Arenas - nomenclature, preparation, chemical properties. Aromatic compounds Chemistry of aromatic compounds

Secondary education Aromatic hydrocarbons

- compounds of carbon and hydrogen, the molecule of which contains a benzene ring. The most important representatives of aromatic hydrocarbons are benzene and its homologues - products of the replacement of one or more hydrogen atoms in a benzene molecule with 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 - C6H6

. If we compare its composition with the composition of a saturated hydrocarbon containing the same number of carbon atoms - hexane (C 6 H 14), then we can see that benzene contains eight less 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 cyclohexanthriene-1,3,5. Thus, a molecule corresponding to the Kekulé formula contains double bonds, therefore, benzene must be unsaturated, i.e., easily undergo addition reactions:

hydrogenation, bromination, hydration, etc. However, data from numerous experiments have shown that benzene undergoes addition reactions only under harsh conditions (at high temperatures and lighting),. resistant to oxidation The most characteristic reactions for it are substitution reactions

Therefore, benzene is closer in character to saturated hydrocarbons.

Trying to explain these discrepancies, many scientists have proposed various options for the structure of benzene. The structure of the benzene molecule was finally confirmed by the reaction of its formation from acetylene. In reality, 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.

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

They overlap each other, forming a single π-system. Thus, the system of alternating double bonds depicted in Kekulé’s formula is a cyclic system of conjugated, overlapping π bonds. This system consists of two toroidal (donut-like) regions of electron density lying on either side 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 cyclohexanthriene-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:

Bond length measurements confirm this assumption. It was found that all C-C bonds in benzene have the same length (0.139 nm). They are slightly shorter than single C-C bonds (0.154 nm) and longer than double bonds (0.132 nm).

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

Isomerism and nomenclature of aromatic hydrocarbons

For benzene homologues isomerism of the position of several substituents is characteristic. The simplest homolog of benzene is toluene(methylbenzene) - has no 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. The atoms in the aromatic ring are numbered, starting from senior deputy to junior:

If the substituents are the same, then numbering is carried out along 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 orthopositions, 4 - para-positions, 3 and 5 - meta-positions.

Physical properties of aromatic hydrocarbons

Benzene and its simplest homologues under normal conditions - very toxic liquids with a characteristic unpleasant odor. They dissolve poorly in water, but well in organic solvents.

Chemical properties of aromatic hydrocarbons

Substitution reactions. Aromatic hydrocarbons undergo 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), the hydrogen atom is replaced by a nitro group - NO 2:

By reducing nitrobenzene we obtain 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 undergo addition reactions to the benzene ring. In this case, cyclohexane and its derivatives are formed.

1. Hydrogenation. Catalytic hydrogenation of benzene occurs at a higher temperature than the hydrogenation of alkenes:

2. Chlorination. The reaction occurs when illuminated with ultraviolet light and is free radical:

Chemical properties of aromatic hydrocarbons - summary

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 location of two identical or different substituents on the benzene ring, the prefixes are used ortho-(abbreviated o-) - substituents are located on neighboring carbon atoms, meta-(m-) - through one carbon atom and pair-(n-) - substituents opposite 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 undergo substitution reactions:

bromination:

nitration:

Toluene is oxidized by permanganate when heated:

Reference material for taking the test:

Mendeleev table

Solubility table

Cyclic conjugated systems are of great interest as a group of compounds with increased thermodynamic stability compared to conjugated open systems. These compounds also have other special properties, the totality of which is united by the general concept aromaticity. These include the ability of such formally unsaturated compounds to undergo substitution rather than addition reactions, resistance to oxidizing agents and temperature.

