1. Classification of aromatic hydrocarbons.

2. Homologous series of monocyclic arenes, nomenclature, preparation.

3. Isomerism, structure of benzene and its homologues.

4. Properties of arenas.

Arenes are carbon-rich cyclic hydrocarbons that contain a benzene ring in the molecule and have special physical and chemical properties. Arenes, based on the number of benzene rings in the molecule and the method of connecting the rings, are divided into monocyclic (benzene and its homologues) and polycyclic (with condensed and isolated rings) compounds.

Arenes of the benzene series can be considered as products of the replacement of hydrogen atoms in the benzene molecule by alkyl radicals. The general formula of such arenes is CnH 2 n- 6. The name of monosubstituted arenes indicates the name of the radical and ring (benzene):

benzene methylbenzene (toluene) ethylbenzene.

In more substituted arenes, the position of the radicals is indicated by the smallest numbers; in disubstituted arenes, the position of the radicals is called: 1,2 - ortho ( o-)-, 1,3 - meta ( m-)- and 1,4 - pair ( n-)-:

1,3-dimethylbenzene 1,2-methylethylbenzene

m-dimethylbenzene ( m-xylene) O-methylethylbenzene ( O-xylene)

Trivial names are common for arenas (some names are given in parentheses).

Being in nature.

Aromatic hydrocarbons are found in plant resins and balms. Phenanthrene, in partially or fully hydrogenated form, is found in the structures of many natural compounds, such as steroids and alkaloids.

Getting arenas:

1. dry distillation of coal;

2. dehydrogenation of cycloalkanes

3. dehydrocyclization of alkanes with 6 or more carbon atoms in the composition

4. alkylation

Isomerism. Benzene homologues are characterized by structural isomerism: different structures of the carbon skeleton of the side radical and different composition and arrangement of radicals in the benzene ring. For example, isomers of aromatic hydrocarbons with the composition C 9 H 12 (propylbenzene, isopropylbenzene, o-methylethylbenzene and 1,2,4-trimethylbenzene):

Structure. Aromatic hydrocarbons have a number of features in the electronic structure of their molecules.

The structural formula of benzene was first proposed by A. Kekule. It is a six-membered ring with alternating double and single bonds, with the double bonds moving around in the structure:

In both formulas, carbon is tetravalent, all carbon atoms are equivalent, and disubstituted benzene exists in the form of three isomers ( ortho-, meta-, pair-). However, this structure of benzene contradicted its properties: benzene did not enter into the addition reactions (for example, bromine) and oxidation (for example, with potassium permanganate) characteristic of unsaturated hydrocarbons; for it and its homologues, the main type of chemical transformation is substitution reactions.

The modern approach to describing the electronic structure of benzene resolves this contradiction as follows. The carbon atoms in the benzene molecule are in sp 2 hybridization. Each of the carbon atoms forms three covalent σ bonds - 2 bonds with neighboring carbon atoms (sp 2 -sp 2 - orbital overlap) and one with a hydrogen atom (sp 2 -s - orbital overlap). Due to lateral overlap, unhybridized p-orbitals form a π-electron conjugated system (π,π-conjugation) containing six electrons. Benzene is a flat regular hexagon with a carbon-carbon bond length of 0.14 nm, a carbon-hydrogen bond of 0.11 nm, and bond angles of 120 0:

The benzene molecule is more stable than cyclic compounds with isolated double bonds, therefore benzene and its homologues are prone to substitution reactions (the benzene ring is retained) rather than addition and oxidation.

Other cyclic compounds also exhibit similarity in structure and properties (aromaticity) to benzene. Aromaticity criteria (E. Hückel, 1931):

a) flat cyclic structure, i.e. the atoms forming the cycle are in sp 2 hybridization; b) coupled electronic system; c) the number of electrons (N) in the ring is 4n+2, where n is any integer value - 0,1,2,3, etc.

The criteria for aromaticity apply to both neutral and charged cyclic conjugated compounds, so aromatic compounds would be, for example:

furan cation of cyclopropenyl.

For benzene and other aromatic compounds, the most typical reactions are substitution of hydrogen atoms at carbon atoms in the ring and less typical are addition reactions at the π-bond in the ring.

