Aromatic hydrocarbons: all about them. Chemical properties. Features of aromatic compounds For aromatic compounds, the most characteristic reactions are

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.

Features of aromatic compounds. Benzene is the first representative of aromatic hydrocarbons. It has a number of peculiar properties that distinguish it from previously studied saturated and unsaturated acyclic hydrocarbons. The aromatic nature of benzene is determined by its structure and manifests itself in chemical properties.

The composition of benzene is expressed by the formula C 6 H 6. The general formula of the homologues of the benzene series C n H 2 n -6. The difference between this formula and the formula for a number of saturated hydrocarbons C n H 2 n +2 is equal to 8H. Therefore, according to the chemical composition, benzene and its homologues are unsaturated compounds. Their unsaturated character is not manifested in typical reactions. One would expect benzene to behave like ethylene, butadiene and other typical unsaturated hydrocarbons. However, it does not decolorize bromine water, i.e., under normal conditions, it does not add bromine. A solution of potassium permanganate does not discolour when shaken with benzene, i.e., benzene is resistant to oxidation under these conditions. Even with prolonged boiling with a solution of KMn0 4, benzene is almost not oxidized. It is mainly characterized by substitution reactions:

but) In the presence of Lewis acid catalysts ( FeCl3, AlCl 3) chlorine and bromine replace hydrogen atoms in the benzene molecule:

b) Concentrated sulfuric acid does not cause polymerization of benzene, as happens in the case of alkadienes, but leads to the production of benzenesulfonic acid:

in) Under the action of a nitrating mixture (concentrated HNO 3 And H2SO4) nitration of the nucleus occurs (the introduction of a nitro group into the nucleus -NO 2) to form nitro derivatives of benzene.

nitrobenzene

Classification of substitution reactions. Upon substitution in the benzene ring, three types of reactions are possible depending on the nature of the attacking species.

1. Radical substitution. If the attacking agent R- a radical carrying an unpaired electron, then the hydrogen bonded to the carbon atom of the nucleus is split off with one of the electrons of the electron pair -bond. This type of substitution is called radical. The radical substitution reaction is rarely used in the aromatic series.

R + H-C 6 H 5 R-C 6 H 5 + H

2. Nucleophilic substitution. Under the action of negatively charged nucleophilic species on substituted benzene C 6 H 5 X(where X- deputy), split-off group X - leaves together with a pair of -electrons that previously carried out its connection with the nucleus:

Z - + X: C 6 H 5 Z-C 6 H 5 + X -

An example is the reaction of the interaction of the sodium salt of benzenesulfonic acid with alkali. This reaction underlies the industrial method for producing phenol:

As a rule, for the successful occurrence of nucleophilic substitution reactions, the nucleus must additionally contain one or better two strong electron-withdrawing substituents ( –NO 2, –SO 3 H, -CF 3).

3. Electrophilic substitution.

Z + + X:C 6 H 5 Z-C 6 H 5 + X +

In all reactions of this type, the attacking reagent ( Y +) carries a positive charge on the atom that enters into bond with the carbon atom of the benzene nucleus, or has a pronounced cationoid character and forms a new bond due to a pair of electrons that previously carried out the bond S-N. The replacing hydrogen atom leaves in the form of a proton ( H+).

Addition reactions to benzene. 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:

cyclohexane

If a solution of chlorine or bromine 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 (hexachloran):

hυ

Thus, the aromatic nature of benzene (and other arenes) is expressed in the fact that this compound, being unsaturated in composition, manifests itself as a limiting compound in a number of chemical reactions; it is characterized by chemical stability, the difficulty of addition reactions. Only under special conditions (catalysts, irradiation) does benzene behave like a triene hydrocarbon.

5.2 EXPERIMENTAL

Aromatic compounds are characterized by aromaticity, i.e. a set of structural, energy properties and features of the reactivity of cyclic structures with a system of conjugated bonds. In a narrower sense, this term refers only to benzoid compounds (arenes), the structure of which is based on a benzene ring, one or more, including fused ones, i.e. having two common carbon atoms.
The main aromatic hydrocarbons of coal tar. Aromatic hydrocarbons contained in coal tar have one or more six-membered rings, which are usually depicted in structural formulas with three alternating double bonds, these are benzene (bp 80 ° C), naphthalene (bp 218 ° C, mp 80°C), diphenyl (bp 259°C, mp 69°C), fluorene (bp 295°C, mp 114°C), phenanthrene (m bp 340°C, mp 101°C), anthracene (bp 354°C, mp 216°C), fluoranthene (mp 110°C), pyrene (mp. mp 151°C), chrysene (mp 255°C) (see also formulas in Table 4, Section III).

