Aromatic hydrocarbons with two isolated rings. Multi-core arenas

Aromatic hydrocarbons (arenes) are compounds containing an aromatic system, which determines their common features in structure and chemical properties.

Methods for obtaining aromatic hydrocarbons
1. Benzene, toluene, xylenes, naphthalene- isolated from coal tar formed during coal coking.
2. Some oils contain benzene and toluene.
But the main way to obtain arenes from oil is its aromatization: catalytic cyclization and dehydrogenation of alkanes. For example:

3. Obtaining alkylbenzenes (Fradel-Crafts reaction)

4. Obtaining diphenyl

Chemical properties of aromatic hydrocarbons

1. Electrophilic substitution reactions (SE)

Effect of Substituents on the Rate and Direction of ReactionsSE.
Different substituents change the electron density in the benzene ring, and it becomes different on different carbon atoms.
This changes the reaction rate SE and makes it different for different positions of the cycle.

A special position is occupied by halogen substituents:

Due to the +M-effect, they orient the reaction to the ortho- and para-positions (as substituents of the first kind), but their –I-effect exceeds the mesomeric one in absolute value: the total electron density in the cycle decreases and the rate of the SE reaction decreases.

Orientation in disubstituted benzene
1. Consistent orientation:

2. In case of inconsistent orientation, the following are taken into account:
a) the influence of a more strongly activating group:

b) spatial difficulties:

Types of electrophilic substitution reactions

1. Halogenation

2. Nitration

3. Sulfonation

Alkylation and acylation according to Friedel-Crafts

4. Alkylation

5. Acylation

2. Reactions of benzene with the destruction of the aromatic system

1.Oxidation

2. Recovery (hydrogenation)

3. Radical chlorination

3. Side chain reactions of alkylbenzenes

1. Radical substitution

Other alkylbenzenes are chlorinated at the α-position:

2. Oxidation

All monoalkylbenzenes, when oxidized with KMnO4 in an alkaline medium, give benzoic acid.

aromatic hydrocarbons- compounds of carbon and hydrogen, in the molecule of which there is a benzene ring. The most important representatives of aromatic hydrocarbons are benzene and its homologues - the products of substitution of one or more hydrogen atoms in the benzene molecule for hydrocarbon residues.

The structure of the benzene molecule

The first aromatic compound, benzene, was discovered in 1825 by M. Faraday. Its molecular formula was established - C 6 H 6. If we compare its composition with the composition of the saturated hydrocarbon containing the same number of carbon atoms - hexane (C 6 H 14), then we can see that benzene contains eight fewer hydrogen atoms. As is known, the appearance of multiple bonds and cycles leads to a decrease in the number of hydrogen atoms in a hydrocarbon molecule. In 1865, F. Kekule proposed its structural formula as cyclohexantriene-1,3,5.

Thus, the molecule corresponding to the Kekule formula contains double bonds, therefore, benzene must have an unsaturated character, i.e., it is easy to enter into addition reactions: hydrogenation, bromination, hydration, etc.

However, numerous experimental data have shown that benzene enters into addition reactions only under harsh conditions(at high temperatures and lighting), resistant to oxidation. The most characteristic of it are the substitution reactions, therefore, benzene is closer in character to saturated hydrocarbons.

Trying to explain these inconsistencies, many scientists have proposed various versions of the structure of benzene. The structure of the benzene molecule was finally confirmed by the reaction of its formation from acetylene. In fact, the carbon-carbon bonds in benzene are equivalent, and their properties are not similar to those of either single or double bonds.

Currently, benzene is denoted either by the Kekule formula, or by a hexagon in which a circle is depicted.

So what is the peculiarity of the structure of benzene?

On the basis of research data and calculations, it was concluded that all six carbon atoms are in the state of sp 2 hybridization and lie in the same plane. The unhybridized p-orbitals of carbon atoms that make up double bonds (Kekule's formula) are perpendicular to the plane of the ring and parallel to each other.

They overlap with each other, forming a single π-system. Thus, the system of alternating double bonds depicted in the Kekule formula is a cyclic system of conjugated, overlapping π-bonds. This system consists of two toroidal (donut-like) regions of electron density lying on both sides of the benzene ring. Thus, it is more logical to depict benzene as a regular hexagon with a circle in the center (π-system) than as cyclohexantriene-1,3,5.

