n(C) = m(C) / M(C);

n(C) = 6 / 12 = 0.5 mol.

Let's calculate the amount of hydrogen substance:

n(H 2) = V(H 2) / V m;

n(H2) = 5.6 / 22.4 = 0.25 mol.

This means that the amount of substance of one hydrogen atom will be equal to:

n(H) = 2 × 0.25 = 0.5 mol.

Let us denote the number of carbon atoms in a hydrocarbon molecule as “x”, and the number of hydrogen atoms as “y”, then the ratio of these atoms in the molecule is:

x: y = 0.5: 0.5 = 1:1.

Then the simplest hydrocarbon formula will be expressed by the composition CH. The molecular weight of a molecule of composition CH is equal to:

M(CH) = 13 g/mol

Let's find the molecular weight of the hydrocarbon based on the conditions of the problem:

M (C x H y) = ρ×V m;

M (C x H y) = 3.482 x 22.4 = 78 g/mol.

Let's determine the true formula of the hydrocarbon:

k= M(C x H y)/ M(CH)= 78/13 =6,

therefore, the coefficients “x” and “y” need to be multiplied by 6 and then the hydrocarbon formula will take the form C 6 H 6. This is benzene.

Aromatic hydrocarbons form an important part of the cyclic series of organic compounds. The simplest representative of such hydrocarbons is benzene. The formula of this substance not only distinguished it from a number of other hydrocarbons, but also gave impetus to the development of a new direction in organic chemistry.

Discovery of aromatic hydrocarbons

Aromatic hydrocarbons were discovered in the early 19th century. In those days, the most common fuel for street lighting was lamp gas. From its condensate, the great English physicist Michael Faraday isolated three grams of an oily substance in 1825, described its properties in detail and named it: carbureted hydrogen. In 1834, the German scientist, chemist Mitscherlich, heated benzoic acid with lime and obtained benzene. The formula for this reaction is presented below:

C6 H5 COOH + CaO fusion of C6 H6 + CaCO3.

At that time, the rare benzoic acid was obtained from the resin of benzoic acid, which can be secreted by some tropical plants. In 1845, a new compound was discovered in coal tar, which was a completely accessible raw material for producing a new substance on an industrial scale. Another source of benzene is petroleum obtained from some fields. To meet the needs of industrial enterprises for benzene, it is also obtained by aromatization of certain groups of acyclic hydrocarbons of oil.

The modern version of the name was proposed by the German scientist Liebig. The root of the word “benzene” should be sought in Arabic languages ​​- there it is translated as “incense”.

Physical properties of benzene

Benzene is a colorless liquid with a specific odor. This substance boils at a temperature of 80.1 o C, hardens at 5.5 o C and turns into a white crystalline powder. Benzene practically does not conduct heat and electricity, is poorly soluble in water and well soluble in various oils. The aromatic properties of benzene reflect the essence of the structure of its internal structure: a relatively stable benzene ring and an uncertain composition.

Chemical classification of benzene

Benzene and its homologues - toluene and ethylbenzene - are an aromatic series of cyclic hydrocarbons. The structure of each of these substances contains a common structure called a benzene ring. The structure of each of the above substances contains a special cyclic group created by six carbon atoms. It is called the benzene aromatic ring.

History of discovery

The establishment of the internal structure of benzene took several decades. The basic principles of the structure (ring model) were proposed in 1865 by the chemist A. Kekule. As the legend tells, a German scientist saw the formula of this element in a dream. Later, a simplified spelling of the structure of a substance called benzene was proposed. The formula of this substance is a hexagon. The symbols for carbon and hydrogen, which should be located at the corners of the hexagon, are omitted. This produces a simple regular hexagon with alternating single and double lines on the sides. The general formula of benzene is shown in the figure below.

Aromatic hydrocarbons and benzene

The chemical formula of this element suggests that addition reactions are not typical for benzene. For it, as for other elements of the aromatic series, substitution reactions of hydrogen atoms in the benzene ring are typical.

