Chemical structure

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 y-bonds. The result is a flat hexagon: all six carbon atoms and all y-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 y-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, Kekule’s formula is also often used indicating double bonds (II)

Aromatic hydrocarbons (Arenas) are organic compounds whose molecules contain one or more benzene rings. The benzene ring, or core, is a cyclic group of carbon atoms with a special character of bonds.

General formula -CnH2n-6

1. Representatives:

MONONUCLEAR

1. C 6 H 6 – benzene, the founder of the homologous series of arenes

2. C 6 H 5 – CH 3 – toluene (methylbenzene)

3. C 6 H 5 – CH = C H 2 – styrene (vinylbenzene)

4. Xylene (ortho-, para-, meta-xylene)

MULTICORE (CONDENSED)

1. Naphthalene

2. Anthracene

2. Structure of aromatic hydrocarbons :

The first structural formula of benzene was proposed in 1865 by the German chemist F.A. Kekule:

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

The above formula correctly reflects the equivalence of six C atoms, but does not explain a number of special properties of benzene. For example, despite being unsaturated, it does not show a tendency to undergo addition reactions: does not discolor bromine water and potassium permanganate solution, i.e. it is not characterized by qualitative reactions typical of unsaturated compounds .

The Kekulé structural formula contains 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. This is explained by the electronic structure of its molecule.

Each C atom in a benzene molecule is in a state of sp 2 hybridization. It is bonded to two neighboring C atoms and the H atom to three σ - connections. The result is a flat hexagon, where all six C atoms and allσ -C–C and C–H bonds lie in the same plane (the angle between the C–C bonds is 120 o).

Rice. Education scheme -bonds in a benzene molecule.

The third p-orbital of the carbon atom does not participate in hybridization. It is shaped like a dumbbell and is oriented perpendicular to the plane of the benzene ring. Such p-orbitals of neighboring C atoms overlap above and below the plane of the ring.

Rice. Non-hybrid 2p carbon orbitals in a benzene molecule

As a result, six p-electrons (from all six C atoms) form a common π -electron cloud and a single chemical bond for all C atoms.

Rice. Benzene molecule. Location π -electron cloud

π -The electron cloud causes a reduction in the distance between C atoms.

In a benzene molecule they are the same and equal to 0.139 nm. In the case of a single and double bond, these distances would be 0.154 and 0.134 nm, respectively. This means that in the benzene molecule there is no alternation of single and double bonds, but there is a special bond - “one and a half” - intermediate between simple and double, the so-called aromatic bond. To show the uniform distribution of the p-electron cloud in the benzene molecule, it is more correct to depict it in the form of a regular hexagon with a circle inside (the circle symbolizes the equivalence of bonds between C atoms):

3. Isomerism, nomenclature

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 neighboring carbon atoms (position 1, 2-), meta- (m-) for separated by one carbon atom (1, 3-) and pair- (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)

Radicals of aromatic hydrocarbons are called aryl radicals . Radical C 6 H 5 - called phenyl.

Benzene was first isolated M. Faraday in 1825 from the condensation that fell from the illuminating gas used to illuminate the city streets of London. Faraday called this liquid, highly mobile substance with a pungent odor “carburated hydrogen.” It is important to note that even then it was established that benzene consists of equal parts carbon and hydrogen.

Somewhat later, in 1834, Mitscherlich prepared benzene by decarboxylation of benzoic acid. He also established the elemental composition of the resulting compound - C 6 H 6 - and proposed his name for it - petrol. However, Liebig did not agree with this name. It seemed to him that this name puts benzene on a par with such distant substances as quinine and strychnine. According to Liebig, a better name for the new compound is benzene, since it shows the similarity of benzene in properties to oils (from the German ol- oil). There were other proposals. Since benzene was isolated by Faraday from illuminating gas, Laurent proposed (1837) a name for it pheno from the Greek “bringer of light.” This name was not established, but it was from it that the name of the monovalent benzene residue came - phenyl.

Faraday's hydrocarbon was unlucky. All the names proposed for it turned out to be flawed. From Liebig's name "benzene" it follows that the compound contains a hydroxyl group, which is not there. Likewise, Mitscherlich's "gasoline" does not contain a nitrogen-containing functional group. Moreover, the existence of different names led to the division of chemists. In German and Russian scientific literature, the name “benzene” was established, and in English and French - “benzene” ( bensene, toluene, xylene).

