aromatic hydrocarbons. Condensed benzoic hydrocarbons Polynuclear aromatic hydrocarbons chemical properties
POLYCYCLIC AROMATIC HYDROCARBONS WITH ISOLATED CYCLES
Aromatic hydrocarbons with multiple benzene rings are divided into:
1. Hydrocarbons with non-condensed cycles. These include biphenyl and di- and triphenylmethanes.
2. Hydrocarbons with condensed cycles. These include naphthalene, anthracene and phenanthrene.
Biphenyl group
Definition: Aromatic compounds in which two (or more) rings (rings) are connected to each other by a single bond are called polycyclic aromatic hydrocarbons with isolated rings.
Biphenyl is considered as an example:
In industry, biphenyl is produced by pyrolysis of benzene:
The laboratory method of preparation is the action of sodium or copper on iodobenzene, or in the presence of electron-withdrawing substituents in the aryl halides, which increase the mobility of the halogen in the nucleus:
Biphenyl is a crystalline substance with T pl. 70 0 C, T b.p. 254 0 C. Thermodynamically stable. It is used in industry as a high-temperature coolant.
Biphenyl participates much more actively than benzene in electrophilic aromatic substitution reactions. Bromination of biphenyl with an equimolar amount of bromine leads to the formation of 4-bromobiphenyl. An excess of bromine leads to the formation of 4,4`-dibromobiphenyl:
Biphenyl nitration reactions, Friedel-Crafts acelation, and other electrophilic aromatic substitution reactions proceed similarly.
Polyphenylmethanes
Definition: Aromatic compounds in which from two to four benzene rings are connected to one carbon atom in the state of sp 3 hybridization.
The founder of the homologous series of polyphenylmethane is toluene, the following compound is diphenylmethane:
Di- and triphenylmethane are produced using benzene by the Friedel-Crafts reaction by two methods:
1. From methylene chloride and chloroform:
2. From benzyl chloride and benzylidene chloride:
Diphenylmethane is a crystalline substance with T pl. 26-27 0 C, has the smell of orange.
When diphenylmethane is oxidized, benzophenone is formed:
The structure of triphenylmethane forms the basis of the so-called dyes of the triphenylmethane series:
1. Malachite green (brilliant green) is obtained by the Friedel-Crafts reaction:
2. Phenolphthalein.
Obtained by the reaction of phenol and phthalic anhydride (phthalic anhydride) in the presence of sulfuric acid:
CONDENSED BENZOID HYDROCARBONS
Hydrocarbons containing two or more benzene rings sharing two carbon atoms are called fused benzenoid hydrocarbons.
Naphthalene
The simplest of the condensed benzoic hydrocarbons is naphthalene:
Positions 1,4,5 and 8 are designated "α", positions 2, 3,6,7 are designated "β".
Ways to get.
The bulk of naphthalene is obtained from coal tar.
In the laboratory, naphthalene can be obtained by passing benzene and acetylene vapors over charcoal:
Dehydrocyclization over platinum of benzene homologues with a side chain of four or more carbon atoms:
By the reaction of the diene synthesis of 1,3-butadiene with P-benzoquinone:
Naphthalene is a crystalline substance with T pl. 80 0 C, characterized by high volatility.
Naphthalene enters into electrophilic substitution reactions more easily than benzene. In this case, the first substituent almost always becomes in the α-position:
Entry of an electrophilic agent into the β-position is less common. As a rule, this occurs in specific conditions. In particular, the sulfonation of naphthalene at 60 0 C proceeds as a kinetically controlled process with the predominant formation of 1-naphthalenesulfonic acid. Sulfonation of naphthalene at 160 0 C proceeds as a thermodynamically controlled process and leads to the formation of 2-naphthalenesulfonic acid:
When a second substituent is introduced into the naphthalene molecule, the orientation is determined by the nature of the substituent already present in it. Electron donor substituents located in the naphthalene molecule direct the attack to the same ring in the 2nd and 4th positions.
By chemical properties, biphenyl is a typical aromatic compound. It is characterized by S E Ar reactions. It is easiest to think of diphenyl as benzene bearing a phenyl substituent. The latter exhibits weak activating properties. All reactions typical of benzene also occur in biphenyl.
