Spatial isomerism of arenes. Physical and chemical properties of arenes

Benzene

The simplest representative of arenes is benzene. Let's take a closer look at its properties.

Physical properties

Benzene is a transparent, colorless, highly volatile liquid with a characteristic odor (it is because of the strong odor that aromatic compounds got their name). Melting point 5.5°C, boiling point - 80°C. Does not mix with water, but mixes well with most organic solvents. It is a solvent for non-polar organic substances. Burns with a smoky flame (incomplete combustion) with the formation, in addition to carbon dioxide and water, of a significant amount of soot. Poisonous both as a liquid and as a vapor if inhaled.

Obtaining benzene

1. In industry, benzene is produced by oil reforming, which is essentially the dehydrogenation of oil alkanes with the formation of a cyclic skeleton. In its “pure” form, the main reforming reaction is the dehydrogenation of hexane:

In addition, benzene is one of the volatile products of coking. Coking is heating coal to 1000°C without air access. This also produces many other valuable reagents for organic synthesis and coke used in metallurgy. Benzene can also be obtained by trimerization of acetylene over activated carbon at 100°C.

2. Of course, benzene is not produced in the laboratory, but theoretically there are methods for its synthesis (they are used to obtain its derivatives). Both industrial and laboratory methods are reflected in the diagram below.

Scheme methods for producing benzene

Industrial methods.

Chemical properties of benzene

The chemical properties of benzene are determined, of course, by its p-system. Just as in the case of alkenes, it can be attacked by an electrophilic species. However, in the case of aromatic compounds, the result of such an attack will be completely different. The high stability of the p-system leads to the fact that at the end of the reaction it is, as a rule, restored and the result of the reaction is not addition (which would destroy

p-system), but electrophilic substitution. Let's take a closer look at its mechanism.

In the first stage, the attack of the AB molecule containing the electrophilic center A leads to the formation of an extremely unstable p-complex (stage 1). In this case, the aromatic system is not disrupted. Next, a covalent bond is formed between one of the atoms of the ring and particle A (stage 2). In this case, firstly, the A-B bond is broken, and secondly, the p-system is destroyed. The resulting unstable positively charged molecule is called an s-complex. As already mentioned, the restoration of the p-system is energetically very favorable, and this leads to the rupture of either the C-A bond (and then the molecule returns to its original state) or the C-H bond (stage 3). In the latter case, the reaction ends, and the product is the replacement of hydrogen by A.

Most reactions of aromatic compounds have this mechanism (electrophilic substitution, abbreviated S E). Let's look at some of them.

1. Halogenation. Occurs only in the presence of catalysts - Lewis acids (see "Lewis Theory"). The task of the catalyst is to polarize the halogen molecule to form a good electrophilic center:

| AlCl 3 + Cl 2 “Cl + [AlCl 4 ] - The resulting particle has an electrophilic chlorine atom, and

reaction occurs:

TO Nitration. It is carried out with a mixture of nitric and sulfuric acids (nitrating mixture). The following reaction occurs in the nitrating mixture:

HNO 3 +H 2 SO 4 “NO + 2 +H 2 O

The resulting nitronium hydrosulfate has a powerful electrophilic center - the nitronium ion NO + 2. Accordingly, a reaction takes place, the general equation of which is:

3. Sulfonation. In concentrated sulfuric acid there is an equilibrium:

2H 2 SO 4 “SO 3 H + - +H 2 O

The molecule on the right side of the equilibrium has a strong electrophile SO 3 H +, which reacts with benzene. Resulting reaction:

Alkylation according to Friedel-Crafts. When benzene reacts with alkyl chlorides or alkenes in the presence of Lewis acids (usually aluminum halides), alkyl-substituted benzenes are obtained. In the case of alkyl halides, the first stage of the process is:

RСl + АlСl 3 «R + [АlСl 4 ] - In the second stage, the electrophilic particle R + attacks the p-system:

In the case of alkenes, the Lewis acid polarizes the double bond of the alkene, and again an electrophilic center is formed on the carbon:

Non-electrophilic reactions include:

1. Hydrogenation of benzene. This reaction involves the destruction of the p-system and requires harsh conditions (high pressure, temperature, catalyst - platinum metals):

2. Radical chlorination. In the absence of Lewis acids and under harsh ultraviolet irradiation, benzene can react with chlorine by a radical mechanism. In this case, the p-system is destroyed and the product of chlorine addition is formed - the solid substance hexachlorane, which was previously used as an insecticide:

Benzene homologues

Nomenclature and isomerism of arenes

All arenas can be roughly divided into two rows. The first row is benzene derivatives (toluene, biphenyl): the second row is condensed (polynuclear) arenes (naphthalene, anthracene).