Resonance in aromatic systems. At first glance, it may seem that these are highly unsaturated compounds, but the double bonds in all of them, with the exception of the 9,10-double bond of phenanthrene, are extremely inert. This lack of reactivity or an abnormally low double-connectivity character is attributed to "resonance". Resonance implies that hypothetical double bonds are not localized in specific or formal bonds. They are delocalized over all the ring carbon atoms, and it is not possible to accurately depict the electronic structure of such molecules with a single formula of the usual type. Wherever it is possible to write for a molecule two (or more) structures which have equal or approximately equal energy and which differ only in the positions attributed to the electrons, it is found that the real molecule is more stable than any of the structures should be, and has the properties intermediate between them. The additional stability acquired in this way is called resonance energy. This principle follows from quantum mechanics and reflects the impossibility of accurately describing many of the microscopic systems, such as atoms and molecules, with simple diagrams. Based on the following evidence, it can be argued that benzene C6H6 is a flat six-membered ring containing three alternating with simple double bonds: hydrogenation under severe conditions turns it into cyclohexane C6H12; ozonolysis yields glyoxal OHC-CHO; the dipole moments of the dichloro derivatives C6H4Cl2 can be calculated exactly from the dipole moment of monochlorobenzene, assuming the ring is a planar regular hexagon. Such a molecule can be assigned the structure

Both of these Kekule structures (named after F. Kekule, who proposed them) are identical in energy and make the same contribution to the true structure. It can be depicted as

ascribing a semi-double bond character to each carbon-carbon bond. A thorough analysis by L. Pauling showed that Dewar structures also make a small contribution:

It was found that the resonance energy of the system is 39 kcal/mol, and, therefore, the benzene double bond is more stable than the olefinic one. Therefore, any reaction consisting of addition to one of the double bonds and leading to the structure

would require a high energy barrier to be overcome, since the two double bonds in cyclohexadiene

Stabilized with a resonance energy of only 5 kcal/mol. For naphthalene, three structures can be written:

Since they all have approximately the same energy, the true structure is the arithmetic mean of all three and can be written as

the fractions indicating the degree of double bonding of each carbon-carbon bond. The resonance energy is 71 kcal/mol. In general, only one Kekul structure is written for benzene, and the first of the structures written above is used to represent naphthalene. The structure of anthracene is depicted in a similar way (see Table 4 in Section III).
A. AROMATIC COMPOUNDS OF THE BENZENE SERIES
1. Hydrocarbons of the benzene series. Benzene and its homologues have the general formula CnH2n - 6. The homologues consist of a benzene ring and one or more aliphatic side chains attached to its carbon atoms instead of hydrogen. The simplest of the homologues, toluene C6H5CH3, is found in coal tar and is essential as a starting compound for the preparation of the explosive trinitrotoluene (see Section IV-3.A.2 "Nitro compounds") and caprolactam. The next formula in the series, C8H10, corresponds to four compounds: ethylbenzene C6H5C2H5 and xylenes C6H4(CH3)2. (Higher homologues are of less interest.) When two substituents are attached to a ring, the possibility of positional isomerism arises; thus, there are three isomeric xylenes: Other important benzene hydrocarbons include the unsaturated hydrocarbon styrene C6H5CH=CH2, used in the production of polymers; stilbene C6H5CH=CHC6H5; diphenylmethane (C6H5)2CH2; triphenylmethane (C6H5)3CH; diphenyl C6H5-C6H5.
Receipt. Benzene hydrocarbons are obtained by the following methods: 1) dehydrogenation and cyclization of paraffins, for example:

2) Wurtz-Fittig synthesis:

3) Friedel-Crafts reaction with alkyl halides or olefins:

4) Friedel-Crafts synthesis of ketones followed by Clemmensen reduction (treatment with zinc amalgam and acid), which converts the carbonyl group into a methylene unit:

5) dehydrogenation of alicyclic hydrocarbons:

7) distillation of phenols with zinc dust (the method is useful for establishing the structure, but rarely used in synthesis) for example:

Also applicable are other methods described above for the production of aliphatic hydrocarbons (eg reduction of halides, alcohols, olefins). The reactions of hydrocarbons of the benzene series can be subdivided into side chain reactions and ring reactions. Except for the position adjacent to the ring, the side chain behaves essentially like a paraffin, olefin, or acetylene, depending on its structure. Carbon-hydrogen bonds on the carbon adjacent to the ring, however, are markedly activated, especially with respect to free radical reactions such as halogenation and oxidation. So, toluene and higher homologues are easily chlorinated and brominated by halogens in sunlight:

In the case of toluene, the second and third halogens can be introduced. These a-chloro compounds are easily hydrolyzed by alkalis:

Toluene is easily oxidized to benzoic acid C6H5COOH. Higher homologs upon oxidation undergo cleavage of the side chain to a carboxyl group, forming benzoic acid. The main ring reaction is aromatic substitution, in which a proton is replaced by a positive atom or group derived from an acidic or "electrophilic" reagent:

Typical examples of such substitution: a) nitration, Ar-H + HNO3 -> Ar-NO2 + H2O; b) halogenation, Ar-H + X2 -> Ar-X + HX; c) Friedel-Crafts alkylation with olefins and alkyl halides (as above); d) Friedel-Crafts acylation,

E) sulfonation, Ar-H + H2SO4 (fuming) -> ArSO3H + H2O. The introduction of the first substituent encounters no complications, since all positions in benzene are equivalent. The introduction of the second substituent occurs in different positions with respect to the first substituent, primarily depending on the nature of the group already present in the ring. The nature of the attacking reagent plays a secondary role. Groups that increase the electron density in the aromatic ring -O-, -NH2, -N(CH3)2, -OH, -CH3, -OCH3, -NHCOCH3 activate the ortho and para positions and direct the next group mainly to these positions . On the contrary, groups that pull the electrons of the ring towards themselves