Sulfonation reaction

By ensuring the interaction of concentrated sulfuric acid and benzene, increasing the reaction temperature, benzosulfonic acid and water can be obtained. The structural formula of benzene in this reaction is as follows:

Halogenation reaction

Bromine or chromium reacts with benzene in the presence of a catalyst. This produces halogen derivatives. But the nitration reaction takes place using concentrated nitric acid. The final result of the reaction is a nitrogenous compound:

Using nitriding, a well-known explosive is produced - TNT, or trinitotoluene. Few people know that tol is based on benzene. Many other benzene ring-based nitro compounds can also be used as explosives

Electronic formula of benzene

The standard formula of the benzene ring does not accurately reflect the internal structure of benzene. According to it, benzene must have three localized p-bonds, each of which must interact with two carbon atoms. But, as experience shows, benzene does not have ordinary double bonds. The molecular formula of benzene allows you to see that all the bonds in the benzene ring are equivalent. Each of them has a length of about 0.140 nm, which is intermediate between the length of a standard single bond (0.154 nm) and an ethylene double bond (0.134 nm). The structural formula of benzene, depicted with alternating bonds, is imperfect. A more plausible three-dimensional model of benzene looks like the image below.

Each of the atoms of the benzene ring is in a state of sp 2 hybridization. It spends three valence electrons on the formation of sigma bonds. These electrons cover two neighboring carbohydrate atoms and one hydrogen atom. In this case, both electrons and C-C, H-H bonds are in the same plane.

The fourth valence electron forms a cloud in the shape of a three-dimensional figure eight, located perpendicular to the plane of the benzene ring. Each such electron cloud overlaps above the plane of the benzene ring and directly below it with the clouds of two neighboring carbon atoms.

The density of the n-electron clouds of this substance is evenly distributed between all carbon bonds. In this way, a single ring electron cloud is formed. In general chemistry, such a structure is called an aromatic electron sextet.

Equivalence of internal bonds of benzene

It is the equivalence of all the faces of the hexagon that explains the uniformity of aromatic bonds, which determine the characteristic chemical and physical properties that benzene possesses. The formula for the uniform distribution of the n-electron cloud and the equivalence of all its internal connections is shown below.

As you can see, instead of alternating single and double lines, the internal structure is depicted as a circle.

The essence of the internal structure of benzene provides the key to understanding the internal structure of cyclic hydrocarbons and expands the possibilities of practical application of these substances.

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:

Mendeleev table

Solubility table

Benzene. Formula 1)

Benzene- organic compound C 6 H 6, the simplest aromatic hydrocarbon; mobile colorless volatile liquid with a peculiar mild odor.

• tnl = 5.5°C;
• t kip = 80.1°C;
• density 879.1 kg/m 3 (0.8791 g/cm 3) at 20°C.

With air in a volume concentration of 1.5-8%, benzene forms explosive mixtures. Benzene is mixed in all proportions with ether, gasoline and other organic solvents; 0.054 g of water dissolves in 100 g of benzene at 26°C; with water it forms an azeotropic (constantly boiling) mixture (91.2% benzene by weight) with t kip = 69.25°C.

Story

Benzene was discovered by M. Faraday. (1825), who isolated it from the liquid condensate of illuminating gas; Benzene was obtained in its pure form in 1833 by E. Mitscherlich by dry distillation of the calcium salt of benzoic acid (hence the name).

In 1865, F.A. Kekule proposed a structural formula for benzene corresponding to cyclohexatriene - a closed chain of 6 carbon atoms with alternating single and double bonds. The Kekule formula is quite widely used, although many facts have accumulated indicating that benzene does not have the structure of cyclohexatriene. Thus, it has long been established that ortho-disubstituted benzenes exist only in one form, while the Kekule formula allows for isomerism of such compounds (substituents on carbon atoms connected by a single or double bond). In 1872, Kekule additionally introduced the hypothesis that the bonds in benzene constantly and very quickly move and oscillate. Other formulas for the structure of benzene were proposed, but they did not receive recognition.