Typical representatives of aromatic systems are arenes and their derivatives. The features of the electronic structure of aromatic hydrocarbons are clearly manifested in the atomic orbital model of the benzene molecule. The benzene framework is formed by six sp 2 -hybridized carbon atoms. All σ bonds (C-C and C-H) lie in the same plane. Six unhybridized p-AOs are located perpendicular to the plane of the molecule and parallel to each other (Fig. 3a). Each R-AO can equally overlap with two neighboring R-AO. As a result of such overlap, a single delocalized π-system arises, the highest electron density in which is located above and below the plane of the σ-skeleton and covers all carbon atoms of the cycle (see Fig. 3, b). The π-Electron density is evenly distributed throughout the cyclic system, which is indicated by a circle or dotted line inside the cycle (see Fig. 3, c). All bonds between carbon atoms in the benzene ring have the same length (0.139 nm), intermediate between the lengths of single and double bonds.

Based on quantum mechanical calculations, it was established that for the formation of such stable molecules, a flat cyclic system must contain (4n + 2) π electrons, where n= 1, 2, 3, etc. (Hückel's rule, 1931). Taking these data into account, the concept of “aromaticity” can be specified.

Aroma systems (molecules)– systems that meet aromaticity criteria :

1) the presence of a flat σ-skeleton consisting of sp 2 -hybridized atoms;

2) delocalization of electrons, leading to the formation of a single π-electron cloud covering all atoms of the cycle (cycles);

3) compliance with E. Hückel’s rule, i.e. the electron cloud should contain 4n+2 π-electrons, where n=1,2,3,4... (usually the number indicates the number of cycles in the molecule);

4) high degree of thermodynamic stability (high conjugation energy).

Rice. 3. Atomic orbital model of the benzene molecule (hydrogen atoms omitted; explanation in text)

Stability of coupled systems. The formation of a conjugated and especially aromatic system is an energetically favorable process, since this increases the degree of overlap of orbitals and delocalization (dispersal) occurs. R-electrons. In this regard, conjugated and aromatic systems have increased thermodynamic stability. They contain a smaller supply of internal energy and in the ground state occupy a lower energy level compared to non-conjugated systems. From the difference between these levels, one can quantify the thermodynamic stability of the conjugated compound, i.e., its conjugation energy (delocalization energy). For butadiene-1,3 it is small and amounts to about 15 kJ/mol. As the length of the conjugated chain increases, the conjugation energy and, accordingly, the thermodynamic stability of the compounds increase. The conjugation energy for benzene is much higher and amounts to 150 kJ/mol.

Examples of non-benzenoid aromatic compounds:

Pyridine Its electronic structure resembles benzene. All carbon atoms and the nitrogen atom are in a state of sp 2 hybridization, and all σ bonds (C-C, C-N and C-H) lie in the same plane (Fig. 4, a). Of the three hybrid orbitals of the nitrogen atom, two are involved in the formation

Rice. 4. Pyridine nitrogen atom (A), (b) and the conjugated system in the pyridine molecule (c) (C-H bonds are omitted to simplify the figure)

σ bonds with carbon atoms (only the axes of these orbitals are shown), and the third orbital contains a lone pair of electrons and is not involved in the formation of the bond. A nitrogen atom with this electronic configuration is called pyridine.

Due to the electron located in the unhybridized p-orbital (see Fig. 4, b), the nitrogen atom participates in the formation of a single electron cloud with R-electrons of five carbon atoms (see Fig. 4, c). Thus, pyridine is a π,π-conjugated system and satisfies the criteria for aromaticity.

As a result of greater electronegativity compared to the carbon atom, the pyridine nitrogen atom lowers the electron density on the carbon atoms of the aromatic ring, therefore systems with a pyridine nitrogen atom are called π-insufficient. In addition to pyridine, an example of such systems is pyrimidine, containing two pyridine nitrogen atoms.

Pyrrole also refers to aromatic compounds. The carbon and nitrogen atoms in it, as in pyridine, are in a state of sp2 hybridization. However, unlike pyridine, the nitrogen atom in pyrrole has a different electronic configuration (Fig. 5, a, b).