The American scientist L. Pauling proposed to represent benzene in the form of two boundary structures that differ in the distribution of electron density and constantly transform into each other:

The measured bond lengths confirm this assumption. It was found that all C-C bonds in benzene have the same length (0.139 nm). They are somewhat shorter than single C-C bonds (0.154 nm) and longer than double ones (0.132 nm).

There are also compounds whose molecules contain several cyclic structures, for example:

Isomerism and nomenclature of aromatic hydrocarbons

For benzene homologues the isomerism of the position of several substituents is characteristic. The simplest homologue of benzene is toluene(methylbenzene) - does not have such isomers; the following homologue is presented as four isomers:

The basis of the name of an aromatic hydrocarbon with small substituents is the word benzene. Atoms in an aromatic ring are numbered starting from senior deputy to junior:

If the substituents are the same, then numbering is carried out according to the shortest path: for example, substance:

called 1,3-dimethylbenzene, not 1,5-dimethylbenzene.

According to the old nomenclature, positions 2 and 6 are called ortho positions, 4 - para-, 3 and 5 - meta positions.

Physical properties of aromatic hydrocarbons

Benzene and its simplest homologues under normal conditions - highly toxic liquids with a characteristic unpleasant odour. They are poorly soluble in water, but well - in organic solvents.

Chemical properties of aromatic hydrocarbons

substitution reactions. Aromatic hydrocarbons enter into substitution reactions.

1. Bromination. When reacting with bromine in the presence of a catalyst, iron (III) bromide, one of the hydrogen atoms in the benzene ring can be replaced by a bromine atom:

2. Nitration of benzene and its homologues. When an aromatic hydrocarbon interacts with nitric acid in the presence of sulfuric acid (a mixture of sulfuric and nitric acids is called a nitrating mixture), a hydrogen atom is replaced by a nitro group - NO 2:

Reduction of nitrobenzene is obtained aniline- a substance that is used to obtain aniline dyes:

This reaction is named after the Russian chemist Zinin.

Addition reactions. Aromatic compounds can also enter into addition reactions to the benzene ring. In this case, cyclohexane and its derivatives are formed.

1. Hydrogenation. The catalytic hydrogenation of benzene proceeds at a higher temperature than the hydrogenation of alkenes:

2. Chlorination. The reaction proceeds under illumination with ultraviolet light and is a free radical:

Chemical properties of aromatic hydrocarbons - compendium

Benzene homologues

The composition of their molecules corresponds to the formula CnH2n-6. The closest homologues of benzene are:

All benzene homologues following toluene have isomers. Isomerism can be associated both with the number and structure of the substituent (1, 2), and with the position of the substituent in the benzene ring (2, 3, 4). Compounds of the general formula C 8 H 10 :

According to the old nomenclature used to indicate the relative position of two identical or different substituents in the benzene ring, prefixes are used ortho-(abbreviated as o-) - substituents are located at neighboring carbon atoms, meta-(m-) - through one carbon atom and pair-(n-) - substituents against each other.

The first members of the homologous series of benzene are liquids with a specific odor. They are lighter than water. They are good solvents. Benzene homologues enter into substitution reactions:

bromination:

nitration:

Toluene is oxidized by permanganate when heated:

Reference material for passing the test:

periodic table

Solubility table

II.3. condensed aromatic hydrocarbons

Hückel's rule on aromaticity of the (4n+2)-electron system was derived for monocyclic systems. On polycyclic fused (i.e., containing several benzene rings with common vertices) systems, it can be transferred for systems having atoms common to two cycles, for example, for naphthalene, anthracene, phenanthrene, biphenylene shown below: (note 12)

For compounds that have at least one atom in common three cycles (for example, for pyrene), Hückel's rule not applicable.

Bicyclic annulenes - naphthalene or azulene are electronic analogues of -annulenes with ten -electrons (see section ii.2). Both of these compounds have aromatic properties, but naphthalene is colorless, and azulene is colored dark blue, since the bipolar structure, which is a combination of nuclei of cyclopentadienyl anion and tropylium cation, makes a significant contribution to its structure:

The reactivity of condensed aromatic hydrocarbons is somewhat increased compared to monocyclic arenes: they are more easily oxidized and reduced, enter into addition and substitution reactions. See Section II.5 for reasons for this difference in reactivity.

II.4. Hydrocarbons with isolated benzene rings. Triphenylmethanes.

