Aromatic hydrocarbons. Benzene and its homologues

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 - S-S connections 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 most accurately reflects structural formula 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 general formula CnH2n-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 Chemical properties 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.

Not happening chemical reaction(similarity 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).

The structure of benzene

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. In addition, the physical and chemical properties of 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 high 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 the 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 Kekulé 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 R- 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 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 benzene ring remains unchanged during the chemical reaction. 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 difficult 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.

Benzene is an unsaturated compound, but we found that its structure does not contain double bonds, but rather an aromatic bond - a delocalized electron cloud. Typical reactions of unsaturated hydrocarbons - electrophilic addition and oxidation - are not characteristic of benzene. Thus, it does not discolor bromine water and does not give the Wagner reaction (oxidation with a solution of potassium permanganate at room temperature). Benzene is characterized by reactions that do not lead to disruption of the closed conjugate system - substitution reactions. To find out what type of substitution (radical, electrophilic, nucleophilic) is characteristic of benzene, remember its electronic structure: the σ-skeleton of the molecule is flat, and an aromatic cloud is located above and below the plane. To react with this aromatic cloud, the reagent must be electrophilic. So, benzene (and aromatic compounds in general) are characterized by electrophilic substitution reactions . Examples of S E reactions are:

In the first stage, the electrophile approaches the benzene molecule and interacts with the entire aromatic cloud (they are attracted to each other). Formed π-complex. A pair of electrons is required to form a new carbon-electrophile covalent bond. The electrophile pulls it out of the aromatic cloud and forms σ-complex. It is not a closed conjugate system, because the carbon atom that formed a new σ bond went into sp 3 hybridization (it left the plane and no longer has a non-hybrid p z orbital). The remaining five carbon atoms continue to participate in conjugation, forming a common electron cloud in which four electrons are delocalized (6-2=4), therefore positive charge in a σ-complex it is not designated on a specific carbon atom, but in the center of an open ring. So, the σ-complex is not an aromatic structure. In order to regain aromaticity, it needs to abstract a hydrogen proton (H+). It is taken up by the nucleophile (Nu -) remaining in the reaction medium. Two electrons C-H bonds return to the aromatic cloud, the carbon atom again becomes
sp 2 -hybridized and can participate in conjugation.

The limiting stage of the electrophilic substitution reaction is the stage of formation of the σ-complex, because in this case, a loss of aromaticity occurs, which requires energy expenditure.

Various electrophilic substitution reactions in benzene proceed according to a general mechanism and differ only in the stage of formation of the electrophilic particle.

Nitration reaction benzene occurs under the action of a mixture of concentrated nitric and sulfuric acids (see reaction diagram above). Let's consider its mechanism.

At the first stage of the reaction Nitric acid interacts with sulfur. In this case, nitric acid acts as a base, accepting a proton from a sulfuric acid molecule (according to Brønsted's theory, an acid is a molecule or ion that donates a proton, and a base is a molecule or ion that accepts a hydrogen proton). Protonated nitric acid is formed, which, by splitting off a water molecule, turns into a nitronium cation, or nitronium cation. This is an electrophilic particle. Thus, sulfuric acid acts as a catalyst, taking part in the formation of an electrophilic reagent. The second role of sulfuric acid is that of a water-removing agent. Water must be removed from the reaction sphere in order to shift its equilibrium to the right.

After the formation of an electrophile - a nitronium cation - the reaction proceeds according to a general mechanism, through the formation of π- and
σ-complexes:

Please note: at the stage of conversion of the σ-complex to nitrobenzene (the stage of returning aromaticity), the hydrogen proton is removed by the action of the sulfuric acid anion, and sulfuric acid is again formed, which proves that it was the catalyst for this reaction.

Catalyst halogenation reactions are the so-called Lewis acids (according to Lewis theory, acids are neutral molecules or ions that can accept a pair of electrons): FeCl 3, FeBr 3, AlCl 3, AlBr 3, etc. A catalyst is needed to polarize the halogen molecule. The Lewis acid displaces the lone electron pair of chlorine onto itself, forming a complex in which a partial positive charge is concentrated on one of the chlorine atoms:

At the stage of formation of the π-complex, further polarization of the Cl-Cl bond occurs, and it breaks heterolytically, and Cl + immediately participates in the formation of the σ-complex.

Proceed similarly alkylation reactions(Friedel-Crafts reaction).

The C-Cl bond in methyl chloride is not polar enough to break heterolytically. Under the action of a Lewis acid, the partial positive charge on the carbon atom increases, and the complex of the reagent with the catalyst is a stronger electrophile than the original methyl chloride.

Sulfonation reaction benzene occurs under the influence of oleum (a solution of sulfuric anhydride SO 3 in concentrated sulfuric acid).

