Polynuclear aromatic hydrocarbons chemical properties. Aromatic hydrocarbons (arenes)

AROMATIC COMPOUNDS

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 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 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 C 6 H 6. 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 sp 2 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 direction of the -bonds. 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 -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 -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) -electrons in a closed conjugation system, where n is an integer, can be called aromatic compounds. The given criteria for aromaticity, known as Hückel's rules, benzene is fully responsible. Its number of six -electrons is the Hückel number for n=1, and therefore, 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 for the existence of only one compound with the formula benzene (C 6 H 6) as well as only one closest homologue - toluene (C 7 H 8). 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.

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.

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

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

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

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 are easily soluble 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	+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=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, it is not characterized by addition reactions. Typical reactions of the benzene ring are reactions of substitution of hydrogen atoms - more precisely, electrophilic substitution reactions.

Let's look at examples of the most typical reactions of this type.

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 most often uses aluminum or iron chlorides.

Nitration. When benzene is exposed to a nitrating mixture, the hydrogen atom is replaced by a nitro group (the 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 water-removing agent.

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.

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

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 – AlCl 3  (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.

+ 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.
Next, the transition of the -complex to the -complex occurs, the formation of which is the most important stage of the reaction. An electrophilic particle “captures” two electrons from the -electron sextet and forms a -bond with one of the carbon atoms of the benzene ring.

A -complex is a cation without an aromatic structure, with four -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 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 abstracted from the -complex and the aromatic system is restored, since the pair of electrons missing from the aromatic sextet returns to the benzene ring.

 + H +

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

H + + AlCl 4 -  HCl + AlCl 3

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.

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.

3H 2 – t  , p , Ni  (cyclohexane)

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.

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.

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, 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 –NH 2 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 + 2H 2 SO 4  (o-toluenesulfonate) + (p-toluenesulfonate) + 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 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.

Cl 2 – h   (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:
–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 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.

In terms of chemical properties, biphenyl is a typical aromatic compound. It is characterized by S E Ar reactions. It is easiest to think of biphenyl as benzene bearing a phenyl substituent. The latter exhibits weak activating properties. All reactions typical for benzene also occur in biphenyl.

Since the aryl group is ortho- And pair-orientant, S E Ar reactions occur predominantly in pair-position. Ortho-isomer is a by-product due to steric hindrance.

Di- and triphenylmethanes

Di- and triphenylmethanes are homologs of benzene, in which the corresponding number of hydrogen atoms are replaced by phenyl residues. Benzene rings separated sp 3-hybridized carbon atom, which prevents conjugation. The rings are completely insulated.

Methods for obtaining diphenylmethane:

S E Ar reactions occur in ortho- And pair-positions of benzene rings of diphenylmethane.

Preparation of triphenylmethane and its derivatives:

A distinctive feature of triphenylmethane derivatives is the high mobility of the hydrogen atom bonded to the tetrahedral carbon.

Triphenylmethane exhibits noticeable acidity, reacting with sodium metal to form the very stable triphenylmethyl anion.

Triphenylchloromethane in aqueous solution dissociates to form a stable carbocation.

In some triphenylmethane derivatives, the cleavage of the C-H bond can occur homolytically with the formation of triphenylmethyl radical, chronologically the first of the discovered stable free radicals.

The reasons for the high stability of the triphenylmethyl cation, anion and radical can be understood by considering the structure of the cation. If we depict the triphenylmethyl cation using boundary structures, it becomes clear that the vacant orbital of the central carbon atom is conjugated with the p-electrons of the benzene rings.

Lecture No. 21

Polynuclear aromatic hydrocarbons and their derivatives.

· Polynuclear aromatic hydrocarbons with condensed nuclei. Linear and angular polycyclic hydrocarbons. Isolating them from coal tar. Carcinogenic properties of polycyclic hydrocarbons. Safety precautions when working with aromatic hydrocarbons.

· Naphthalene. Isomerism and nomenclature of derivatives. Structure, aromaticity. Chemical properties of naphthalene and its derivatives: oxidation, catalytic hydrogenation and reduction with sodium in liquid ammonia, aromatic electrophilic substitution reactions. (effect of substituents on orientation, activity of a-position).

· Anthracene. Nomenclature, structure, aromaticity (in comparison with benzene and naphthalene), isomerism of derivatives. Reactions of oxidation and reduction, electrophilic addition and substitution. Meso position activity.

· Phenanthrene. Nomenclature, structure, aromaticity (in comparison with benzene and naphthalene). Reactions of oxidation, reduction, electrophilic substitution and addition.

Condensed aromatic hydrocarbons

Polycyclic aromatic compounds can be linear, angular or pericyclic.

Polycyclic compounds are isolated from coal tar. Many of them have a pronounced carcinogenic effect. The more cycles, the more likely it is carcinogenic.

