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

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:

The most important aromatic hydrocarbons include benzene C 6 H 6 and its homologues: toluene C 6 H 5 CH 3, xylene C 6 H 4 (CH 3) 2, etc.; naphthalene C 10 H 8 , anthracene C 14 H 10 and their derivatives.

The carbon atoms in the benzene molecule form a regular flat hexagon, although it is usually drawn elongated.

The structure of the benzene molecule was finally confirmed by the reaction of its formation from acetylene. The structural formula shows three single and three double alternating carbon-carbon bonds. But such an image does not convey the true structure of the molecule. In fact, the carbon-carbon bonds in benzene are equivalent, and they have properties that are not similar to those of either single or double bonds. These features are explained by the electronic structure of the benzene molecule.

Electronic structure of benzene

Each carbon atom in the benzene molecule is in a state of sp 2 hybridization. It is linked to two adjacent carbon atoms and a hydrogen atom by three σ-bonds. As a result, a flat hexagon is formed: all six carbon atoms and all C-C and C-H σ-bonds lie in the same plane. The electron cloud of the fourth electron (p-electron), not participating in hybridization, has the shape of a dumbbell and is oriented perpendicular to the plane of the benzene ring. Such p-electron clouds of neighboring carbon atoms overlap above and below the plane of the ring.

As a result, six p-electrons form a common electron cloud and a single chemical bond for all carbon atoms. Two regions of the large electronic plane are located on both sides of the plane of σ-bonds.

The p-electron cloud causes a reduction in the distance between carbon atoms. In the benzene molecule, they are the same and equal to 0.14 nm. In the case of a single and double bond, these distances would be 0.154 and 0.134 nm, respectively. This means that there are no single and double bonds in the benzene molecule. The benzene molecule is a stable six-membered cycle of identical CH-groups lying in the same plane. All bonds between carbon atoms in benzene are equivalent, which determines the characteristic properties of the benzene nucleus. This is most accurately reflected by the structural formula of benzene in the form of a regular hexagon with a circle inside (I). (The circle symbolizes the equivalence of bonds between carbon atoms.) However, the Kekule formula is often used, indicating double bonds (II):

The benzene nucleus has a certain set of properties, which is commonly called aromaticity.

Homologous series, isomerism, nomenclature

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):

The homologous series of benzene has the general formula C n H 2 n -6 . Homologues can be considered as derivatives of benzene, in which one or more hydrogen atoms are replaced by various hydrocarbon radicals. For example, C 6 H 5 -CH 3 - methylbenzene or toluene, C 6 H 4 (CH 3) 2 - dimethylbenzene or xylene, C 6 H 5 -C 2 H 5 - ethylbenzene, etc.

Since all carbon atoms in benzene are equivalent, its first homologue, toluene, has no isomers. The second homologue, dimethylbenzene, has three isomers that differ in the mutual arrangement of methyl groups (substituents). This is an ortho- (abbreviated as o-), or 1,2-isomer, in which substituents are located at neighboring carbon atoms. If the substituents are separated by one carbon atom, then it is the meta (abbreviated m-) or 1,3-isomer, and if they are separated by two carbon atoms, then it is the para- (abbreviated p-) or 1,4-isomer. In the names, substituents are indicated by letters (o-, m-, p-) or numbers.

Physical properties

The first members of the homologous series of benzene are colorless liquids with a specific odor. Their density is less than 1 (lighter than water). Insoluble in water. Benzene and its homologues are themselves good solvents for many organic substances. Arenas burn with a smoky flame due to the high carbon content in their molecules.

Chemical properties

Aromaticity determines the chemical properties of benzene and its homologues. The six-electron π-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. Thus, aromatic hydrocarbons in their chemical properties occupy an intermediate position between saturated and unsaturated hydrocarbons.

I. Substitution reactions

1. Halogenation (with Cl 2, Br 2)

2. Nitration

3. Sulfonation

4. Alkylation (benzene homologs are formed) - Friedel-Crafts reactions

Alkylation of benzene also occurs when it interacts with alkenes:

Dehydrogenation of ethylbenzene produces styrene (vinylbenzene):

II. Addition reactions

1. Hydrogenation

2. Chlorination

III. Oxidation reactions

1. Combustion

2C 6 H 6 + 15O 2 → 12CO 2 + 6H 2 O

2. Oxidation under the action of KMnO 4, K 2 Cr 2 O 7, HNO 3, etc.

No chemical reaction occurs (similar to alkanes).

