Benzene with a double bond. Benzene: formula

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).

Aromatic compounds are those whose molecules contain a cyclic group of atoms with a special bonding pattern - a benzene ring. The international name for aromatic hydrocarbons is arenes.

The simplest representative of arenes is benzene C 6 H 6 . The formula reflecting the structure of the benzene molecule was first proposed by the German chemist Kekule (1865):

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

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 ring plane . 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 both sides of the σ bond plane ./>/>

p-The 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 often used indicating double bonds ( II

METHODOLOGICAL INSTRUCTIONS

organic chemistry course

« AROMATIC HYDROCARBONS»

Rostov-on-Don

Guidelines for the course of organic chemistry “Aromatic hydrocarbons”. - Rostov n/a: Rost. state builds. univ., 2007. - 12 p.

Theoretical principles on the topic “Aromatic hydrocarbons” are presented. A definition of aromatic hydrocarbons is given, as well as the concept of “aromaticity”. The structure of the benzene molecule is described. The nomenclature and isomerism of aromatic compounds with one benzene ring are considered. The main methods for producing arenes are presented, and the physical and chemical properties of aromatic hydrocarbons are also considered.

Designed for first- and second-year full-time and part-time students of the PSM, ZChS, SSP, BTP and AS specialties.

Compiled by: Ph.D. chem. Sciences, Associate Professor

M.N. Mitskaya,

Ph.D. chem. Sciences, Asst.

E.A. Levinskaya

Reviewer: Ph.D. chem. Sciences, Associate Professor

L.M. Astakhova

Construction University, 2007

Aromatic compounds (arenes) - organic compounds with a planar cyclic structure in which all carbon atoms create a single delocalized π-electron system containing (4n+2) π-electrons.

Aromatic compounds include primarily benzene C 6 H 6 and its numerous homologues and derivatives. Aromatic compounds may contain one or more benzene rings per molecule (polynuclear aromatic compounds). But we will look at aromatic compounds with one benzene ring.

The structure of the benzene molecule

Benzene was discovered by M. Faraday in 1825 in illuminating (coke oven) gas, and the structure of the benzene molecule is most often expressed by the formula proposed by the German chemist A. Kekule (1865)

According to modern concepts, the benzene molecule has the structure of a flat hexagon, the sides of which are equal to each other and amount to 0.14 nm. This distance is the average value between 0.154 nm (single bond length) and 0.134 nm (double bond length). Not only the carbon atoms, but also the six hydrogen atoms associated with them lie in the same plane. The angles formed by the H-C-C and C-C-C bonds are 120°:

All carbon atoms in a benzene molecule are in a state of sp 2 hybridization. Each of them is connected by its three hybrid orbitals with two of the same orbitals of two neighboring carbon atoms and one orbital of the H atom, forming three σ bonds (see figure). The fourth, unhybridized 2p orbital of the carbon atom, whose axis is perpendicular to the plane of the benzene ring, overlaps with similar orbitals of two neighboring carbon atoms located on the right and left.

Scheme of formation of σ-bonds and π-bonds in a benzene molecule

This overlap occurs above and below the plane of the benzene ring. As a result, a single closed system of π-electrons is formed. As a result of such uniform overlap of the 2p orbitals of all six carbon atoms, “alignment” of single and double bonds occurs, i.e. the benzene ring lacks classical double and single bonds. The uniform distribution of π-electron density between all carbon atoms, due to π-electron delocalization, is the reason for the high stability of the benzene molecule. Currently, there is no single way to graphically depict a benzene molecule taking into account its real properties. But to emphasize the uniformity of the π-electron density in the benzene molecule, they resort to the following formulas:

It is necessary, however, to remember that none of these formulas corresponds to the actual physical state of the molecule, much less can reflect the whole variety of its properties. Kekule's formula is currently only a symbol of the benzene molecule. However, it is widely used, while keeping in mind its disadvantages.

DEFINITION

Benzene(cyclohexatriene - 1,3,5) is an organic substance, the simplest representative of a number of aromatic hydrocarbons.

Formula – C 6 H 6 (structural formula – Fig. 1). Molecular weight – 78.11.

Rice. 1. Structural and spatial formulas of benzene.