Chemical properties

Benzene. Formula (2)

The chemical properties of benzene formally correspond to some extent to formula (1). So, under certain conditions, 3 molecules of chlorine or 3 molecules of hydrogen are added to a benzene molecule; benzene is formed by the condensation of 3 acetylene molecules. However, benzene is characterized mainly not by addition reactions typical of unsaturated compounds, but by electrophilic substitution reactions. In addition, the benzene ring is very resistant to oxidizing agents such as potassium permanganate, which also contradicts the presence of localized double bonds in benzene. Special, so-called The aromatic properties of benzene are explained by the fact that all the bonds in its molecule are aligned, i.e. the distances between neighboring carbon atoms are the same and equal to 0.14 nm, the length of a single C-C bond is 0.154 nm and a double C=C bond is 0.132 nm. The benzene molecule has a symmetry axis of six order; Benzene as an aromatic compound is characterized by the presence of a sextet of p-electrons, forming a single closed stable electronic system. However, there is still no generally accepted formula reflecting its structure; Formula (2) is often used.

Effect on the body

Benzene can cause acute and chronic poisoning. Penetrates into the body mainly through the respiratory system, but can also be absorbed through intact skin. The maximum permissible concentration of benzene vapor in the air of working premises is 20 mg/m 3 . It is excreted through the lungs and in the urine. Acute poisoning usually occurs during accidents; their most characteristic signs are: headache, dizziness, nausea, vomiting, agitation followed by a depressed state, rapid pulse, drop in blood pressure, in severe cases - convulsions, loss of consciousness. Chronic benzene poisoning is manifested by changes in the blood (bone marrow dysfunction), dizziness, general weakness, sleep disturbance, fatigue; in women - menstrual dysfunction. A reliable measure against benzene vapor poisoning is good ventilation of industrial premises.

Treatment for acute poisoning: rest, warmth, bromide drugs, cardiovascular drugs; for chronic poisoning with severe anemia: transfusion of red blood cells, vitamin B12, iron supplements.

Sources

• Omelyanenko L. M., Senkevich N. A., Clinic and prevention of benzene poisoning, M., 1957;

The concept of “benzene ring” immediately requires decoding. To do this, it is necessary to at least briefly consider the structure of the benzene molecule. The first structure of benzene was proposed in 1865 by the German scientist A. Kekule:

The most important aromatic hydrocarbons include benzene C 6 H 6 and its homologues: toluene C 6 H 5 CH 3, xylene C 6 H 4 (CH 3) 2, etc.; naphthalene C 10 H 8, anthracene C 14 H 10 and their derivatives.

The carbon atoms in the benzene molecule form a regular flat hexagon, although it is usually drawn as an elongated one.

The structure of the benzene molecule was finally confirmed by the reaction of its formation from acetylene. The structural formula depicts three single and three double alternating carbon-carbon bonds. But such an image does not convey the true structure of the molecule. In reality, the carbon-carbon bonds in benzene are equivalent, and they have properties that are unlike those of either single or double bonds. These features are explained by the electronic structure of the benzene molecule.

Electronic structure of benzene

Each carbon atom in a benzene molecule is in a state of sp 2 hybridization. It is connected to two neighboring carbon atoms and a hydrogen atom by three σ bonds. The result is a flat hexagon: all six carbon atoms and all σ-bonds C-C and C-H lie in the same plane. The electron cloud of the fourth electron (p-electron), which is not involved in hybridization, has the shape of a dumbbell and is oriented perpendicular to the plane of the benzene ring. Such p-electron clouds of neighboring carbon atoms overlap above and below the plane of the ring.

As a result, six p-electrons form a common electron cloud and a single chemical bond for all carbon atoms. Two regions of the large electron plane are located on either side of the σ bond plane.