Physical properties.

Benzene and its homologues are colorless liquids and crystalline substances with a peculiar odor. They are lighter than water and do not dissolve well in it. Benzene is a non-polar compound (μ=0), alkylbenzenes -

polar compounds (μ≠0).

Chemical properties.

Electrophilic substitution. The most characteristic transformation for arenes is electrophilic substitution - S E. The reaction proceeds in two stages with the formation of an intermediate σ-complex:

Reaction conditions: temperature 60-80 0 C, catalysts - Lews acids or mineral acids.

Typical S E - reactions:

a) halogenation(Cl 2, Br 2):

b) nitration:

V) sulfonation(H 2 SO 4, SO 3, oleum) :

d) alkylation according to Friedel-Crafts (1877)(RNal, ROH, alkenes) :

e) alkylation according to Friedel-Crafts(acid halides, carboxylic acid anhydrides) :

In benzene homologues, as a result of the influence of the side radical (+I-effect, electron-donating group), the π-electron density of the benzene ring is unevenly distributed, increasing in the 2,4,6-positions. Therefore, S E reactions proceed directionally (in 2,4,6- or O- And p- provisions). Homologs of benzene, compared to benzene, exhibit greater reactivity in reactions of this type.

toluene n-chlorotoluene O-chlorotoluene

Reactions of side radicals in alkylbenzenes (radical substitution -S R and oxidation).

Radical substitution reactions proceed, as in saturated hydrocarbons, by a chain mechanism and include the stages of initiation, growth and chain termination. The chlorination reaction proceeds undirectionally, the bromination reaction is regioselective - the replacement of hydrogen occurs at the α-carbon atom.

In alkylbenzenes, the side chain is oxidized by potassium permanganate and potassium dichromate to form carboxylic acids. Regardless of the length of the side chain, the carbon atom associated with the benzene ring (α-carbon or benzyl carbon atom) is oxidized, the remaining carbon atoms are oxidized to CO 2 or carboxylic acids.

ethylbenzene benzoic acid

n-methylethylbenzene terephthalic acid

Reactions of benzene with disruption of the aromatic system.

Aromatic hydrocarbons have a strong cycle, so reactions that disrupt the aromatic system (oxidation, radical addition) occur under harsh conditions (high temperatures, strong oxidizing agents).

a) radical accession:

1. hydrogenation

toluene cyclohexane

2. chlorination

benzene 1,2,3,4,5,6-hexachlorocyclohexane (hexachlorane).

The product of this reaction is a mixture of spatial isomers.

Orientation of electrophilic substitution in aromatic compounds. Substituents on the benzene ring are divided into two types according to their orienting influence: ortho-, pair-orientants (substituents of the 1st kind) and meta-orientants (substituents of the 2nd kind).

Substituents of the 1st kind are electron-donating groups that increase the electron density of the ring, increase the rate of electrophilic substitution reaction and activate the benzene ring in these reactions:

D(+I-effect): - R, -CH 2 OH, -CH 2 NH 2, etc.

D(-I,+M-effects): -NH 2 , -OH, -OR, -NR 2 , -SH, etc.

Substituents of the 2nd kind are electron-withdrawing groups that reduce the electron density of the ring, reduce the rate of electrophilic substitution reaction and deactivate the benzene ring in these reactions:

A (-I-effect): -SO 3 H, -CF 3, -CCl 3, etc.

A (-I, -M -effect): -HC=O, -COOH, -NO 2, etc.

Halogen atoms occupy an intermediate position - they lower the electron density of the ring, reduce the rate of electrophilic substitution reaction and deactivate the benzene ring in these reactions, however this O-,n-orientators.

If there are two substituents on the benzene ring, then their orienting action may coincide ( consistent orientation) or not match ( inconsistent orientation). In electrophilic substitution reactions, compounds with coordinated orientation form a smaller number of isomers; in the second case, a mixture of a larger number of isomers is formed. For example:

n- hydroxybenzoic acid m- hydroxybenzoic acid

(consistent orientation) (inconsistent orientation)

Polycyclic condensed aromatic hydrocarbons (naphthalene, anthracene, phenanthrene, etc.) are basically similar in properties to benzene, but at the same time they have some differences.