All six carbon atoms in the benzene molecule are in the sp 2 hybrid state. Each carbon atom forms 3σ bonds with two other carbon atoms and one hydrogen atom, lying in the same plane. Six carbon atoms form a regular hexagon (σ-skeleton of the benzene molecule). Each carbon atom has one unhybridized p orbital containing one electron. Six p-electrons form a single π-electron cloud (aromatic system), which is depicted as a circle inside a six-membered ring. The hydrocarbon radical obtained from benzene is called C 6 H 5 - - phenyl (Ph-).

Chemical properties of benzene

Benzene is characterized by substitution reactions that occur via an electrophilic mechanism:

- halogenation (benzene reacts with chlorine and bromine in the presence of catalysts - anhydrous AlCl 3, FeCl 3, AlBr 3)

C 6 H 6 + Cl 2 = C 6 H 5 -Cl + HCl;

- nitration (benzene easily reacts with the nitrating mixture - a mixture of concentrated nitric and sulfuric acids)

- alkylation with alkenes

C 6 H 6 + CH 2 = CH-CH 3 → C 6 H 5 -CH(CH 3) 2;

Addition reactions to benzene lead to the destruction of the aromatic system and occur only under harsh conditions:

— hydrogenation (the reaction occurs when heated, the catalyst is Pt)

- addition of chlorine (occurs under the influence of UV radiation with the formation of a solid product - hexachlorocyclohexane (hexachlorane) - C 6 H 6 Cl 6)

Like any organic compound, benzene undergoes a combustion reaction with the formation of carbon dioxide and water as reaction products (burns with a smoky flame):

2C 6 H 6 +15O 2 → 12CO 2 + 6H 2 O.

Physical properties of benzene

Benzene is a colorless liquid, but has a specific pungent odor. Forms an azeotropic mixture with water, mixes well with ethers, gasoline and various organic solvents. Boiling point – 80.1C, melting point – 5.5C. Toxic, carcinogen (i.e. promotes the development of cancer).

Preparation and use of benzene

The main methods of obtaining benzene:

— dehydrocyclization of hexane (catalysts – Pt, Cr 3 O 2)

CH 3 –(CH 2) 4 -CH 3 → C 6 H 6 + 4H 2;

— dehydrogenation of cyclohexane (the reaction occurs when heated, the catalyst is Pt)

C 6 H 12 → C 6 H 6 + 4H 2;

— trimerization of acetylene (the reaction occurs when heated to 600C, the catalyst is activated carbon)

3HC≡CH → C 6 H 6 .

Benzene serves as a raw material for the production of homologues (ethylbenzene, cumene), cyclohexane, nitrobenzene, chlorobenzene and other substances. Previously, benzene was used as an additive to gasoline to increase its octane number, however, now, due to its high toxicity, the benzene content in fuel is strictly regulated. Benzene is sometimes used as a solvent.

Examples of problem solving

EXAMPLE 1

Exercise

Write down the equations that can be used to carry out the following transformations: CH 4 → C 2 H 2 → C 6 H 6 → C 6 H 5 Cl.

Solution

To produce acetylene from methane, the following reaction is used:

2CH 4 → C 2 H 2 + 3H 2 (t = 1400C).

The production of benzene from acetylene is possible by the trimerization reaction of acetylene, which occurs when heated (t = 600C) and in the presence of activated carbon:

3C 2 H 2 → C 6 H 6.

The chlorination reaction of benzene to produce chlorobenzene as a product is carried out in the presence of iron (III) chloride:

C 6 H 6 + Cl 2 → C 6 H 5 Cl + HCl.

EXAMPLE 2

Exercise

To 39 g of benzene in the presence of iron (III) chloride, 1 mol of bromine water was added. What amount of substance and how many grams of what products was produced?

Solution

Let us write the equation for the reaction of benzene bromination in the presence of iron (III) chloride:

C 6 H 6 + Br 2 → C 6 H 5 Br + HBr.

The reaction products are bromobenzene and hydrogen bromide. Molar mass of benzene, calculated using the table of chemical elements by D.I. Mendeleev – 78 g/mol. Let's find the amount of benzene:

n(C 6 H 6) = m(C 6 H 6) / M(C 6 H 6);

n(C 6 H 6) = 39 / 78 = 0.5 mol.