Let's consider the homologous series of benzene; compounds of this series have the general formula C n H 2 n. 6. 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 positional isomers, since all atoms in the benzene ring are equivalent,

I Group C 6 H 5 is called phenyl. The phenyl and substituted phenyl groups are called aryl. Some benzene derivatives are shown below:

Benzene reaction scheme

Isomers with two substituents at positions 1,2; 1,3 and 1,4 are called ortho-, meta- and para-isomers:

Nomenclature of aromatic compounds

Below are the names of some aromatic compounds:

C 6 H 5 NH 3 + Cl - Phenylammonium chloride (anilinium chloride)

C b H 5 CO 2 H Benzenecarboxylic acid (benzoic acid)

C 6 H 5 CO 2 C 2 H 5 Benzene carboxylic acid ethyl ester (ethyl benzoate)

C 6 H 5 COCl Benzenecarbonyl chloride (benzoyl chloride)

C 6 H 5 CONH 2 Benzenecarboxamide (benzamide)

C 6 H 5 CN Benzenecarbonitrile (benzonitrile)

C6H5CHO Benzenecarbaldehyde (benzaldehyde)

C 6 H 5 COCH 3 Acetophenone

C6H5OH Phenol

C 6 H 5 NH 2 Phenylamine (aniline)

C 6 H 5 OCH 3 Methoxybenzene (anisole)

These names follow IUPAC nomenclature. In parentheses are traditional names that are still widespread and quite acceptable.

Arena nomenclature

The name of a benzene derivative with two or more substituents on the benzene ring is constructed in this way. The carbon atom of the benzene ring to which the substituent closest to the beginning of the above list is attached receives the number 1. Next, the carbon atoms of the benzene ring are numbered so that the locant - the number of the second substituent - is the smallest.

3-Hydroxybenzenecarboxylic acid (3-hydroxybenzoic acid)

The carboxyl group is treated as the main group and is assigned a locant of "1". The ring numbering is constructed so that the hydroxyl group receives a smaller (“3” rather than “5”) locant.

2-aminobenzenecarb aldehyde (2-aminobenzaldehyde)

The -CHO group is considered as the main one. She receives a locant of "1". Group-NH 2 is in position "2" rather than "6". In addition, the name o-aminobenzaldehyde is acceptable.

1-bromo-2-nitro-4-chlorobenzene These groups are listed in alphabetical order.

Getting arenas

Preparation from aliphatic hydrocarbons. When straight-chain alkanes with at least 6 carbon atoms per molecule are passed over heated platinum or chromium (III) oxide, dehydrocyclization occurs - the formation of an arene with the release of hydrogen. For example:

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

|. Preparation of benzene by trimerization of acetylene. 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 100°C. The whole process can be represented by a diagram:

4. Preparation of benzene homologues using the Friedel-Crafts reaction(see Chemical properties of benzene).

5. Fusion of salts of aromatic acids with alkali: C 6 H 6 -COONa+NaOH ®C 6 H 6 +Na 2 CO 3

Application of arenas

Arenas are used as chemical raw materials for the production of medicines, plastics, dyes, pesticides and many other organic substances. Arenes are widely used as solvents.

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.

ARENES

Aromatic hydrocarbons (arenes) – cyclic hydrocarbons, united by the concept of aromaticity, which determines common characteristics in structure and chemical properties.

Classification

Based on the number of benzene rings in the molecule, arenes are divided into on the:

mononuclear

multi-core

Nomenclature and isomerism

The structural ancestor of benzene series hydrocarbons is benzene C 6 H 6 from which the systematic names of homologues are derived.

For monocyclic compounds, the following non-systematic (trivial) names are retained:

The position of the substituents is indicated in the smallest numbers (the direction of numbering does not matter),

and for di-substituted compounds you can use the notation ortho, meta, pair.

If there are three substituents in the ring, they should receive the lowest numbers, i.e. the row “1,2,4” has an advantage over “1,3,4”.

1,2-dimethyl-4-ethylbenzene (correct name) 3,4-dimethyl-1-ethylbenzene (incorrect name)

The isomerism of monosubstituted arenes is due to the structure of the carbon skeleton of the substituent; in di- and polysubstituted benzene homologues, additional isomerism is added, caused by the different arrangement of substituents in the nucleus.