Rice. 5. Pyrrole nitrogen atom (A), distribution of electrons among orbitals (b) and the conjugated system in the pyrrole molecule (c) (C-H bonds are omitted to simplify the figure)

On unhybridized R The -orbital of the nitrogen atom contains a lone pair of electrons. She is involved in pairing with R-electrons of four carbon atoms to form a single six-electron cloud (see Fig. 5, c). Three sp 2 hybrid orbitals form three σ bonds - two with carbon atoms, one with a hydrogen atom. The nitrogen atom in this electronic state is called pyrrole.

Six-electron cloud in pyrrole thanks to p,p-conjugation is delocalized on five ring atoms, so pyrrole is π-excess system.

IN furane And thiophene the aromatic sextet also includes a lone pair of electrons from the unhybridized p-AO of oxygen or sulfur, respectively. IN imidazole And pyrazole The two nitrogen atoms make different contributions to the formation of a delocalized electron cloud: the pyrrole nitrogen atom supplies a pair of π electrons, and the pyridine nitrogen atom supplies one p electron.

It also has aromatic properties purine, representing a condensed system of two heterocycles - pyrimidine and imidazole.

The delocalized electron cloud in purine includes 8 π double bond electrons and a lone pair of electrons from the N=9 atom. The total number of electrons in conjugation, equal to ten, corresponds to the Hückel formula (4n + 2, where n = 2).

Heterocyclic aromatic compounds have high thermodynamic stability. It is not surprising that they serve as structural units of the most important biopolymers - nucleic acids.

AROMATIC HYDROCARBONS (ARENES)

Typical representatives of aromatic hydrocarbons are benzene derivatives, i.e. These are carbocyclic compounds whose molecules contain a special cyclic group of six carbon atoms, called a benzene or aromatic ring.
The general formula of aromatic hydrocarbons is CnH2n-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 only available on CD-ROM). The text accompanying this film has been transferred in full 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 C6H6. This compound, now called benzene, is the simplest aromatic hydrocarbon.
The common structural formula of benzene, proposed in 1865 by the German scientist Kekule, is a cycle with alternating double and single bonds between carbon atoms:

However, physical, chemical, and quantum mechanical studies have established that the benzene molecule does not contain the usual double and single carbon-carbon bonds. All these connections in it are equivalent, equivalent, i.e. are, as it were, intermediate “one and a half” bonds, characteristic only of the benzene aromatic ring. It turned out, in addition, that in a 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 a state of sp2 hybridization. This means that each carbon atom has one s and two p orbitals that are hybridized, and one p orbital that is nonhybridized. Three hybrid orbitals overlap: two of them with the same orbitals of two adjacent carbon atoms, and the third with the s orbital of a hydrogen atom. Similar overlaps of the corresponding orbitals are observed on 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 s-bond direction. It consists of two identical lobes, 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 pairing of electrons and the formation of an additional p-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 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 shaped like a donut or a lifebuoy). In other words, six p-electrons, interacting with each other, form a single p-electron cloud, which is represented 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) p-electrons in a closed conjugation system, where n is an integer, can be called aromatic compounds. Benzene fully meets the above criteria for aromaticity, known as Hückel’s rule. Its number of six p-electrons is the Hückel number for n=1, and therefore, the six p-electrons of the benzene molecule are called an aromatic sextet."
An example of aromatic systems with 10 and 14 p-electrons are representatives of polynuclear aromatic compounds -
naphthalene and
anthracene.

Isomerism

The theory of structure allows for the existence of only one compound with the formula benzene (C6H6) and also only one closest homologue - toluene (C7H8). However, subsequent homologs may already exist in the form of several isomers. Isomerism is due to the isomerism of the carbon skeleton of the existing radicals and their relative position in the benzene ring. The position of two substituents is indicated using prefixes: ortho- (o-), if they are located at adjacent carbon atoms (position 1, 2-), meta- (m-) for those separated by one carbon atom (1, 3-) and para- (n-) 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 (method of B.A. Kazansky and A.F. Plate). The reaction is carried out at elevated temperature using a catalyst such as chromium oxide.

heptane--500°C® + 4H2 toluene

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

cyclohexane --300°C,Pd®+ 3H2

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

3НCєСН--600°C®

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

NaOH--t°®+ Na2CO3

5) Alkylation of benzene itself 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 arenas are presented in the table.

