What compounds are aromatic. aromatic compounds
AROMATIC HYDROCARBONS
For aromatic compounds or arenes, refers to a large group of compounds whose molecules contain a stable cyclic group (benzene ring) with special physical and chemical properties.
These compounds primarily include benzene and its numerous derivatives.
The term "aromatic" was originally used in relation to products of natural origin, which had an aromatic smell. Since among these compounds there were many that included benzene rings, the term "aromatic" began to apply to any compounds (including those with an unpleasant odor) containing a benzene ring.
Benzene, its electronic structure
According to the benzene formula C 6 H 6, it can be assumed that benzene is a highly unsaturated compound, similar, for example, to acetylene. However, the chemical properties of benzene do not support this assumption. So, under normal conditions, benzene does not give reactions characteristic of unsaturated hydrocarbons: it does not enter into addition reactions with hydrogen halides, it does not decolorize a solution of potassium permanganate. At the same time, benzene enters into substitution reactions similarly to saturated hydrocarbons.
These facts indicate that benzene is partly similar to saturated, partly to unsaturated hydrocarbons and at the same time differs from both. Therefore, for a long time, there were lively discussions between scientists on the question of the structure of benzene.
In the 60s. of the last century, most chemists accepted the theory of the cyclic structure of benzene based on the fact that monosubstituted benzene derivatives (for example, bromobenzene) do not have isomers.
The most recognized formula of benzene, proposed in 1865 by the German chemist Kekule, in which double bonds in the ring of carbon atoms of benzene alternate with simple ones, and, according to Kekule's hypothesis, simple and double bonds move continuously:
However, the Kekule formula cannot explain why benzene does not exhibit the properties of unsaturated compounds.
According to modern concepts, the benzene molecule has the structure of a flat hexagon, the sides of which are equal to each other and are 0.140 nm. This distance is an average 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 bonds H - C - C and C - C - C are 120 °.
The carbon atoms in benzene are in sp 2 hybridization, i.e. of the four orbitals of the carbon atom, only three are hybridized (one 2s- and two 2p-), which take part in the formation of σ-bonds between carbon atoms. The fourth 2 p-orbital overlaps with the 2 p-orbitals of two neighboring carbon atoms (right and left), six delocalized π-electrons located in dumbbell-shaped orbitals, the axes of which are perpendicular to the plane of the benzene ring, form a single stable closed electronic system.
As a result of the formation of a closed electronic system by all six carbon atoms, the "alignment" of single and double bonds occurs, i.e. in the benzene molecule there are no classical double and single bonds. The uniform distribution of the π-electron density between all carbon atoms is the reason for the high stability of the benzene molecule. To emphasize the uniformity of the π-electron density in the benzene molecule, one resorts to the following formula:
Nomenclature and isomerism of aromatic hydrocarbons of the benzene series
The general formula for the homologous series of benzene C n H 2 n -6.
The first homologue of benzene is methylbenzene, or toluene, C 7 H 8
has no position isomers, like all other monosubstituted derivatives.
The second homologue C 8 H 10 can exist in four isomeric forms: ethylbenzene C 6 H 5 -C 2 H 5 and three dimethylbenzenes, or xylene, C b H 4 (CH 3) 2 (ortho-, meta- and pair-xylenes, or 1,2-, 1,3- and 1,4-dimethylbenzenes):
The radical (residue) of benzene C 6 H 5 - is called phenyl; the names of radicals of benzene homologues are derived from the names of the corresponding hydrocarbons by adding the suffix to the root -silt(tolyl, xylyl, etc.) and lettering (o-, m-, p-) or digits the position of the side chains. Generic name for all aromatic radicals aryls similar to the title alkyls for alkane radicals. The radical C 6 H 5 -CH 2 - is called benzyl.
When naming more complex benzene derivatives, from the possible numbering orders, one is chosen in which the sum of the digits of the substituent numbers will be the smallest. For example, dimethyl ethyl benzene of the structure
should be called 1,4-dimethyl-2-ethylbenzene (the sum of the digits is 7), not 1,4-dimethyl-6-ethylbenzene (the sum of the digits is 11).