Isomerism of aromatic hydrocarbons with the composition C 9 H 12:

Physical properties

The boiling and melting points of arenes are higher than those of alkanes, alkenes, alkynes, they are slightly polar, insoluble in water and highly soluble in non-polar organic solvents. Arenas are liquids or solids that have specific odors. Benzenes and many condensed arenes are toxic, some of them exhibit carcinogenic properties. Intermediate products of the oxidation of condensed arenes in the body are epoxides, which either themselves directly cause cancer or are precursors of carcinogens.

Getting arenas

Many aromatic hydrocarbons are of great practical importance and are produced on a large industrial scale. A number of industrial methods are based on the processing of coal and oil.

Oil consists mainly of aliphatic and alicyclic hydrocarbons; to convert aliphatic or acyclic hydrocarbons into aromatic ones, methods for aromatizing oil have been developed, the chemical basis of which was developed by N.D. Zelinsky, B.A. Kazansky.

1. Cyclization and dehydrogenation:

2. Hydrodesmethylation:

3. Benzene homologues are prepared by alkylation or acylation followed by reduction of the carbonyl group.

a) Friedel-Crafts alkylation:

b) Friedel-Crafts acylation:

4. Preparation of biphenyl by the Wurtz-Fitting reaction:

5. Preparation of diphenylmethane by the Friedel-Crafts reaction:

Structure and chemical properties.

Aromaticity criteria:

Based on theoretical calculations and experimental studies of cyclic conjugated systems, it was found that a compound is aromatic if it has:

• Flat cyclic σ-skeleton;
• A conjugated closed π-electron system, covering all atoms of the ring and containing 4n + 2, where n = 0, 1, 2, 3, etc. This formulation is known as Hückel's rule. Aromaticity criteria allow one to distinguish conjugated aromatic systems from all others. Benzene contains a sextet of π electrons and follows Hückel's rule at n = 1.

What does aromaticity give:

Despite the high degree of unsaturation, aromatic compounds are resistant to oxidizing agents and temperature, and they are more prone to undergo substitution reactions rather than addition reactions. These compounds have increased thermodynamic stability, which is ensured by the high conjugation energy of the aromatic ring system (150 kJ/mol); therefore, arenes preferably enter into substitution reactions, as a result of which they retain aromaticity.

Mechanism of electrophilic substitution reactions in the aromatic ring:

The electron density of the π-conjugated system of the benzene ring is a convenient target for attack by electrophilic reagents.

Typically, electrophilic reagents are generated during a reaction using catalysts and appropriate conditions.

E – Y → E δ + – Y δ - → E + + Y -

Formation of a π-complex. The initial attack by the electrophile of the π-electron cloud of the ring leads to coordination of the reagent with the π-system and the formation of a donor-acceptor type complex called π-complex. The aroma system is not disrupted:

Formation of the σ-complex. The limiting stage, in which the electrophile forms a covalent bond with a carbon atom due to two electrons of the π-system of the ring, which is accompanied by the transition of this carbon atom from sp 2 - V sp 3 - hybrid state and aromatic disruption, the molecule turns into a carbocation.

Stabilization of the σ-complex. It is carried out by abstraction of a proton from the σ-complex using a base. In this case, due to the two electrons of the breaking covalent bond C–H, the closed π-system of the ring is restored, i.e. the molecule returns to the aromatic state:

Effect of substituents on reactivity and orientation of electrophilic substitution

Substituents on the benzene ring disrupt the distribution uniformity π- electron cloud of the ring and thereby influence the reactivity of the ring.

• Electron-donating substituents (D) increase the electron density of the ring and increase the rate of electrophilic substitution; such substituents are called activating.
• Electron-withdrawing substituents (A) reduce the electron density of the ring and reduce the reaction rate, called decontaminating.

ARENES (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 a number of substances have a pleasant odor. However, nowadays the concept of “aromaticity” has a completely different meaning.

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

Arene 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 conjugate connections contains

4n+2π electrons ( n– integer).

The benzene molecule fully meets these criteria. C 6 H 6.

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

INAll bonds between carbon atoms in benzene are identical (there are no double or single bonds as such) and have a length of 0.139 nm. This value is intermediate between the length of a single bond in alkanes (0.154 nm) and the length of a double bond in alkenes (0.133 nm).

The equivalence of connections is usually represented by a circle inside a cycle

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

General formula: CnH2n-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- (O-) substituents on neighboring 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 ring (1,4-).

aryl

C 6 H 5- (phenyl) And C6H Aromatic monovalent radicals have the common name " aryl". Of these, two are the most common in the nomenclature of organic compounds:

C 6 H 5- (phenyl) And C6H5CH2- (benzyl). 5 CH 2- (benzyl).