Table. Physical properties of some arenas

Name	Formula	t°.pl., °C	t°.b.p., °C	d 4 20
Benzene	C6H6		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)	C6H5CH(CH3)2	96,0	152,39	0,8618
Styrene (vinylbenzene)	C 6 H 5 CH=CH 2	30,6	145,2	0,9060

Chemical properties

The benzene ring is highly durable, which explains the tendency of aromatic hydrocarbons to undergo substitution reactions. Unlike alkanes, which are also prone to substitution reactions, aromatic hydrocarbons are characterized by high mobility of hydrogen atoms in the nucleus, therefore the reactions of halogenation, nitration, sulfonation, etc. occur under much milder conditions than for alkanes.

Electrophilic substitution in benzene

Despite the fact that benzene is an unsaturated compound in composition, addition reactions are not typical for it. Typical reactions of the benzene ring are substitution reactions of hydrogen atoms - more precisely, electrophilic substitution reactions.
Let's look at examples of the most typical 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.

Cl2 -AlCl3® (chlorobenzene) + H2O

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

2) Nitration. When benzene is exposed to a nitrating mixture, the 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).

HNO3 -H2SO4® (nitrobenzene) + H2O

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

3) 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 sulfonic acid group, resulting in a monosulfonic acid.

H2SO4 -SO3® (benzenesulfonic acid) + H2O

4) Alkylation (Friedel-Crafts reaction). When benzene is exposed to alkyl halides in the presence of a catalyst (aluminum chloride), alkyl replaces the hydrogen atom of the benzene ring.

R-Cl -AlCl3® (R-hydrocarbon radical) + HCl

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

Let us consider the mechanism of the electrophilic substitution reaction in the benzene series using the example of the chlorination reaction.
The primary step is the generation of an electrophilic species. 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.

AlCl3 ® Cl+ + AlCl4-

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

P-complex

In other words, a p-complex is a simple electrostatic interaction between an electrophile and the p-electron cloud of an aromatic nucleus.
Next, the transition of the p-complex to the s-complex occurs, the formation of which is the most important stage of the reaction. The electrophilic particle “captures” two electrons of the s-electron sextet and forms an s-bond with one of the carbon atoms of the benzene ring.

s-complex

An s-complex is a cation without an aromatic structure, with four p-electrons delocalized (in other words, distributed) in the sphere of influence of the nuclei of five carbon atoms. The sixth carbon atom changes the hybrid state of its electron shell from sp2- to sp3-, 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 abstracted from the s-complex and the aromatic system is restored, since the pair of electrons missing from the aromatic sextet returns to the benzene ring.

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

H+ + AlCl4- ® HCl + AlCl3

It is thanks 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 undergo substitution reactions, under harsh conditions it also enters into addition reactions.

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

3H2 -t°,p,Ni® (cyclohexane)

2) In sunlight, under the influence of ultraviolet radiation, benzene combines with chlorine and bromine to form hexahalides, which, when heated, lose three molecules of hydrogen halide and lead to trihalobenzenes.

3Cl2 -hn®

hexachlorocyclohexane

sim-trichlorobenzene

3) Oxidation. The benzene ring is more resistant to oxidation than alkanes. Even potassium permanganate, nitric acid, and hydrogen peroxide have no effect on benzene under normal conditions. When oxidizing agents act 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.

2KMnO4 ® (potassium salt of benzoic acid) + 2MnO2 + KOH + H2O

4KMnO4 ® + K2CO3 + 4MnO2 + 2H2O + KOH

In all cases, as can be seen, benzoic acid is formed, regardless of the length of the side chain.
If there are several substituents on the benzene ring, all existing chains can be oxidized sequentially. This reaction is used to determine the structure of aromatic hydrocarbons.

