Chemical properties. Reactivity of benzene homologues Chemical properties of benzene

The first group of reactions is substitution reactions. We said that arenes do not have multiple bonds in the molecular structure, but contain a conjugated system of six electrons, which is very stable and gives additional strength to the benzene ring. Therefore, in chemical reactions, first of all, the substitution of hydrogen atoms occurs, and not the destruction of the benzene ring.

We have already encountered substitution reactions when talking about alkanes, but for them these reactions proceeded according to a radical mechanism, while arenes are characterized by an ionic mechanism of substitution reactions.

First chemical property - halogenation. Substitution of a hydrogen atom for a halogen atom - chlorine or bromine.

The reaction proceeds when heated and always with the participation of a catalyst. In the case of chlorine, it can be aluminum chloride or iron chloride three. The catalyst polarizes the halogen molecule, resulting in heterolytic bond breaking and ions are obtained.

The positively charged chloride ion reacts with benzene.

If the reaction occurs with bromine, then iron tribromide or aluminum bromide acts as a catalyst.

It is important to note that the reaction occurs with molecular bromine and not with bromine water. Benzene does not react with bromine water.

The halogenation of benzene homologues has its own characteristics. In the toluene molecule, the methyl group facilitates substitution in the ring, the reactivity increases, and the reaction proceeds under milder conditions, that is, already at room temperature.

It is important to note that the substitution always occurs in the ortho and para positions, so a mixture of isomers is obtained.

Second property - nitration of benzene, the introduction of a nitro group into the benzene ring.

A heavy yellowish liquid with the smell of bitter almonds is formed - nitrobenzene, so the reaction can be qualitative for benzene. For nitration, a nitrating mixture of concentrated nitric and sulfuric acids is used. The reaction is carried out by heating.

Let me remind you that for the nitration of alkanes in the Konovalov reaction, dilute nitric acid was used without the addition of sulfuric acid.

In the nitration of toluene, as well as in the halogenation, a mixture of ortho- and para-isomers is formed.

Third property - alkylation of benzene with haloalkanes.

This reaction allows the introduction of a hydrocarbon radical into the benzene ring and can be considered a method for obtaining benzene homologues. Aluminum chloride is used as a catalyst, which promotes the decomposition of the haloalkane molecule into ions. It also needs heating.

Fourth property - alkylation of benzene with alkenes.

In this way, for example, cumene or ethylbenzene can be obtained. The catalyst is aluminum chloride.

2. Reactions of addition to benzene

The second group of reactions is addition reactions. We said that these reactions are not characteristic, but they are possible under rather harsh conditions with the destruction of the pi-electron cloud and the formation of six sigma bonds.

Fifth property in the general list - hydrogenation, addition of hydrogen.

Temperature, pressure, catalyst nickel or platinum. Toluene is able to react in the same way.

sixth property - chlorination. Please note that we are talking specifically about the interaction with chlorine, since bromine does not enter into this reaction.

The reaction proceeds under hard ultraviolet irradiation. Hexachlorocyclohexane, another name for hexachlorane, is formed, a solid.

It is important to remember that for benzene not possible addition reactions of hydrogen halides (hydrohalogenation) and addition of water (hydration).

3. Substitution in the side chain of benzene homologues

The third group of reactions concerns only benzene homologues - this is a substitution in the side chain.

seventh a property in the general list is halogenation at the alpha carbon atom in the side chain.

The reaction occurs when heated or irradiated, and always only at the alpha carbon. As the halogenation continues, the second halogen atom will return to the alpha position.

4. Oxidation of benzene homologues

The fourth group of reactions is oxidation.

The benzene ring is too strong, so benzene does not oxidize potassium permanganate - does not discolor its solution. This is very important to remember.

On the other hand, benzene homologues are oxidized with an acidified solution of potassium permanganate when heated. And this is the eighth chemical property.

It turns out benzoic acid. Discoloration of the solution is observed. In this case, no matter how long the carbon chain of the substituent is, it always breaks after the first carbon atom and the alpha atom is oxidized to a carboxyl group with the formation of benzoic acid. The rest of the molecule is oxidized to the corresponding acid or, if it is only one carbon atom, to carbon dioxide.