Isomerism:

structural:

1) positions of substituents for di-, three- And tetra-substituted benzenes (for example, O-, m- And P-xylenes);

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

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

Chemical properties:

For arenes, reactions proceeding with preservation of the aromatic system, namely, substitution reactions hydrogen atoms associated with the ring.

2. Nitration

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

3. Alkylation

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

Substitution in alkylbenzenes:

Benzene homologs (alkylbenzenes) undergo substitution reactions more actively than benzene.

For example, during the nitration of 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 it becomes more active in oxidation and radical substitution reactions compared to methane CH 4.

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

Radical substitution reactions occur more easily than in alkanes. side chain alkylbenzenes:

This is explained by the fact that at the limiting stage, stable intermediate radicals are easily formed (at a low activation energy). For example, in case toluene a radical is formed benzyl Ċ H 2 -C 6 H 5 . It is more stable than alkyl free radicals ( Ċ N 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 on the benzene ring direct the newly introduced group to certain positions, i.e. have an orienting effect.

2. According to their directing action, all substituents are divided into two groups:orientants of the first kind And orientants of the second kind.

   Orientants of the 1st kind(ortho-para-orientators) direct subsequent substitution predominantly toortho- And pair- provisions.

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

R ( +I); - OH(+M,-I); - OR(+M,-I); - NH 2(+M,-I); - NR 2(+M,-I) The +M effect is stronger than the -I effect in these groups.

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

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

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

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

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

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

-NO 2 (–M, –I); -COOH (–M, –I); -CH=O (–M, –I); -SO3H (–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 slightly higher.
Example:

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

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

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

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

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

Arene molecules may 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 C 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 that it accurately reflects the composition and equivalence of all C and H atoms in the ring. However, it 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, today the formula for benzene is often written in the manner proposed by Kekule. Firstly, with its help it is convenient to write equations of chemical reactions. Secondly, modern chemists see in it only a symbol, and not a real structure. The structure of the benzene molecule is today conveyed by various types of structural formulas.

Structure of the benzene ring

The main feature of the benzene ring is the absence of single and double bonds in it in the traditional sense. In accordance with modern concepts, the benzene molecule appears as 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 also 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 C atoms located nearby and one H atom. That is, it forms three s-bonds. Another, but already unhybridized, 2p orbital overlaps with the same orbitals of neighboring C atoms (to the 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 equal overlap of the 2p orbitals of the 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 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 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 relative position of substituents, it is customary to use the prefixes ortho-, 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 H 5 - phenyl, -C 6 H 4 - phenylene, C 6 H 5 -C H 2 - 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 are practically insoluble, but they dissolve well in most organic solvents.

All aromatic hydrocarbons burn with a smoky flame, which is explained by the high C content in the molecules. Their melting and boiling points increase with increasing 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 that occur under special conditions and oxidation processes.

Substitution reactions

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

• First stage. The appearance of the π-complex is due to the binding of the π-electron system of the benzene ring to the X + particle, which binds to 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 the production of 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 a 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 corresponding to this chemical property of arenes is:

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 occur 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 light, chlorine molecules can join a benzene molecule:

C 6 Η 6 + 3Cl 2 —> C 6 Η 6 Cl 6 .

Oxidation reactions

Benzene is very resistant to oxidizing agents. Thus, it does not discolor 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 = CΗ-COOΗ.

In air, benzene burns with the appearance of soot:

2C 6 Η 6 + 3O2 → 12C + 6 Η 2 O.

Chemical properties of arenes

1. Substitution.

Orientation rules

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

• if there is already any substituent in the benzene ring, then it is this substituent that 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 ortho- and para-positions (-NH 2, -OH, -CH 3, -C 2 H 5, halogens); orientants of the second kind direct the entering substituents to the meta position (-NO 2, -SO 3 H, -COHO, -COOH).

The orientations are listed here in order of decreasing directional force.

It is worth noting that this division of group substituents is conditional, due to the fact that in most reactions the formation of all three isomers is observed. Orientants only influence which of the isomers will be obtained in greater quantities.

Getting arenas

The main sources of arenes are dry distillation of coal and oil refining. Coal tar contains a huge amount of all kinds of aromatic hydrocarbons. Some types of oil contain up to 60% arenes, which can be easily isolated by simple distillation, pyrolysis or cracking.

Methods of synthetic preparation and chemical properties of arenes are often interrelated. Benzene, like its homologues, is obtained in one of the following ways.

1. Reforming of petroleum products. 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 + 4 Η 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, like its homologues, is capable of forming arenes when heated with a catalyst:

3C 2 Η 2 -> C 6 Η 6.

4. Friedel-Crafts reaction. The method of obtaining and converting benzene homologues has already been discussed above in the chemical properties of arenes.