The sulfuric anhydride molecule is an electrophile due to the large partial positive charge on the sulfur atom.

When a π-complex is formed, the S=O bond (primarily the π-bond) is polarized and broken in a heterolytic manner, therefore, when a σ-complex is formed, a total negative charge appears on the oxygen atom. To restore aromaticity, a hydrogen proton is split off from the carbon atom of the ring and goes to negatively charged oxygen. Benzenesulfonic acid is formed.

When we consider electrophilic substitution reactions in benzene, we are not faced with the question of in what position the reaction occurs, because all carbon atoms are absolutely equal. It’s another matter if the benzene ring already has a substituent. In this case, as a result of electrophilic substitution, it is fundamentally possible education of three isomers:

To answer the question of which of these possible products is predominant, it is necessary to consider the electronic effects of the substituent.

Let's abstract from electrophilic substitution reactions in benzene and its derivatives and consider electronic effects in general.

Mutual influence of atoms in organic molecules
connections. Electronic effects

Atoms and atomic groups in the molecules of organic compounds influence each other, and not only the atoms directly connected to each other. This influence is somehow transmitted through the molecule. The transfer of the influence of atoms in molecules due to the polarization of bonds is called electronic effects . There are two types of electronic effects: inductive and mesomeric effects.

Inductive effect- this is the transfer of the influence of substituents along a chain of σ-bonds due to their polarization. The inductive effect is denoted by the symbol I. Let's consider it using 1-chlorobutane as an example:

The C-Cl bond is polar due to the higher electronegativity of chlorine. A partial positive charge (δ+) appears on the carbon atom. The electron pair of the next σ bond is shifted towards the electron-deficient carbon atom, i.e. polarized. Due to this, a partial positive charge (δ+’) also appears on the next carbon atom, etc. So chlorine induces polarization of not only the “own” σ bond, but also subsequent ones in the chain. Please note that each subsequent partial positive charge is smaller in magnitude than the previous one (δ+>δ+’>δ+’’>δ+’’’), i.e. the inductive effect is transmitted through the circuit with attenuation. This can be explained by the low polarizability of σ bonds. It is generally accepted that the inductive effect extends to 3-4 σ bonds. In the example given, the chlorine atom shifts electron density along a chain of bonds to myself. This effect is called the negative inductive effect and is denoted –I Cl.

Most substituents exhibit a negative inductive effect, because their structure contains atoms that are more electronegative than hydrogen (the inductive effect of hydrogen is assumed to be zero). For example: -F, -Cl, -Br, -I, -OH, -NH 2, -NO 2,
-COOH, >C=O.

If a substituent shifts the electron density along a chain of σ bonds Push, it exhibits a positive inductive effect (+I). For example:

Oxygen with a total negative charge exhibits a positive inductive effect.

In the propene molecule, the carbon of the methyl group is sp 3 -hybridized, and the carbon atoms at the double bond are sp 2 -hybridized, i.e. more electronegative. Therefore, the methyl group shifts the electron density away from itself, exhibiting a positive inductive effect (+I CH 3).

So, the inductive effect can manifest itself in any molecule in which there are atoms of different electronegativity.

Mesomeric effect– this is the transfer of the electronic influence of substituents in conjugated systems through the polarization of π bonds. The mesomeric effect is transmitted without attenuation, because π bonds are easily polarized. Please note: only those substituents that are themselves part of the conjugated system have a mesomeric effect. For example:

The mesomeric effect can be either positive (+M) or negative (-M).

In the vinyl chloride molecule, the lone electron pair of chlorine participates in p,π-conjugation, i.e. the contribution of chlorine to the conjugated system is greater than that of each of the carbon atoms. Therefore, chlorine exhibits a positive mesomeric effect.

The acrylic aldehyde molecule is
π.π-conjugate system. The oxygen atom gives up one electron to conjugation - the same as each carbon atom, but at the same time the electronegativity of oxygen is higher than that of carbon, therefore oxygen shifts the electron density of the conjugated system towards itself, the aldehyde group as a whole exhibits a negative mesomeric effect.

So, substituents that donate two electrons to conjugation have a positive mesomeric effect. These include:

a) substituents with a complete negative charge, for example, –O - ;

b) substituents, in the structure of which there are atoms with unshared electron pairs in p z orbitals, for example: -NH 2, -OH,
-F, -Cl, -Br-, -I, -OR (-OCH 3, -OC 2 H 5).

Substituents that shift the electron density toward themselves along the conjugated system exhibit a negative mesomeric effect. These include substituents whose structure contains double bonds, for example:

A substituent can exhibit both inductive and mesomeric effects simultaneously. In some cases, the direction of these effects is the same (for example, -I and –M), in others they act in opposite directions (for example, -I and +M). In these cases, how can we determine the overall effect of the substituent on the rest of the molecule (in other words, how can we determine whether a given substituent is electron-donating or electron-withdrawing)? Substituents that increase the electron density in the rest of the molecule are called electron-donating, and substituents that lower the electron density in the rest of the molecule are called electron-withdrawing.