If the benzene homologue has more than one hydrocarbon substituent on the aromatic ring, then the oxidation occurs according to the same rules - the carbon in the alpha position is oxidized.

In this example, a dibasic aromatic acid is obtained, which is called phthalic acid.

In a special way, I note the oxidation of cumene, isopropylbenzene, with atmospheric oxygen in the presence of sulfuric acid.

This is the so-called cumene method for producing phenol. As a rule, one has to deal with this reaction in matters relating to the production of phenol. This is the industrial way.

ninth property - combustion, complete oxidation with oxygen. Benzene and its homologues burn to carbon dioxide and water.

Let us write the equation for the combustion of benzene in a general form.

According to the law of conservation of mass, there should be as many atoms on the left as there are atoms on the right. Because, after all, in chemical reactions, atoms do not go anywhere, but the order of bonds between them simply changes. So there will be as many carbon dioxide molecules as there are carbon atoms in an arene molecule, since the molecule contains one carbon atom. That is n CO 2 molecules. There will be half as many water molecules as hydrogen atoms, that is, (2n-6) / 2, which means n-3.

There are the same number of oxygen atoms on the left and on the right. On the right, there are 2n from carbon dioxide, because there are two oxygen atoms in each molecule, plus n-3 from water, for a total of 3n-3. On the left, there are the same number of oxygen atoms - 3n-3, which means there are half as many molecules, because the molecule contains two atoms. That is (3n-3)/2 oxygen molecules.

Thus, we have compiled the equation for the combustion of benzene homologues in a general form.

DEFINITION

Benzene(cyclohexatriene - 1,3,5) - 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, which contains one electron. Six p-electrons form a single π-electron cloud (aromatic system), which is depicted as a circle inside a six-membered cycle. The hydrocarbon radical derived from benzene is called C 6 H 5 - - phenyl (Ph-).

Chemical properties of benzene

Benzene is characterized by substitution reactions proceeding according to the electrophilic mechanism:

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

C 6 H 6 + Cl 2 \u003d C 6 H 5 -Cl + HCl;

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

- alkylation with alkenes

C 6 H 6 + CH 2 \u003d CH-CH 3 → C 6 H 5 -CH (CH 3) 2;

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

- hydrogenation (the reaction proceeds when heated, the catalyst is Pt)

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

Like any organic compound, benzene enters into a combustion reaction with the formation of carbon dioxide and water as reaction products (it 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. contributes to the development of cancer).

Obtaining and using benzene

The main methods for 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 proceeds when heated, the catalyst is Pt)

C 6 H 12 → C 6 H 6 + 4H 2;

– trimerization of acetylene (the reaction proceeds 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 content of benzene in fuel is strictly regulated. Sometimes benzene is used as a solvent.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

Such a compound as benzene, Mrs. Chemistry in her household finally and irrevocably acquired only in 1833. Benzene is a compound that has a quick-tempered, one might say, even explosive character. How did you find out?

Story

Johann Glauber in 1649 turned his attention to a compound that had been successfully formed while the chemist was working on coal tar. But it wished to remain incognito.

After about 170 years, and to be much more precise, in the mid-twenties of the XIX century, by chance, benzene was extracted from the lighting gas, namely from the released condensate. Mankind owes such efforts to Michael Faraday, a scientist from England.

The baton for the acquisition of benzene was intercepted by the German Eilgard Mitscherlich. This happened during the processing of anhydrous calcium salts of benzoic acid. Perhaps that is why the compound was given such a name - benzene. Still, as an option, the scientist called it gasoline. Incense, if translated from Arabic.

Benzene burns beautifully and brightly, in connection with these observations, Auguste Laurent advised to call it "fen" or "benzene". Bright, shining - if translated from Greek.

Based on the opinion of the concept of the nature of the electronic bond, the qualities of benzene, the scientist provided the molecule of the compound in the form of the following image. This is a hexagon. A circle is inscribed in it. The foregoing suggests that benzene has an integral electron cloud, which safely contains six (without exception) carbon atoms of the cycle. There are no fixed binary bonds.

Benzene was previously treated as a solvent. But basically, as they say, he was not a member, did not participate, was not involved. But this is in the 19th century. Significant changes took place in the XX. The properties of benzene express the most valuable qualities that helped him become more popular. The octane number, which turned out to be high, made it possible to use it as a fuel element for refueling cars. This action was the impetus for the extensive withdrawal of benzene, the extraction of which is carried out as a by-product of the coking of steel production.