Properties of benzene homologues

In benzene homologues, a core and a side chain (alkyl radicals) are distinguished. In terms of chemical properties, alkyl radicals are similar to alkanes; the influence of the benzene nucleus on them is manifested in the fact that hydrogen atoms always participate in substitution reactions at the carbon atom directly bonded to the benzene nucleus, as well as in the easier oxidizability of C-H bonds.

The effect of an electron-donating alkyl radical (for example, -CH 3) on the benzene core is manifested in an increase in the effective negative charges on carbon atoms in the ortho and para positions; as a result, the substitution of their associated hydrogen atoms is facilitated. Therefore, benzene homologues can form trisubstituted products (and benzene usually forms monosubstituted derivatives).

Benzene is obtained from coal tar formed during the coking of coal, oil, by synthetic methods.

1. Obtaining from aliphatic hydrocarbons. When straight-chain alkanes having at least six carbon atoms per molecule are passed over heated platinum or chromium oxide, dehydrocyclization- formation of an arene with the release of hydrogen: the method of B.A. Kazansky and A.F. Plate

2. Dehydrogenationcycloalkanes (N.D. Zelinsky) The reaction occurs by passing vapors of cyclohexane and its homologues over heated platinum at 3000 0 .

3. Obtaining benzene trimerization of acetylene over activated carbon at 600 0(N.D. Zelinsky )

3HC?CH

-- 600?C?

4. Fusion of salts of aromatic acids with alkali or soda lime:

5. Chemical properties of arenes.

The benzene core has high strength. For arenes, the most typical reactions proceed according to the mechanism electrophilic substitution, denoted by the symbol S E (from the English substitution electrophilic).

Chemical properties of benzene.

1. Substitution reactions:

Halogenation . Benzene does not interact with chlorine or bromine under normal conditions. The reaction can proceed only in the presence of catalysts - anhydrous AlCl 3 , FeCl 3 , AlBr 3 . 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:

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

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

2. Friedel-Crafts Alkylation. 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 RCl on benzene in the presence of catalysts - aluminum halides. The role of the catalyst is reduced to the polarization of the RСl 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:

Alkylation with alkenes. These reactions are widely used in industry to produce ethylbenzene and isopropylbenzene (cumene). Alkylation is carried out in the presence of AlCl 3 catalyst. The reaction mechanism is similar to that of the previous reaction:

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

3. Addition reactions proceeding with bond breaking:

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 turning to cyclohexane, a benzene homologues - into cyclohexane derivatives:

Radical halogenation. The interaction of benzene vapor with chlorine proceeds according to the radical mechanism only under the influence of hard ultraviolet radiation. In this case, benzene adds three molecules of chlorine and forms solid product - hexachlorocyclohexane (hexachlorane) C 6 H 6 Cl 6:

4. 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 V 2 O 5 catalyst, a mixture of maleic acid and its anhydride is obtained:

5. Benzene is on fire. (View experience) The flame of benzene is smoky due to the high carbon content in the molecule.

2 C 6 H 6 + 15 O 2 → 12CO 2 + 6H 2 O

6. The use of arenes.

Benzene and its homologues are used as chemical raw materials for the production of medicines, plastics, dyes, acetone, phenol, and formaldehyde plastics. pesticides and many other organic substances. Widely used as solvents. Benzene as an additive improves the quality of motor fuel. Ethylene is used to produce ethyl alcohol, polyethylene. It accelerates the ripening of fruits (tomatoes, citrus fruits) with the introduction of small amounts of it into the air of greenhouses. Propylene is used for the synthesis of glycerin, alcohol, for the extraction of polypropylene, which is used for the manufacture of ropes, ropes, and packaging material. Based on 1-butene, synthetic rubber is produced.

Acetylene is used for autogenous welding of metals. Polyethylene is used as a packaging material, for the manufacture of bags, toys, household utensils (bottles, buckets, bowls, etc.). Aromatic hydrocarbons are widely used in the production of dyes, plastics, chemical pharmaceuticals, explosives, synthetic fibers, motor fuels, and others. are the products of coal coking. From 1 t kam.-ug. resins can be isolated on average: 3.5 kg benzene, 1.5 kg toluene, 2 kg naphthalene. Of great importance is the production of A. at. from fatty hydrocarbons. For some A. at. purely synthetic methods are of practical importance. Thus, ethylbenzene is produced from benzene and ethylene, the dehydrogenation of which leads to styrene.