Application:

1. aromatic hydrocarbons - raw materials for the synthesis of dyes, explosives, drugs, polymers, surfactants, carboxylic acids, amines;

2. liquid aromatic hydrocarbons are good solvents of organic compounds;

3. arenas - additives for producing high-octane gasoline.

Did you know that-In 1649, the German chemist Johann Glauber first obtained benzene.

In 1825, M. Faraday isolated hydrocarbon from illuminating gas and established its composition - C 6 H 6.

In 1830, Justus Liebig named the resulting compound benzene (from Arabic: Ben-aroma + zoa-juice + Latin: ol-oil).

In 1837, Auguste Laurent named the benzene radical C 6 H 5 - phenyl (from the Greek phenix - to illuminate).

In 1865, the German organic chemist Friedrich August Kekule proposed a formula for benzene with alternating double and single bonds in a six-membered ring.

In the 1865-70s, V. Kerner proposed using prefixes to indicate the relative position of two substituents: 1,2 position - ortho-(orthos - straight);1,3- meta(meta - after) and 1.4- pair(para - opposite).

Aromatic hydrocarbons are highly toxic substances that cause poisoning and damage to some organs, such as the kidneys and liver.

Some aromatic hydrocarbons are carcinogens (substances that cause cancer), for example benzene (causes leukemia), one of the strongest is benzopyrene (found in tobacco smoke).

Aromatic hydrocarbons– compounds of carbon and hydrogen, the molecule of which contains a benzene ring. The most important representatives of aromatic hydrocarbons are benzene and its homologues - products of the replacement of one or more hydrogen atoms in a benzene molecule with hydrocarbon residues.

The structure of the benzene molecule

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 a saturated hydrocarbon containing the same number of carbon atoms - hexane (C 6 H 14), then we can see that benzene contains eight less hydrogen atoms. 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 cyclohexanthriene - 1, 3, 5.

Thus, the molecule corresponding Kekule's formula, contains double bonds, therefore, benzene must be unsaturated, i.e., it must easily undergo addition reactions: hydrogenation, bromination, hydration, etc.

However, data from numerous experiments have shown that benzene enters into addition reactions only under harsh conditions (at high temperatures and lighting) and is resistant to oxidation. The most characteristic reactions for it are substitution reactions; therefore, benzene is closer in character to marginal hydrocarbons.

Trying to explain these discrepancies, 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 reality, 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 special about the structure of benzene? Based on the researchers' data and calculations, it was concluded that all six carbon atoms are in a 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 each other, forming a single π-system. Thus, the system of alternating double bonds depicted in Kekulé’s formula is a cyclic system of conjugated, overlapping bonds. This system consists of two toroidal (donut-like) regions of electron density lying on either side 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 in the form of two boundary structures that differ in the distribution of electron density and constantly transform into each other, i.e., consider it an intermediate compound, “averaging” of two structures.

Bond length measurements confirm these assumptions. It was found that all C-C bonds in benzene have the same length (0.139 nm). They are slightly shorter than single C-C bonds (0.154 nm) and longer than double bonds (0.132 nm).

There are also compounds whose molecules contain several cyclic structures.

Isomerism and nomenclature

Benzene homologues are characterized by isomerism of the position of several substituents. The simplest homolog of benzene - 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. The atoms in the aromatic ring are numbered from highest to lowest substituent:

According to the old nomenclature, positions 2 and 6 are called orthopositions, 4 - pair-, and 3 and 5 - meta-provisions.

Physical properties
Under normal conditions, benzene and its simplest homologues are very toxic liquids with a characteristic unpleasant odor. They dissolve poorly in water, but well in organic solvents.

Chemical properties of benzene

Substitution reactions. Aromatic hydrocarbons undergo 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), the hydrogen atom is replaced by a nitro group -NO2:

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 undergo addition reactions to the benzene ring. In this case, cyclohexane or its derivatives are formed.
1. Hydrogenation. Catalytic hydrogenation of benzene occurs at a higher temperature than the hydrogenation of alkenes:

2. Chlorination. The reaction occurs when illuminated with ultraviolet light and is free radical:

Benzene homologues

The composition of their molecules corresponds to the formula C n H 2 n-6. The closest homologues of benzene are:

All homologues of benzene 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 location of two identical or different substituents on the benzene ring, the prefixes are used ortho- (abbreviated o-) - substituents are located at neighboring carbon atoms, meta-(m-) – through one carbon atom and pair— (n-) – substituents 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 substitutions ( bromination, nitration). Toluene is oxidized by permanganate when heated:

Benzene homologues are used as solvents to produce dyes, plant protection products, plastics, and medicines.