By the forties, in the chemical field, benzene began to be consumed in the manufacture of substances that explode quickly. The 20th century crowned itself with the fact that the oil refining industry produced so much benzene that it began to supply the chemical industry.

Characterization of benzene

Unsaturated hydrocarbons are very similar to benzene. For example, the hydrocarbon series of ethylene characterizes itself as an unsaturated hydrocarbon. It has an addition reaction. Benzene willingly enters into All this thanks to the atoms that are in the same plane. And as a fact - the conjugated electron cloud.

If there is a benzene ring in the formula, then we can come to the elementary conclusion that this is benzene, the structural formula of which looks exactly like this.

Physical properties

Benzene is a colorless liquid, but has an unfortunate smell. Benzene melts when the temperature reaches 5.52 degrees Celsius. Boils at 80.1. The density is 0.879 g / cm 3, the molar mass is 78.11 g / mol. When burning, it smokes heavily. Forms explosive compounds when air enters. rocks (gasoline, ether and others) are combined with the described substance without problems. An azeotropic compound creates with water. Heating before the start of vaporization occurs at 69.25 degrees (91% benzene). At 25 degrees Celsius, 1.79 g / l can be dissolved in water.

Chemical properties

Benzene reacts with sulfuric and nitric acid. And also with alkenes, halogens, chloroalkanes. The substitution reaction is what is characteristic of him. The pressure temperature affects the breakthrough of the benzene ring, which takes place under rather harsh conditions.

We can consider each benzene reaction equation in more detail.

1. Electrophilic substitution. Bromine, in the presence of a catalyst, interacts with chlorine. The result is chlorobenzene:

С6H6+3Cl2 → C6H5Cl + HCl

2. Friedel-Crafts reaction, or benzene alkylation. The appearance of alkylbenzenes occurs due to the combination with alkanes, which are halogen derivatives:

C6H6 + C2H5Br → C6H5C2H5 + HBr

3. Electrophilic substitution. Here is the reaction of nitration and sulfonation. The benzene equation will look like this:

C6H6 + H2SO4 → C6H5SO3H + H2O

C6H6 + HNO3 → C6H5NO2 + H2O

4. Benzene when burning:

2C6H6 + 15O2 → 12CO2 + 6H2O

Under certain conditions, it exhibits a character characteristic of saturated hydrocarbons. The p-electron cloud, which is in the structure of the substance under consideration, explains these reactions.

Various types of benzene depend on special technology. This is where the labeling of petroleum benzene comes from. For example, purified and higher purification, for synthesis. I would like to separately note the homologues of benzene, and more specifically, their chemical properties. These are alkylbenzenes.

Benzene homologues are much more likely to react. But the above reactions of benzene, namely homologues, take place with some difference.

Halogenation of alkylbenzenes

The form of the equation is as follows:

С6H5-CH3 + Br = C6H5-CH2Br + HBr.

The aspiration of bromine into the benzene ring is not observed. It goes into the chain on the side. But thanks to the Al(+3) salt catalyst, bromine boldly enters the ring.

Nitration of alkylbenzenes

Thanks to sulfuric and nitric acids, benzenes and alkylbenzenes are nitrated. Reactive alkylbenzenes. Two products are obtained from the three presented - these are para- and ortho-isomers. You can write one of the formulas:

C6H5 - CH3 + 3HNO3 → C6H2CH3 (NO2)3.

Oxidation

For benzene, this is unacceptable. But alkylbenzenes react willingly. For example, benzoic acid. The formula is below:

C6H5CH3 + [O] → C6H5COOH.

Alkylbenzene and benzene, their hydrogenation

In the presence of an enhancer, hydrogen begins to react with benzene, resulting in the formation of cyclohexane, as discussed above. Similarly, alkylbenzenes are easily converted to alkylcyclohexanes. To obtain alkylcyclohexane, it is required to subject the desired alkylbenzene to hydrogenation. Basically, it is a necessary procedure for the production of a pure product. And this is not all the reactions of benzene and alkylbenzene.