Since the aryl group is ortho- And pair-orientant, S E Ar reactions proceed predominantly in pair-position. Ortho-isomer is a by-product due to steric hindrance.
Di- and triphenylmethanes
Di- and triphenylmethanes are benzene homologues in which the corresponding number of hydrogen atoms is replaced by phenyl residues. Benzene cycles are separated sp 3-hybridized carbon atom, which prevents conjugation. The rings are completely isolated.
Methods for obtaining diphenylmethane:
S E Ar reactions proceed in ortho- And pair- positions of the benzene rings of diphenylmethane.
Obtaining triphenylmethane and its derivatives:
A distinctive feature of triphenylmethane derivatives is the high mobility of the hydrogen atom bonded to the tetrahedral carbon.
Triphenylmethane exhibits marked acidity, reacting with sodium metal to form a very stable triphenylmethyl anion.
Triphenylchloromethane in aqueous solution dissociates to form a stable carbocation.
In some derivatives of triphenylmethane gap S-N bonds can proceed homolytically with the formation of triphenylmethyl radical - chronologically the first of the discovered stable free radicals.
The reasons for the high stability of the triphenylmethyl cation, anion, and radical can be understood by considering the structure of the cation. If we depict the triphenylmethyl cation using boundary structures, it becomes clear that the free orbital of the central carbon atom is in conjugation with the p-electrons of the benzene rings.
Lecture #21
Polynuclear aromatic hydrocarbons and their derivatives.
· Polynuclear aromatic hydrocarbons with condensed nuclei. Linear and angular polycyclic hydrocarbons. Extraction from coal tar. Carcinogenic properties of polycyclic hydrocarbons. Safety precautions when working with aromatic hydrocarbons.
Naphthalene. Isomerism and nomenclature of derivatives. Structure, aroma. Chemical properties of naphthalene and its derivatives: oxidation, catalytic hydrogenation and reduction with sodium in liquid ammonia, aromatic electrophilic substitution reactions. (influence of substituents on orientation, activity of a-position).
Anthracene. Nomenclature, structure, aromaticity (in comparison with benzene and naphthalene), isomerism of derivatives. Reactions of oxidation and reduction, electrophilic addition and substitution. Meso position activity.
Phenantrene. Nomenclature, structure, aromaticity (in comparison with benzene and naphthalene). Reactions of oxidation, reduction, electrophilic substitution and addition.
condensed aromatic hydrocarbons
Polycyclic aromatic compounds can be linear, angular, or pericyclic.
Polycyclic compounds are isolated from coal tar. Many of them have a pronounced carcinogenic effect. The more cycles, the more likely carcinogenicity.
Naphthalene
The simplest bicyclic aromatic compound.
Although the molecular formula indicates the unsaturated nature of naphthalene, its properties are typical of aromatic compounds. Naphthalene satisfies the structural criteria for aromaticity. cyclic flat system, which has a continuous conjugation chain, in which 10 p-electrons participate. It should be remembered that Hückel formulated his rule (4n + 2) for monocyclic systems. In the case of naphthalene, it is believed that there are 6 delocalized electrons in each cycle, and one of the pairs is common to both rings. Conjugation is shown using canonical structures:
As a result: above and below the plane of cycles there are p-electron clouds having the shape of "eight" Fig. 20.1.
Rice. 20.1. The shape of p-electron clouds of the naphthalene molecule
In naphthalene, not all C-C bonds are the same. So, the length of C 1 -C 2 is 1.365 Å, and C 2 -C 3 is 1.404 Å. The conjugation energy of naphthalene is 61 kcal/mol, which is less than twice the delocalization energy of benzene (2x36 kcal/mol). The second cycle contributes less to the conjugation than the first. Naphthalene is less aromatic than benzene. Violation of the aromaticity of one of its cycles requires only 25 kcal / mol, which is manifested in its reactions.
Reactions
The oxidation of naphthalene proceeds similarly to the oxidation of benzene.
The resulting phthalic acid under the reaction conditions is converted into phthalic anhydride, which is isolated as a result of the reaction.
The reduction reactions also illustrate the lower aromaticity of naphthalene compared to benzene. Naphthalene can be hydrogenated with chemical reducing agents under mild conditions.