Of the hydrocarbons with isolated benzene rings, the most interesting are di- and tri-phenylmethanes, as well as biphenyl. (Note 13) The properties of benzene rings in di- and triphenylmethanes are the same as in ordinary alkylbenzenes. Features of their chemical behavior are manifested in properties of the C-H bond of the aliphatic ("methane") part of the molecule. The ease of hetero- or homolytic rupture of this bond depends primarily on the possibility of delocalization of the emerging positive or negative charge (in the case of a heterolytic rupture) or the unpaired electron (in the case of a homolytic rupture). In the di- and especially in the tri-phenylmethane system, the possibility of such a delocalization is extremely high.

Consider first the ability of phenylated methanes to dissociation of C-H bonds with the elimination of a proton( CH-acidity ). The strength of CH-acids, as well as ordinary protic OH-acids, is determined by the stability, and hence the ease of formation, of the corresponding anions (in the case under consideration, carbanions). The stability and ease of formation of anions, in turn, are determined by the possibility of delocalization of the negative charge in them. Each benzene nucleus associated with the benzyl carbon atom can take part in the delocalization of the negative charge arising on it, which can be represented using boundary (resonant) structures:

For diphenylmethane, seven boundary structures can already be depicted:

and for triphenylmethane, ten:

Since the ability to delocalize increases with the number of possible boundary structures, di- and especially triphenylmethyl anions should be especially stable. take part in charge delocalization on the central carbon atom, i.e. rise in a row

CH 4< С 6 Н 5 СН 3 < (С 6 Н 5) 2 СН 2 < (С 6 Н 5) 3 СН

p-values K a of these hydrocarbons, determined by special methods, confirm this assumption. Diphenylmethane (p K a 33) is approximately equal in acidity to ammonia, and triphenylmethane (p K a 31.5) - tert- butanol; triphenylmethane more than 10 10 times acidic than methane (p K a~ 40).(note 15)

Cherry-colored triphenylmethylsodium is usually prepared by reducing triphenylchloromethane with sodium amalgam:

Unlike conventional CH bonds sp 3-hybrid carbon atom, benzyl C-H bond tri- pair- nitrophenylmethane is already heterolytically cleaved with alcohol alkali:

In the latter case, in addition to three benzene nuclei, three nitro groups additionally participate in the delocalization of the negative charge in the anion.

Another type of heterolytic cleavage of the benzyl CH bond is the abstraction of the hydride anion with the formation of the corresponding carbocations benzyl type:

Since benzene nuclei are capable of stabilizing both positive and negative charges, phenylated methanes on hydride mobility hydrogen in the aliphatic part will be the same row as by proton mobility, i.e. CH 4< С 6 Н 5 СН 3 < (С 6 Н 5) 2 СН 2 < (С 6 Н 5) 3 СН.

However, it is usually difficult to experimentally compare the ease of abstraction of the hydride anion, since very active Lewis acids are usually used to carry out such abstraction. Comparative estimates can easily be made by comparing the mobility of a halogen (usually chlorine) under conditions S N 1 reactions, since in this case, as in the case of the elimination of the hydride anion, the stage that determines the rate of transformation is the formation of the corresponding carbocation. Indeed, it turned out that under these conditions, chlorine has the highest mobility in triphenylchloromethane, and the lowest in benzyl chloride:

Ar-CR 2 -Cl ArCR 2 + + Cl - ; R=H or R=Ar

reaction rate: (C 6 H 5) 3 C-Cl > (C 6 H 5) 2 CH-Cl > C 6 H 5 CH 2 -Cl

The reactivity of chlorine in the first of them resembles that in carboxylic acid chlorides, and in the second - in allyl chloride. Below are data on the relative rates of solvolysis of R-Cl chlorides in formic acid at 25 o C:

R-Cl + HCOOH R-O-C(O)H + HCl

Comparative stability of triphenylmethyl ( trityl ) of the cation is also confirmed by many other experimental data. An example is the ease of formation of its salts with non-nucleophilic anions, solutions of which in polar aprotic solvents are electrically conductive (and, therefore, have an ionic structure) and are characteristically colored yellow:

The same is evidenced by the ability of triphenylchloromethane to dissociate into triphenylmethyl cation and chloride anion in a solution of liquid sulfur dioxide:

The stability of the triphenylmethyl cation can be further increased by introducing into benzene rings electron donor groups(for example, amino-, alkyl- and dialkylamino-, hydroxyl, alkoxy). A further increase in the stability of the carbocation leads to a situation where it becomes stable in aqueous solution, that is, the equilibrium of the reaction

shifted to the left. Similar trityl cations not only resistant, but also painted. An example is the intensely purple tri(4-dimethylaminophenyl)methyl cation. Its chloride is used as a dye called " crystal violet ". In crystal violet, the positive charge is dispersed between the three nitrogen atoms and the nine carbon atoms of the benzene nuclei. Participation of one of the three pair-dimethylaminophenyl substituents in the positive charge delocalization can be reflected using the following boundary structures:

All triphenylmethane dyes containing amine or substituted amine groups in the benzene ring acquire a color in an acid medium, which, as shown above with the example of crystal violet, contributes to the formation of a structure with an extended conjugation chain (structure I in the diagram) - the so-called quinoid structure . Below are the formulas for the most common triphenylmethane dyes.

Similar to that considered above for the triphenylmethyl anion and cation, the benzene rings should also have an effect on the stability triphenylmethyl radical . In the latter case, the ease of breaking the bond formed by the central carbon atom with a "non-phenyl" substituent is due, to a certain extent, to other reasons. The fact is that in triphenylmethane, triphenylchloromethane, triphenylcarbinol, etc. the central carbon atom is in sp 3-hybrid state and, accordingly, has a tetrahedral configuration. For this reason, the phenyl nuclei are not located in the same plane and not conjugated. When passing to a triphenylmethyl cation (heterolytic gap) or a radical (homolytic gap), the central carbon atom is in sp 2- hybrid state; as a result of this, the structure is flattened (note 17) and the interaction (conjugation) between the three phenyl nuclei is enhanced. This partially compensates for the energy costs associated with the dissociation under consideration, and thus facilitates it.

Triphenylmethyl radical

can be generated from the corresponding chloride by the action of zinc, copper or silver, which in this case act as electron donors:

This radical is quite stable and dimerizes only partially in dilute solutions (in ether, benzene). For a long time, the structure of hexaphenylethylene was attributed to this dimer, but it turned out that, in fact, during dimerization, a bond arises between the central carbon atom of one radical and pair- the position of one of the phenyl nuclei of the other radical:

Apparently, in the case under consideration, one triphenylmethyl radical attacks least spatially obstructed place another, and, naturally, one of those places that participates in the delocalization of the unpaired electron.

The degree of dissociation of such dimers strongly depends on the nature of the aryl radicals. Thus, in a 0.1 M benzene solution at 25°, the triphenylmethyl radical dimerizes by 97%, while the tri-4-nitrophenylmethyl radical does not dimerize at all.

S.Yu. Eliseev

The concept of aromatic hydrocarbons, their application, physico-chemical and fire-explosive properties.

Modern understanding of the structure of the benzene molecule. Homologous series of benzene, nomenclature, isomerism. Arene toxicity.

Basic chemical reactions:

substitutions (halogenation, nitration, sulfonation, alkylation)

additions (hydrogen and halogens);

oxidation (incomplete oxidation, features of the combustion process, tendency to spontaneous combustion upon contact with strong oxidizing agents);

Rules of substitution in the benzene ring. Deputy first and second row.

Industrial methods for obtaining aromatic hydrocarbons.

Brief description of the main aromatic hydrocarbons: toluene, benzene, xylene, ethylbenzene, isopropylbenzene, styrene, etc.

Aromatic nitro compounds, physicochemical and fire hazardous properties of nitrobenzene, toluene. Reactions to receive them.

Aromatic amines: nomenclature, isomerism, production methods, individual representatives (aniline, diphenylamine, dimethylaniline).

Aromatic hydrocarbons (arenes)

Aromatic compounds are usually called carbocyclic compounds, in the molecules of which there is a special cyclic group of six carbon atoms - the benzene ring. The simplest substance containing such a group is the hydrocarbon benzene; all other aromatic compounds of this type are considered to be derivatives of benzene.

Due to the presence of a benzene ring in aromatic compounds, they differ significantly in some properties from saturated and unsaturated alicyclic compounds, as well as from compounds with an open chain. The distinctive properties of aromatic substances, due to the presence of a benzene nucleus in them, are usually called aromatic properties, and the benzene nucleus, respectively, the aromatic nucleus.

It should be noted that the very name "aromatic compounds" no longer has its original direct meaning. This was the name of the first studied benzene derivatives, because they had an aroma or were isolated from natural aromatic substances. At present, aromatic compounds include many substances that have both unpleasant odors or no smell at all if their molecule contains a flat ring with (4n + 2) generalized electrons, where n can take on the values ​​0, 1, 2, 3, etc. .d., is Hückel's rule.