The names of the higher homologues of benzene are often derived not from the name of the aromatic nucleus, but from the name of the side chain, that is, they are considered as derivatives of alkanes:
Physical properties of aromatic hydrocarbons of the benzene series
The lower members of the benzene homologous series are colorless liquids with a characteristic odor. Their density and refractive index are much higher than those of alkanes and alkenes. The melting point is also noticeably higher. Due to the high carbon content, all aromatic compounds burn with a very smoky flame. All aromatic hydrocarbons are insoluble in water and highly soluble in most organic solvents: many of them are readily steam distillable.
Chemical properties of aromatic hydrocarbons of the benzene series
For aromatic hydrocarbons, the most typical reactions are the substitution of hydrogen in the aromatic ring. Aromatic hydrocarbons enter into addition reactions with great difficulty under harsh conditions. A distinctive feature of benzene is its significant resistance to oxidizing agents.
Addition reactions
Addition of hydrogen
In some rare cases, benzene is capable of addition reactions. Hydrogenation, i.e., the addition of hydrogen, occurs under the action of hydrogen under harsh conditions in the presence of catalysts (Ni, Pt, Pd). In this case, a benzene molecule adds three hydrogen molecules to form cyclohexane:
Addition of halogens
If a solution of chlorine in benzene is exposed to sunlight or ultraviolet rays, then three halogen molecules are radically added to form a complex mixture of stereoisomers of hexachlorocyclohexane:
Hexachlorocyclohexai (trade name hexachloran) is currently used as an insecticide - substances that destroy insects that are pests of agriculture.
Oxidation reactions
Benzene is even more resistant to oxidizing agents than saturated hydrocarbons. It is not oxidized by dilute nitric acid, KMnO 4 solution, etc. Benzene homologues are oxidized much more easily. But even in them, the benzene core is relatively more resistant to the action of oxidizing agents than the hydrocarbon radicals associated with it. There is a rule: any benzene homologue with one side chain is oxidized to a monobasic (benzoic) acid:
Benzene homologues with multiple side chains of any complexity are oxidized to form polybasic aromatic acids:
Substitution reactions
1. Halogenation
Under normal conditions, aromatic hydrocarbons practically do not react with halogens; benzene does not decolorize bromine water, but in the presence of catalysts (FeCl 3, FeBr 3, AlCl 3) in an anhydrous medium, chlorine and bromine vigorously react with benzene at room temperature:
Nitration reaction
For the reaction, concentrated nitric acid is used, often mixed with concentrated sulfuric acid (catalyst):
In unsubstituted benzene, the reactivity of all six carbon atoms in substitution reactions is the same; substituents may attach to any carbon atom. If there is already a substituent in the benzene nucleus, then under its influence the state of the nucleus changes, and the position into which any new substituent enters depends on the nature of the first substituent. It follows from this that each substituent in the benzene nucleus exhibits a certain guiding (orienting) effect and contributes to the introduction of new substituents only in certain positions in relation to itself.
According to the guiding influence, various substituents are divided into two groups:
a) substituents of the first kind:
They direct any new substituent into ortho and para positions with respect to themselves. At the same time, almost all of them reduce the stability of the aromatic group and facilitate both substitution reactions and reactions of the benzene ring:
b) substituents of the second kind:
They direct any new substitute to a meta position in relation to themselves. They increase the stability of the aromatic group and hinder substitution reactions:
Thus, the aromatic nature of benzene (and other arenes) is expressed in the fact that this compound, being unsaturated in composition, in a number of chemical reactions manifests itself as a limiting compound, it is characterized by chemical stability, the difficulty of addition reactions. Only under special conditions (catalysts, irradiation) does benzene behave as if it had three double bonds in its molecule.
Chemistry is a very fascinating science. It studies all substances that exist in nature, and there are a lot of them. They are divided into inorganic and organic. In this article, we will look at aromatic hydrocarbons, which belong to the last group.
What it is?
These are organic substances that have one or more benzene nuclei in their composition - stable structures of six carbon atoms connected in a polygon. These chemical compounds have a specific smell, which can be understood from their name. Hydrocarbons of this group are cyclic, in contrast to alkanes, alkynes, etc.
aromatic hydrocarbons. Benzene
This is the simplest chemical compound from this group of substances. The composition of its molecules includes six carbon atoms and the same amount of hydrogen. All other aromatic hydrocarbons are derivatives of benzene and can be obtained using it. This substance under normal conditions is in a liquid state, it is colorless, has a specific sweet smell, and does not dissolve in water. It begins to boil at a temperature of +80 degrees Celsius, and freeze - at +5.