At first glance, it seems that establishing the structure of benzene does not present much difficulty. The benzene molecule contains only two elements; for every six carbon atoms there are six hydrogen atoms. Moreover, physical and chemical properties benzene have been studied in great detail. However, this work dragged on for many decades and was completed only in 1931.

The most difficult barriers to understanding the structure of benzene were overcome by the outstanding German chemist Kekule. From above modern knowledge it is difficult to understand and evaluate the significance of the hypothesis he put forward, according to which the benzene molecule has a cyclic structure (1865). However, it was precisely this assumption, when taken together with the number of isomers in mono- and disubstituted benzenes, that led Kekule to well-known formula. According to Kekulé, benzene is a six-membered cyclic compound with three alternating double bonds, i.e. cyclohexatriene

It is this structure that is consistent with the existence of one and only one monosubstituted benzene and three isomers of disubstituted benzenes

From the moment the Kekule structure appeared, criticism began, which, unfortunately, it fully deserved. It has already been noted that characteristic feature aromatic compounds- their inherent aromatic character. The Kekule structure for benzene was unable to explain this feature of aromatic compounds. In a number of cases, it also could not explain the absence of isomers, while the cyclohexatriene formula for benzene allowed their existence. So, ortho-substituted benzenes can have two isomers

however, they could not be found. Let us immediately note that to overcome this difficulty, Kekule proposed to consider benzene as a cyclohexatriene with mobile, unfixed, double bonds. As a result of the rapid transformation I in II and vice versa, benzene behaves as a structure as if consisting of equal quantities I And II.

So, the main disadvantage of Kekule benzene is the inability to explain on its basis the aromatic character of compounds containing a benzene ring in their molecule. If benzene were cyclohexatriene, i.e. compound with three double bonds, then it would have to:

Easy to oxidize even when cold aqueous solution KMnO 4,

Already at room temperature, add bromine and easily enter into other electrophilic addition reactions,

Rapidly hydrogenated with hydrogen in the presence of nickel at room temperature,

Benzene enters into these reactions reluctantly, unlike alkenes. But substitution reactions are very typical for aromatic compounds. It follows that benzene cannot be a cyclohexatriene and Kekule's formula does not reflect the true structure of benzene. The main disadvantage of Kekule benzene is the presence of double bonds in it. If they did not exist, then one would not expect benzene to exhibit properties characteristic of alkenes. In this regard, it becomes clear why all further attempts to “improve” Kekule’s formula took the form of depriving it of double bonds, while retaining the cyclic structure of benzene. These are the formulas III – VII, proposed by Claus (1867), Dewar (1867), Armstrong–Bayer (1887), Thiele (1899) and Ladenburg (1869)

None of these formulas could explain all the properties inherent in benzene. This became possible only with the development of quantum chemistry.

According to modern ideas about the structure of benzene, its molecule is flat regular hexagon, at the tops of which there are carbon atoms located in sp 2 – hybrid state. Each of the six carbon atoms, due to three trigonal hybrid orbitals, forms two σ -bonds with neighboring carbons and another bond with hydrogen. All these bonds are located in the same plane at an angle of 120 0 to each other. Only two out of three are involved in hybridization r-electrons of carbon atoms. Therefore, after education σ -bonds on each of the six carbons of the benzene ring still have one more r-electron. From the history of establishing the structure of benzene, which stretched over many decades, it is clear how difficult it was for the idea that r-electrons are capable of overlapping with each other not only in pairs with the formation π - connections. Under some circumstances Possible cloud cover p- electrons with both a neighbor on the right and a neighbor on the left

This becomes possible if the molecule has a cyclic structure, the distances between the carbons are the same and the axes r-electrons are parallel to each other. The last condition is met if the molecule has a flat structure.

With this construction of the benzene molecule, the carbon atoms are connected to each other by neither single nor double bonds. These connections, most likely, should be classified as “one-and-a-half”. It is worth mentioning that according to the results of X-ray diffraction analysis of crystalline benzene, all carbon-carbon bonds in benzene have the same length of 0.14 nm, which is intermediate between single (0.154 nm) and double (0.134 nm) bonds.