The ortho- and para-positions are most deactivated with respect to electrophilic attack, so the substitution is directed mainly to the meta-position. Intermediate in their behavior are some groups that, due to opposite electronic influences, deactivate the ring with respect to further substitution, but remain ortho-para-orientants: -Cl, -Br, -I and -CH=CHCOOH. These principles are important for synthesis in the aromatic series. So to get p-nitrobenzene

,

you must first brominate the ring and then nitrate it. The reverse order of the operation gives the meta isomer. Under harsh conditions, the ring can be "forced" to reveal its latent unsaturated character. With very active platinum catalysts, hydrogenation of benzene to cyclohexane can be achieved at a hydrogen pressure of several atmospheres (but partial hydrogenation products like cyclohexadiene can never be obtained). Prolonged treatment with chlorine and bromine in sunlight leads to the formation of hexahalocyclohexanes.
2. Substituted benzene. Nomenclature.
1) Monosubstituted benzenes can be considered as benzene derivatives, for example ethylbenzene C6H5-C2H5, or as phenyl derivatives of aliphatic hydrocarbons, for example 2-phenylbutane C6H5-CH(CH3)C2H5, if they do not have trivial names (for example, toluene, xylene). Halogen and nitro derivatives are called benzene derivatives, for example, nitrobenzene C6H5NO2, bromobenzene C6H5Br. Other monosubstituted benzenes are designated by special names: phenol C6H5OH, anisole C6H5OCH3, aniline C6H5NH2, benzaldehyde C6H5CH=O. 2) In disubstituted compounds, indicate the relative positions of the substituents ortho (o), meta (m) and para (p), as in xylenes (see section IV-3.A.1). The order of precedence in the selection of the first substituent is: COOH, CHO, COR, SO3H, OH, R, NH2, halogen and NO2. For example

Some trivial names are widely used, for example,

3) In the case of three or more substituents, numbers (from 1 to 6) are used to indicate the positions. When choosing a first alternate, the same rules of precedence apply, for example:

4) Substituents in the side chain: such compounds are usually referred to as aryl derivatives of aliphatic compounds. Examples are a-phenylethylamine (C6H5)CH(NH2)CH3 and a-phenylbutyric acid C2H5CH(C6H5)COOH. There are numerous trivial names (eg mandelic acid C6H5CH(OH)COOH) which will be considered when discussing the respective compounds. Halogen derivatives are obtained by the following methods: 1) direct halogenation of the ring

(Br2 reacts in a similar way); 2) substitution of the diazonium group (see "Aromatic amines" below) with a halide ion:

(at X = Cl- and Br- copper or CuX should be used as catalysts). The halogen atoms in aromatic halides are very inert to the action of bases. Therefore, substitution reactions analogous to those of aliphatic halides are rarely useful in practice in the case of aryl halides. In industry, the hydrolysis and ammonolysis of chlorobenzene is achieved under harsh conditions. Substitution with a nitro group in the p- or o-position activates the halogen towards bases. From bromine and iodobenzenes, a Grignard reagent can be prepared. Chlorobenzene does not form Grignard reagents, but phenyllithium can be obtained from it. These aromatic organometallic compounds are similar in properties to their aliphatic counterparts. Nitro compounds are usually prepared by direct nitration of the ring (see Section IV-3.A.1, "Reactions") with a mixture of concentrated nitric and sulfuric acids. Less commonly, they are prepared by the oxidation of nitroso compounds (C6H5NO). The introduction of one nitro group into benzene proceeds relatively simply. The second enters more slowly. The third can be introduced only with prolonged treatment with a mixture of fuming nitric and sulfuric acids. This is the general effect of m-orienting groups; they always reduce the ring's ability to be further replaced. Trinitrobenzenes are valued as explosives. To carry out their synthesis, nitration is usually carried out not on benzene itself, but on its derivatives such as toluene or phenol, in which o,p-orienting substituents can activate the ring. Well-known examples are 2,4,6-trinitrophenol (picric acid) and 2,4,6-trinitrotoluene (TNT). The only useful reactions of nitro compounds are their reduction reactions. Strong reducing agents (catalyst-activated hydrogen, tin and hydrochloric acid, bisulfide ion) convert them directly to amines. Controlled electrolytic reduction makes it possible to distinguish the following intermediate stages:

Ammonium bisulfide is a specific reagent for the conversion of dinitro compounds to nitroanilines, for example:

aromatic amines. Primary amines are obtained by reduction of the corresponding nitro compounds. They are very weak bases (K = 10-10). N-alkylanilines can be prepared by alkylation of primary amines. They resemble aliphatic amines in most reactions, with the exception of the interaction with nitrous acid and with oxidizing agents. With nitrous acid in an acidic medium (at 0-5°C), primary amines give stable diazonium salts (C6H5N=N+X-), which have many important synthetic applications. The substitution of a diazonium group by a halogen has already been discussed. This group can also be replaced by cyanide ion (with CuCN as a catalyst) to give aromatic nitriles (C6H5CN). Boiling water converts diazonium salts into phenols. In boiling alcohol, this group is replaced by hydrogen:

In nearly neutral solution, diazonium salts combine with phenols (and many amines) to give azo dyes:

This reaction is of great importance for the synthetic dye industry. Reduction with bisulfite leads to arylhydrazines C6H5NHNH2. Secondary arylamines, like aliphatic secondary amines, give N-nitroso compounds. Tertiary arylamines C6H5NRRў, however, give p-nitrosoarylamines (e.g. p-ON-C6H4NRR"). These compounds are of some importance for the preparation of pure secondary aliphatic amines because they easily hydrolyze to the secondary amine RRўNH and p-nitrosophenol. Oxidation of aromatic amines can affect not only the amino group, but also the p-position of the ring.Thus, aniline during oxidation turns into many products, including azobenzene, nitrobenzene, quinone (

and aniline black dye). Arylalkylamines (eg benzylamine C6H5CH2NH2) exhibit the same properties and reactions as alkylamines of the same molecular weight. Phenols are aromatic hydroxy compounds in which the hydroxyl group is attached directly to the ring. They are much more acidic than alcohols, ranging in strength between carbonic acid and bicarbonate ion (for phenol, Ka = 10-10). The most common method for their preparation is the decomposition of diazonium salts. Their salts can be obtained by fusing salts of arylsulfonic acids with alkali:

In addition to these methods, phenol is produced industrially by the direct oxidation of benzene and by the hydrolysis of chlorobenzene under harsh conditions - with a solution of sodium hydroxide at high temperature under pressure. Phenol and some of its simplest homologues, methylphenols (cresols) and dimethylphenols (xylenols), are found in coal tar. The reactions of phenols are notable for the lability of the hydroxyl hydrogen and the resistance of the hydroxyl group to substitution. In addition, the para position (and the ortho positions if the para position is blocked) are very sensitive to attack by aromatic substitution reagents and oxidizing agents. Phenols easily form sodium salts when treated with caustic soda and soda, but not with sodium bicarbonate. These salts readily react with anhydride and acid chloride to give esters (eg C6H5OOCCH3) and with alkyl halides and alkyl sulfates to form ethers (eg anisole C6H5OCH3). Phenol esters can also be prepared by the action of acylating agents in the presence of pyridine. Phenolic hydroxyl groups can be removed by distillation of phenols with zinc dust, but they are not replaced by heating with hydrohalic acids like alcohol hydroxyl groups. The hydroxyl group activates the ortho and para positions so strongly that the reactions of nitration, sulfonation, halogenation, and the like proceed violently even at low temperatures. The action of bromine water on phenol leads to 2,4,6-tribromophenol, but p-bromophenol can be obtained by bromination in solvents such as carbon disulfide at low temperatures. Solventless halogenation gives a mixture of o- and p-halophenols. Dilute nitric acid easily nitrates phenol, giving a mixture of o- and p-nitrophenols, from which o-nitrophenol can be steam stripped. Phenol and cresols are used as disinfectants. Among other phenols, the following are important: a) carvacrol (2-methyl-5-isopropylphenol) and thymol (3-methyl-6-isopropylphenol), which are found in many essential oils as products of chemical transformations of terpenes; b) anol (p-propenylphenol), which occurs as the corresponding anethole methyl ester in anise oil; close to it havikol (p-allylphenol) is found in oils from betel and laurel leaves and in the form of methyl ester, estragole, in anise oil; c) pyrocatechin (2-hydroxyphenol), which is found in many plants; in industry, it is obtained by hydrolysis (under harsh conditions) of o-dichlorobenzene or o-chlorophenol, as well as demethylation of guaiacol (pyrocatechol monomethyl ether) contained in the dry distillation products of beech; catechol is easily oxidized to o-quinone

And it is widely used as a reducing agent in photographic developers; d) resorcinol (m-hydroxyphenol); it is obtained by alkaline melting of m-benzenedisulfonic acid and used for the preparation of dyes; it is easily substituted in position 4 and reduced to dihydroresorcinol (cyclohexanedione-1,3), which is cleaved with dilute alkali into d-ketocaproic acid; its 4-n-hexyl derivative is a useful antiseptic; e) hydroquinone (p-hydroxyphenol), which is found in some plants in the form of arbutin glycoside; it is obtained by the reduction of quinone (see above "Aromatic amines"), a product of the oxidation of aniline; it is an easily reversible reaction; at 50% its flow, a stable equimolecular compound of quinone and hydroquinone, quinhydrone, is formed; The quinhydrone electrode is often used in potentiometric analysis; due to the reducing properties of hydroquinone, it, like catechol, is used in photographic developers; e) pyrogallol (2,3-dihydroxyphenol), which is obtained from gallic acid (see "Aromatic acids" below) by distillation over pumice stone in an atmosphere of carbon dioxide; being a powerful reducing agent, pyrogallol finds use as an oxygen scavenger in gas analysis and as a photographic developer. Aromatic alcohols are compounds which, like benzyl alcohol C6H5CH2OH, contain a hydroxyl group in the side chain (not in the ring like phenols). If the hydroxyl group is located at the carbon atom adjacent to the ring, it is especially easily replaced by a halogen under the action of hydrogen halides on hydrogen (above platinum) and is easily cleaved off during dehydration (in C6H5CHOHR). Simple aromatic alcohols such as benzyl, phenethyl (C6H5CH2CH2OH), phenylpropyl (C6H5CH2CH2CH2OH) and cinnamon (C6H5CH=CHCH2OH) are used in the perfume industry and occur naturally in many essential oils. They can be obtained by any of the general reactions described above for the preparation of aliphatic alcohols.
aromatic aldehydes. Benzaldehyde C6H5CHO, the simplest aromatic aldehyde, is formed in bitter almond oil as a result of enzymatic hydrolysis of amygdalin glycoside C6H5CH(CN)-O-C12H21O10. It is widely used as an intermediate in the synthesis of dyes and other aromatic compounds, as well as a fragrance and perfume base. In industry, it is obtained by hydrolysis of benzylidene chloride C6H5CHCl2, a product of the chlorination of toluene, or by direct oxidation of toluene in gas (over V2O5) or in liquid phase with MnO2 in 65% sulfuric acid at 40 ° C. The following general methods are used to prepare aromatic aldehydes: 1 ) Guttermann-Koch synthesis:

2) Guttermann synthesis:

3) Reimer-Timan synthesis (to obtain aromatic hydroxyaldehydes):

Benzaldehyde is oxidized by atmospheric oxygen to benzoic acid; this can also be achieved by using other oxidizing agents, such as permanganate or dichromate. In general, benzaldehyde and other aromatic aldehydes enter into carbonyl condensation reactions (see Section IV-1.A.4) somewhat less actively than aliphatic aldehydes. The absence of an a-hydrogen atom prevents the entry of aromatic aldehydes into aldol self-condensation. Nevertheless, mixed aldol condensation is used in the synthesis:

The following reactions are typical for aromatic aldehydes: 1) Cannizzaro reaction:

2) benzoin condensation:

3) Perkin's reaction:

The following aromatic aldehydes are of some importance: 1) Salicylaldehyde (o-hydroxybenzaldehyde) occurs naturally in meadowsweet fragrant oil. It is obtained from phenol by the Reimer-Timan synthesis. It finds application in the synthesis of coumarin (see Section IV-4.D) and some dyes. 2) Cinnamaldehyde C6H5CH=CHCHO is found in cinnamon and cassia oil. It is obtained by crotonic condensation (see Section IV-1.A.4) of benzaldehyde with acetaldehyde. 3) Anisaldehyde (p-methoxybenzaldehyde) is found in cassia oil and is used in perfumes and fragrances. It is obtained by the Guttermann synthesis from anisole. 4) Vanillin (3-methoxy-4-hydroxybenzaldehyde) is the main aromatic component of vanilla extracts. It can be obtained by the Reimer-Tieman reaction from guaiacol or by treating eugenol (2-methoxy-4-allylphenol) with alkali followed by oxidation. 5) Piperonal has a heliotrope odor. It is obtained from safrole (American laurel oil) in a similar way to how vanillin is obtained from eugenol.

aromatic ketones. These substances are usually obtained from aromatic compounds and acid chlorides by the Friedel-Crafts reaction. General methods for the preparation of aliphatic ketones are also used. A specific method for preparing hydroxy ketones is the Fries rearrangement in phenol esters:

(at elevated temperatures of the order of 165-170 ° C, the o-isomer predominates). In general, aromatic ketones undergo the same reactions as aliphatic ketones, but much more slowly. a-Diketonebenzyl C6H5CO-COC6H5, obtained by the oxidation of benzoin (see previous section "Aromatic aldehydes"), undergoes a characteristic rearrangement when treated with alkali, forming benzyl acid (C6H5)2C(OH)COOH.
aromatic acids. The simplest aromatic carboxylic acid is benzoic C6H5COOH, which, together with its esters, occurs naturally in many resins and balms. It is widely used as a food preservative, especially in the form of the sodium salt. Like aliphatic acids, benzoic acid and other aromatic acids can be prepared by the action of carbon dioxide on a Grignard reagent (eg C6H5MgBr). They can also be prepared by hydrolysis of the corresponding nitriles, which in the aromatic series are obtained from diazonium salts, or by fusing the sodium salts of aromatic sulfonic acids with sodium cyanide:

Other methods for their preparation include: 1) oxidative cleavage of aliphatic side chains

2) hydrolysis of trichloromethylarenes

3) synthesis of hydroxy acids according to Kolbe

4) oxidation of acetophenones by hypohalogenites

Some of the most important aromatic carboxylic acids are listed below: 1) Salicylic (o-hydroxybenzoic) acid o-C6H4(COOH)OH is prepared from phenol by the Kolbe synthesis. Its methyl ester is a fragrant component of the oil of winter love (gaulteria), and the sodium salt of the acetyl derivative is aspirin (sodium o-acetoxybenzoate). 2) Phthalic (o-carboxybenzoic) acid is obtained by the oxidation of naphthalene. It easily forms an anhydride, and the latter, under the action of ammonia, gives phthalimide, an important intermediate in the synthesis of many compounds, including indigo dye