Aromatic hydrocarbons of the benzene series.

The first representative of aromatic hydrocarbons - benzene - has the composition C6H6. This substance was discovered by M. Faraday in 1825 in a liquid formed during compression or cooling of the so-called. lighting gas, which is obtained during the dry distillation of coal. Subsequently, benzene was discovered (A. Hoffman, 1845) in another product of the dry distillation of coal - in coal tar. It turned out to be a very valuable substance and found wide application. Then it was found that very many organic compounds are derivatives of benzene.

The structure of benzene.

For a long time the question of the chemical nature and structure of benzene remained unclear. It would seem that it is a strongly unsaturated compound. After all, its composition C6H6 according to the ratio of carbon and hydrogen atoms corresponds to the formula CnH2n-6, while the hexane corresponding to the number of carbon atoms has the composition C6H14 and corresponds to the formula CnH2n+2. However, benzene does not give reactions characteristic of unsaturated compounds; for example, it does not provide bromine water and KMnO4 solution; under normal conditions, it is not prone to addition reactions, it does not oxidize. On the contrary, benzene in the presence of catalysts enters into substitution reactions characteristic of saturated hydrocarbons, for example, with halogens:

C6H6 + Cl2 ® C6H5Cl + HCl

It turned out, however, that under certain conditions, benzene can also enter into addition reactions. There, in the presence of catalysts, it is hydrogenated, adding 6 hydrogen atoms:

C6H6 + 3H2 ® C6H12

Under the action of light, benzene slowly adds 6 halogen atoms:

C6H6 + 3Cl2 ® C6H6Cl6

Some other addition reactions are also possible, but they all proceed with difficulty, many times less actively than addition to double bonds in substances with an open goal or in alicyclic compounds.

Further, it was found that monosubstituted derivatives of benzene C6H5X do not have isomers. This showed that all hydrogen and all carbon atoms in its molecule are equivalent in their position, which also did not find an explanation for a long time.

For the first time, the formula for the structure of benzene was proposed in 1865. German chemist August Kekule. He suggested that the 6 carbon atoms in benzene form a cycle, connecting to each other by alternating single and double bonds, and, in addition, each of them is connected to one hydrogen atom: CH CH CH CH CH Kekule suggested that the double bonds in benzene not motionless; according to him, they continuously move (oscillate) in the ring, which can be represented by the scheme: CH (I) CH (II) Formulas I and II, according to Kekule, CH CH CH CH are completely equivalent and only ½½<=>½½ express 2 mutually passing CH CH CH CH CH phases of the compound of the benzene molecule. CH CH

Kekule came to this conclusion on the basis that if the position of double bonds in benzene was fixed, then its disubstituted derivatives C6H4X2 with substituents at neighboring carbons should exist in the form of isomers at the position of single and double bonds:

½ (III) ½ (IV)

C C

NS S-X NS S-X

½½½<=>½½½

The Kekule formula has become widespread. It is consistent with the concept of tetravalent carbon, explains the equivalence of hydrogen atoms in benzene. The presence of a six-membered cycle in the latter has been proven; in particular, it is confirmed by the fact that during hydrogenation benzene forms cyclohexane, in turn, cyclohexane turns into benzene by dehydrogenation.

However, the Kekule formula has significant drawbacks. Assuming that there are three double bonds in benzene, she cannot explain why benzene in this case hardly enters into addition reactions, is resistant to the action of oxidizing agents, i.e. does not show the properties of unsaturated compounds.

The study of benzene using the latest methods indicates that in its molecule there are neither ordinary single nor ordinary double bonds between carbon atoms. For example, the study of aromatic compounds using X-rays showed that 6 carbon atoms in benzene, which form a cycle, lie in the same plane at the vertices of a regular hexagon and their centers are at equal distances from each other, constituting 1.40 A. These distances are smaller, than the distances between the centers of carbon atoms connected by a single bond (1.54 A), and more than m. connected by a double bond (1.34 A). Thus, in benzene, carbon atoms are connected using special, equivalent bonds, which were called aromatic bonds. By their nature, they differ from double and single bonds; their presence determines the characteristic properties of benzene. From the point of view of modern electronic concepts, the nature of aromatic bonds is explained as follows.