5. Preparation from the corresponding salts. Benzene can be isolated by distilling benzoic acid salts with alkali:

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

6. Reduction 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 arenas

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

Benzene C 6 × 6 is used mainly in the synthesis of ethylbenzene, cumene and cyclohexane. On its basis, intermediate products are obtained for the production of various polymers: rubbers, plastics, fibers, dyes, surfactants, insecticides, and medicines.

Toluene C 6 H 5 -CH 3 is used in the production of dyes, medicines and explosives.

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

Isopropylbenzene (or cumene) C 6 H 4 -C H (C H 3) 2 is the starting reagent for the synthesis of phenol and acetone.

Vinylbenzene (styrene) C 6 Η 5 -CΗ=CΗ 2 is the raw material for the production of the most important polymer material - polystyrene.

DEFINITION

Aromatic hydrocarbons (arenes)– substances whose molecules contain one or more benzene rings. General formula of the homologous series of benzene C n H 2 n -6

The simplest representatives of aromatic hydrocarbons are benzene - C 6 H 6 and toluene - C 6 H 5 -CH 3. Hydrocarbon radicals obtained from arenes are called: C 6 H 5 - - phenyl (Ph-) and C 6 H 5 -CH 2 - - benzyl.

All six carbon atoms in the benzene molecule are in the sp 2 hybrid state. Each carbon atom forms 3σ bonds with two other carbon atoms and one hydrogen atom, lying in the same plane. Six carbon atoms form a regular hexagon (σ-skeleton of the benzene molecule).

Each carbon atom has one unhybridized p orbital containing one electron. Six p-electrons form a single π-electron cloud (aromatic system), which is depicted as a circle inside a six-membered ring.

Chemical properties of arenes

Benzene and its homologues are characterized by substitution reactions that occur via an electrophilic mechanism:

- halogenation (benzene reacts with chlorine and bromine in the presence of catalysts - anhydrous AlCl 3, FeCl 3, AlBr 3)

C 6 H 6 + Cl 2 = C 6 H 5 -Cl + HCl

- nitration (benzene easily reacts with the nitrating mixture - a mixture of concentrated nitric and sulfuric acids)

- alkylation with alkenes

C 6 H 6 + CH 2 = CH-CH 3 → C 6 H 5 -CH(CH 3) 2

Addition reactions to benzene lead to the destruction of the aromatic system and occur only under harsh conditions:

— hydrogenation (the reaction occurs when heated, the catalyst is Pt)

- addition of chlorine (occurs under the influence of UV radiation with the formation of a solid product - hexachlorocyclohexane (hexachlorane) - C 6 H 6 Cl 6)

Physical properties of arenas

The first members of the homologous series of benzene are colorless liquids with a specific odor. They are lighter than water and practically insoluble in it. They dissolve well in organic solvents and are themselves good solvents.

Getting arenas

The main methods for obtaining benzene and its homologues:

— dehydrocyclization of alkanes (catalysts – Pt, Cr 3 O 2)

— dehydrogenation of cycloalkanes (the reaction occurs when heated, the catalyst is Pt)

— trimerization of acetylene (the reaction occurs when heated to 600C, the catalyst is activated carbon)

3HC≡CH → C6H6

- alkylation of benzenes (Friedel-Crafts reaction) (catalyst - aluminum chloride or phosphoric acid)

Examples of problem solving

EXAMPLE 1

Exercise

The vapor density of the substance is 3.482 g/l. Its pyrolysis yielded 6g of soot and 5.6l of hydrogen. Determine the formula of this substance.

Solution

Let's find the amount of soot substance (carbon): v(C) = m(C)/M(C) v(C) = 6/12 = 0.5 mol Let's find the amount of hydrogen substance: v(H 2) = V(H 2)/V m v(H 2) = 5.6/22.4 = 0.25 mol Therefore, the amount of substance of one hydrogen atom will be equal to: v(H) = 2×0.25 = 0.5 mol Let us denote the number of carbon atoms in a hydrocarbon molecule as x, and the number of hydrogen atoms as y, then the ratio of these atoms in the molecule is: x:y = 0.5: 0.5 = 1:1 The simplest formula of hydrocarbon CH The molecular weight of a hydrocarbon is: M(C x H y) = ρ×V m = 3.482×22.4 = 78 g/mol The molecular weight of a molecule of composition CH is equal to: M(CH) = 13 g/mol n = M(C x H y)/ M(CH) = 78/13 = 6, therefore, the coefficients x and y need to be multiplied by 6, then the desired hydrocarbon has the composition C 6 H 6 - this is benzene