Benzene production. Industry

The foundation of such production is based on the processing of components: toluene, naphtha, tar, which is released during coal cracking, and others. Therefore, benzene is produced at petrochemical, metallurgical enterprises. It is important to know how to get benzene of varying degrees of purification, because the brand of this substance is directly dependent on the principle of manufacture and purpose.

The lion's share is made by thermocatalytic reforming of the caustobiolite part, boiling off at 65 degrees, having the effect of an extract, distillation with dimethylformamide.

During the production of ethylene and propylene, liquid products are obtained, which are formed during the decomposition of inorganic and organic compounds under the influence of heat. Of these, benzene is isolated. But, unfortunately, there is not so much source material for this benzene production option. Therefore, the substance of interest to us is produced by reforming. By this method, the volume of benzene is increased.

By dealkylation at a temperature of 610-830 degrees with a plus sign, in the presence of steam formed during the boiling of water and hydrogen, benzene is obtained from toluene. There is another option - catalytic. When the presence of zeolites, or, alternatively, oxide catalysts, is observed, subject to a temperature regime of 227-627 degrees.

There is another, older, way to develop benzene. With the help of absorption by organic absorbers, it is isolated from the final result of coal coking. The product is steam-gas and has been pre-cooled. For example, oil is used, the source of which is oil or coal. When the distillation is carried out with steam, the scavenger is separated. Hydrotreating helps to free crude benzene from excess substances.

Coal raw materials

In metallurgy, when coal is used, or, to be more precise, its dry distillation, coke is obtained. During this procedure, the air supply is limited. Do not forget that coal is heated to a temperature of 1200-1500 Celsius.

Coal-chemical benzene needs thorough purification. It is necessary to get rid of methyl cyclohexane and its companion n-heptane without fail. should also be removed. Benzene is to undergo a process of separation, purification, which will be carried out more than once.

The method described above is the oldest, but after time it loses its high position.

Oil fractions

0.3-1.2% - such indicators of the composition of our hero in crude oil. Scanty indicators to invest finances and forces. It is best to use an industrial procedure for processing petroleum fractions. That is catalytic reforming. In the presence of an aluminum-platinum-rhenium amplifier, the percentage of inclusion of aromatic carbohydrates increases, and the indicator that determines the ability of the fuel not to ignite spontaneously during its compression increases.

Pyrolysis resins

If our oil product is extracted from non-solid raw materials, namely by pyrolysis of propylene and ethylene arising in the manufacture, then this approach will be the most acceptable. To be precise, benzene is released from the pyrocondensate. Decomposition of certain fractions requires hydrotreatment. Sulfur and unsaturated mixtures are removed during cleaning. In the initial result, the content of xylene, toluene, benzene was noted. With the help of a distillation, which is extractive, the BTX group is separated and benzene is obtained.

Hydrodealkylation of toluene

The protagonists of the process, a cocktail of hydrogen flow and toluene, are fed heated into the reactor. Toluene passes through the catalyst bed. During this process, the methyl group is separated to form benzene. There is a certain way of purification here. The result is a highly pure substance (for nitration).

Toluene disproportionation

As a result of the rejection of the methyl class, creation to benzene takes place, xylene is oxidized. In this process, transalkylation has been observed. The catalytic action is due to palladium, platinum and neodymium, which are on aluminum oxide.

Toluene and hydrogen are supplied to the reactor with a stable catalyst bed. Its purpose is to keep hydrocarbons from settling onto the catalyst plane. The stream that exits the reactor is cooled, and hydrogen is safely recovered for recycle. What is left is distilled three times. At the initial stage, compounds that are non-aromatic are withdrawn. Benzene is extracted second, and the last step is the extraction of xylenes.

Acetylene trimerization

Thanks to the work of the French physical chemist Marcelin Berthelot, benzene began to be produced from acetylene. But at the same time, a heavy cocktail stood out from many other elements. The question was how to lower the reaction temperature. The answer was received only at the end of the forties of the XX century. V. Reppe found an appropriate catalyst, it turned out to be nickel. Trimerization is the only way to get benzene from acetylene.