AROMATIC COMPOUNDS

AROMATIC HYDROCARBONS (ARENES)

Typical representatives of aromatic hydrocarbons are benzene derivatives, i.e. such carbocyclic compounds, in the molecules of which there is a special cyclic group of six carbon atoms, called the benzene or aromatic ring.

The general formula for aromatic hydrocarbons is C n H 2 n -6 .

The structure of benzene

To study the structure of benzene, you need to watch the animated film "The structure of benzene" (This video is available only on CD-ROM). The text accompanying this film has been moved in its entirety to this subsection and follows below.

"In 1825, the English researcher Michael Faraday, during the thermal decomposition of blubber, isolated an odorous substance that had the molecular formula C 6 H 6. This compound, now called benzene, is the simplest aromatic hydrocarbon.

The common structural formula for benzene, proposed in 1865 by the German scientist Kekule, is a cycle with alternating double and single bonds between carbon atoms:

However, physical, chemical, as well as quantum mechanical studies have established that there are no ordinary double and single carbon–carbon bonds in the benzene molecule. All these links in it are equal, equivalent, i.e. are, as it were, intermediate "one-and-a-half" bonds, characteristic only of the benzene aromatic nucleus. It turned out, moreover, that in the benzene molecule all carbon and hydrogen atoms lie in the same plane, and the carbon atoms are located at the vertices of a regular hexagon with the same bond length between them, equal to 0.139 nm, and all bond angles are equal to 120 °. This arrangement of the carbon skeleton is due to the fact that all carbon atoms in the benzene ring have the same electron density and are in the state of sp 2 - hybridization. This means that each carbon atom has one s and two p orbitals hybridized, and one p orbital is non-hybrid. Three hybrid orbitals overlap: two of them with the same orbitals of two adjacent carbon atoms, and the third one with the s-orbital of the hydrogen atom. Similar overlaps of the corresponding orbitals are observed for all carbon atoms of the benzene ring, resulting in the formation of twelve s-bonds located in the same plane.

The fourth non-hybrid dumbbell-shaped p-orbital of carbon atoms is located perpendicular to the plane of direction of -bonds. It consists of two identical shares, one of which lies above and the other below the mentioned plane. Each p orbital is occupied by one electron. The p-orbital of one carbon atom overlaps with the p-orbital of the neighboring carbon atom, which leads, as in the case of ethylene, to the pairing of electrons and the formation of an additional -bond. However, in the case of benzene, the overlap is not limited to just two orbitals, as in ethylene: the p-orbital of each carbon atom overlaps equally with the p-orbitals of the two adjacent carbon atoms. As a result, two continuous electron clouds are formed in the form of tori, one of which lies above and the other below the plane of atoms (a torus is a spatial figure that has the shape of a donut or a lifebuoy). In other words, six p-electrons, interacting with each other, form a single -electron cloud, which is depicted by a circle inside a six-membered cycle:

From a theoretical point of view, only those cyclic compounds that have a planar structure and contain (4n + 2) -electrons in a closed conjugation system, where n is an integer, can be called aromatic compounds. These criteria for aromaticity, known as Hückel's rules, fully meets benzene. Its number of six -electrons is the Hückel number for n=1, in connection with which, the six -electrons of the benzene molecule are called an aromatic sextet.

An example of aromatic systems with 10 and 14 -electrons are representatives of polynuclear aromatic compounds -
naphthalene and
anthracene .

isomerism

The theory of structure allows the existence of only one compound with the formula benzene (C 6 H 6) and also only one closest homologue - toluene (C 7 H 8). However, subsequent homologues may already exist as several isomers. Isomerism is due to the isomerism of the carbon skeleton of the existing radicals and their mutual position in the benzene ring. The position of two substituents is indicated using prefixes: ortho- (o-), if they are located on adjacent carbon atoms (position 1, 2-), meta- (m-) for separated by one carbon atom (1, 3-) and para- (p-) for those opposite each other (1, 4-).

For example, for dimethylbenzene (xylene):

ortho-xylene (1,2-dimethylbenzene)

meta-xylene (1,3-dimethylbenzene)

para-xylene (1,4-dimethylbenzene)

Receipt

The following methods for producing aromatic hydrocarbons are known.

1. 
   Catalytic dehydrocyclization of alkanes, i.e. elimination of hydrogen with simultaneous cyclization (the method of B.A. Kazansky and A.F. Plate). The reaction is carried out at elevated temperature using a catalyst such as chromium oxide.