AROMATIC COMPOUNDS (from the Greek ?ρωμα - incense), organic compounds characterized mainly by the presence of a closed system of conjugated bonds, which includes, according to Hückel’s rule, (4n + 2) π-electrons (n ​​= 0, 1, 2, ... ); satisfy all or more of the aromaticity criteria. The most famous and important are: aromatic hydrocarbons (arenes), including monocyclic - benzene and its homologues (for example, xylenes, cumene, toluene, ethylbenzene) and polycyclic, built from benzene rings directly connected to each other (for example, biphenyl), bound through any group (for example, diphenylmethane), condensed (for example, anthracene, naphthalene); arene derivatives (eg phenols); heteroaromatic compounds, i.e. heterocyclic systems with aromaticity (for example, pyridine, pyrimidine, thiophene, furan). Aromatic compounds also include some macrocyclic annulenes (for example, annulene), organoelement compounds (for example, ferrocene), tropylium compounds, etc.

Aromatic compounds are liquids or solids. They are characterized by the presence of a so-called magnetic ring current and resonance in the low-field (“aromatic”) part of the NMR spectrum (6.5-8.0 ppm for 1 H and 110-170 ppm for 13 C). Aromatic compounds undergo electrophilic substitution reactions (for example, halogenation, nitration, sulfonation, alkylation and Friedel-Crafts acylation). The introduction of electrophile E + into the molecule of an aromatic compound is facilitated and the electrophile is directed predominantly to the ortho- and para positions of the ring in the presence of substituents in the molecule of the aromatic compound that are type I orientants (alkyl, aryl, halogen atoms, groups OR, NR 2, SR, where R - organic radical), is hindered and directed predominantly to the meta position of the ring by substituents - orientants of the second kind (COR, COOR, CN, NO 2, SO 2 R, SO 3 H). Electrophilic substitution occurs via the addition-elimination mechanism through the cationic σ-complex - Ueland intermediate (X - substituent):

Aromatic compounds also undergo nucleophilic substitution reactions under the action of Nu - nucleophiles, for example R2N -, RO -, RS -, (RCO)2CH -, halide anions. In this case, halogen atoms, groups NO 2, NR 2, OR, SR, SO 3 H, and less often hydrogen atoms are replaced in the molecule of the aromatic compound. Such reactions often occur under harsh conditions, such as elevated temperatures. They are facilitated in the presence of copper compounds and, especially in the presence of a substituent in the ortho- or para-position to the leaving group - an orienting agent of the second kind. Nucleophilic substitution occurs mainly through the addition-elimination mechanism, through the formation of an anionic σ-complex - Meisenheimer intermediate (Y - substituted group):

Of lesser importance for an aromatic compound is hemolytic substitution, for example, arylation with diazo compounds and hydroxylation using Fenton's reagent (H 2 O 2 + CuSO 4 + H 2 SO 4). Aromatic compounds undergo metalation (direct replacement of hydrogen or exchange of halogen for metal under the action of metals or organometallic compounds). The reactions of an aromatic compound with respect to substituent groups are generally similar to the reactions of the corresponding aliphatic compounds. The main features are the formation of stable diazo compounds by aromatic amines with HNO 2, capable of azo coupling and converted under the action of nucleophiles into various substituted aromatic compounds. Of the addition reactions for aromatic compounds, the most important is catalytic hydrogenation, a general method for the synthesis of compounds of the cyclohexane series. Aromatic compounds are resistant to oxidation. Alkylaromatic compounds are usually oxidized at the carbon atom of the alkyl substituent adjacent to the aromatic ring. This method produces aromatic acids (for example, terephthalic acid from n-xylene), aldehydes (n-nitrobenzaldehyde from n-nitrotoluene), ketones (acetophenone from ethylbenzene) and alcohols (triphenylcarbinol from triphenylmethane).