Thus, according to modern ideas Benzene does not have the typical double bonds between carbons. Consequently, such a compound should not be expected to exhibit properties due to double bonds. At the same time, the significant unsaturation of the benzene molecule cannot be denied. A six-carbon cycloalkane (cyclohexane) contains 12 hydrogen atoms, while benzene has only 6. It follows that formally benzene could have three double bonds and behave like a cyclotriene in addition reactions. Indeed, under the conditions of addition reactions, benzene adds three molecules of hydrogen, halogens or ozone.

Currently, scientific and technical literature uses two graphic images benzene

One of them emphasizes the unsaturated nature of benzene, and the other emphasizes its aromaticity.

How can we relate the structure of benzene with its characteristic properties, mainly with its aromatic character? Why does benzene exhibit unique thermodynamic stability?

At one time it was shown that alkenes quite easily add a hydrogen molecule and turn into alkanes. This reaction proceeds with the release of heat, about 125.61 kJ for each double bond, and is called the heat of hydrogenation. Let's try to use the heat of hydrogenation to evaluate the thermodynamic stability of benzene.

Real-life cyclohexene, cyclohexadiene and benzene are hydrogenated into cyclohexane

The heat of hydrogenation of cyclohexene was 119.75 kJ. Then the expected value for cyclohexadiene should be 119.75 x 2 = 239.50 kJ (actually 231.96 kJ). If benzene had three double bonds (Kekule's cyclohexatriene), then its heat of hydrogenation would have to be 119.75 x 3 = 359.25 kJ. The experimental value in the latter case is strikingly different from the expected one. During the hydrogenation of benzene, only 208.51 kJ of heat is released, which is less than the expected value by 359.25 - 208.51 = 150.73 kJ. This energy is called resonance energy. If the hydrogenation of benzene releases 150.73 kJ less energy than the expected value, then this only means that benzene itself initially contains 150.73 kJ less energy than the hypothetical cyclohexatriene. It follows that benzene cannot have the structure of cyclohexatriene. The stability of the benzene molecule at the resonance energy is the result of the absence of isolated double bonds in it and the presence of a single electron cloud of the sextet r-electrons.

Having acquired high thermodynamic stability due to the benefits of its structure, benzene strives in every possible way to maintain this stability during chemical reactions. It is clear that this can only be realized if the chemical reaction benzene ring unchanged. This possibility is provided only by substitution reactions, and it is for this reason that substitution reactions are more typical for aromatic compounds than addition reactions. During electrophilic addition reactions, an aromatic compound ceases to be aromatic and loses exceptional stability along with the resonance energy that determines precisely this stability. For this reason, aromatic compounds undergo addition reactions much more difficultly than, for example, alkenes. Another feature of addition reactions involving aromatic compounds is their uncompromising nature. They either do not enter into addition reactions or add everything at once. This is evidenced by the fact that it is not possible to obtain partial hydrogenation or chlorination products from benzene. If these reactions already take place, they proceed in such a way that the products of complete hydrogenation or chlorination are immediately obtained

This development of events is due to the fact that a single electron cloud of six r-electrons in benzene either exist or do not exist, intermediate options for it are excluded.

Electronic and spatial
benzene structure

Grade 10 (professional level)

Target. Form a concept of aromatic bonds, features of the electronic structure and the resulting chemical properties of benzene.

Tasks. Comprehensively consider the structure of benzene as the most important representative of aromatic hydrocarbons; find out the nature of aromaticity.

Lesson type. Problem lecture.

In order to increase students' motivation for more successful learning new topic, you can prepare cards in advance with the names of the students, mix them and announce that at the end of the lesson, several schoolchildren present will get questions, but the children will find out who later.

PROGRESS OF THE CLASS

Teacher. Where do you think the name “aromatic hydrocarbons” comes from?

The first representatives of the class of aromatic hydrocarbons (arenes), isolated from natural objects, had a peculiar pleasant odor and were called “aromatic”. However, today the concept of “aromatic hydrocarbon” has a completely different meaning.

It is advisable to start the lesson with a consideration of the chemical properties, comparison and analysis of the results obtained and their generalization.