1. 
   Catalytic dehydrogenation of cyclohexane and its derivatives (N.D. Zelinsky). Palladium black or platinum at 300°C is used as a catalyst.

1. 
   Cyclic trimerization of acetylene and its homologues over activated carbon at 600°C (N.D. Zelinskii).

1. 
   Fusion of salts of aromatic acids with alkali or soda lime.

1. 
   Alkylation of benzene proper with halogen derivatives (Friedel-Crafts reaction) or olefins.

^

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, but they easily dissolve in organic solvents - alcohol, ether, acetone.

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	d4 20
Benzene	C 6 H 6	+5,5	80,1	0,8790
Toluene (methylbenzene)	C 6 H 5 CH 3	-95,0	110,6	0,8669
Ethylbenzene	C 6 H 5 C 2 H 5	-95,0	136,2	0,8670
Xylene (dimethylbenzene)	C 6 H 4 (CH 3) 2
ortho-		-25,18	144,41	0,8802
meta-		-47,87	139,10	0,8642
pair-		13,26	138,35	0,8611
Propylbenzene	C 6 H 5 (CH 2) 2 CH 3	-99,0	159,20	0,8610
Cumene (isopropylbenzene)	C 6 H 5 CH(CH 3) 2	-96,0	152,39	0,8618
Styrene (vinylbenzene)	C 6 H 5 CH \u003d CH 2	-30,6	145,2	0,9060

^

Chemical properties

The benzene core has a high strength, which explains the tendency of aromatic hydrocarbons to substitution reactions. Unlike alkanes, which are also prone to substitution reactions, aromatic hydrocarbons are characterized by high mobility of hydrogen atoms in the nucleus, so the reactions of halogenation, nitration, sulfonation, etc. proceed under much milder conditions than alkanes.

^

Electrophilic substitution in benzene

Despite the fact that benzene is an unsaturated compound in composition, addition reactions are not characteristic of it. Typical reactions of the benzene ring are hydrogen substitution reactions - more precisely, electrophilic substitution reactions.

Let us consider examples of the most characteristic reactions of this type.

1. 
   Halogenation. When benzene reacts with a halogen (in this case, chlorine), the hydrogen atom of the nucleus is replaced by a halogen.

Cl 2 - AlCl 3  (chlorobenzene) + H 2 O

Halogenation reactions are carried out in the presence of a catalyst, which is most often aluminum or iron chlorides.

1. 
   Nitration. When a nitrating mixture acts on benzene, a hydrogen atom is replaced by a nitro group (a nitrating mixture is a mixture of concentrated nitric and sulfuric acids in a ratio of 1: 2, respectively).

HNO 3 - H 2 SO 4  (nitrobenzene) + H 2 O

Sulfuric acid in this reaction plays the role of a catalyst and a water-removing agent.

1. 
   Sulfonation. The sulfonation reaction is carried out with concentrated sulfuric acid or oleum (oleum is a solution of sulfuric anhydride in anhydrous sulfuric acid). During the reaction, the hydrogen atom is replaced by a sulfo group, resulting in a monosulfonic acid.

H 2 SO 4 - SO 3  (benzenesulfonic acid) + H 2 O

1. 
   Alkylation (Friedel-Crafts reaction). When benzene is treated with alkyl halides in the presence of a catalyst (aluminum chloride), the hydrogen atom of the benzene ring is replaced by alkyl.

R–Cl - AlCl 3  (R-hydrocarbon radical) + HCl

It should be noted that the alkylation reaction is a general method for obtaining benzene homologues - alkylbenzenes.

Let us consider the mechanism of the electrophilic substitution reaction in the benzene series using the chlorination reaction as an example.
The primary step is the generation of an electrophilic particle. It is formed as a result of heterolytic cleavage of a covalent bond in a halogen molecule under the action of a catalyst and is a chloride cation.

+ AlCl 3  Cl + + AlCl 4 -

The resulting electrophilic particle attacks the benzene ring, leading to the rapid formation of an unstable -complex, in which the electrophilic particle is attracted to the electron cloud of the benzene ring.

In other words, the -complex is a simple electrostatic interaction between the electrophile and the -electron cloud of the aromatic nucleus.
Then the -complex passes into the -complex, the formation of which is the most important stage of the reaction. The electrophilic particle "captures" two electrons of the -electronic sextet and forms a -bond with one of the carbon atoms of the benzene ring.