3) Anthranilic (o-aminobenzoic) acid o-C6H4(NH2)COOH is obtained by the action of sodium hypochlorite on phthalimide (Hoffmann reaction). Its methyl ester is a perfume ingredient and is found naturally in jasmine and orange leaf oils. 4) Gallic (3,4,5-trihydroxybenzoic) acid is formed together with glucose during the hydrolysis of some complex plant substances known as tannins. Sulfonic acids. Benzenesulfonic acid C6H5SO3H is obtained by the action of fuming sulfuric acid on benzene. She and other sulfonic acids are strong acids (K > 0.1). Sulfonic acids are easily soluble in water, hygroscopic; they are difficult to obtain in a free state. Most often they are used in the form of sodium salts. The most important reactions of salts, namely fusion with alkalis (to form phenols) and with sodium cyanide (to form nitriles), have already been discussed. Under the action of phosphorus pentachloride, they give arylsulfonic chlorides (for example, C6H5SO2Cl), which are used in aliphatic and alicyclic syntheses. The arylsulfochloride most commonly used in this manner is p-toluenesulfochloride (p-CH3C6H4SO2Cl), often referred to in the literature as tosyl chloride (TsCl). Heating sulfonic acids in 50-60% sulfuric acid at 150 ° C causes their hydrolysis to sulfuric acid and initial hydrocarbons:

An important sulfonic acid is sulfanilic acid p-H2NC6H4SO3H (or p-H3N+C6H4SO3-), the amide (sulfanilamide) and other derivatives of which are important chemotherapeutic agents. Sulfanilic acid is produced by the action of fuming sulfuric acid on aniline. Many detergents are salts of long chain sulfonic acids such as NaO3S-C6H4-C12H25.
B. AROMATIC COMPOUNDS OF THE NAPHTHALENE RANGE
1. Synthesis of a- and b-substituted naphthalene derivatives. Naphthalene is the main component of coal tar. It is of exceptional importance in the synthesis of many industrial products, including indigo and azo dyes. However, its use as a moth repellant has declined with the introduction of new agents such as p-dichlorobenzene. Its monosubstituted derivatives are designated as a- or b- in accordance with the position of the substituent (see Table 4 in section III). Positions in polysubstituted derivatives are indicated by numbers. Generally speaking, the a-position is more reactive. Nitration, halogenation and low-temperature sulfonation lead to a-derivatives. Access to the b-position is achieved mainly through high-temperature sulfonation. Under these conditions, the a-sulfonic acid rearranges into the more stable b-form. The introduction of other substituents in the b-position then becomes possible using the Bucherer reaction: first, b-naphthol b-C10H7OH is obtained from b-naphthalenesulfonic acid by alkaline melting, which then, when treated with ammonium bisulfite at 150 ° C and 6 atm, gives b-naphthylamine b- C10H7NH2; through the diazonium compounds obtained from this amine in the usual way, it is now possible to introduce a halogen or a cyano group into the b-position. The Friedel-Crafts reaction between naphthalene and acid chloride also gives b-acyl derivatives of b-C10H7COR.
2. Substitution reactions of naphthalene derivatives. The reactions of naphthalene derivatives are the same as those of benzene derivatives. Thus, naphthalenesulfonic acids serve as a source of naphthols; naphthylamines are converted through diazonium salts into halo- and cyano-naphthalenes. Therefore, a specific discussion of the reactions of naphthalene compounds will be omitted. However, substitution reactions in naphthalene derivatives are of particular interest. 1) In the presence of an o,p-orientant (-CH3, -OH) in the 1(a)-position, the attack is directed mainly to position 4 and then to position 2. 2) In the presence of an m-orientant (-NO2) in position 1, the attack goes to position 8 (peri) and then to position 5. 3) In the presence of an o,n orientant in position 2 (b), position 1 is predominantly attacked, although sulfonation may occur in position 6. It is especially important that it is never attacked position 3. This is explained by the low degree of double bonding of the carbon-carbon bond 2-3. In naphthalene, substitution proceeds under milder conditions than in benzene. Naphthalene is also easier to recover. Thus, sodium amalgam reduces it to tetralin (tetrahydronaphthalene; see the formula in Table 4, Section III). It is also more sensitive to oxidation. Hot concentrated sulfuric acid in the presence of mercury ions converts it into phthalic acid (see section IV-3.A.2 "Aromatic acids"). Although in toluene the methyl group is oxidized before the ring, in p-methylnaphthalene the 1,4 positions are more susceptible to oxidation, so that the first product is 2-methyl-1,4-naphthoquinone:

B. DERIVATIVES OF POLYNUCLEAR AROMATIC HYDROCARBONS
1. Anthracene and its derivatives. Anthracene (see the formula in Table 4, Section III) is found in significant amounts in coal tar and is widely used in industry as an intermediate in the synthesis of dyes. Positions 9,10 are highly reactive in addition reactions. Thus, hydrogen and bromine are easily added, giving respectively 9,10-dihydro- and 9,10-dibromoanthracene. Oxidation with chromic acid converts anthracene to anthraquinone.