-[O]® (terephthalic acid)

Rules for orientation in the benzene ring

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 ring has a certain directing (or orienting) effect. The laws that determine the direction of substitution reactions in the benzene ring are called orientation rules.
All substituents, according to the nature of their orienting action, are divided into two groups.
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 -NH2 groups, as well as halogens. The listed substituents (except for halogens) increase the activity of the benzene ring. Substituents of the first kind orient the new substituent predominantly to the ortho and para positions.

2 + 2H2SO4 ® (o-toluenesulfonate) + (p-toluenesulfonate) + 2H2O

2 + 2Cl2 -AlCl3® (o-chlorotoluene) + (p-chlorotoluene) + 2HCl

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

Cl2 -hn® (benzyl chloride) + HCl

Substituents of the second kind (meta-orientants) are electron-withdrawing groups capable of withdrawing and accepting electrons from the benzene ring. These include:
-NO2, -COOH, -CHO, -COR, -SO3H.
Substituents of the second kind reduce the activity of the benzene ring; they direct the new substituent to the meta position.

HNO3 -H2SO4® (m-dinitrobenzene) + H2O

HNO3 -H2SO4® (m-nitrobenzoic acid) + H2O

Application

Aromatic hydrocarbons are important raw materials for the production of various synthetic materials, dyes, and physiologically active substances. Thus, 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 polymer material - polystyrene.

AROMATIC HYDROCARBONS

For aromatic compounds, or arenes, refers to a large group of compounds whose molecules contain a stable cyclic group (benzene ring), which has special physical and chemical properties.

These compounds include primarily benzene and its numerous derivatives.

The term "aromatic" was first used to refer to naturally occurring products that had an aromatic odor. Since among these compounds there were many that included benzene rings, the term “aromatic” began to be applied to any compounds (including those with an unpleasant odor) containing a benzene ring.

Benzene, its electronic structure

Based on the formula of benzene C 6 H 6, it can be assumed that benzene is a highly unsaturated compound, similar, for example, to acetylene.

However, the chemical properties of benzene do not support this assumption.

In the 60s. last century, most chemists accepted the theory of the cyclic structure of benzene based on the fact that monosubstituted benzene derivatives (for example, bromobenzene) do not have isomers.

The most widely recognized formula for benzene was proposed in 1865 by the German chemist Kekule, in which double bonds in the ring of carbon atoms of benzene alternate with simple ones, and, according to Kekule’s hypothesis, single and double bonds are continuously moving:

However, Kekule's formula cannot explain why benzene does not exhibit the properties of unsaturated compounds.

According to modern concepts, the benzene molecule has the structure of a flat hexagon, the sides of which are equal to each other and amount to 0.140 nm. This distance is the average value between 0.154 nm (single bond length) and 0.134 nm (double bond length). Not only the carbon atoms, but also the six hydrogen atoms associated with them lie in the same plane. The angles formed by the bonds H - C - C and C - C - C are equal to 120 °.

The carbon atoms in benzene are in sp 2 -hybridization, i.e. Of the four orbitals of the carbon atom, only three are hybridized (one 2s- and two 2 p-), which take part in the formation of σ bonds between carbon atoms. The fourth 2p orbital overlaps with the 2p orbitals of two neighboring carbon atoms (on the right and left), six delocalized π-electrons located in dumbbell-shaped orbitals, the axes of which are perpendicular to the plane of the benzene ring, form a single stable closed electronic system.

As a result of the formation of a closed electronic system by all six carbon atoms, the “alignment” of single and double bonds occurs, i.e. the benzene molecule lacks classical double and single bonds.