Chemical properties of benzene and other aromatic hydrocarbons
The first thing you need to pay attention to is halogenation and nitration.
Substitution reactions
The first of these is halogenation. In this case, in order for the chemical reaction to take place, a catalyst, namely iron trichloride, must be used. Thus, if we add chlorine (Cl 2) to benzene (C 6 H 6), we will get chlorobenzene (C 6 H 5 Cl) and hydrogen chloride (HCl), which will be released as a clear gas with a pungent odor. That is, as a result of this reaction, one hydrogen atom is replaced by a chlorine atom. The same thing can happen when other halogens (iodine, bromine, etc.) are added to benzene. The second substitution reaction - nitration - proceeds according to a similar principle. Here, a concentrated solution of sulfuric acid acts as a catalyst. To carry out this kind of chemical reaction, it is necessary to add nitrate acid (HNO 3), also concentrated, to benzene, as a result of which nitrobenzene (C 6 H 5 NO 2) and water are formed. In this case, the hydrogen atom is replaced by a group of a nitrogen atom and two oxygens.
Addition reactions
This is the second type of chemical interactions that aromatic hydrocarbons are capable of entering into. They also exist in two forms: halogenation and hydrogenation. The first occurs only in the presence of solar energy, which acts as a catalyst. To carry out this reaction, chlorine must also be added to benzene, but in a larger amount than for substitution. There should be three chlorine per molecule of benzene. As a result, we get hexachlorocyclohexane (C 6 H 6 Cl 6), that is, six more chlorine will join the existing atoms.
Hydrogenation occurs only in the presence of nickel. To do this, mix benzene and hydrogen (H 2). The proportions are the same as in the previous reaction. As a result, cyclohexane (C 6 H 12) is formed. All other aromatic hydrocarbons can also enter into this type of reaction. They occur according to the same principle as in the case of benzene, only with the formation of more complex substances.
Obtaining chemicals of this group
Let's start with benzene. It can be obtained using a reagent such as acetylene (C 2 H 2). Of the three molecules of a given substance, under the influence of high temperature and a catalyst, one molecule of the desired chemical compound is formed.
Also, benzene and some other aromatic hydrocarbons can be extracted from coal tar, which is formed during the production of metallurgical coke. Toluene, o-xylene, m-xylene, phenanthrene, naphthalene, anthracene, fluorene, chrysene, diphenyl and others can be attributed to those obtained in this way. In addition, substances of this group are often extracted from petroleum products.
What do the various chemical compounds of this class look like?
Styrene is a colorless liquid with a pleasant odor, slightly soluble in water, the boiling point is +145 degrees Celsius. Naphthalene is a crystalline substance, also slightly soluble in water, melts at a temperature of +80 degrees, and boils at +217. Anthracene under normal conditions is also presented in the form of crystals, however, no longer colorless, but yellow in color. This substance is insoluble neither in water nor in organic solvents. Melting point - +216 degrees Celsius, boiling point - +342. Phenantrene looks like shiny crystals that dissolve only in organic solvents. Melting point - +101 degrees, boiling point - +340 degrees. Fluorene, as the name implies, is capable of fluorescence. This, like many other substances of this group, are colorless crystals, insoluble in water. Melting point - +116, boiling point - +294.
Application of aromatic hydrocarbons
Benzene is used in the production of dyes as a raw material. It is also used in the production of explosives, pesticides, and some drugs. Styrene is used in the production of polystyrene (polystyrene) by polymerization of the starting material. The latter is widely used in construction: as a heat and sound insulating, electrical insulating material. Naphthalene, like benzene, is involved in the production of pesticides, dyes, and drugs. In addition, it is used in the chemical industry to produce many organic compounds. Anthracene is also used in the manufacture of dyes. Fluorene plays the role of a polymer stabilizer. Phenantrene, like the previous substance and many other aromatic hydrocarbons, is one of the components of dyes. Toluene is widely used in the chemical industry for the extraction of organic substances, as well as for the production of explosives.
Characterization and use of substances extracted with aromatic hydrocarbons
These include, first of all, the products of the considered chemical reactions of benzene. Chlorobenzene, for example, is an organic solvent, also used in the production of phenol, pesticides, organic substances. Nitrobenzene is a component of metal polishing agents, is used in the manufacture of some dyes and flavors, and can play the role of a solvent and oxidizing agent. Hexachlorocyclohexane is used as a poison for pest control and also in the chemical industry. Cyclohexane is used in the production of paints and varnishes, in the production of many organic compounds, in the pharmaceutical industry.
