aromatic hydrocarbons- compounds of carbon and hydrogen, in the molecule of which there is a benzene ring. The most important representatives of aromatic hydrocarbons are benzene and its homologues - the products of substitution of one or more hydrogen atoms in the benzene molecule for hydrocarbon residues.
The first aromatic compound, benzene, was discovered in 1825 by M. Faraday. Its molecular formula was established - C 6 H 6. If we compare its composition with the composition of the saturated hydrocarbon containing the same number of carbon atoms, hexane (C 6 H 14), we can see that benzene contains eight hydrogen atoms less. As is known, the appearance of multiple bonds and cycles leads to a decrease in the number of hydrogen atoms in a hydrocarbon molecule. In 1865, F. Kekule proposed its structural formula as cyclohexantriene - 1, 3, 5.
So the molecule corresponding to Kekule formula, contains double bonds, therefore, benzene must have an unsaturated character, i.e., it is easy to enter into addition reactions: hydrogenation, bromination, hydration, etc.
However, the data of numerous experiments have shown that benzene enters into addition reactions only under harsh conditions (at high temperatures and light), and is resistant to oxidation. The most characteristic of it are substitution reactions, therefore, benzene is closer in character to the marginal hydrocarbons.
Trying to explain these inconsistencies, many scientists have proposed various options for the structure of benzene. The structure of the benzene molecule was finally confirmed by the reaction of its formation from acetylene. In fact, the carbon-carbon bonds in benzene are equivalent, and their properties are not similar to those of either single or double bonds.
Currently, benzene is denoted either by the Kekule formula, or by a hexagon in which a circle is depicted.
So what is the peculiarity of the structure of benzene? Based on the researchers' data and calculations, it was concluded that all six carbon atoms are in the state sp 2 hybridization and lie in the same plane. unhybridized p-orbitals of carbon atoms that make up double bonds (Kekule formula) are perpendicular to the plane of the ring and parallel to each other.
They overlap with each other, forming a single π-system. Thus, the system of alternating double bonds depicted in the Kekule formula is a cyclic system of conjugated, overlapping α-bonds. This system consists of two toroidal (donut-like) regions of electron density lying on both sides of the benzene ring. Thus, it is more logical to depict benzene as a regular hexagon with a circle in the center (π-system) than as cyclohexatriene-1,3,5. 
The American scientist L. Pauling proposed to represent benzene as two boundary structures that differ in the distribution of electron density and constantly transform into each other, that is, to consider it an intermediate compound, an "averaging" of two structures.
The measured bond lengths confirm these assumptions. It was found that all C-C bonds in benzene have the same length (0.139 nm). They are somewhat shorter than single C-C bonds (0.154 nm) and longer than double ones (0.132 nm).
There are also compounds whose molecules contain several cyclic structures.
Isomerism and nomenclature
The benzene homologues are characterized by position isomerism of several substituents. The simplest benzene homologue, toluene (methylbenzene), does not have such isomers; the following homologue is presented as four isomers:
The basis of the name of an aromatic hydrocarbon with small substituents is the word benzene. Atoms in an aromatic ring are numbered from the highest substituent to the youngest:
According to the old nomenclature, positions 2 and 6 are called ortho positions, 4 - pair-, and 3 and 5 - metapositions.
Physical Properties
Benzene and its simplest homologues under normal conditions are very toxic liquids with a characteristic unpleasant odor. They are poorly soluble in water, but well - in organic solvents.
Substitution reactions. Aromatic hydrocarbons enter into substitution reactions.
1. Bromination. When reacting with bromine in the presence of a catalyst, iron bromide (ΙΙΙ), one of the hydrogen atoms in the benzene ring can be replaced by a bromine atom:
2. Nitration of benzene and its homologues. When an aromatic hydrocarbon interacts with nitric acid in the presence of sulfuric acid (a mixture of sulfuric and nitric acids is called a nitrating mixture), a hydrogen atom is replaced by a nitro group -NO 2:
By reducing the nitrobenzene formed in this reaction, aniline is obtained - a substance that is used to obtain aniline dyes:
This reaction is named after the Russian chemist Zinin.
Addition reactions. Aromatic compounds can also enter into addition reactions to the benzene ring. In this case, cyclohexane or its derivatives are formed.
1. hydrogenation. The catalytic hydrogenation of benzene proceeds at a higher temperature than the hydrogenation of alkenes:
2. Chlorination. The reaction proceeds under illumination with ultraviolet light and is a free radical:
The composition of their molecules corresponds to the formula C n H 2 n-6. The closest homologues of benzene are:
All benzene homologues following toluene have isomers. Isomerism can be associated both with the number and structure of the substituent (1, 2), and with the position of the substituent in the benzene ring (2, 3, 4). Compounds of the general formula C 8 H 10:
According to the old nomenclature used to indicate the relative position of two identical or different substituents in the benzene ring, prefixes are used ortho- (abbreviated o-) - substituents are located at neighboring carbon atoms, meta-(m-) - through one carbon atom and pair— (P-) - substitutes against each other.
The first members of the homologous series of benzene are liquids with a specific odor. They are lighter than water. They are good solvents.