According to the conditions of the problem, benzene reacted with 1 mole of bromine. Consequently, benzene is in short supply and further calculations will be made using benzene. According to the reaction equation n(C 6 H 6): n(C 6 H 5 Br) : n(HBr) = 1:1:1, therefore n(C 6 H 6) = n(C 6 H 5 Br) =: n(HBr) = 0.5 mol. Then, the masses of bromobenzene and hydrogen bromide will be equal:

m(C 6 H 5 Br) = n(C 6 H 5 Br)×M(C 6 H 5 Br);

m(HBr) = n(HBr)×M(HBr).

Molar masses of bromobenzene and hydrogen bromide, calculated using the table of chemical elements by D.I. Mendeleev - 157 and 81 g/mol, respectively.

m(C 6 H 5 Br) = 0.5 × 157 = 78.5 g;

m(HBr) = 0.5×81 = 40.5 g.

Answer

The reaction products are bromobenzene and hydrogen bromide. The masses of bromobenzene and hydrogen bromide are 78.5 and 40.5 g, respectively.

Lesson objectives:

give an idea of the aromatic bond, its characteristics, establish the relationship between the structure of benzene and its properties;
consolidate the ability to compare the composition and structure of hydrocarbons of different series;
introduce the physical properties of benzene;
show the toxic effects of arenes on human health.

Lecture outline

Derivation of the molecular and structural formula of benzene.
History of the discovery of benzene.
Kekule's formula.
The structure of benzene.
The concept of “aromaticity”.
The emergence of the term “aromatic compounds”.
Physical properties of benzene.
Toxic effects of arenes on the human body.
Reinforcing the material covered.
Homework.

At the beginning of the lesson, I ask students to solve a problem to derive the formula of a substance.

Task. When 2.5 g of the substance was burned, 8.46 g of carbon dioxide and 1.73 g of water were released. The mass of 1 liter of the substance is 3.5 g. Determine the molecular and possible structural formula of the substance.

When solving the problem, students derive the molecular formula of the substance – C 6 H 6 . A problematic situation arises: “What structure can a benzene molecule have?” Based on knowledge about unsaturated hydrocarbons, students propose possible structural formulas for them:

NS C-CH 2 -CH 2 - C CH

H 2 C = CH -C C-CH = CH 2 and others.

Students conclude that benzene is a highly unsaturated compound and recall qualitative reactions to unsaturation.

I invite students to test the hypothesis about the unsaturation of benzene during an experiment. Having carried out the reactions of benzene with bromine water and a solution of potassium permanganate, students come to the conclusion that benzene, being an unsaturated system, does not give high-quality reactions to unsaturation, therefore, it cannot be classified as an unsaturated hydrocarbon.

What structure does the benzene molecule have, and to what class of hydrocarbons can it be classified?

Before answering this question, I introduce students to the history of the discovery of benzene, which is very interesting. Gas lighting first appeared in London between 1812 and 1815. Illuminating gas, extracted from the fat of sea animals, was delivered in iron cylinders. These cylinders were usually placed in the basement of the house, from which gas was distributed through tubes throughout the room. Soon, an extremely unpleasant circumstance was noticed - in extreme cold, the gas lost its ability to produce bright light when burning. The owners of a gas plant in 1825 turned to Faraday for advice, who found that those components that are capable of burning with a bright flame are collected at the bottom of the cylinder in the form of a transparent liquid layer. While studying this liquid, Faraday discovered a new hydrocarbon - benzene. The name of this substance was given by Liebig - (the suffix -ol indicates its oily nature, from the Latin oleum - oil).

In 1865, the German scientist Kekule proposed the structure of the benzene molecule, which he dreamed of as a snake biting its own tail:

But this formula, while corresponding to the elemental composition of benzene, does not correspond to many of its features:

benzene does not give qualitative reactions to unsaturation;
benzene is characterized by substitution rather than addition reactions;
Kekule's formula is not able to explain the equality of distances between carbon atoms, which occurs in a real benzene molecule.

To get out of this difficulty, Kekule admitted that in benzene there is a continuous movement of double bonds.

The use of modern physical and quantum research methods has made it possible to create a comprehensive understanding of the structure of benzene.

