H 5 (CH 2) 2 CH 3	99,0	159,20
Cumene (isopropylbenzene)	C 6 H 5 CH(CH 3) 2	96,0	152,39
Styrene (vinylbenzene)	C 6 H 5 CH \u003d CH 2	30,6	145,2

Benzene - low-boiling ( tkip= 80.1°C), colorless liquid, insoluble in water

Attention! Benzene - poison, acts on the kidneys, changes the blood formula (with prolonged exposure), can disrupt the structure of chromosomes.

Most aromatic hydrocarbons are life threatening and toxic.

Obtaining arenes (benzene and its homologues)

In the laboratory

1. Fusion of salts of benzoic acid with solid alkalis

C 6 H 5 -COONa + NaOH t → C 6 H 6 + Na 2 CO 3

sodium benzoate

2. Wurtz-Fitting reaction: (here G is halogen)

From 6H 5 -G+2Na + R-G →C 6 H 5 - R + 2 NaG

FROM 6 H 5 -Cl + 2Na + CH 3 -Cl → C 6 H 5 -CH 3 + 2NaCl

In industry

• isolated from oil and coal by fractional distillation, reforming;
• from coal tar and coke oven gas

1. Dehydrocyclization of alkanes with more than 6 carbon atoms:

C 6 H 14 t , kat→C 6 H 6 + 4H 2

2. Trimerization of acetylene(only for benzene) – R. Zelinsky:

3C 2 H2 600°C, Act. coal→C 6 H 6

3. Dehydrogenation cyclohexane and its homologues:

Soviet Academician Nikolai Dmitrievich Zelinsky established that benzene is formed from cyclohexane (dehydrogenation of cycloalkanes

C 6 H 12 t, cat→C 6 H 6 + 3H 2

C 6 H 11 -CH 3 t , kat→C 6 H 5 -CH 3 + 3H 2

methylcyclohexanetoluene

4. Alkylation of benzene(obtaining homologues of benzene) – r Friedel-Crafts.

C 6 H 6 + C 2 H 5 -Cl t, AlCl3→C 6 H 5 -C 2 H 5 + HCl

chloroethane ethylbenzene

Chemical properties of arenes

I. OXIDATION REACTIONS

1. Combustion (smoky flame):

2C 6 H 6 + 15O 2 t→12CO 2 + 6H 2 O + Q

2. Benzene under normal conditions does not decolorize bromine water and an aqueous solution of potassium permanganate

3. Benzene homologues are oxidized by potassium permanganate (discolor potassium permanganate):

A) in an acidic environment to benzoic acid

Under the action of potassium permanganate and other strong oxidants on the homologues of benzene, the side chains are oxidized. No matter how complex the chain of the substituent is, it is destroyed, with the exception of the a -carbon atom, which is oxidized into a carboxyl group.

Homologues of benzene with one side chain give benzoic acid:

Homologues containing two side chains give dibasic acids:

5C 6 H 5 -C 2 H 5 + 12KMnO 4 + 18H 2 SO 4 → 5C 6 H 5 COOH + 5CO 2 + 6K 2 SO 4 + 12MnSO 4 + 28H 2 O

5C 6 H 5 -CH 3 + 6KMnO 4 + 9H 2 SO 4 → 5C 6 H 5 COOH + 3K 2 SO 4 + 6MnSO 4 + 14H 2 O

Simplified :

C 6 H 5 -CH 3 + 3O KMnO4→C 6 H 5 COOH + H 2 O

B) in neutral and slightly alkaline to salts of benzoic acid

C 6 H 5 -CH 3 + 2KMnO 4 → C 6 H 5 COO K + K OH + 2MnO 2 + H 2 O

II. ADDITION REACTIONS (harder than alkenes)

1. Halogenation

C 6 H 6 + 3Cl 2 h ν → C 6 H 6 Cl 6 (hexachlorocyclohexane - hexachloran)

2. Hydrogenation

C 6 H 6 + 3H 2 t , PtorNi→C 6 H 12 (cyclohexane)

3. Polymerization

III. SUBSTITUTION REACTIONS – ionic mechanism (lighter than alkanes)

b) benzene homologues upon irradiation or heating

In terms of chemical properties, alkyl radicals are similar to alkanes. Hydrogen atoms in them are replaced by halogens by a free radical mechanism. Therefore, in the absence of a catalyst, heating or UV irradiation leads to a radical substitution reaction in the side chain. The influence of the benzene ring on alkyl substituents leads to the fact that the hydrogen atom is always replaced at the carbon atom directly bonded to the benzene ring (a-carbon atom).

1) C 6 H 5 -CH 3 + Cl 2 h ν → C 6 H 5 -CH 2 -Cl + HCl

c) benzene homologues in the presence of a catalyst

C 6 H 5 -CH 3 + Cl 2 AlCl 3 → (mixture of orta, pair of derivatives) +HCl

2. Nitration (with nitric acid)

C 6 H 6 + HO-NO 2 t, H2SO4→C 6 H 5 -NO 2 + H 2 O

nitrobenzene - smell almond!

C 6 H 5 -CH 3 + 3HO-NO 2 t, H2SO4→ FROM H 3 -C 6 H 2 (NO 2) 3 + 3H 2 O

2,4,6-trinitrotoluene (tol, trotyl)

The use of benzene and its homologues

Benzene C 6 H 6 is a good solvent. Benzene as an additive improves the quality of motor fuel. It serves as a raw material for the production of many aromatic organic compounds - nitrobenzene C 6 H 5 NO 2 (solvent, aniline is obtained from it), chlorobenzene C 6 H 5 Cl, phenol C 6 H 5 OH, styrene, etc.

Toluene C 6 H 5 -CH 3 - a solvent used in the manufacture of dyes, drugs and explosives (trotyl (tol), or 2,4,6-trinitrotoluene TNT).

Xylene C 6 H 4 (CH 3) 2 . Technical xylene is a mixture of three isomers ( ortho-, meta- And pair-xylene) - is used as a solvent and starting product for the synthesis of many organic compounds.

Isopropylbenzene C 6 H 5 -CH (CH 3) 2 serves to obtain phenol and acetone.

Chlorine derivatives of benzene used for plant protection. Thus, the product of substitution of H atoms in benzene with chlorine atoms is hexachlorobenzene C 6 Cl 6 - a fungicide; it is used for dry seed dressing of wheat and rye against hard smut. The product of the addition of chlorine to benzene is hexachlorocyclohexane (hexachloran) C 6 H 6 Cl 6 - an insecticide; it is used to control harmful insects. These substances refer to pesticides - chemical means of combating microorganisms, plants and animals.

Styrene C 6 H 5 - CH \u003d CH 2 polymerizes very easily, forming polystyrene, and copolymerizing with butadiene - styrene-butadiene rubbers.

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