Conclusion
After reading this article, we can conclude that all aromatic hydrocarbons have the same chemical structure, which allows us to combine them into one class of compounds. In addition, their physical and chemical properties are also quite similar. The appearance, boiling and melting points of all chemicals in this group do not differ much. Many aromatic hydrocarbons find their use in the same industries. Substances that can be obtained as a result of halogenation, nitration, hydrogenation reactions also have similar properties and are used for similar purposes.
Aromatic hydrocarbons (otherwise called arenes) are organic biocompounds whose molecules contain one or more cycles with six carbon atoms. The benzene ring is characterized by special physical and chemical properties. The name "arena" entered organic and general chemistry at the end of the 18th - at the beginning of the 19th century. These included substances that consisted of two chemical compounds - Carbon and Hydrogen and had a pleasant smell (resins, balms, ethers). Over time, the name "aromatic hydrocarbons" lost its meaning, since aromatic substances were also found among other classes of organic substances, and most aromatic compounds have an unpleasant or specific odor. For the first time, benzene was isolated from lighting gas at the beginning of the 19th century. A little later (1845) A.F. Hoffman isolated from coal tar. Nowadays, the class of arenes (according to the IUPAC classification) combines compounds, based on molecules of which have benzel nuclei.So, such compounds are divided into single- and multi-nuclear arenes, as well as aromatic hydrocarbons with condensed nuclei.
Mononuclear arenas are those that contain one benzene ring. The structure of the benzene molecule, a typical representative of arenes, is most often shown by the Kekule formula in the form of a cycle of six carbon atoms, which are alternately connected by simple C-C and double C=C bonds. This structure is confirmed by the data of modern physical and chemical analysis.
Kekule's basic ideas about the structure of benzene are as follows: 1) benzene has the structure of a hexagonal ring; 2) there are three single and three double bonds in the benzel ring; 3) all six carbon atoms in the benzene ring are equivalent to each other. The formula reflects the elemental composition of benzene, the ratio of carbon and hydrogen atoms in the molecule, the absence of isomers for monosubstituted benzene derivatives.
Aromatic hydrocarbons are quite common in nature. They are an integral part of coal tar, which is obtained after the dry distillation of coal. Arenas are part of many varieties of oil and other natural products (resins, balms, etc.). In the process of dry distillation of coal, on average, about three percent of coal tar, or coal tar, is obtained. And a number of fractions are obtained from coal tar during fractional distillation: light oil (contains xylenes, toluene, benzene, thiophene), carbolic oil (contains cresols, phenols and naphthalene), creosote oil (contains naphthalene), (contains phenanthrene, anthracene and other higher arenas) and pitch, which is used to cover roads and as a building material.
The use of aromatic hydrocarbons. Benzene has a specific odor and is practically insoluble in water. It is a good solvent for organic biocompounds. Synthesized from coal tar. Benzene is a valuable raw material for the production of dyes, medicines, explosives, herbicides, insecticides, etc.
Toluene is highly soluble in organic solvents. Obtained from coal tar, as well as from some types of oil. Benzene alcohol, benzaldehyde, dyes, medicines, saccharin, trinitrotoluene are synthesized from toluene.
Xylenes are good. They are obtained from coal oil, as well as by fractional distillation of coal tar. Phthalic anhydride, xylene, artificial fiber lavsan are synthesized from xylenes. Xylenes are sometimes added to gasolines.
AROMATIC COMPOUNDS (from the Greek? ρωμα - incense), organic compounds characterized mainly by the presence of a closed system of conjugated bonds, which includes, according to Hückel's rule, (4n + 2) π-electrons (n = 0, 1, 2, ... ); meet all or more of the criteria for aromaticity. The most famous and important are: aromatic hydrocarbons (arenes), including monocyclic ones - benzene and its homologues (for example, xylenes, cumene, toluene, ethylbenzene) and polycyclic ones built from benzene rings directly connected to each other (for example, biphenyl), connected through any group (for example, diphenylmethane), condensed (for example, anthracene, naphthalene); arene derivatives (eg phenols); heteroaromatic compounds, i.e. aromatic heterocyclic systems (for example, pyridine, pyrimidine, thiophene, furan). Aromatic compounds also include some macrocyclic annulenes (for example, annulenes), organoelement compounds (for example, ferrocene), tropylium compounds, etc.