The uniform distribution of π-electron density between all carbon atoms is the reason for the high stability of the benzene molecule. To emphasize the uniformity of the π-electron density in the benzene molecule, they resort to the following formula:

Nomenclature and isomerism of aromatic hydrocarbons of the benzene series

The general formula of the homologous series of benzene is C n H 2 n -6. The first homolog of benzene is methylbenzene, or toluene,

C 7 H 8

has no positional isomers, like all other monosubstituted derivatives. The second homolog of C 8 H 10 can exist in four isomeric forms: ethylbenzene C 6 H 5 -C 2 H 5 and three dimethylbenzenes, or xylene, S b H 4 (CH 3) 2(ortho-, meta- And-xylenes, or 1,2-, 1,3- and 1,4-dimethylbenzenes):

The radical (residue) of benzene C 6 H 5 is called phenyl; the names of the radicals of benzene homologues are derived from the names of the corresponding hydrocarbons by adding a suffix to the root-il (tolyl, xylyl, etc.) and denoted by letters (o-, m-, p-) or numbers the position of the side chains. General name for all aromatic radicals aryls similar to the name alkyls

for alkane radicals. The radical C 6 H 5 -CH 2 is called

benzyl.

When naming more complex benzene derivatives, from the possible numbering orders, choose the one in which the sum of the digits of the substituent numbers is the smallest.

For example, dimethyl ethyl benzene structure

should be called 1,4-dimethyl-2-ethylbenzene (sum of digits is 7), not 1,4-dimethyl-6-ethylbenzene (sum of digits is 11).

The names of higher homologs of benzene are often derived not from the name of the aromatic ring, but from the name of the side chain, i.e. they are considered as derivatives of alkanes:

Physical properties of aromatic hydrocarbons of the benzene series

The lower members of the homologous series of benzene are colorless liquids with a characteristic odor.

Their density and refractive index are much higher than those of alkanes and alkenes. The melting point is also noticeably higher. Due to the high carbon content, all aromatic compounds burn with a highly smoky flame. All aromatic hydrocarbons are insoluble in water and highly soluble in most organic solvents: many of them are easily distilled with steam.

Chemical properties of aromatic hydrocarbons of the benzene series

For aromatic hydrocarbons, the most typical reactions are substitution of hydrogen in the aromatic ring. Aromatic hydrocarbons undergo addition reactions with great difficulty under harsh conditions. A distinctive feature of benzene is its significant resistance to oxidizing agents.

Addition reactions

Hexachlorocyclohexai (trade name hexachlorane) is currently used as an insecticide - substances that destroy insects that are agricultural pests.

Oxidation reactions

Benzene is even more resistant to oxidizing agents than saturated hydrocarbons. It is not oxidized by dilute nitric acid, KMnO 4 solution, etc. Benzene homologues are oxidized much more easily. But even in them, the benzene ring is relatively more resistant to the action of oxidizing agents than the hydrocarbon radicals associated with it. There is a rule: any benzene homolog with one side chain is oxidized to a monobasic (benzoic) acid:

Benzene homologs with multiple side chains of any complexity are oxidized to form polybasic aromatic acids:

Substitution reactions

1. Halogenation

Under normal conditions, aromatic hydrocarbons practically do not react with halogens; benzene does not decolorize bromine water, but in the presence of catalysts (FeCl 3, FeBr 3, AlCl 3) in an anhydrous environment, chlorine and bromine react vigorously with benzene at room temperature:

Nitration reaction

Concentrated nitric acid is used for the reaction, often mixed with concentrated sulfuric acid (catalyst):

In unsubstituted benzene, the reactivity of all six carbon atoms in substitution reactions is the same; substituents can attach to any carbon atom. If there is already a substituent in the benzene ring, then under its influence the state of the nucleus changes, and the position into which any new substituent enters depends on the nature of the first substituent. It follows from this that each substituent in the benzene ring exhibits a certain directing (orienting) influence and contributes to the introduction of new substituents only in positions specific to itself.