Benzene homologues react substitution ( bromination, nitration). Toluene is oxidized by permanganate when heated:
Benzene homologues are used as solvents, for the production of dyes, plant protection products, plastics, and medicines.
These are cyclic compounds, unsaturated in composition, not showing the typical properties of unsaturated compounds, but having a special set of properties, united by the term “aromatic character” of the ring.
1) Quantum-chemical criterion - compliance of the structure with Hückel's rules
a) the presence of 4n+2 (n-integer, including 0)(p)-electrons in a closed conjugation circuit;
b) flat structure of the ring.
2) Physical criterion - high values of conjugation energy (delocalization). The more 
E, the more aromatic.
3) Alignment of the lengths of single and double bonds in the ring.
4) Chemical criterion - the presence of a complex of chemical properties that characterize the "aromatic character".
a) the stability of the double bonds of the ring in addition and oxidation reactions;
b) the ability to easily enter into substitution reactions (according to the ionic mechanism);
c) the ability to be easily formed in various reactions, i.e. high thermodynamic stability of the ring.
subdivided into:
compounds of a benzoid structure, contain a ring of cyclohexatriene (benzene) in the molecule.
compounds of non-benzenoid structure:
a) some heterocyclic compounds;
b) some derivatives of unsaturated cyclic compounds with 3, 5, 7, etc. carbon atoms in the cycle.
The simplest representative - benzene C 6 H 6 - should correspond in structure to cyclohexatriene, because. it can be obtained by dehydrogenation of 1,3-cyclohexadiene.
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This formula for the structure of benzene was proposed by Kekule. However, this formula does not describe all the features of the properties of benzene.
1. They do not give qualitative reactions to the double bond - they do not discolor bromine water and potassium permanganate, they do not polymerize, i.e. stable in addition and oxidation reactions.
2. With a more energetic effect than on unsaturated hydrocarbons, they enter into addition reactions of the most active reagents, for example, hydrogen and chlorine, while the ring is immediately completely saturated, no intermediate products of addition along one or two bonds were found. This means that in the benzene ring the entire system of double bonds behaves as a single whole.
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3H2 |
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3. The most typical for aromatic hydrocarbons are substitution reactions in which double bonds are not affected. This confirms the strength of the aromatic ring.
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4. For orthodisubstituted homologues, there is only 1 isomer, i.e. formulas (1) and (2) of o-xylene are equivalent.
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This is also confirmed by the ozonation reaction. The decomposition of ozonide yielded a mixture of glyoxal, methylglyoxal, and dimethylglyoxal. This is possible if the reaction proceeds with the participation of compounds of formulas (1) and (2).
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3O3 |
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dimethylglyoxal |
glyoxal | ||||||||||
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3O3 + 3H2O |
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methylglyoxal |
glyoxal | ||||||||||
This means that the position of double bonds in the benzene molecule cannot be considered fixed. Now the peculiar properties of benzene have been explained in the light of electronic concepts.
Valence angles and bond lengths are found. The carbon atoms in the benzene molecule are located at the corners of a regular hexagon. The angles of the hexagon are 120 0 С. Hydrogen atoms are located in the same plane at an angle of 120 0 С to carbon-carbon bonds.
Angle (1.54+1.34)/2
This geometry of the molecule takes place during the sp 2 hybridization of carbon atoms. Unhybridized p-electrons occupy dumbbell-shaped orbits, the axes of which are perpendicular to the plane of the hexagon and parallel to each other, so each of them is equally interspersed with two neighboring ones. Above and below the ring, a single six-electron cloud forms, an "aromatic sextet".
The lengths of bonds between carbon atoms in the aromatic ring have a value of 1.4A 0, intermediate between the lengths of single and double bonds, but somewhat less than the arithmetic mean: C-C 1.54A 0, C=C 1.34 A 0. This is evidence of a higher electron density between carbon atoms compared to unsaturated ones, which determines the greater strength of the aromatic ring. Confirmation is a comparison of the energy of formation of benzene with that calculated for cyclohexatriene; E exp. 39.6 kcal/mol less than E calc. This difference (E calc - E exp. = 
E) is called the conjugation energy.
The Kekule formula thus does not accurately describe the state of the bonds in the benzene molecule. This was understood by Kekule himself. For clarification, he introduced the concept of “valency oscillations”, according to which it was believed that the double bonds in the benzene molecule are not fixed, that is, that formulas (1) and (2) are equivalent.
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With this amendment, the Kekule formula is still used today. The Armstrong-Bayer formula is also used, which reflects the alignment of the electron density in the ring, and some others.
aromatic compounds- cyclic organic compounds that have an aromatic system in their composition. The main distinguishing properties are the increased stability of the aromatic system and, despite the unsaturation, the tendency to substitution reactions, rather than addition.
There are benzoid (arenes and structural derivatives of arenes, contain benzene rings) and non-benzenoid (all others) aromatic compounds. Azulene, annulenes, hetarenes (pyridine, pyrrole, furan, thiophene), and ferrocene are well known among non-benzenoid aromatic compounds. Inorganic aromatic compounds are also known, such as borazole ("inorganic benzene").