Naphthalene

The simplest bicyclic aromatic compound.

Although the molecular formula indicates the unsaturated nature of naphthalene, its properties are typical of aromatic compounds. Naphthalene satisfies the structural criteria of aromaticity. A cyclic planar system having a continuous conjugation chain, in which 10 p-electrons participate. It should be remembered that Hückel formulated his rule (4n + 2) for monocyclic systems. In the case of naphthalene, it is believed that each ring contains 6 delocalized electrons, and one of the pairs is common to both rings. Conjugation is shown using canonical structures:

As a result: above and below the plane of cycles there are p-electron clouds shaped like a figure of eight. Fig. 20.1.

Rice. 20.1. Shape of p-electron clouds of naphthalene molecule

In mothballs, not all C-C bonds are the same. Thus, the length of C 1 -C 2 is 1.365 Å, and C 2 -C 3 is 1.404 Å. The conjugation energy of naphthalene is 61 kcal/mol, which is less than twice the delocalization energy of benzene (2x36 kcal/mol). The second cycle contributes less to the conjugation than the first. Naphthalene is less aromatic than benzene. Disrupting the aromaticity of one of its cycles requires only 25 kcal/mol, which is reflected in its reactions.

Reactions

The oxidation of naphthalene proceeds similarly to the oxidation of benzene.

The resulting phthalic acid under the reaction conditions turns into phthalic anhydride, which is released as a result of the reaction.

Reduction reactions also illustrate the lower aromaticity of naphthalene compared to benzene. Naphthalene can be hydrogenated with chemical reducing agents under mild conditions.

Aromatic electrophilic substitution reactions

In general, S E Ar reactions in naphthalene proceed according to the general mechanism discussed earlier. The peculiarity of reactions in the naphthalene series is that monosubstituted naphthalenes exist in the form of two isomers (1- and 2-derivatives). The features of S E Ar reactions are considered using the example of a nitration reaction, the main product of which is 1-nitronaphthalene (2-isomers are traces).

The key stage of the reaction is the formation of an s-complex, of which there can be two. It is necessary to determine the structural factors that stabilize or destabilize the intermediate. On this basis, the course of substitution can be predicted and explained. Let us consider the structure of possible intermediate products.

When an electrophile attacks position 1 of naphthalene, an s-complex is formed, the structure of which can be described by two boundary structures in which the benzene ring is retained. Such structures are more stable due to benzene conjugation. When an electrophile attacks position 2, only one energetically favorable structure can be drawn.

It can be concluded that the electrophilic attack at position 1 of naphthalene leads to a more stable s-complex than the reaction at position 2.

S.Yu. Eliseev

The concept of aromatic hydrocarbons, their application, physicochemical and fire and explosion properties.

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

Basic chemical reactions:

substitutions (halogenation, nitration, sulfonation, alkylation)

addition (hydrogen and halogens);

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

Rules for substitution in the benzene ring. First and second row deputies.

Industrial methods for the production of aromatic hydrocarbons.

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

Nitro compounds of the aromatic series, physicochemical and fire hazardous properties of nitrobenzene, toluene. Reactions for their production.

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

Aromatic hydrocarbons (arenes)

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

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 open-chain compounds. The distinctive properties of aromatic substances due to the presence of a benzene ring in them are usually called aromatic properties, and the benzene ring is, accordingly, an aromatic ring.

It should be noted that the very name “aromatic compounds” no longer has its original direct meaning. The first benzene derivatives studied were named this way because they had an aroma or were isolated from natural aromatic substances. Currently, aromatic compounds include many substances that have unpleasant odors or no smell at all, if their molecule contains a flat ring with (4n + 2) generalized electrons, where n can take values 0, 1, 2, 3, etc. .d., - 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 by compression or cooling of the so-called. illuminating gas, which is obtained from the dry distillation of coal. Subsequently, benzene was discovered (A. Hoffman, 1845) in another product of dry distillation of coal - in coal tar. It turned out to be a very valuable substance and has found wide application. It was then discovered that 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 highly unsaturated compound. After all, its composition C6H6 in terms of the ratio of carbon and hydrogen atoms corresponds to the formula CnH2n-6, while the saturated hydrocarbon 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; it, for example, does not provide bromine water and KMnO4 solution, i.e. under normal conditions it is not prone to addition reactions and does not oxidize. On the contrary, benzene, in the presence of catalysts, undergoes 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 undergo addition reactions. There, in the presence of catalysts, it is hydrogenated, adding 6 hydrogen atoms:

C6H6 + 3H2 ® C6H12

When exposed to light, benzene slowly adds 6 halogen atoms:

C6H6 + 3Cl2 ® C6H6Cl6

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

Further, it was found that monosubstituted benzene derivatives C6H5X do not have isomers. This showed that all hydrogen and all carbon atoms in its molecule are equivalent in position, which also could not be explained for a long time.