Aromatic compounds are contained in oil, but they are mainly obtained industrially from the products of coking coal and aromatization of hydrocarbons; The aromatic compounds are then converted into various derivatives. Aromatic compounds are important intermediate and target products of industrial organic synthesis; They are used in the production of dyes, medicines, plant protection products, explosives, and polymeric materials.

Aromatic hydrocarbons are components of high-octane gasoline.

Lit.: Gorelik M.V., Efros L.S. Fundamentals of chemistry and technology of aromatic compounds. M., 1992.

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 under 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. The 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 ones r-AO. As a result of such overlap, a single delocalized π-system arises, the highest electron density in which is located above and below the plane of the σ-skeleton and covers all carbon atoms of the cycle (see Fig. 3, b). The π-Electron density is evenly distributed throughout the cyclic system, which is indicated by a circle or 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 was established that for the formation of such stable molecules, a flat cyclic system must contain (4n + 2) π electrons, where n= 1, 2, 3, etc. (Hückel's rule, 1931). Taking these data into account, the concept of “aromaticity” can be specified.

Aroma systems (molecules)– systems that meet 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 contain 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; explanation in text)

Stability of coupled systems. The formation of a conjugated and especially aromatic system is an energetically favorable process, since this increases the degree of overlap of orbitals and delocalization (dispersal) occurs. r-electrons. In this regard, conjugated and aromatic systems have increased thermodynamic stability. They contain a smaller supply of internal energy and in the ground state occupy a lower energy level compared to non-conjugated systems. From the difference between these levels, one can 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. As the length of the conjugated chain increases, 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 Its electronic structure resembles benzene. All carbon atoms and the nitrogen atom are in a state of sp 2 hybridization, and all σ bonds (C-C, C-N and C-H) lie in the same plane (Fig. 4, a). Of the three hybrid orbitals of the nitrogen atom, two are involved in the formation

Rice. 4. Pyridine nitrogen atom (A), (b) and the 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 is not involved in the formation of the bond. A nitrogen atom with this electron 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 criteria for aromaticity.

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

Rice. 5. Pyrrole nitrogen atom (A), distribution of electrons among orbitals (b) and the conjugated system in the pyrrole molecule (c) (C-H bonds are omitted to simplify the figure)

On unhybridized r The -orbital of the nitrogen atom contains a lone pair of electrons. She is involved in pairing with r-electrons of four carbon atoms to form 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 thanks to p, p-conjugation is delocalized on five ring atoms, so pyrrole is π-excess system.

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

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

The delocalized electron cloud in purine includes 8 π double bond electrons and a lone pair of electrons from 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 serve as structural units of the most important biopolymers - nucleic acids.

Aromatic compounds are characterized by aromaticity, i.e. a set of structural, energetic properties and features of the reactivity of cyclic structures with a system of conjugated bonds. In a narrower sense, this term refers only to benzenoid compounds (arenes), the structure of which is based on one or more benzene rings, including fused ones, i.e. having two common carbon atoms.
The main aromatic hydrocarbons of coal tar. The 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), biphenyl (bp. 259°C, mp. 69°C), fluorene (bp. 295°C, mp. 114°C), phenanthrene (t. b. 340°C, mp. 101°C), anthracene (bp. 354°C, mp. 216°C), fluoranthene (mp. 110°C), pyrene (mp. pl. 151° C), chrysene (mp 255° C) (see also formulas in Table 4, Section III).

Resonance in aromatic systems. At first glance these may seem to be 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 abnormally low doubly connectedness is attributed to "resonance". Resonance implies that the hypothetical double bonds are not localized in specific or formal bonds. They are delocalized over all ring carbon atoms, and it is impossible to accurately represent 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 assigned to the electrons, it is found that the real molecule is more stable than either structure would have to be and has the properties , intermediate between them. The additional stability thus acquired is called resonance energy. This principle follows from quantum mechanics and reflects the impossibility of accurately describing many of these 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 double bonds alternating with single ones: hydrogenation under severe conditions transforms it into cyclohexane C6H12; ozonolysis produces glyoxal OHC-CHO; The dipole moments of the dichloro derivatives of C6H4Cl2 can be accurately calculated from the dipole moment of monochlorobenzene, assuming that 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

attributing a semi-doubly bonded character to each carbon-carbon bond. A thorough analysis carried out by L. Pauling showed that Dewar structures also make a small contribution:

