Aromatic compounds. Aromatic hydrocarbons: main representatives and their use in the national economy. Chemical properties of benzene and other aromatic hydrocarbons

Let us recall that all organic compounds are divided into two large groups:

open chain compounds (aliphatic) And
cyclic compounds.

Cyclic compounds are characterized by the presence of so-called cycles in their molecules.

A cycle is a closed chain, that is, a chain that, starting at a certain vertex, ends at the same vertex.

Cyclic compounds, in turn, are divided into:

Carbocyclic compounds

Carbocyclic compounds- these are compounds in the molecules of which there are cycles consisting only of carbon atoms.

In addition to bonding with each other, carbon atoms are also bonded with other atoms (hydrogen, oxygen, etc.), but the cycle itself is made up of carbon atoms.

This circumstance is reflected in their name (Carboneum in Latin - carbon).

These are cyclic compounds, in the cycles of which, in addition to carbon atoms, there are atoms of other elements (oxygen, nitrogen, sulfur, etc.). And this is also reflected in their name (from the Greek ετερος - “different”, “different”).

Carbocyclic compounds

In the figure above (right), Pyridine is shown as an example of a heterocyclic compound.

Carbocyclic compounds are divided into alicyclic and aromatic.

Alicyclic compounds are one of two subtypes of carbocyclic compounds.

They are called so because their chemical properties are closest to aliphatic compounds, although they are ring-shaped in structure.

They differ in the number of carbon atoms in the cycle and, depending on the nature of the bond between these atoms, can be saturated or unsaturated.

In the molecules of saturated cyclic hydrocarbons, the carbon atoms are connected by simple bonds, as in the molecules of saturated hydrocarbons with an open chain, which makes them similar in properties to the latter.

Examples of such compounds are cycloparaffins:

The names of cyclic compounds are constructed similarly to the names of compounds of the fatty (aliphatic) series with the addition of the prefix “cyclo”.

The second subtype of carbocyclic compounds are aromatic compounds. Aromatic series covers all carbocyclic compounds in the molecules of which there is a specific grouping of atoms -. This grouping of atoms determines certain physical and chemical properties of aromatic compounds.

The simplest of them are benzene C 6 H 6 and its homologues, for example, toluene(methylbenzene) C 6 H 5 -CH 3, ethylbenzene C 6 H 5 -CH 2 CH 3. General formula of these compounds.

C n H 2n-2 A characteristic feature of the structure of the benzene ring is three single and three double bonds alternating with each other . For ease of writing, the benzene ring is depicted in a simplified form as a hexagon, in which the symbols WITH And

N , related to the ring, do not write: Benzene monovalent radical C 6 H 5 -.

, formed by subtracting one hydrogen atom from any carbon atom of the benzene ring, is called phenyl For ease of writing, the benzene ring is depicted in a simplified form as a hexagon, in which the symbols Aromatic hydrocarbons with multiple bonds in side chains are known, for example styrene, as well as polynuclear hydrocarbons containing several benzene nuclei, for example:

naphthalene

anthracene

Or simplified:

Preparation of aromatic compounds and their use.

Aromatic hydrocarbons are contained in coal tar, obtained by coking coal.

Another important source of their production is oil from some fields.

Aromatic hydrocarbons are also obtained by catalytic aromatization of acyclic petroleum hydrocarbons.

Some aromatic compounds can be isolated from essential oils of plants. They are used to produce fragrant substances.

Aromatic hydrocarbons and their derivatives are widely used to produce plastics, synthetic dyes, medicinal and explosive substances, synthetic rubbers, and detergents.

Origin of name.

Thus, unlike unsaturated compounds with ethylene double bonds, benzene is resistant to oxidizing agents. For example, like saturated hydrocarbons, it does not discolor potassium permanganate. Addition reactions are not typical for benzene. On the contrary, it, like other aromatic compounds, is characterized by substitution reactions for hydrogen atoms in the benzene ring.

From the above it follows that the formula of benzene with alternating single and double bonds does not accurately express the nature of the bonds between the carbon atoms in the benzene ring.

According to this formula, benzene must have three localized pi bonds, i.e. three pairs of pi electrons, each of which is fixed between two carbon atoms. If we designate these pi electrons as dots, then the structure can be represented by the diagram:

However, experience shows that in the benzene ring there are no ordinary double bonds alternating with single ones, and that all bonds between .-atoms are equivalent.

This equivalence is explained as follows.

Each of the carbon atoms in the benzene ring is in the state sp 2-hybridization and spends three valence electrons on the formation of sigma bonds with two neighboring carbon atoms and one hydrogen atom.

Moreover, all six carbon atoms and all sigma bonds S-S For ease of writing, the benzene ring is depicted in a simplified form as a hexagon, in which the symbols S-N lie in the same plane:

The cloud of the fourth valence electron of each carbon atom (i.e. the cloud R-electron not involved in hybridization) has the shape of a three-dimensional figure eight (“dumbbell”) and is oriented perpendicular to the plane of the benzene ring.