The p-electron cloud causes a reduction in the distance between carbon atoms. In a benzene molecule they are the same and equal to 0.14 nm. In the case of a single and double bond, these distances would be 0.154 and 0.134 nm, respectively. This means that there are no single or double bonds in the benzene molecule. The benzene molecule is a stable six-membered cycle of identical CH groups lying in the same plane. All bonds between carbon atoms in benzene are equivalent, which determines the characteristic properties of the benzene ring. This is most accurately reflected by the structural formula of benzene in the form of a regular hexagon with a circle inside (I). (The circle symbolizes the equivalence of bonds between carbon atoms.) However, Kekulé’s formula indicating double bonds (II) is also often used:

The benzene ring has a certain set of properties, which is commonly called aromaticity.

Homologous series, isomerism, nomenclature

Conventionally, the arenas can be divided into two rows. The first includes benzene derivatives (for example, toluene or biphenyl), the second includes condensed (polynuclear) arenes (the simplest of them is naphthalene):

The homologous series of benzene has the general formula C n H 2 n -6. Homologues can be considered as benzene derivatives in which one or more hydrogen atoms are replaced by various hydrocarbon radicals. For example, C 6 H 5 -CH 3 - methylbenzene or toluene, C 6 H 4 (CH 3) 2 - dimethylbenzene or xylene, C 6 H 5 -C 2 H 5 - ethylbenzene, etc.

Since all carbon atoms in benzene are equivalent, its first homologue, toluene, has no isomers. The second homologue, dimethylbenzene, has three isomers that differ in the relative arrangement of methyl groups (substituents). This is an ortho- (abbreviated o-), or 1,2-isomer, in which the substituents are located on neighboring carbon atoms. If the substituents are separated by one carbon atom, then it is a meta- (abbreviated m-) or 1,3-isomer, and if they are separated by two carbon atoms, then it is a para- (abbreviated p-) or 1,4-isomer. In names, substituents are designated by letters (o-, m-, p-) or numbers.

Physical properties

The first members of the homologous series of benzene are colorless liquids with a specific odor. Their density is less than 1 (lighter than water). Insoluble in water. Benzene and its homologues are themselves good solvents for many organic substances. Arenas burn with a smoky flame due to the high carbon content in their molecules.

Chemical properties

Aromaticity determines the chemical properties of benzene and its homologues. The six-electron π system is more stable than ordinary two-electron π bonds. Therefore, addition reactions are less common for aromatic hydrocarbons than for unsaturated hydrocarbons. The most characteristic reactions for arenes are substitution reactions. Thus, aromatic hydrocarbons, in their chemical properties, occupy an intermediate position between saturated and unsaturated hydrocarbons.

I. Substitution reactions

1. Halogenation (with Cl 2, Br 2)

2. Nitration

3. Sulfonation

4. Alkylation (benzene homologues are formed) - Friedel-Crafts reactions

Alkylation of benzene also occurs when it reacts with alkenes:

Styrene (vinylbenzene) is obtained by dehydrogenation of ethylbenzene:

II. Addition reactions

1. Hydrogenation

2. Chlorination

III. Oxidation reactions

1. Combustion

2C 6 H 6 + 15O 2 → 12CO 2 + 6H 2 O

2. Oxidation under the influence of KMnO 4, K 2 Cr 2 O 7, HNO 3, etc.

No chemical reaction occurs (similar to alkanes).

Properties of benzene homologues

In benzene homologues, a core and a side chain (alkyl radicals) are distinguished. The chemical properties of alkyl radicals are similar to alkanes; the influence of the benzene ring on them is manifested in the fact that substitution reactions always involve hydrogen atoms at the carbon atom directly bonded to the benzene ring, as well as in the easier oxidation of C-H bonds.

The effect of an electron-donating alkyl radical (for example, -CH 3) on the benzene ring is manifested in an increase in the effective negative charges on carbon atoms in the ortho and para positions; as a result, the replacement of associated hydrogen atoms is facilitated. Therefore, homologues of benzene can form trisubstituted products (and benzene usually forms monosubstituted derivatives).

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