TASKS FOR SELF-CONTROL:

1. What compounds are called arenas?

2. What are the characteristic physical properties?

3. A task. From 7.8 g of benzene, 8.61 g of nitrobenzene was obtained. Determine the yield (in%) of the reaction product.

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 С 6 Η 6 . 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, -C 6 Η 5 - phenyl, -C 6 Η 4 - phenylene, C 6 Η 5 -CH 2 - benzyl.

Physical properties

The first representatives in the homologous series of benzene are colorless liquids with a specific smell. 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. Such an 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 is associated with six π-electrons.

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

C 6 Η 6 + Br 2 -> C 6 Η 5 -Br + ΗBr.

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

C 6 Η 6 + ΗONO 2 -> C 6 Η 5 -NO 2 + Η 2 O.

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

3Η 2 SO 4 ⇄ SO 3 Η + + Η 3 O + + 2ΗSO 4 - ,

or sulfur trioxide.

The reaction corresponds to this chemical property of arenes:

C 6 H 6 + SO 3 H + -> C 6 H 5 -SO 3 H + H +.

Alkyl and acyl substitution reactions, or Friedel-Crafts reactions, are carried out in the presence of anhydrous AlCl 3 .

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:

C 6 Η 6 + 3H 2 -> C 6 H 12.

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

С 6 Η 6 + 3Cl 2 —> C 6 Η 6 Cl 6 .

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:

C 6 H 6 + 4O -> COOΗ-CΗ \u003d CΗ-COOΗ.

In air, benzene burns with the appearance of soot:

2C 6 H 6 + 3O2 → 12C + 6H 2 O.

Chemical properties of arenes

1. Substitution.

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 (-NH 2, -OH, -CH 3, -C 2 H 5, halogens); orientants of the second kind direct the incoming substituents to the meta position (—NO 2 , —SO 3 Η, —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, Cr 2 O 3, Mo and V oxides) at t = 350-450 o C:

C 6 H 14 -> C 6 Η 6 + 4Η 2.

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:

3С 2 Η 2 —> С 6 Η 6 .

4. Friedel-Crafts reaction. The method of obtaining and converting benzene homologues has already been considered above 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:

C 6 Η 5 -COONa + NaOΗ -> C 6 Η 6 + Na 2 CO 3.

6. Recovery of ketones:

C 6 Η 5 -CO-CΗ 3 + Zn + 2ΗCl -> C 6 Η 5 -CΗ 2 -CΗ 3 + Η 2 O + ZnCl 2;

CΗ 3 -C 6 Η 5 -CO-CΗ 3 + NΗ 2 -NΗ 2 -> CΗ 3 -C 6 Η 5 -CΗ 2 -CΗ 3 + Η 2 O.

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 C 6 Η 6 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 C 6 H 5 -CH 3 is used in the manufacture of dyes, drugs and explosives.

Xylenes С 6 Η 4 (СН 3) 2 in a mixed form (technical xylene) are used as a solvent or initial preparation for the synthesis of organic substances.

Isopropylbenzene (or cumene) С 6 Η 4 -СН (СН 3) 2 is the initial reagent for the synthesis of phenol and acetone.

Vinylbenzene (styrene) C 6 Η 5 -CΗ=СΗ 2 is a raw material for obtaining the most important polymeric material - polystyrene.

ARENA (aromatic hydrocarbons)

Arenes or aromatic hydrocarbons - these are compounds whose molecules contain stable cyclic groups of atoms (benzene nuclei) with a closed system of conjugated bonds.

Why "Aromatic"? Because some of the substances have a pleasant smell. However, at present, a completely different meaning is put into the concept of "aromaticity".

Aromaticity of a molecule means its increased stability due to the delocalization of π-electrons in a cyclic system.

Arenes aromaticity criteria:

1. carbon atoms in sp 2 -hybridized state form a cycle.
2. The carbon atoms are arranged in one plane(the cycle has a flat structure).

A closed system of conjugated bonds contains

4n+2π electrons ( n is an integer).

The benzene molecule fully complies with these criteria. C 6 H 6.

The concept “ benzene ring” requires decryption. To do this, it is necessary to consider the structure of the benzene molecule.

ATAll bonds between carbon atoms in benzene are the same (there are no double or single bonds as such) 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 links is usually depicted as a circle inside the cycle

Circular conjugation gives an energy gain of 150 kJ/mol. This value is conjugation energy - the amount of energy that must be expended to break the aromatic system of benzene.