Aromatic compounds are liquids or solids. They are characterized by the presence of the so-called magnetic ring current and resonance in the low-field ("aromatic") part of the NMR spectrum (6.5-8.0 ppm for 1 H and 110-170 ppm for 13 C). Aromatic compounds enter into electrophilic substitution reactions (for example, halogenation, nitration, sulfonation, alkylation and Friedel-Crafts acylation). The introduction of the electrophile E + into the molecule of the aromatic compound is facilitated and the electrophile is directed mainly to the ortho- and para-positions of the cycle in the presence of substituents in the molecule of the aromatic compound, which are orientants of the first kind (alkyls, aryls, halogen atoms, groups OR, NR 2, SR, where R - organic radical), is hampered and directed mainly to the meta position of the cycle by substituents - orientants of the second kind (COR, COOR, CN, NO 2, SO 2 R, SO 3 H). Electrophilic substitution proceeds by the mechanism of addition - elimination through the cationic σ-complex - Ueland intermediate (X - substituent):
Aromatic compounds also enter into nucleophilic substitution reactions under the action of Nu - nucleophiles, for example R2N - , RO - , RS - , (RCO) 2CH - , halide anions. In this case, halogen atoms, NO 2, NR 2, OR, SR, SO 3 H groups are replaced in the aromatic compound molecule, less often hydrogen atoms. Such reactions often take place under harsh conditions, such as elevated temperatures. They are facilitated in the presence of copper compounds and, especially, in the presence of a substituent, an orienting agent of the second kind, in the ortho or para position to the leaving group. Nucleophilic substitution proceeds mainly by the mechanism of addition - elimination, through the formation of an anionic σ-complex - Meisenheimer's intermediate (Y - substitutable group):
Of lesser importance for an aromatic compound is hemolytic substitution, for example, arylation with diazo compounds and hydroxylation using Fenton's reagent (H 2 O 2 + CuSO 4 + H 2 SO 4). Aromatic compounds undergo metalation (direct replacement of hydrogen or exchange of a halogen for a metal under the action of metals or organometallic compounds). The reactions of an aromatic compound on substituent groups are generally similar to the reactions of the corresponding aliphatic compounds. The main features are the formation of stable diazo compounds by aromatic amines with HNO 2, capable of azo coupling and turning into various substituted aromatic compounds under the action of nucleophiles. Of the addition reactions for aromatic compounds, catalytic hydrogenation is the most important - a general method for the synthesis of compounds of the cyclohexane series. Aromatic compounds are resistant to oxidation. Alkylaromatic compounds are usually oxidized at the carbon atom of the alkyl substituent adjacent to the aromatic ring. Aromatic acids (eg terephthalic acid from p-xylene), aldehydes (p-nitrobenzaldehyde from p-nitrotoluene), ketones (acetophenone from ethylbenzene) and alcohols (triphenylcarbinol from triphenylmethane) are obtained in this way.
Aromatic compounds are found in oil, but they are mainly obtained in industry from the products of coal coking and aromatization of hydrocarbons; the aromatic compounds are then converted into various derivatives. Aromatic compounds are important intermediate and target products of industrial organic synthesis; they are used in the production of dyes, medicines, plant protection products, explosives, polymeric materials.
Aromatic hydrocarbons are components of high-octane gasolines.
Lit .: Gorelik M. V., Efros L. S. Fundamentals of chemistry and technology of aromatic compounds. M., 1992.
Electrophilic substitution reactions are characteristic of aromatic, carbocyclic, and heterocyclic systems. As a result of the delocalization of p-electrons in the benzene molecule (and other aromatic systems), the p-electron density is distributed uniformly on both sides of the cycle. Such screening of the carbon atoms of the cycle by p-electrons protects them from attack by nucleophilic reagents and, conversely, facilitates the possibility of attack by electrophilic reagents.
But unlike the reactions of alkenes with electrophilic reagents, the interaction of aromatic compounds with them does not lead to the formation of addition products, since in this case the aromaticity of the compound would be violated and its stability would decrease. The preservation of aromaticity is possible if the electrophilic particle replaces the hydrogen cation.
The mechanism of electrophilic substitution reactions is similar to the mechanism of electrophilic addition reactions, since there are general patterns of reactions.