Aromatic electrophilic substitution reactions
In general, S E Ar reactions in naphthalene proceed according to the general mechanism discussed earlier. A feature of reactions in the naphthalene series is that monosubstituted naphthalenes exist in the form of two isomers (1- and 2-derivatives). The features of the S E Ar reactions are considered using the example of the nitration reaction, the main product of which is 1-nitronaphthalene (2-isomers are traces).
The key stage of the reaction is the formation of an s-complex, of which there may be two. It is necessary to determine the structural factors that stabilize or destabilize the intermediate. On this basis, it is possible to predict and explain the course of substitution. Consider the structure of possible intermediate products.
When an electrophile is attacked at position 1 of naphthalene, an s-complex is formed, the structure of which can be described by two boundary structures in which the benzene ring is preserved. Such structures are more stable due to benzene conjugation. When attacking an electrophile at position 2, only one energetically favorable structure can be drawn.
It can be concluded that an electrophilic attack at position 1 of naphthalene leads to a more stable s-complex than a reaction at position 2.
primate hydrocarbons containing two or more benzene nuclei are referred to as polynuclear arenes.
Depending on how the benzene rings are linked, polynuclear arenes are divided into two groups:
Arenas with condensed (anelated) benzene rings;
Arenas with non-condensed (isolated) benzene rings.
MULTIPLE ARENA WITH CONDENSED BENZENE CYCLES
Fused multinuclear arenes contain two or more benzene nuclei sharing common carbon atoms.
the most important representatives of the condensed are-
Niv are naphthalene, anthracene and phenanthrene.
NAPHTHALENE
paphthalene consists of two fused benzene rings. Two carbon atoms (9 and 10) are shared by two rings.
Multi-core arenas
and unlike benzene, the carbon atoms in the naphthalene molecule are not equivalent. Positions 1, 4, 5, 8 are equivalent, they are usually denoted by the letter a and called a-positions. Positions 2, 3, 6 and 7 are also equivalent, they are denoted by the letter p and are called p-positions.
For monosubstituted naphthalene, two isomers are possible (a- and p-), and in the presence of two identical substituents, 10 isomers:
In the nomenclature of disubstituted naphthalenes, along with the numerical designation of the positions of the substituents, prefixes are also used: ortho-pylon - 1.2; zheta - 1.3; lora - 1.4; light - 1.8; - 2.6. For example:
^ii^iish nil^mspil. ±. naphthalene is mainly obtained from coal tar, where its content is about 10%. Along with naphthalene, some of its mono- and dimethyl derivatives are also isolated from coal tar.
2. Obtaining naphthalene from acetylene. When acetylene is passed through tubes heated to 700–800 °C, naphthalene is also formed along with benzene.
^ilichssis kiiik!va. naphthatin - is ^ tsvsshis crystalline substance with a characteristic odor, sublimes at 81 ° C. Insoluble in water, soluble in organic solvents. It is used in everyday life to fight moths, in the chemistry of dyes, to obtain medicines, plastics, glyptal resins.
The structure of naphthalene. The electronic structure of naphthalene is similar to that of benzene. Its molecule is flat. The dipole moment is zero, but the electron density is not distributed as uniformly as in the benzene molecule. The higher electron density of the a-positions of naphthalene makes them more reactive than the p-positions.
The bond lengths in the naphthalene molecule are different.
(in alkanes) 0.154 nm
(in benzene) 0.140 nm
(in naphthalene) 0.136 nm
(in naphthalene) 0.143 nm
chemical properties of naphthalene. Naphthalene, like benzene, exhibits the properties of aromatic compounds; it is characterized primarily by electrophilic substitution reactions, but it also easily enters into addition and oxidation reactions.
A. Electrophilic substitution reactions. In the reaction of electrophilic substitution (nitration, sulfonation, halogenation), naphthalene enters much easier than benzene. In this case, mainly a-substitution products are formed. This is due to the fact that the electron density is higher in the a-position of the naphthalene nucleus and, when attacked in the a-position, a more stable h-complex is formed than in the p-position:
Multi-core arenas
1. Apparently, when the a-position is attacked, the delocalization of the positive charge in the c-complex occurs with the preservation of the aromaticity of one of the benzene nuclei in possible resonance structures.