Anthraquinone (mp. 285 ° C) is a yellow crystalline substance. The most common way to obtain anthraquinone and its derivatives is the cyclization of o-benzoylbenzoic acids under the action of sulfuric acid

o-Benzoylbenzoic acids are obtained by the action of phthalic anhydride on benzene (or its corresponding derivative) in the presence of aluminum chloride. Anthraquinone is extremely resistant to oxidation. Reducing agents such as zinc dust and alkali or sodium bisulfite convert it to anthrahydroquinone (9,10-dihydroxyanthracene), a white substance that dissolves in alkali to form blood-red solutions. Tin and hydrochloric acid reduce one keto group to methylene, forming anthrone. Nitration under stringent conditions gives mainly a(1)-derivative together with a noticeable amount of 1,5- and 1,8-dinitroanthraquinones. Sulfonation with sulfuric acid produces mainly b(2)-sulfonic acid, but in the presence of small amounts of mercury sulfate, the main product is a-sulfonic acid. Disulfation in the presence of mercury sulfate gives mainly 1,5- and 1,8-disulfonic acids. In the absence of mercury, 2,6- and 2,7-disulfonic acids are formed. Anthraquinone sulfonic acids are of great importance, since hydroxyanthraquinones are obtained from them by alkaline melting, many of which are valuable dyes. So, oxidative alkaline melting of b-sulfonic acid gives the dye alizarin (1,2-dihydroxyanthraquinone), which is naturally found in madder roots. The sulfonic acid groups in anthraquinone can also be directly replaced by amino groups to form aminoanthraquinones, which are valuable dyes. In this reaction, the sodium salt of a sulfonic acid is treated with ammonia at 175-200° C. in the presence of a mild oxidizing agent (eg sodium arsenate) added to destroy the resulting sulfite.
2. Phenantrene and its derivatives. In nature, phenanthrene is found in coal tar. It itself and its derivatives can be obtained from o-nitrostilbenecarboxylic acid, which is formed by the condensation of o-nitrobenzaldehyde and phenylacetic acid according to the Pschorr method:

The double bond at position 9,10 is highly reactive; it readily adds bromine and hydrogen and undergoes oxidation first to 9,10-phenanthraquinone and then to diphenic acid

Substitution reactions in phenanthrene usually go to positions 2, 3, 6 and 7.
3. Higher polynuclear hydrocarbons attracted attention mainly due to their high carcinogenic activity. Here are some examples:

Dyes pyrantrone, idantrene yellow and violantrone are keto derivatives of complex polynuclear hydrocarbons.

• - fragrant essential oil plants., possessing b. or m. a strong odor from an essential oil contained in plants or their parts, used in medicine and industry ...

  Agricultural dictionary-reference book

• - essential oil plants with a strong smell ...

  Glossary of botanical terms

• - carbocyclic. Comm., not containing benzene nuclei, but characterized by aromaticity. These include, for example, annulene, tropylium compounds, tropolones, cyclopentadienide anion...

  Chemical Encyclopedia

• - compounds whose molecules are characterized by the presence of an aromatic system of bonds. In a narrower sense, aromatic hydrocarbons include benzene and polycyclic compounds based on it ...

  The Beginnings of Modern Natural Science

• - In ancient times, herbal products with a pleasant aroma, such as frankincense, myrrh, cassia, cinnamon and lavender, were used in religious rituals and everyday life ...

  Dictionary of antiquity

• - AROMATIC compounds - organic compounds, the molecules of which contain cycles of 6 carbon atoms involved in the formation of a single system of conjugated bonds. Includes hydrocarbons and their derivatives...

  Big encyclopedic dictionary

• - organic Comm., molecules to-rykh contain cycles of 6 carbon atoms involved in the formation of a single system of conjugated bonds. They include hydrocarbons and their derivatives ... - Most modern chemists divide the entire mass of organic substances into two large classes: fatty compounds and aromatic ... Great Soviet Encyclopedia

  Aroma oils

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  From the book Daily Life of the Army of Alexander the Great the author Fort Paul

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  Aromatic Substances Fruits, vegetables and herbs contain aromatic substances that give them a unique taste and aroma characteristic of each plant species and variety. Most of the aromatic substances are concentrated in that part of the plant,

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  From the book The Great Encyclopedia of Health by Paul Bragg author Moskin A. V.

  Aromatic baths In terms of their effect on the body, none of the types of baths can be compared with aromatic ones. Indeed, in this case, not only temperature and mechanical factors act, but also the healing properties of aromatic oils and medicinal infusions. Their action

Cyclic conjugated systems are of great interest as a group of compounds with increased thermodynamic stability compared to conjugated open systems. These compounds also have other special properties, the totality of which is united by the general concept aromaticity. These include the ability of such formally unsaturated compounds to undergo substitution rather than addition reactions, resistance to oxidizing agents, and temperature.

Typical representatives of aromatic systems are arenes and their derivatives. Features of the electronic structure of aromatic hydrocarbons are clearly manifested in the atomic orbital model of the benzene molecule. The benzene framework is formed by six sp 2 hybridized carbon atoms. All σ-bonds (C-C and C-H) lie in the same plane. Six unhybridized p-AOs are located perpendicular to the plane of the molecule and parallel to each other (Fig. 3a). Each R-AO can equally overlap with two neighboring R-AO. As a result of this overlap, a single delocalized π-system arises, in which the highest electron density is located above and below the σ-skeleton plane and covers all carbon atoms of the cycle (see Fig. 3b). The π-electron density is uniformly distributed over the entire cyclic system, which is indicated by a circle or a dotted line inside the cycle (see Fig. 3, c). All bonds between carbon atoms in the benzene ring have the same length (0.139 nm), intermediate between the lengths of single and double bonds.