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

would require overcoming a high energy barrier, since the two double bonds in cyclohexadiene

Stabilized by 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 average of all three and can be written as

wherein the fractions indicate the degree of biconnectivity 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 structure 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. Homologues consist of a benzene ring and one or more aliphatic side chains attached to its carbon atoms in place of hydrogen. The simplest of homologues - toluene C6H5CH3 - is found in coal tar and is essential as a starting compound for the production 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 homologs 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 is rarely used in synthesis), for example:

Other methods described above for the production of aliphatic hydrocarbons (eg, reduction of halides, alcohols, olefins) are also applicable. The reactions of benzene hydrocarbons can be divided 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 are, however, markedly activated, especially with respect to free radical reactions such as halogenation and oxidation. Thus, toluene and higher homologues are easily chlorinated and brominated by halogens in sunlight:

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

Toluene can be easily oxidized to benzoic acid C6H5COOH. Higher homologues, 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) alkylation with olefins and alkyl halides according to Friedel-Crafts (as indicated above); d) Friedel-Crafts acylation,

E) sulfonation, Ar-H + H2SO4 (fuming) -> ArSO3H + H2O. The introduction of the first substituent does not encounter complications, since all positions in benzene are equivalent. The introduction of the second substituent occurs at different positions relative to the first substituent depending primarily on the nature of the group already present on 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 attract ring electrons

The ortho- and para-positions are most strongly 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-nitrobromobenzene

,

Some trivial names are widely used, e.g.

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

4) Side chain substituents: Such compounds are usually referred to as aryl derivatives of aliphatic compounds. Examples include 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 covered when discussing the corresponding compounds. Halogen derivatives are obtained by the following methods: 1) direct halogenation of the ring

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

(with X = Cl- and Br-, copper or CuX should be used as catalysts). The halogen atoms in aromatic halides are very inert to bases. Therefore, substitution reactions similar to those of aliphatic halides are rarely useful in the case of aryl halides. In industry, hydrolysis and ammonolysis of chlorobenzene is achieved under harsh conditions. Substitution with a nitro group at the p- or o-position activates the halogen towards bases. Grignard reagent can be prepared from bromo- and iodobenzenes. Chlorobenzene does not form Grignard reagents, but phenyllithium can be obtained from it. These aromatic organometallic compounds have properties similar 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 oxidation of nitroso compounds (C6H5NO). The introduction of one nitro group into benzene is relatively simple. The second one enters more slowly. The third can be introduced only with prolonged treatment with a mixture of fuming nitric and sulfuric acids. This is a general effect of m-orienting groups; they always reduce the ability of the ring to undergo further substitution. 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 (catalyzed hydrogen, tin and hydrochloric acid, bisulfide ion) convert them directly into amines. Controlled electrolytic reduction allows the following intermediate stages to be distinguished:

Ammonium bisulfide is a specific reagent for the conversion of dinitro compounds into 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. In most reactions they resemble aliphatic amines, with the exception of interaction with nitrous acid and oxidizing agents. With nitrous acid in an acidic environment (at 0-5°C), primary amines give stable diazonium salts (C6H5N=N+X-), which have many important synthetic applications. The replacement of the diazonium group with a halogen has already been discussed. This group can also be replaced with a 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, since they are easily hydrolyzed 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 is converted into many products, including azobenzene, nitrobenzene, quinone (