Each of these R-electron clouds overlap above and below the plane of the ring with R-electron clouds of two neighboring carbon atoms.

Cloud density pi-electrons in benzene are evenly distributed between all bonds S-S. In other words, six pi-electrons are generalized by all carbon atoms of the ring and form a single ring cloud ( aromatic electronic sextet).

For this reason, in structural formulas, instead of the generally accepted symbol of a benzene ring with alternating double and single bonds, a hexagon with a circle inside is used:

Heterocyclic compounds are compounds with a closed chain, including not only carbon atoms, but also atoms of other elements.

Shown in the picture Pyridine can be considered as benzene, in which the group -CH replaced by a nitrogen atom.

– the most numerous class of compounds. These include many vitamins, pigments, antibiotics, most alkaloids, some amino acids, etc.

Elements that participate together with carbon atoms in the formation of a cycle are called heteroatoms. The most common and studied are heterocyclic compounds of oxygen, sulfur and nitrogen.

A heteromolecule can contain either one heteroatom or a larger number:

Heterocycles can contain three, four, five, six or more atoms.

Similar to carbocyclic compounds, five- and six-membered heterocycles are the most stable.

The presence of a heteroatom leads to a violation of the uniform distribution of electron density in the cycle. This determines the ability of heterocyclic compounds to react with both electrophilic and nucleophilic reagents (i.e., to be both a donor and an acceptor of an electron pair), and also undergo ring cleavage relatively easily. 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. sp 2 All 6 carbon atoms are in σ -hybridization. Each carbon atom forms 2 R-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 π -the habitation on which the unpaired electron is located. This orbital is perpendicular to the plane, and therefore

-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.

can be divided into 2 groups:

benzene derivatives:

condensed arenas: .The general formula of arenes isn 2 The general formula of arenes is -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 - 6 n C

5, which is called the aryl radical. Protozoa:

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

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 passing 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.

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 preparing 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, it also undergoes addition reactions under harsh conditions.

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 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 (otherwise called arenes) are organic biocompounds whose molecules contain one or more rings with six carbon atoms. The benzene ring is characterized by special physical and chemical properties. The name “arena” entered organic and general chemistry in the late 18th and early 19th centuries. These included substances that consisted of two chemical compounds - Carbon and Hydrogen and had a pleasant odor (resins, balms, essential oils). Over time, the name "aromatic hydrocarbons" lost its meaning, since aromatic substances were also found among other classes of organic substances, and most aromatic compounds have an unpleasant or specific odor. Benzene was first isolated in the early 19th century from lamp gas. A little later (1845) A.F. isolated from coal tar. In our time, the class of arenes (according to the IUPAC classification) combines compounds based on molecules that have benzene nuclei. So, such compounds are divided into mono- and multinuclear arenes, as well as aromatic hydrocarbons with condensed nuclei.

Mononuclear arenes are those that contain one benzene ring. The structure of the benzene molecule, a typical representative of the arenes, is most often demonstrated by the Kekulé formula in the form of a cycle of six Carbon atoms, which are alternately connected by single C-C and double C=C bonds. This structure is confirmed by modern physicochemical analysis.

Kekule's basic ideas about the structure of benzene are as follows: 1) benzene has the structure of a hexagonal ring; 2) the benzene ring has three single and three double bonds; 3) all six carbon atoms in the benzene ring are equivalent to each other. The formula displays the elemental composition of benzene, the ratio of Carbon and Hydrogen atoms in the molecule, the absence of isomers for monosubstituted benzene derivatives.

Aromatic hydrocarbons are quite common in nature. They are a component of coal tar, which is obtained after the dry distillation of coal. Arenas are included in many types of oil and other natural products (resins, balms, etc.). The process of dry distillation of coal produces an average of about three percent coal tar, or coal tar. And from coal tar during fractional distillation a number of fractions are obtained: light oil (contains xylenes, toluene, benzene, thiophene), carbolic oil (contains cresols, phenols and naphthalene), creosote oil (contains naphthalene), (contains phenanthrene, anthracene and other higher arenas) and pitch, which is used to cover roads and as a building material.

Application of aromatic hydrocarbons. Benzene has a specific odor and is practically insoluble in water. It is a good solvent for organic biocompounds. Synthesized from coal tar. Benzene is a valuable raw material for the production of dyes, medicines, explosives, herbicides, insecticides, etc.

Toluene is highly soluble in organic solvents. It is obtained from coal tar, as well as from some types of petroleum. Benzene alcohol, benzaldehyde, dyes, medicines, saccharin, and trinitrotoluene are synthesized from toluene.

Xylenes are good. They are obtained from coal oil, as well as from the fractional distillation of coal tar. Phthalic anhydride, xylene, and artificial fiber lavsan are synthesized from xylenes. Sometimes xylenes are added to gasoline.

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.

So what is special about the structure of benzene?

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 If one proton is “taken away” from benzene, a radical is formed -The general formula of arenes isn2n-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 If one proton is “taken away” from benzene, a radical is formed - 8 n 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