General scheme of the mechanism of electrophilic substitution reactions S E:
In the first step of the reaction, p-complex with an electrophilic particle (fast stage), which then turns into s-complex(slow stage) due to the formation s- bonds of one of the carbon atoms with an electrophilic particle. For education s- In connection with the electrophilic particle, a pair of electrons “breaks out” from the conjugation, and the resulting product acquires a positive charge. AT s-complex aromaticity is broken, since one of the carbon atoms is in sp 3 hybridization, and four electrons and a positive charge are delocalized on five other carbon atoms.
To regenerate a thermodynamically favorable aromatic system, heterolytic cleavage of the C sp 3 -H bond occurs. As a result, the H + ion is split off, and a pair of bond electrons goes to restore the conjugation system, while the carbon atom that split off the proton changes the hybridization of atomic orbitals from sp 3 to sp 2 . The mechanism of reactions of nitration, sulfonation, halogenation, alkylation, acylation of aromatic compounds includes an additional stage not indicated in the general scheme - the stage of generating an electrophilic particle.
Reaction equationnitrationbenzene looks like:
In nitration reactions, the generation of an electrophilic particle occurs as a result of the interaction of nitric and sulfuric acids, which leads to the formation of the nitronium cation NO 2 +, which then reacts with an aromatic compound:
In the benzene molecule, all carbon atoms are equivalent, substitution occurs at one of them. If substituents are present in the molecule, then the reactivity and direction of the electrophilic attack is determined by the nature of this substituent. According to the influence on the reactivity and direction of attack, all substituents are divided into two groups.
Orientants of the first kind. These substituents facilitate electrophilic substitution compared to benzene and direct the incoming group to the ortho and para positions. These include electron-donating substituents that increase the electron density in the benzene nucleus. As a result of its redistribution to positions 2,4,6 (ortho- and para-positions), partial negative charges arise, which facilitates the attachment of an electrophilic particle to these positions with the formation s-complex.
Orientants of the second kind. These substituents make electrophilic substitution reactions more difficult than benzene and direct the incoming group to one of the meta positions. These include electron-withdrawing substituents that reduce the electron density in the benzene ring. As a result of its redistribution in positions 3,5 (meta-positions), partial negative charges arise and the addition of an electrophilic particle with the formation s-complex going under tough conditions.
Halogen atoms direct the electrophilic particle to the ortho- or para-positions (due to the positive mesomeric effect), but at the same time hinder the reaction, since they are electron-withdrawing substituents (-I>+M). Reactions of halogen derivatives of benzene with electrophilic reagents proceed under harsh conditions.
In reactions sulfonation the role of the electrophilic particle is played by the SO 3 molecule, which is formed as a result of the reaction: 2H 2 SO 4 «SO 3 + H 3 O + + HSO 4 -. The sulfur atoms in this molecule are characterized by a strong deficit of electron density and the presence of a partial positive charge, and, therefore, it is the S atom that, as an electrophile, must bind to the carbon atom of the benzene ring of toluene.
The methyl group in toluene is an orientant of the first kind, and as an electron-donating substituent, it facilitates the substitution reaction and directs the incoming group to the ortho and para positions. In practice, substitution products are also formed in the meta position, but their amount is significantly less than the amount of substitution products in the ortho-para positions.
Halogenation benzene and many aromatic compounds, the action of the halogen itself proceeds only in the presence of catalysts such as ZnCl 2 , AlCl 3 , FeBr 3 , etc. The catalysts are usually Lewis acids. A bond is formed between the metal atom and the halogen atom by the donor-acceptor mechanism, which causes the polarization of the halogen molecule, enhancing its electrophilic character. The resulting adduct can undergo dissociation with the formation of a complex anion and a halogen cation, which further acts as an electrophilic particle:
Aqueous solutions of HO-Hal in the presence of strong acids can also be used as halogenating agents. The formation of an electrophilic particle in this case can be explained by the following reactions:
The mechanism of further interaction of Br + or Cl + cations is no different from the mechanism of nitration with NO 2 + cations. Let us consider the reaction mechanism using the example of aniline bromination (we restrict ourselves to the formation of monosubstituted products). As is known, aniline devalues bromine water, eventually forming 2,4,6-tribromaniline, which is released as a white precipitate:
The resulting electrophilic species attacks the p-electrons of the benzene ring, forming a p-complex. From the resulting p-complex, two main s-complexes in which the carbon-bromine bond occurs in the ortho- and para-positions of the cycle. At the next stage, the elimination of a proton occurs, which leads to the formation of monosubstituted aniline derivatives. In excess of the reagent, these processes are repeated, leading to the formation of aniline dibromo and tribromo derivatives.