Aromaticity criteria
There is no single characteristic that can reliably classify a compound as aromatic or non-aromatic. The main characteristics of aromatic compounds are:
1. tendency to substitution reactions, not addition (it is most easily determined, historically the first sign, an example is benzene, unlike ethylene, it does not discolor bromine water)
2. energy gain, in comparison with the system of non-conjugated double bonds. Also called Resonance Energy (an improved method - Dewar Resonance Energy) (the gain is so great that the molecule undergoes significant transformations to achieve an aromatic state, for example, cyclohexadiene is easily dehydrogenated to benzene, di and trihydric phenols exist mainly in the form of phenols (enols) and not ketones etc.).
3. the presence of a ring magnetic current (observation requires complex equipment), this current provides a shift of the chemical shifts of protons associated with the aromatic ring to a weak field (7-8 ppm for the benzene ring), and protons located above / below the aromatic plane systems - in a strong field (NMR spectrum).
4. the presence of the plane itself (minimally distorted), in which all (or not all - homoaromatic) atoms lie, forming an aromatic system. In this case, the rings of pi-electrons formed during the conjugation of double bonds (or electrons of heteroatoms included in the ring) lie above and below the plane of the aromatic system.
5. Hückel's Rule is almost always observed: only a system containing (in the ring) 4n+2 electrons (where n = 0, 1, 2, …) can be aromatic. A system containing 4n electrons is antiaromatic (in a simplified sense, this means an excess of energy in a molecule, inequality of bond lengths, low stability - a tendency to addition reactions). At the same time, in the case of a peri-junction (there is an atom (s) that belongs to (e) simultaneously 3 cycles, that is, there are no hydrogen atoms or substituents near it), the total number of pi electrons does not correspond to the Hückel rule (phenalene, pyrene, crowned). It is also predicted that if it is possible to synthesize molecules in the form of a Möbius strip (a ring large enough so that there is little twisting in each pair of atomic orbitals), then for such molecules a system of 4n electrons will be aromatic, and of 4n + 2 electrons - antiaromatic.
Receipt
1. Catalytic dehydrocyclization of alkanes, i.e. elimination of hydrogen with simultaneous cyclization. The reaction is carried out at elevated temperature using a catalyst such as chromium oxide.
2. Catalytic dehydrogenation of cyclohexane and its derivatives. Palladium black or platinum at 300°C is used as a catalyst. (N. D. Zelinsky)
3. Cyclic trimerization of acetylene and its homologues over activated carbon at 600°C. (N. D. Zelinsky)
4. Alkylation of benzene with halogen derivatives or olefins. (Friedel-Crafts reaction)
Classification
In general, aromatic hydrocarbons can be classified as follows:
Systems with 2 π-electrons.
They are derivatives of the cyclopropenylium cation and cyclobutadiene dication. An example is cyclopropenylium perchlorate.
Systems with 6 π-electrons.
1. Benzene and its homologues
2. Cyclopentadienyl anion
3. Cycloheptatrienyl cation
4. Dianion of cyclobutadiene, dication of cyclooctatetraene
5. Five- and six-membered rings containing one or more heteroatoms, usually nitrogen, oxygen or sulfur. The most famous among them are pyrrole, furan, thiophene, pyridine.
Systems with 10 π-electrons.
1. Naphthalene. Widely found in nature, fused benzene rings.
2. Azulene. Isomer of naphthalene, contains 5- and 7-membered rings. Found in essential oils.
3. Dianion of cyclooctatetraene, anion of cyclononatetraene, azonine, 1,6-substituted--annulenes (bridged).
4. Indole, quinoline, isoquinoline, quinazoline, quinoxaline, other systems based on a benzene ring fused to another ring containing a heteroatom. Widely distributed in nature.
5. Quinolizidine, pyrrolizidine, purine, pteridine (their analogues) - bicyclic derivatives of pyrrole, pyridine, etc. Contain nitrogen atoms (less often, oxygen at the conjugation point or several heteroatoms in both rings). Widely distributed in nature.
Systems with 14 π-electrons.
1. Anthracene, phenanthrene, in a certain sense - phenalene - condensed benzene rings. Compounds of this type are called polycenes (the next one is tetracene).
2. -cancelled. Both by itself and its bridging variations (trans-15,16-dimethylhydropyrene, syn-1,6:8,13-bisoxidoannulene). Dehydroannulene is also aromatic.
Systems with more than 14 π-electrons.
1. 18-Annulen, kekulen.
2. Coronene is an aromatic polycyclic hydrocarbon containing 24 π-electrons, which means, according to the Hückel rule, its antiaromaticity. However, the π electron system of coronene consists of two concentric rings containing 18 (outer) and 6 (inner) electrons.
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 manufacture 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
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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 of their reactions, with the exception of 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 acid anhydride and acid chloride to give esters (eg C6H5OOCCH3) and with alkyl halides and alkyl sulfates to form ethers (eg C6H5OCH3 anisole). 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-Timan 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 the 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 sulfite formed.
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.
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