The formation of benzene occurs with the help of activated carbon. At high heat rates, acetylene passes over the coal. Benzene is released if the temperature is at least 410 degrees. At the same time, a variety of aromatic hydrocarbons are still born. Therefore, good equipment is needed that is capable of purifying acetylene in a qualitative manner. With such a laborious method as trimerization, a lot of acetylene is consumed. To get 15 ml of benzene, 20 liters of acetylene are taken. You can see how it looks in the reaction will not take long.

3C2H2 → C6H6 (Zelinsky equation).

3CH → CH = (t, kat) = C6H6.

Where is benzene used

Benzene is a fairly popular brainchild of chemistry. Especially often it was noticed how benzene was used in the manufacture of cumene, cyclohexane, ethylbenzene. To create styrene, ethylbenzene is indispensable. The starting material for the production of caprolactam is cyclohexane. When making a thermoplastic resin, it is caprolactam that is used. The described substance is indispensable in the manufacture of various paints and varnishes.

How dangerous is benzene

Benzene is a toxic substance. The manifestation of a feeling of malaise, which is accompanied by nausea and severe dizziness, is a sign of poisoning. Even death is not ruled out. The feeling of indescribable delight is no less disturbing bells in case of benzene poisoning.

Liquid benzene causes skin irritation. Benzene vapors easily penetrate even through intact skin. With the most short-term contact with a substance in a small dose, but on a regular basis, unpleasant consequences will not be long in coming. This may be a lesion of the bone marrow and acute leukemia of various types.

In addition, the substance is addictive in humans. Benzene acts like a dope. Tobacco smoke produces a tar-like product. When they studied it, they came to the conclusion that the content of the latter is not safe for humans. In addition to the presence of nicotine, the presence of aromatic carbohydrates of the benzpyrene type was also found. A distinctive feature of benzpyrene are carcinogens. They have a very harmful effect. For example, they cause cancer.

Despite the foregoing, benzene is a starting raw material for the production of various drugs, plastics, synthetic rubber and, of course, dyes. This is the most common brainchild of chemistry and aromatic compound.

The cyclic structure of benzene was first proposed by F.A. Kekule in 1865

Friedrich August Kekule von Stradonitz was an outstanding German chemist of the 19th century. In 1854, he discovered the first organic compound containing sulfur - thioacetic acid (thioethanoic acid). In addition, he established the structure of diazo compounds. However, his most famous contribution to the development of chemistry is the establishment of the structure of benzene (1866). Kekule showed that the double bonds of benzene alternate around the ring (this idea first occurred to him in a dream). He later showed that the two possible double bond arrangements are identical and that the benzene ring is a hybrid between the two structures. Thus, he anticipated the concept of resonance (mesomerism), which appeared in the theory of chemical bonding in the early 1930s.

If benzene really had such a structure, then its 1,2-disubstituted derivatives should have two isomers each. For example,

However, none of the 1,2-disubstituted benzenes can isolate two isomers.

Therefore, Kekule subsequently suggested that the benzene molecule exists as two structures rapidly passing into each other:

Note that such schematic representations of benzene molecules and their derivatives usually do not indicate the hydrogen atoms attached to the carbon atoms of the benzene ring.

In modern chemistry, the benzene molecule is considered as a resonant hybrid of these two limiting resonant forms (see Section 2.1). Another description of the benzene molecule is based on a consideration of its molecular orbitals. In sec. 3.1, it was indicated that the -electrons located in the -bonding orbitals are delocalized between all carbon atoms of the benzene ring and form an -electron cloud. In accordance with this representation, the benzene molecule can be conventionally depicted as follows:

Experimental data confirm the presence of just such a structure in benzene. If benzene had the structure that Kekule originally proposed, with three conjugated double bonds, then benzene would have to enter into addition reactions like alkenes. However, as mentioned above, benzene does not enter into addition reactions. In addition, benzene is more stable than if it had three isolated double bonds. In sec. 5.3 it was indicated that the enthalpy of hydrogenation of benzene with the formation of cyclohexane has a larger negative

Table 18.3. Length of various carbon-carbon bonds

Rice. 18.6. The geometric structure of the benzene molecule.

value than three times the enthalpy of hydrogenation of cyclohexene. The difference between these values ​​is usually called the delocalization enthalpy, resonant energy, or benzene stabilization energy.

All carbon-carbon bonds in the benzene ring have the same length, which is less than the length of C-C bonds in alkanes, but longer than the length of C=C bonds in alkenes (Table 18.3). This confirms that the carbon-carbon bonds in benzene are a hybrid between single and double bonds.