Alkylation(replacement of a hydrogen atom by an alkyl radical) of aromatic compounds is carried out by their interaction with haloalkanes (Friedel-Crafts reaction). The interaction of primary alkyl halides, for example, CH 3 Cl, with aromatic compounds in the presence of Lewis acids differs little in its mechanism from halogenation reactions. Consider the mechanism using the example of methylation of nitrobenzene. The nitro group, as an orienting agent of the second kind, deactivates the benzene ring in electrophilic substitution reactions and directs the incoming group to one of the meta positions.
In general, the reaction equation has the form:
The generation of an electrophilic particle occurs as a result of the interaction of a haloalkane with a Lewis acid:
The resulting methyl cation attacks the p-electrons of the benzene ring, resulting in the formation of a p-complex. The resulting p-complex then slowly turns into s-complex (carbocation), in which the bond between the methyl cation and the carbon atom of the cycle occurs mainly in positions 3 or 5 (i.e., in meta positions, in which partial negative charges arise due to the electronic effects of the nitro group). The final step is the elimination of a proton from s-complex and restoration of the conjugated system.
Alkenes or alcohols can also be used as alkylating agents in the alkylation of benzene instead of alkyl halides. For the formation of an electrophilic particle - a carbocation - the presence of an acid is necessary. The reaction mechanism in this case will differ only at the stage of generating an electrophilic particle. Consider this using the example of benzene alkylation with propylene and propanol-2:
Electrophilic particle generation:
In the case of using propylene as a reagent, the formation of a carbocation occurs as a result of the addition of a proton (according to the Markovnikov rule). When propanol-2 is used as a reagent, the formation of a carbocation occurs as a result of the elimination of a water molecule from protonated alcohol.
The resulting isopropyl cation attacks the p-electrons of the benzene ring, which leads to the formation of a p-complex, which then turns into s- complex with disturbed aromaticity. The subsequent elimination of a proton leads to the regeneration of the aromatic system:
Reactions acylation(substitution of the H + cation by the acyl group R-C + =O) occur in a similar way. Consider the example of the acylation reaction of methoxybenzene, the equation of which can be represented as follows:
As in the previous cases, an electrophilic particle is generated as a result of the interaction of acetic acid chloride with a Lewis acid:
The resulting acylium cation first forms a p-complex, from which mainly two s-complexes in which the formation s- the bonds between the cycle and the electrophilic particle occur predominantly in the ortho and para positions, since partial negative charges arise in these positions due to the electronic influence of the methoxy group.
Aromatic heterocycles also enter into electrophilic substitution reactions. At the same time, five-membered heterocycles - pyrrole, furan and thiophene - more easily enter into S E reactions, since they are p-excess systems. However, when carrying out reactions with these compounds, it is necessary to take into account their acidophobicity. The instability of these compounds in an acidic environment is explained by the violation of aromaticity as a result of the addition of a proton.
When carrying out reactions, an electrophilic particle replaces a proton in the a-position; if both a-positions are occupied, then the substitution proceeds at the b-position. Otherwise, the mechanism of electrophilic substitution reactions is similar to the cases considered above. As an example, we give the bromination of pyrrole:
The reaction mechanism involving aromatic heterocycles includes all the stages discussed above - the generation of an electrophilic particle, the formation of a p-complex, its transformation into s- complex (carbocation), the removal of a proton, leading to the formation of an aromatic product.
When carrying out electrophilic substitution reactions involving p-deficient aromatic systems, such as pyridine and pyrimidine, one must take into account their initially lower reactivity (the deficit of p-electron density hinders the formation of the p-complex and its transformation into s- complex), which decreases even more when reactions are carried out in an acidic medium. Although the aromaticity of these compounds is not disturbed in an acidic medium, the protonation of the nitrogen atom leads to an increase in the deficit of p-electron density in the cycle.
Pyridine can be alkylated, sulfonated, nitrated, acylated, and halogenated. However, in most cases, the more nucleophilic nitrogen atom, rather than the pyridine carbon atoms, forms a bond with the electrophilic particle.
In the case of a reaction in the pyridine ring, the substitution occurs at one of the b-positions, in which partial negative charges arise.