And aniline black dye). Arylalkylamines (for example, benzylamine C6H5CH2NH2) exhibit the same properties and reactions as alkylamines with the same molecular weight. Phenols, aromatic hydroxy compounds in which the hydroxyl group is attached directly to the ring. They are significantly 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 direct oxidation of benzene and hydrolysis of chlorobenzene under harsh conditions - sodium hydroxide solution 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 position if the para position is blocked) are very susceptible 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 react readily with acid anhydrides and acid chlorides to form esters (e.g., C6H5OOCCH3), and with alkyl halides and alkyl sulfates to form ethers (e.g., 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 distilling 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 nitration, sulfonation, halogenation and the like reactions proceed vigorously even at low temperatures. The action of bromine water on phenol leads to 2,4,6-tribromophenol, but p-bromophenol can be prepared by bromination in solvents such as carbon disulfide at low temperatures. Solvent-free halogenation produces 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 distilled off with steam. 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 methyl ester of anethole in anise oil; a related chavicol (p-allylphenol) is found in oils from betel and laurel leaves and in the form of methyl ester, estragole, in anise oil; c) pyrocatechol (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 by demethylation of guaiacol (pyrocatechol monomethyl ether) contained in the products of dry distillation of beech; pyrocatechin 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-benzene disulfonic acid and is used to prepare dyes; it is easily replaced at position 4 and reduced to dihydroresorcinol (cyclohexanedione-1,3), which is cleaved by 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"), the oxidation product of aniline; this is an easily reversible reaction; with 50% flow, a stable equimolecular compound of quinone and hydroquinone is formed - quinhydrone; The quinhydrone electrode is often used in potentiometric analysis; due to the reducing properties of hydroquinone, it, like pyrocatechin, is used in photographic developers; f) pyrogallol (2,3-dihydroxyphenol), which is obtained from gallic acid (see below "Aromatic acids") by distillation over pumice in an atmosphere of carbon dioxide; Being a powerful reducing agent, pyrogallol is used as an oxygen scavenger in gas analysis and as a photographic developer. Aromatic alcohols are compounds that, like benzyl alcohol C6H5CH2OH, contain a hydroxyl group on the side chain (rather than on the ring like phenols). If the hydroxyl group is located at a carbon atom adjacent to the ring, it is especially easily replaced by a halogen when hydrogen halides act on hydrogen (over platinum) and is easily cleaved off during dehydration (in C6H5CHOHR). Simple aromatic alcohols such as benzyl, phenethyl (C6H5CH2CH2OH), phenylpropyl (C6H5CH2CH2CH2OH) and cinnamic (C6H5CH=CHCH2OH) are used in the perfume industry and occur naturally in many essential oils. They can be prepared 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 the enzymatic hydrolysis of the 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 as a fragrance and base for perfumes. In industry, it is obtained by hydrolysis of benzylidene chloride C6H5CHCl2, a product of toluene chlorination, or by direct oxidation of toluene in the gas (over V2O5) or liquid phase with MnO2 in 65% sulfuric acid at 40 ° C. The following general methods are used for the preparation of aromatic aldehydes: 1 ) Gattermann-Koch synthesis:

2) Gutterman synthesis:

3) Reimer-Tiemann synthesis (for the production of aromatic hydroxyaldehydes):

Benzaldehyde is oxidized by atmospheric oxygen into 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 aromatic aldehydes from entering aldol self-condensation. However, mixed aldol condensation is used in synthesis:

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

2) benzoin condensation:

3) Perkin reaction:

The following aromatic aldehydes are of some importance: 1) Salicylic aldehyde (o-hydroxybenzaldehyde) occurs naturally in the aromatic oil of meadowsweet. It is obtained from phenol by the Reimer-Tiemann synthesis. It finds use 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 croton 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 flavorings. It is obtained by the Gattermann synthesis from anisole. 4) Vanillin (3-methoxy-4-hydroxybenzaldehyde) is the main aroma component of vanilla extracts. It can be obtained by the Reimer-Tiemann reaction from guaiacol or by treating eugenol (2-methoxy-4-allylphenol) with an alkali followed by oxidation. 5) Piperonal has the smell of heliotrope. 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 using the Friedel-Crafts reaction. General methods for the preparation of aliphatic ketones are also used. A specific method for obtaining hydroxyketones 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 acid 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 sodium salt. Like aliphatic acids, benzoic acid and other aromatic acids can be prepared by reacting carbon dioxide with a Grignard reagent (for example, 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 hypohalites