Demonstration experiments

1) Burning of paper soaked in benzene: indicates the possible unsaturation of the benzene molecule, since the flame is smoky, like the flame of acetylene.

2) Adding bromine water and a solution of potassium permanganate to benzene: does not confirm the unsaturated nature of the benzene molecule.

Based on this, students come to the conclusion about the specificity of the chemical properties of benzene, and therefore the structure of the molecule.

Questions for students

1) Describe the chemical properties of benzene.

2) What determine the properties of a substance?

3) What does the smoky flame indicate?

4) What does the lack of reaction with potassium permanganate and bromine water indicate?

The teacher writes down the gross formula of benzene (C 6 H 6) and suggests drawing up possible variants of structural formulas of linear and cyclic structure.

Historical background

It can be prepared in advance by one of the students, or it can be made by several students in the form of a presentation.

In 1825, M. Faraday isolated hydrocarbon from illuminating gas and studied its composition and properties. In 1835, E. Mitscherlich obtained a hydrocarbon by heating benzoic acid with quicklime, which turned out to be identical to the substance obtained by Faraday. Mitscherlich established its formula - C 6 H 6, Liebig later called it benzene.

Particular attention to this hydrocarbon for more than a century and a half is explained by its specific properties.

The first attempt to explain such properties of benzene was made in 1865 by A. Kekule (Fig. 1).

Along with the Kekulé formula, other benzene formulas have been proposed (Fig. 2).

The teacher talks about the interaction of benzene with three hydrogen molecules to form cyclohexane and about the production of benzene by passing acetylene through iron shavings heated to 500 ° C, notes that the structural formula of benzene should correspond to a hexagon with alternating double and single bonds.

The equations are written on the board:

C 6 H 6 + 3 H 2 -> C 6 H 12,

3C 2 H 2 -> C 6 H 6.

Next, the teacher reports some data on the cyclic structure of benzene, focusing on the following points: the location of all atoms in the same plane and the same distance between the nuclei of neighboring carbon atoms. Thanks to the discovery of the X-ray diffraction method, it became possible to explain the structure of the benzene molecule: when sp 2-hybridization from one s-orbitals and two p-orbitals, three hybrid orbitals are formed and one non-hybrid one remains r-orbital.

Hybrid orbitals form three -bonds, while non-hybrid orbitals are located perpendicular to the plane and form a single -electron cloud.

The concept of the electronic structure of benzene is supported by tables and models from the disk " Electronic textbook. Open chemistry 2.5"; volumetric models from the disk “Educational electronic edition. Virtual laboratory. Chemistry. grades 8–11.”

Teacher. Let's think, if there is an -bond in the molecule, then why don't the reactions characteristic of alkenes occur (addition of bromine and oxidation with potassium permanganate)?

ANSWER Combination of six-bonds with a single -electronic system are called aromatic bonds. The electron density is evenly distributed. Therefore, there are neither single nor double bonds in the benzene molecule. All bonds between carbon atoms in benzene are equivalent, which determines the properties characteristic of benzene. A cycle of six carbon atoms linked by six-bonds and a single -electron cloud are called a benzene ring or a benzene nucleus.

Physical methods Research has shown the following (see table).

Table

The structure of the benzene molecule

Let me remind you that a bond angle of 120° corresponds to sp 2 hybridization of carbon atoms.

So, let's summarize the study of the structure of the benzene molecule.

Here you can use prepared cards with surnames, or you can try asking a question at the address. The address is written on prepared cards in advance, for example: “street 1, house 3, apartment 1,” where street is the row number, house is the desk number, apartment is an option. A card is pulled out, the student “living” at this address is identified, and a question is asked, then the next card is pulled out.

Sample questions

1) What is the formula of the benzene molecule?

2) What type of hybridization does the carbon atoms in this molecule have?

3) What is aromaticity?

4) How does the structure of a molecule affect the properties of a substance?

5) In what year and by whom was benzene first obtained?

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 found 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 its structure internal structure: relatively stable benzene ring and 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. Structural formula 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 nitration, 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. Molecular formula 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. At the same time, both electrons and S-S connections, H-H 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 This 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 equalization of aromatic bonds, which determine the characteristic chemical and physical properties, which benzene has. The formula for the uniform distribution of the n-electron cloud and the equivalence of all of it internal connections 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.