He first proposed the formula for the structure of benzene in 1865. German chemist August Kekule. He proposed that the 6 carbon atoms in benzene form a ring, connected 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 proposed that the double bonds in benzene not motionless; according to his ideas, they continuously move (oscillate) in the ring, which can be represented by the diagram: CH (I) CH (II) Formulas I and II, according to Kekule, CH CH CH CH are completely equivalent and only ½½<=>½½ expresses 2 mutually transferring 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 had been fixed, then its disubstituted derivatives C6H4X2 with substituents at adjacent carbons would have to exist in the form of isomers based on the position of single and double bonds:

½ (III) ½ (IV)

C C

NS S-X NS S-X

½½½<=>½½½

Kekule's formula has become widespread. It is consistent with the concept of tetravalency of carbon and 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 upon hydrogenation, benzene forms cyclohexane, and cyclohexane, in turn, is converted into benzene by dehydrogenation.

However, Kekule's formula has significant drawbacks. Assuming that benzene has three double bonds, she cannot explain why benzene in this case hardly enters into addition reactions and is resistant to the action of oxidizing agents, i.e. does not exhibit 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 the carbon atoms. For example, the study of aromatic compounds using X-rays showed that the 6 carbon atoms in benzene, forming a ring, lie in the same plane at the vertices of a regular hexagon and their centers are at equal distances from each other, amounting to 1.40 A. These distances are less than than the distances between the centers of carbon atoms connected by a single bond (1.54 A), and greater than those connected by a double bond (1.34 A). Thus, in benzene, carbon atoms are connected using special, equivalent bonds, which were called aromatic bonds. They differ in nature 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 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 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:

Periodic table

Solubility table

II.3. Condensed aromatic hydrocarbons

Hückel's rule on the aromaticity of a (4n+2) electron system was derived for monocyclic systems. To polycyclic fused (i.e. containing several benzene rings with common vertices) systems, it can be transferred for systems that have 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 the 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 a significant contribution to its structure is made by a bipolar structure, which is a combination of cyclopentadienyl anion nuclei and tropylium cation:

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

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

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

Let us first consider the ability of phenylated methanes to dissociation of C-H bonds with proton abstraction( CH-acidity ). The strength of CH acids, like ordinary protic OH acids, is determined by the stability, and therefore the ease of formation, of the corresponding anions (in this case, 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 ring associated with a benzyl carbon atom can take part in the delocalization of the negative charge arising on it, which can be represented using boundary (resonance) structures:

For diphenylmethane, seven boundary structures can 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 particularly stable. (Note 14) In this regard, it can be expected that the CH acidity of methanes will increase with an increase in the number of phenyl rings, which can take part in the delocalization of the charge on the central carbon atom, i.e. rise in rank

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) - rubs-butanol; triphenylmethane more than 10 10 times more acidic than methane (p K a~ 40).(note 15)

Cherry-colored triphenylmethyl sodium is usually prepared by reducing triphenyl chloromethane with sodium amalgam:

Unlike conventional CH bonds sp 3-hybrid carbon atom, benzyl C-H bond tri- pair- nitrophenylmethane is cleaved heterolytically by 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 a benzyl CH bond is the abstraction of a hydride anion with the formation of the corresponding carbocations benzyl type:

Since benzene rings are capable of stabilizing both positive and negative charges, phenylated methanes By hydride mobility hydrogen in the aliphatic part will form the same series 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 generally difficult to experimentally compare the ease of abstraction of a hydride anion, since highly active Lewis acids are usually used to accomplish 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 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 the indicated conditions, chlorine has the greatest mobility in triphenylchloromethane, and the least 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 acid chlorides of carboxylic acids, 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 ) 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 it into benzene rings electron-donating 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 reaction equilibrium

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

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

Benzene nuclei should have an effect similar to that discussed above for triphenylmethyl anions and cation on the stability triphenylmethyl radical . In the latter case, the ease of breaking the bond formed by the central carbon atom with the “non-phenyl” substituent is, to a certain extent, due to other reasons. The fact is that in triphenylmethane, triphenylchloromethane, triphenylcarbinol, etc. the central carbon atom is located 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 paired. When going to a triphenylmethyl cation (heterolytic cleavage) or a radical (homolytic cleavage), the central carbon atom ends up in sp 2-hybrid state; As a result, 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 in question 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 another radical:

Apparently, in the case under consideration, one triphenylmethyl radical attacks least spatially difficult place another, and, naturally, one of those places that is involved 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 o, the triphenylmethyl radical is dimerized by 97%, and the tri-4-nitrophenylmethyl radical does not dimerize at all.