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 Kolbe synthesis. Its methyl ester is the fragrant component of wintergreen oil, 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, when exposed to 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 component of perfumes and occurs naturally in jasmine and orange leaf oils. 4) Gallic (3,4,5-trihydroxybenzoic) acid is formed together with glucose during the hydrolysis of certain complex substances of plant origin, known as tannins. Sulfonic acids. Benzenesulfonic acid C6H5SO3H is obtained by the action of fuming sulfuric acid on benzene. It and other sulfonic acids are strong acids (K > 0.1). Sulfonic acids are easily soluble in water and 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 produce nitriles), have already been discussed. When exposed to phosphorus pentachloride, they give arylsulfonyl chlorides (for example, C6H5SO2Cl), which are used in aliphatic and alicyclic syntheses. The most commonly used arylsulfonyl chloride in this manner is p-toluenesulfonyl chloride (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 parent hydrocarbons:

An important sulfonic acid is sulfanilic acid p-H2NC6H4SO3H (or p-H3N+C6H4SO3-), the amide (sulfonamide) and other derivatives of which are important chemotherapeutic agents. Sulfanilic acid is obtained by reacting fuming sulfuric acid with aniline. Many detergents are salts of long-chain sulfonic acids, for example NaO3S-C6H4-C12H25.
B. AROMATIC COMPOUNDS OF THE NAPTHALINE SERIES
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 repellent has declined with the introduction of new products 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 exhibits higher reactivity. Nitration, halogenation and low-temperature sulfonation lead to a-derivatives. Access to the b-position is achieved primarily through high-temperature sulfonation. Under these conditions, the a-sulfonic acid rearranges into the more stable b-form. The introduction of other substituents into the b-position then becomes possible using the Bucherer reaction: first, b-naphthol b-C10H7OH is obtained from b-naphthalene sulfonic acid by alkaline melting, which then, when treated with ammonium bisulfite at 150 ° C and 6 atm, gives b-naphthylamine b- C10H7NH2; Through diazonium compounds obtained from this amine in the usual way, it is now possible to introduce a halogen or cyano group into the b-position. The Friedel-Crafts reaction between naphthalene and the acid chloride also produces the b-acyl derivatives 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 halogen and cyannaphthalenes. Therefore, specific discussion of the reactions of naphthalene compounds will be omitted. However, substitution reactions in naphthalene derivatives are of some interest. 1) In the presence of an o,p-orientant (-CH3, -OH) in the 1(a)-position, the attack is directed predominantly to position 4 and then to position 2. 2) In the presence of an m-orientant (-NO2) in position 1, attack is directed to position 8 (peri) and then to position 5. 3) In the presence of an o,n-orientator at position 2 (b), position 1 is predominantly attacked, although sulfonation can occur at position 6. It is especially important that it is never attacked position 3. This is explained by the low degree of double connectivity of the carbon-carbon bond 2-3. In naphthalene, substitution occurs under milder conditions than in benzene. Naphthalene is also easier to reduce. Thus, sodium amalgam reduces it to tetralin (tetrahydronaphthalene; see the formula in Table 4 in 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 b-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 (formula see Table 4, Section III) is found in significant quantities in coal tar and is widely used in industry as an intermediate in the synthesis of dyes. Positions 9 and 10 are highly reactive in addition reactions. Thus, hydrogen and bromine easily add, giving 9,10-dihydro- and 9,10-dibromomanthracene, respectively. Oxidation with chromic acid converts anthracene to anthraquinone.

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

o-Benzoylbenzoic acids are prepared 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 a methylene group, forming an anthrone. Nitration under severe conditions produces mainly the a(1)-derivative along with appreciable amounts 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 mercuric sulfate the main product is a-sulfonic acid. Disulfonation in the presence of mercuric sulfate produces 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. Thus, oxidative alkaline melting of b-sulfonic acid produces 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 the sulfonic acid is treated with ammonia at 175-200°C in the presence of a mild oxidizing agent (for example, sodium arsenic acid) added to destroy the sulfite formed.
2. Phenanthrene and its derivatives. In nature, phenanthrene is found in coal tar. It itself and its derivatives can be obtained from o-nitrostilbenecarboxylic acid, 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 easily adds bromine and hydrogen and undergoes oxidation first to 9,10-phenanthraquinone and then to diphenic acid

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

The dyes pyranthrone, idanthrene yellow and violanthrone are keto derivatives of complex polynuclear hydrocarbons.

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