Based on quantum mechanical calculations, it has been established that for the formation of such stable molecules, a planar cyclic system must contain (4n + 2) π electrons, where n= 1, 2, 3 etc. (Hückel's rule, 1931). Taking into account these data, it is possible to concretize the concept of "aromaticity".

Aromatic systems (molecules)- systems that respond aromaticity criteria :

1) the presence of a flat σ-skeleton consisting of sp 2 hybridized atoms;

2) delocalization of electrons, leading to the formation of a single π-electron cloud, covering all atoms of the cycle (cycles);

3) compliance with E. Hückel's rule, i.e. the electron cloud should have 4n + 2 π-electrons, where n = 1,2,3,4 ... (usually the number indicates the number of cycles in the molecule);

4) high degree of thermodynamic stability (high conjugation energy).

Rice. 3. Atomic orbital model of the benzene molecule (hydrogen atoms omitted; see text for explanation)

Stability of coupled systems. The formation of a conjugated and especially aromatic system is an energetically favorable process, since the degree of overlapping of the orbitals increases and delocalization (dispersal) occurs. R-electrons. In this regard, conjugated and aromatic systems have increased thermodynamic stability. They contain a smaller amount of internal energy and in the ground state occupy a lower energy level compared to non-conjugated systems. The difference between these levels can be used to quantify the thermodynamic stability of the conjugated compound, i.e., its conjugation energy (delocalization energy). For butadiene-1,3, it is small and amounts to about 15 kJ/mol. With an increase in the length of the conjugated chain, the conjugation energy and, accordingly, the thermodynamic stability of the compounds increase. The conjugation energy for benzene is much higher and amounts to 150 kJ/mol.

Examples of non-benzenoid aromatic compounds:

pyridine electronically resembles benzene. All carbon atoms and the nitrogen atom are in the state of sp 2 hybridization, and all σ-bonds (C-C, C-N and C-H) lie in the same plane (Fig. 4a). Of the three hybrid orbitals of the nitrogen atom, two are involved in the formation

Rice. 4. Pyridine nitrogen atom (but), (b) and a conjugated system in the pyridine molecule (c) (C-H bonds are omitted to simplify the figure)

σ-bonds with carbon atoms (only the axes of these orbitals are shown), and the third orbital contains a lone pair of electrons and does not participate in bond formation. A nitrogen atom with this electronic configuration is called pyridine.

Due to the electron located in the unhybridized p-orbital (see Fig. 4, b), the nitrogen atom participates in the formation of a single electron cloud with R-electrons of five carbon atoms (see Fig. 4, c). Thus, pyridine is a π,π-conjugated system and satisfies the aromaticity criteria.

As a result of greater electronegativity compared to the carbon atom, the pyridine nitrogen atom lowers the electron density on the carbon atoms of the aromatic ring, so systems with a pyridine nitrogen atom are called π-insufficient. In addition to pyridine, an example of such systems is pyrimidine containing two pyridine nitrogen atoms.

pyrrole also applies to aromatic compounds. The carbon and nitrogen atoms in it, as in pyridine, are in the state of sp2 hybridization. However, unlike pyridine, the nitrogen atom in pyrrole has a different electronic configuration (Fig. 5, a, b).

Rice. five. Pyrrole nitrogen atom (but), distribution of electrons in orbits (b) and a conjugated system in the pyrrole molecule (c) (C-H bonds are omitted to simplify the figure)

On unhybridized R-orbital of the nitrogen atom is a lone pair of electrons. She participates in the connection with R-electrons of four carbon atoms with the formation of a single six-electron cloud (see Fig. 5, c). Three sp 2 -hybrid orbitals form three σ-bonds - two with carbon atoms, one with a hydrogen atom. The nitrogen atom in this electronic state is called pyrrole.

Six-electron cloud in pyrrole due to p,p-conjugation is delocalized on five ring atoms, so pyrrole is π-excess system.

IN furan And thiophene the aromatic sextet also includes the lone pair of electrons of the unhybridized p-AO of oxygen or sulfur, respectively. IN imidazole And pyrazole two nitrogen atoms contribute differently to the formation of a delocalized electron cloud: the pyrrole nitrogen atom supplies a pair of π-electrons, and the pyridine one - one p-electron.

It also has aromatic purine, which is a condensed system of two heterocycles - pyrimidine and imidazole.

The delocalized electron cloud in purine includes 8 π-electrons of double bonds and the lone pair of electrons of the N=9 atom. The total number of electrons in conjugation, equal to ten, corresponds to the Hückel formula (4n + 2, where n = 2).

Heterocyclic aromatic compounds have high thermodynamic stability. It is not surprising that they are the structural units of the most important biopolymers - nucleic acids.

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 benzene carbon atoms 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 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.