According to their directing influence, various substituents are divided into two groups:

a) substituents of the first kind:

They direct any new substituent into ortho and para positions relative to themselves. At the same time, they almost all reduce the stability of the aromatic group and facilitate both substitution reactions and reactions of the benzene ring:

b) substituents of the second kind:

They direct any new substitute to a meta-position in relation to themselves. They increase the stability of the aromatic group and complicate substitution reactions:

Thus, the aromatic character of benzene (and other arenes) is expressed in the fact that this compound, being unsaturated in composition, manifests itself as a saturated compound in a number of chemical reactions; it is characterized by chemical stability and the difficulty of addition reactions. Only under special conditions (catalysts, irradiation) does benzene behave as if its molecule had three double bonds.

Arenas(aromatic hydrocarbons) - compounds whose molecules contain one or more benzene rings - cyclic groups of carbon atoms with a specific nature of bonds.

Benzene - molecular formula C 6 H 6. It was first proposed by A. Kekule:

Arena structure.

All 6 carbon atoms are in sp 2-hybridization. Each carbon atom forms 2 σ -bonds with two neighboring carbon atoms and one hydrogen atom, which are in the same plane. The angles are 120°. Those. All carbon atoms lie in the same plane and form a hexagon. Each atom has a non-hybrid R-the habitation on which the unpaired electron is located. This orbital is perpendicular to the plane, and therefore π -the electron cloud is “spread” over all carbon atoms:

All connections are equal. Conjugation energy is the amount of energy that must be expended to destroy an aromatic system.

This is what determines the specific properties of benzene - the manifestation of aromaticity. This phenomenon was discovered by Hückel, and is called Hückel's rule.

Arene isomerism.

Arenas can be divided into 2 groups:

• benzene derivatives:

• condensed arenas:

The general formula of arenes is WITHnH 2 n -6 .

Arenes are characterized by structural isomerism, which is explained by the mutual arrangement of substituents in the ring. If there are 2 substituents in the ring, then they can be in 3 different positions - ortho (o-), meta (m-), para (p-):

If one proton is “taken away” from benzene, a radical is formed - C 6 H 5, which is called the aryl radical. Protozoa:

Arenes are called the word “benzene”, indicating the substituents in the ring and their positions:

Physical properties of arenas.

The first members of the series are colorless liquids with a characteristic odor. They are highly soluble in organic solvents, but insoluble in water. Benzene is toxic, but has a pleasant smell. Causes headaches and dizziness; inhalation of large quantities of vapor can cause loss of consciousness. Irritating to mucous membranes and eyes.

Getting arenas.

1. From aliphatic hydrocarbons using the “aromatization” of saturated hydrocarbons that make up the oil. When passed over platinum or chromium oxide, dihydrocyclization occurs:

2. Dehydrogenation of cycloalkanes:

3. From acetylene (trimerization) when passed over hot coal at 600°C:

4. Friedel-Crafts reaction in the presence of aluminum chloride:

5. Fusion of salts of aromatic acids with alkali:

Chemical properties of arenes.

Arene substitution reactions.

The arene core has a mobile π - a system that is affected by electrophilic reagents. Arenes are characterized by electrophilic substitution, which can be represented as follows:

An electrophilic particle is attracted to π -ring system, then a strong bond is formed between the reagent X and one of the carbon atoms, in which case the unity of the ring is disrupted. To restore aromaticity, a proton is emitted and 2 electrons S-N pass into the π-system of the ring.

1. Halogenation occurs in the presence of catalysts - anhydrous chlorides and bromides of aluminum and iron:

2. Nitration of arenes. Benzene reacts very slowly with concentrated nitric acid when heated. But if you add sulfuric acid, the reaction proceeds very easily:

3. Sulfonation occurs under the influence of 100% sulfuric acid - oleum:

4. Alkylation with alkenes. As a result, chain elongation occurs; the reaction occurs in the presence of a catalyst - aluminum chloride.