The carbon atoms in the benzene molecule are in the second valence state (sp 2). Each carbon atom forms -bonds with two other carbon atoms and one hydrogen atom lying in the same plane. The bond angles between the three -bonds are 120°. Thus, all six carbon atoms lie in the same plane, forming a regular hexagon (Fig. 1):

Rice. 1. Scheme of formation of -connections
in a benzene molecule

Each carbon atom has one non-hybrid p orbital. Six such orbitals are located perpendicular to the -bond plane and parallel to each other (Fig. 2). All six p-electrons interact with each other, forming a single -electron cloud. Thus, circular conjugation occurs in the benzene molecule. The highest electron density in this conjugated system is located above and below the plane of the ring (Fig. 3):

As a result of such uniform overlap of the 2p orbitals of all six carbon atoms, the “alignment” of single and double bonds occurs - the bond length is 0.139 nm. This value is intermediate between the length of a single bond in alkanes (0.154 nm) and the length of a double bond in alkenes (0.133 nm). That is, the benzene molecule lacks classical double and single bonds.

Circular conjugation gives an energy gain of 150 kJ/mol. This value constitutes the conjugation energy - the amount of energy that must be expended to disrupt the aromatic system of benzene.

This electronic structure explains all the features of benzene. In particular, why benzene is difficult to enter into addition reactions - this leads to a violation of conjugation. Such reactions are possible under harsh conditions.

Currently, there is no single way to graphically depict a benzene molecule taking into account its real properties. But in order to emphasize the uniformity of the electron density in the benzene molecule, they resort to the following formulas:

They also use the Kekule formula, keeping in mind its shortcomings.

The set of properties of benzene is usually called aromaticity. In general terms, the phenomenon of aromaticity was formulated by the German physicist Hückel: a compound must exhibit aromatic properties if its molecule contains a flat ring with (4n+2) electrons, where n can take values 0, 1, 2, 3, etc. According to this rule, systems containing 6, 10, 14 electrons are aromatic.

Examples of such compounds are naphthalene (n=2) and anthracene (n=3).

After reviewing the structure of benzene, we discuss the answers to the questions with students:

Can benzene be classified as unsaturated hydrocarbons? Justify your answer.
What class of hydrocarbons does benzene belong to?
What is meant by the term “aromatic compound”?
What hydrocarbons are called aromatic?

Next, I introduce students to the origin of the term “aromatic compounds.” I inform you that this name arose in the initial period of the development of chemistry. It was noticed that benzene compounds are obtained during the distillation of some pleasant-smelling (aromatic) substances - natural resins and balms. However, most aromatic compounds are odorless or unpleasant-smelling. But this term has been preserved in chemistry. Aromatic hydrocarbons (arenes) are substances whose molecules contain one or more benzene rings - cyclic groups of carbon atoms with a special character of bonds.

Next, students become familiar with the physical properties of benzene by working with educational literature. They know that benzene is a liquid and can also be in a vapor state (during odor testing). I introduce students to benzene in solid form. The melting point of benzene is 5.5°C. Based on this information, I demonstrate the transformation of liquid benzene into a white crystalline mass. To do this, I put 4-5 ml of benzene in a test tube into a vessel filled with snow or ice. After a few minutes, students observe a change in the state of aggregation of benzene. Based on observations, students suggest that this substance must have a molecular crystal lattice.

I draw students' attention to the fact that benzene is a highly toxic substance. Inhaling its vapors causes dizziness and headaches. At high concentrations of benzene, cases of loss of consciousness are possible. Its vapors irritate the eyes and mucous membranes.

Liquid benzene easily penetrates the body through the skin, which can lead to poisoning. Therefore, working with benzene and its homologues requires special care.

I use the material on the topic “Benzene” to explain the harm of smoking. Studies of a tar-like substance obtained from tobacco smoke have shown that, in addition to nicotine, it contains aromatic hydrocarbons such as benzopyrene,

having strong carcinogenic properties, i.e. these substances act as cancer causative agents. Tobacco tar, when it comes into contact with the skin and lungs, causes the formation of cancerous tumors. Smokers are more likely to develop cancer of the lip, tongue, larynx, and esophagus. They are much more likely to suffer from angina pectoris and myocardial infarction. I note that a smoker releases about 50% of toxic substances into the surrounding space, creating around himself a ring of “passive smokers” who quickly develop headaches, nausea, general malaise, and then may develop chronic diseases.

At the end of the lesson I conduct a frontal survey on the following questions:

Homework: pp. 55-58, pp. 61 No. 1, 2 according to the textbook by E. E. Nifantiev, L. A. Tsvetkova “Chemistry 10-11”.