The benzene molecule has a flat structure, which is shown in Fig. 18.6.

Physical properties

Under normal conditions, benzene is a colorless liquid that freezes at 5.5°C and boils at 80°C. It has a characteristic pleasant smell, but, as mentioned above, is highly toxic. Benzene is immiscible with water, and in the benzene system, water forms the top of the two layers. However, it is soluble in non-polar organic solvents and is itself a good solvent for other organic compounds.

Chemical properties

Although benzene enters into certain addition reactions (see below), it does not exhibit the reactivity typical of alkenes in them. For example, it does not decolorize bromine water or α-ion solutions. In addition, benzene

enters into addition reactions with strong acids, such as hydrochloric or sulfuric acid.

At the same time, benzene takes part in a number of electrophilic substitution reactions. Aromatic compounds are the products of reactions of this type, since the delocalized -electron system of benzene is preserved in these reactions. The general mechanism of substitution of a hydrogen atom on a benzene ring by some electrophile is described in Sec. 17.3. Examples of electrophilic substitution of benzene are its nitration, halogenation, sulfonation, and Friedel-Crafts reactions.

Nitration. Benzene can be nitrated (introducing a group into it) by treating it with a mixture of concentrated nitric and sulfuric acids:

Nitrobenzene

The conditions for this reaction and its mechanism are described in Sec. 17.3.

Nitrobenzene is a pale yellow liquid with a characteristic almond odor. During the nitration of benzene, in addition to nitrobenzene, crystals of 1,3-dinitrobenzene are also formed, which is the product of the following reaction:

Halogenation. If you mix benzene in the dark with chlorine or bromine, no cancer will occur. However, in the presence of catalysts with the properties of Lewis acids, electrophilic substitution reactions occur in such mixtures. Typical catalysts for these reactions are iron(III) bromide and aluminum chloride. The action of these catalysts is that they create polarization in the halogen molecules, which then form a complex with the catalyst:

although there is no direct evidence that free ions are formed in this case. The mechanism of benzene bromination using iron (III) bromide as an ion carrier can be represented as follows:

Sulfonation. Benzene can be sulfonated (replacing a hydrogen atom in it with a sulfo group) by refluxing its mixture with concentrated sulfuric acid for several hours. Instead, benzene can be gently heated mixed with fuming sulfuric acid. Fuming sulfuric acid contains sulfur trioxide. The mechanism of this reaction can be represented by the scheme

Friedel-Crafts reactions. Friedel-Crafts reactions were originally called condensation reactions between aromatic compounds and alkyl halides in the presence of an anhydrous aluminum chloride catalyst.

In condensation reactions, two molecules of reactants (or one reactant) are combined with each other, forming a molecule of a new compound, while a molecule of some simple compound, such as water or hydrogen chloride, is split off (eliminates) from them.

Currently, the Friedel-Crafts reaction is any electrophilic substitution of an aromatic compound in which a carbocation or a highly polarized complex with a positively charged carbon atom plays the role of an electrophile. The electrophilic agent is usually an alkyl halide or chloride of a carboxylic acid, although it can also be, for example, an alkene or an alcohol. Anhydrous aluminum chloride is usually used as a catalyst for these reactions. Friedel-Crafts reactions are usually divided into two types: alkylation and acylation.

Alkylation. In Friedel-Crafts reactions of this type, one or more hydrogen atoms in the benzene ring are replaced by alkyl groups. For example, when a mixture of benzene and chloromethane is heated carefully in the presence of anhydrous aluminum chloride, methylbenzene is formed. Chloromethane plays the role of an electrophilic agent in this reaction. It is polarized by aluminum chloride in the same way as it happens with halogen molecules:

The mechanism of the reaction under consideration can be represented as follows:

It should be noted that in this condensation reaction between benzene and chloromethane, a hydrogen chloride molecule is split off. We also note that the real existence of a metal carbocation in the form of a free ion is doubtful.

Alkylation of benzene with chloromethane in the presence of a catalyst - anhydrous aluminum chloride does not end with the formation of methylbenzene. In this reaction, further alkylation of the benzene ring occurs, leading to the formation of 1,2-dimethylbenzene:

Acylation. In Friedel-Crafts reactions of this type, a hydrogen atom in the benzene ring is replaced by an acyl group, resulting in the formation of an aromatic ketone.