To determine the overall effect of a substituent, it is necessary to compare its electronic effects in magnitude. If the effect is positive in sign, the substituent is electron-donating. If an effect with a negative sign predominates, the substituent is electron-withdrawing. It should be noted that, as a rule, the mesomeric effect is more pronounced than the inductive effect (due to the greater ability of π bonds to polarize). However, there are exceptions to this rule: the inductive effect of halogens is stronger than the mesomeric effect.

Let's consider specific examples:

In this compound, the amino group is an electron-donating substituent, because its positive mesomeric effect is stronger than the negative inductive effect.

In this compound, the amino group is an electron-withdrawing site, because exhibits only a negative inductive effect.

In the phenol molecule, the hydroxyl group is an electron-donating substituent due to the predominance of the positive mesomeric effect over the negative inductive effect.

In the benzyl alcohol molecule, the hydroxyl group does not participate in conjugation and exhibits only a negative inductive effect. Therefore, it is an electron-withdrawing substituent.

These examples show that one cannot consider the influence of any substituent in general, but must consider its influence in a specific molecule.

Only halogens are always electron-withdrawing substituents, because their negative inductive effect is stronger than the positive mesomeric effect. For example:

Now let's return to electrophilic substitution reactions in benzene derivatives. So, we have found that the substituent already present in the ring affects the course of electrophilic substitution reactions. What is this influence expressed in?

The substituent affects the reaction rate S E and the position of the second substituent introduced into the ring. Let's look at both of these aspects of influence.

Effect on reaction speed. The higher the electron density in the ring, the easier electrophilic substitution reactions occur. It is clear that electron-donating substituents facilitate S E reactions (they are cycle activators), and electron-withdrawing substituents hinder them (they deactivate the cycle). Therefore, electrophilic substitution reactions in benzene derivatives containing electron-withdrawing substituents are carried out under more stringent conditions.

Let's compare the activity of phenol, toluene, benzene, chlorobenzene and nitrobenzene in the nitration reaction.

Since phenol and toluene contain electron-donating substituents, they are more active in SE reactions than benzene. On the contrary, chlorobenzene and nitrobenzene are less active in these reactions than benzene, because contain electron-withdrawing substituents. Phenol is more active than toluene due to the positive mesomeric effect of the OH group. Chlorine is not as strong an electron-withdrawing substituent as the nitro group, because the nitro group exhibits both negative inductive and negative mesomeric effects. So, in this series, activity in electrophilic substitution reactions decreases from phenol to nitrobenzene. It has been experimentally established that if the reaction rate of benzene nitration is taken to be 1, then this series will look like this:

The second aspect of the influence of a substituent on the aromatic ring on the course of electrophilic substitution reactions is the so-called orienting action of substituents. All substituents can be divided into two groups: ortho-, para-orientants (substituents of the 1st kind) and meta-orientants (substituents of the 2nd kind).

TO deputies of the 1st kind include: -OH, -O -, -NH 2, alkyl groups (-CH 3, -C 2 H 5, etc.) and halogens. You can see that all of these substituents exhibit a positive inductive effect and/or a positive mesomeric effect. All of them, except the halogens, increase the electron density in the ring, especially in the ortho and para positions. Therefore, the electrophile is directed to these positions. Let's look at this using phenol as an example:

Due to the positive mesomeric effect of the hydroxyl group, the electron density is redistributed throughout the conjugated system, and in the ortho- and para-positions it is especially increased.

When phenol is brominated, a mixture of ortho- and para-bromophenol is formed:

If bromination is carried out in a polar solvent (bromine water) and an excess of bromine is used, the reaction proceeds in three stages at once:

Substitutes of the 2nd kind are: -NH 3 + , -COOH, -CHO (aldehyde group), -NO 2 , -SO 3 H. All these substituents reduce the electron density in the aromatic ring, but due to its redistribution in meta positions, it is not so reduced strongly, as in ortho- and para-. Let's look at this using benzoic acid as an example:

The carboxyl group exhibits negative inductive and negative mesomeric effects. Due to redistribution throughout the conjugated system in the meta positions, the electron density remains higher than in the ortho and para positions, so the electrophile will attack the meta positions:

DEFINITION

Benzene(cyclohexatriene – 1,3,5) – organic matter, the simplest representative of a number of aromatic hydrocarbons.

Formula – C 6 H 6 (structural formula – Fig. 1). Molecular mass – 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 to form reaction products carbon dioxide and water (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

EXAMPLE 2