General formula: C n H 2n-6(n ≥ 6)

Homologous series:

Benzene homologues are compounds formed by replacing one or more hydrogen atoms in a benzene molecule with hydrocarbon radicals (R):

ortho- (about-) substituents at adjacent carbon atoms of the ring, i.e. 1,2-;
meta- (m-) substituents through one carbon atom (1,3-);
pair- (P-) substituents on opposite sides of the (1,4-) ring.

aryl

C 6H5- (phenyl) and C6H Aromatic monovalent radicals have the common name " aryl". Of these, two are most common in the nomenclature of organic compounds:

C 6H5- (phenyl) and C 6 H 5 CH 2- (benzyl). 5 CH 2- (benzyl).

Isomerism:

structural:

1) positions of deputies for di-, three- and tetra-substituted benzenes (for example, about-, m- and P-xylenes);

2) carbon skeleton in the side chain containing at least 3 carbon atoms:

3) isomerism of substituents R, starting from R = C 2 H 5 .

Chemical properties:

Arenes are more characteristic of reactions going with preservation of the aromatic system, namely, substitution reactions hydrogen atoms associated with the cycle.

2. Nitration

Benzene reacts with a nitrating mixture (a mixture of concentrated nitric and sulfuric acids):

3. Alkylation

Substitution of a hydrogen atom in the benzene ring with an alkyl group ( alkylation) occurs under the action alkyl halides or alkenes in the presence of catalysts AlCl 3 , AlBr 3 , FeCl 3 .

Substitution in alkylbenzenes:

Benzene homologues (alkylbenzenes) are more active in substitution reactions than benzene.

For example, when nitrating toluene C 6 H 5 CH 3 substitution of not one, but three hydrogen atoms can occur with the formation of 2,4,6-trinitrotoluene:

and facilitates substitution in these positions.

On the other hand, under the influence of the benzene ring, the methyl group CH 3 in toluene becomes more active in oxidation and radical substitution reactions compared to methane CH 4.

Toluene, unlike methane, oxidizes under mild conditions (discolors the acidified solution of KMnO 4 when heated):

Easier than in alkanes, radical substitution reactions proceed in side chain alkylbenzenes:

This is explained by the fact that stable intermediate radicals are easily (at a low activation energy) formed at the limiting stage. For example, in the case toluene a radical is formed benzyl Ċ H 2 -C 6 H 5 . It is more stable than alkyl free radicals ( Ċ H 3 Ċ H 2 R), because its unpaired electron is delocalized due to interaction with the π-electron system of the benzene ring:

Orientation rules

1. The substituents present in the benzene ring direct the newly entering group to certain positions, i.e. have an orienting effect.

2. According to their guiding action, all substituents are divided into two groups:orientators of the first kind and orientators of the second kind.

   Orientants of the 1st kind(ortho pair-orientants) direct the subsequent substitution mainly inortho- and pair-provisions.

   These include electron donor groups (electronic effects of groups are indicated in brackets):

R( +I); - Oh(+M,-I); - OR(+M,-I); - NH2(+M,-I); - NR 2(+M,-I) +M-effect in these groups is stronger than -I-effect.

Orientants of the 1st kind increase the electron density in the benzene ring, especially on carbon atoms inortho- and pair-positions, which favors the interaction of these atoms with electrophilic reagents.

Orientants of the 1st kind, by increasing the electron density in the benzene ring, increase its activity in electrophilic substitution reactions compared to unsubstituted benzene.

A special place among the orientants of the 1st kind is occupied by halogens, which exhibitelectron-withdrawing properties:

-F (+M<–I ), -Cl (+M<–I ), -Br (+M<–I ).

Being ortho pair-orientants, they slow down electrophilic substitution. Reason is strong –I-the effect of electronegative halogen atoms, which lowers the electron density in the ring.

Orientators of the 2nd kind ( meta-orientants) direct subsequent substitution predominantly to meta-position.
These include electron-withdrawing groups:

-NO 2 (-M, -I); -COOH (-M, -I); -CH=O (-M, -I); -SO 3 H (–I); -NH3+ (–I); -CCl 3 (–I).

Orientants of the 2nd kind reduce the electron density in the benzene ring, especially in ortho- and pair-provisions. Therefore, the electrophile attacks carbon atoms not in these positions, but in meta-position, where the electron density is somewhat higher.
Example:

All orientants of the 2nd kind, reducing the overall electron density in the benzene ring, reduce its activity in electrophilic substitution reactions.

Thus, the ease of electrophilic substitution for compounds (given as examples) decreases in the series:

toluene C 6 H 5 CH Unlike benzene, its homologues are oxidized quite easily.