The acyl group has the general formula

The systematic name of an acyl compound is formed by replacing the suffix and ending -ova in the name of the corresponding carboxylic acid, of which the given acyl compound is a derivative, with the suffix -(o)yl. for example

Acylation of benzene is carried out using a chloride or anhydride of a carboxylic acid in the presence of an anhydrous aluminum chloride catalyst. for example

This reaction is a condensation in which the elimination of a hydrogen chloride molecule occurs. Note also that the name "phenyl" is often used to denote the benzene ring in compounds where benzene is not the main group:

Addition reactions. Although benzene is most characteristic of electrophilic substitution reactions, it also enters into some addition reactions. We have already met one of them. We are talking about the hydrogenation of benzene (see Section 5.3). When a mixture of benzene and hydrogen is passed over the surface of a finely ground nickel catalyst at a temperature of 150–160 °C, a whole sequence of reactions occurs, which ends with the formation of cyclohexane. The overall stoichiometric equation for this reaction can be represented as follows:

Under the influence of ultraviolet radiation or direct sunlight, benzene also reacts with chlorine. This reaction is carried out by a complex radical mechanism. Its final product is 1,2,3,4,5,6-hexachlorocyclohexane:

A similar reaction takes place between benzene and bromine under the action of ultraviolet radiation or sunlight.

Oxidation. Benzene and the benzene ring in other aromatic compounds are generally resistant to oxidation even by such strong oxidizing agents as an acidic or alkaline solution of potassium permanganate. However, benzene and other aromatics burn in air or oxygen to form a very smoky flame, which is typical for hydrocarbons with a high relative carbon content.

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, however, they easily dissolve in organic solvents - alcohol, ether, acetone.

Benzene and its homologues are themselves good solvents for many organic substances. All arenas burn with a smoky flame due to the high carbon content in their molecules.

The physical properties of some arenes are presented in the table.

Table. Physical properties of some arenas

Benzene - low-boiling ( tkip= 80.1°C), colorless liquid, insoluble in water

Attention! Benzene - poison, acts on the kidneys, changes the blood formula (with prolonged exposure), can disrupt the structure of chromosomes.

Most aromatic hydrocarbons are life threatening and toxic.

Obtaining arenes (benzene and its homologues)

In the laboratory

1. Fusion of salts of benzoic acid with solid alkalis

C 6 H 5 -COONa + NaOH t → C 6 H 6 + Na 2 CO 3

sodium benzoate

2. Wurtz-Fitting reaction: (here G is halogen)

From 6H 5 -G+2Na + R-G →C 6 H 5 - R + 2 NaG

With 6 H 5 -Cl + 2Na + CH 3 -Cl → C 6 H 5 -CH 3 + 2NaCl

In industry

• isolated from oil and coal by fractional distillation, reforming;
• from coal tar and coke oven gas

1. Dehydrocyclization of alkanes with more than 6 carbon atoms:

C 6 H 14 t , kat→C 6 H 6 + 4H 2

2. Trimerization of acetylene(only for benzene) – R. Zelinsky:

3С 2 H2 600°C, Act. coal→C 6 H 6

3. Dehydrogenation cyclohexane and its homologues:

Soviet Academician Nikolai Dmitrievich Zelinsky established that benzene is formed from cyclohexane (dehydrogenation of cycloalkanes

C 6 H 12 t, cat→C 6 H 6 + 3H 2

C 6 H 11 -CH 3 t , kat→C 6 H 5 -CH 3 + 3H 2

methylcyclohexanetoluene

4. Alkylation of benzene(obtaining homologues of benzene) – r Friedel-Crafts.

C 6 H 6 + C 2 H 5 -Cl t, AlCl3→C 6 H 5 -C 2 H 5 + HCl

chloroethane ethylbenzene

Chemical properties of arenes

I. OXIDATION REACTIONS

1. Combustion (smoky flame):

2C 6 H 6 + 15O 2 t→12CO 2 + 6H 2 O + Q

2. Benzene under normal conditions does not decolorize bromine water and an aqueous solution of potassium permanganate

3. Benzene homologues are oxidized by potassium permanganate (discolor potassium permanganate):

A) in an acidic environment to benzoic acid

Under the action of potassium permanganate and other strong oxidants on the homologues of benzene, the side chains are oxidized. No matter how complex the chain of the substituent is, it is destroyed, with the exception of the a -carbon atom, which is oxidized into a carboxyl group.