-Complex is a cation devoid of aromatic structure, with four -electrons delocalized (in other words, distributed) in the sphere of action of the nuclei of five carbon atoms. The sixth carbon atom changes the hybrid state of its electron shell from sp 2 - to sp 3 -, leaves the plane of the ring and acquires tetrahedral symmetry. Both substituents - hydrogen and chlorine atoms are located in a plane perpendicular to the plane of the ring.
At the final stage of the reaction, a proton is split off from the -complex and the aromatic system is restored, since the pair of electrons missing from the aromatic sextet returns to the benzene nucleus.

+H+

The split off proton binds to the aluminum tetrachloride anion to form hydrogen chloride and regenerate aluminum chloride.

H + + AlCl 4 -  HCl + AlCl 3

It is due to this regeneration of aluminum chloride that a very small (catalytic) amount of it is needed to start the reaction.

Despite the tendency of benzene to substitution reactions, it also enters into addition reactions under harsh conditions.

1. 
   Hydrogenation. Hydrogen addition is carried out only in the presence of catalysts and at elevated temperatures. Benzene is hydrogenated to form cyclohexane, and benzene derivatives give cyclohexane derivatives.

3H 2 - t , p, Ni  (cyclohexane)

1. 
   In sunlight, under the influence of ultraviolet radiation, benzene adds chlorine and bromine to form hexahalides, which, when heated, lose three molecules of hydrogen halide and lead to trihalogenbenzenes.

1. 
   Oxidation. The benzene nucleus is more resistant to oxidation than alkanes. Even potassium permanganate, nitric acid, hydrogen peroxide do not act on benzene under normal conditions. Under the action of oxidizing agents on benzene homologues, the carbon atom of the side chain closest to the nucleus is oxidized to a carboxyl group and gives an aromatic acid.

2KMnO 4  (potassium salt of benzoic acid) + 2MnO 2 + KOH + H 2 O

4KMnO 4  + K 2 CO 3 + 4MnO 2 + 2H 2 O + KOH

In all cases, as can be seen, regardless of the length of the side chain, benzoic acid is formed.

If there are several substituents in the benzene ring, all existing chains can be oxidized sequentially. This reaction is used to determine the structure of aromatic hydrocarbons.

– [ O ]  (terephthalic acid)

^

Orientation rules in the benzene nucleus

Like benzene itself, benzene homologues also undergo electrophilic substitution reactions. However, an essential feature of these reactions is that new substituents enter the benzene ring in certain positions relative to the existing substituents. In other words, each substituent of the benzene nucleus has a certain guiding (or orienting) action. The patterns that determine the direction of substitution reactions in the benzene nucleus are called orientation rules.

All substituents are divided into two groups according to the nature of their orienting action.

Substituents of the first kind (or ortho-para-orientants) are atoms or groups of atoms capable of donating electrons (electron donor). These include hydrocarbon radicals, –OH and –NH 2 groups, and halogens. The listed substituents (except for halogens) increase the activity of the benzene ring. Substituents of the first kind orient the new substituent predominantly in the ortho and para positions.

2 + 2H 2 SO 4  (o-toluenesulfonic acid) + (p-toluenesulfonic acid) + 2H 2 O

2 + 2Cl 2 - AlCl 3  (o-chlorotoluene) + (p-chlorotoluene) + 2HCl

Considering the last reaction, it should be noted that in the absence of catalysts in the light or on heating (ie, under the same conditions as for alkanes), halogen can be introduced into the side chain. The mechanism of the substitution reaction in this case is radical.

Cl 2 - h   (benzyl chloride) + HCl

Substituents of the second kind (meta-orienting agents) are electron-withdrawing groups capable of withdrawing and accepting electrons from the benzene nucleus. These include:
–NO 2 , –COOH, –CHO, –COR, –SO 3 H.

Substituents of the second kind reduce the activity of the benzene ring, they direct the new substituent to the meta position.

HNO 3 - H 2 SO 4  (m-dinitrobenzene) + H 2 O

HNO 3 - H 2 SO 4  (m-nitrobenzoic acid) + H 2 O

Application

Aromatic hydrocarbons are an important raw material for the production of various synthetic materials, dyes, and physiologically active substances. So, benzene is a product for the production of dyes, medicines, plant protection products, etc. Toluene is used as a raw material in the production of explosives, pharmaceuticals, and also as a solvent. Vinylbenzene (styrene) is used to produce a polymeric material - polystyrene.