Homologues of benzene with one side chain give benzoic acid:

Homologues containing two side chains give dibasic acids:

5C 6 H 5 -C 2 H 5 + 12KMnO 4 + 18H 2 SO 4 → 5C 6 H 5 COOH + 5CO 2 + 6K 2 SO 4 + 12MnSO 4 + 28H 2 O

5C 6 H 5 -CH 3 + 6KMnO 4 + 9H 2 SO 4 → 5C 6 H 5 COOH + 3K 2 SO 4 + 6MnSO 4 + 14H 2 O

Simplified :

C 6 H 5 -CH 3 + 3O KMnO4→C 6 H 5 COOH + H 2 O

B) in neutral and slightly alkaline to salts of benzoic acid

C 6 H 5 -CH 3 + 2KMnO 4 → C 6 H 5 COO K + K OH + 2MnO 2 + H 2 O

II. ADDITION REACTIONS (harder than alkenes)

1. Halogenation

C 6 H 6 + 3Cl 2 h ν → C 6 H 6 Cl 6 (hexachlorocyclohexane - hexachloran)

2. Hydrogenation

C 6 H 6 + 3H 2 t , PtorNi→C 6 H 12 (cyclohexane)

3. Polymerization

III. SUBSTITUTION REACTIONS – ionic mechanism (lighter than alkanes)

b) benzene homologues upon irradiation or heating

In terms of chemical properties, alkyl radicals are similar to alkanes. Hydrogen atoms in them are replaced by halogens by a free radical mechanism. Therefore, in the absence of a catalyst, heating or UV irradiation leads to a radical substitution reaction in the side chain. The influence of the benzene ring on alkyl substituents leads to the fact that the hydrogen atom is always replaced at the carbon atom directly bonded to the benzene ring (a-carbon atom).

1) C 6 H 5 -CH 3 + Cl 2 h ν → C 6 H 5 -CH 2 -Cl + HCl

c) benzene homologues in the presence of a catalyst

C 6 H 5 -CH 3 + Cl 2 AlCl 3 → (mixture of orta, pair of derivatives) +HCl

2. Nitration (with nitric acid)

C 6 H 6 + HO-NO 2 t, H2SO4→C 6 H 5 -NO 2 + H 2 O

nitrobenzene - smell almond!

2,4,6-trinitrotoluene (tol, trotyl)

The use of benzene and its homologues

Benzene C 6 H 6 is a good solvent. Benzene as an additive improves the quality of motor fuel. It serves as a raw material for the production of many aromatic organic compounds - nitrobenzene C 6 H 5 NO 2 (solvent, aniline is obtained from it), chlorobenzene C 6 H 5 Cl, phenol C 6 H 5 OH, styrene, etc.

Toluene C 6 H 5 -CH 3 - a solvent used in the manufacture of dyes, drugs and explosives (trotyl (tol), or 2,4,6-trinitrotoluene TNT).

Xylene C 6 H 4 (CH 3) 2 . Technical xylene is a mixture of three isomers ( ortho-, meta- and pair-xylenes) - is used as a solvent and starting product for the synthesis of many organic compounds.

Isopropylbenzene C 6 H 5 -CH (CH 3) 2 serves to obtain phenol and acetone.

Chlorine derivatives of benzene used for plant protection. Thus, the product of substitution of H atoms in benzene with chlorine atoms is hexachlorobenzene C 6 Cl 6 - a fungicide; it is used for dry seed dressing of wheat and rye against hard smut. The product of the addition of chlorine to benzene is hexachlorocyclohexane (hexachloran) C 6 H 6 Cl 6 - an insecticide; it is used to control harmful insects. These substances refer to pesticides - chemical means of combating microorganisms, plants and animals.

Styrene C 6 H 5 - CH \u003d CH 2 polymerizes very easily, forming polystyrene, and copolymerizing with butadiene - styrene-butadiene rubbers.

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