In the case of an attack by an electrophile on the p-position, only in one case is it possible to preserve the aromaticity of the benzene ring. Therefore, substitution at the a-position is energetically more favorable.
1. Nitration. Naphthalene is quite easily nitrated by nitrous
schey mixture with the formation of mainly a-isomer:
a-nitronaphtapine
2. ^allocation. For sulfonation of naphthalene, concentrated H2SO4 is used, and, depending on the temperature at which the reaction is carried out, a- or p-substitution products are obtained. At a temperature of 80 °C of the reaction medium, a-naphthalenesulfonic acid is formed, and at 160 °C, p-naphthalenesulfonic acid is formed:
p-naphthalenesulfonic acid
When the a-isomer is heated to a temperature of 160 °C, it is completely converted into p-naphthalenesulfonic acid.
3. Halogenation. At a temperature of 90–110 °C, in the presence of a FeCl3 catalyst, naphthalene is chlorinated to form predominantly a-chloronaphthalene. The reaction proceeds according to the mechanism £_:
a-chloronaphthalene
12. Multinuclear arenas with condensed benzene rings
first, the halogen addition reaction proceeds, and then the elimination of the hydrogen halide. For example, during the interaction of bromine with naphthalene, bromine is added to positions 1 and 4, and then the addition product splits off a hydrogen bromide molecule, forming a-bromonaphthalene:
hkhravmla orientation in the naphthalene core. orientation rules in the naphthalene core have their own characteristics.
The direction of electrophilic substitution in monosubstituted naphthalene derivatives is determined by the electronic nature of the already existing substituent and the greater reactivity of the a-position.
In the presence of an electron-donating substituent in the naphthalene core, the electron density increases primarily in the ring to which the substituent is bound, and therefore the electrophilic substitution reaction occurs precisely on this ring. The substitution primarily occurs at position 4 (a-position).
For example, nitration of 1-methylnaphthalene leads to the formation of 1-methyl-4-nitronaphthalene:
^lekironoacceptor substituent lowers ^lekironnush
density in the naphthalene core and, above all, in the ring with which it is associated. Therefore, reactions proceed with the participation of carbon atoms of the second benzene nucleus.
Multi-core arenas
Since the a-positions are more reactive, the substitution occurs mainly in positions 5 and 8.
For example, nitration of 1-nitronaphthalene with a nitrating mixture results in the formation of a mixture of isomers:
m. addition reactions. I easy recovery. hydrogen addition occurs first at the a-position. Then 1,2,3,4-tetrahydronaphthalene is formed, which is hydrogenated at 200 ° C to form decahydronaphthalene - decalin:
khetratin is used as a fuel and solvent for fats
and resin. Decalin is used as a solvent for varnishes, it is a substitute for turpentine.
B. Oxidation reactions. Unlike benzene, the naphthalene core is easily oxidized, and in the oxidation of naphthalene homologues, the core itself is oxidized first of all.
For example:
12. Multinuclear arenas with condensed benzene rings
ANTHRACENE
it is a polynuclear compound consisting of three linearly fused benzene rings.
In the anthracene molecule, only the carbon atoms bearing hydrogen are numbered. Positions 1, 4, 5, 8 are called a-positions; 2, 3, 6, 7 - p-positions; 9, 10 - y-positions or 1- (meso - middle) positions.
With one substituent in the nucleus, three isomers are possible - a, p, y.
Ways to get. Anthracene is found in coal tar, namely in anthracene oil, in the amount of 0.5%, from where it is extracted industrially.
In the laboratory, anthracene can be obtained by the Friede-la-Crafts reaction from benzene and 1,1,2,2-tetrabromoethane in the presence of A1Br
physical properties. Anthracene is a colorless crystalline substance, melting point 217 ° C, distilled with water vapor, soluble in benzene, ether, insoluble in water. Solutions exhibit blue fluorescence.
Multi-core arenas
lymic svoillva. Anthracene is less aromatic than benzene and naphthalene. It is more of an "unsaturated" compound, and is more characteristic of addition reactions than electrophilic substitution reactions. The most reactive in the anthracene molecule are the leso positions (positions 9 and 10).
1. Recovery reaction. Anthracene readily adds hydrogen to form 9,10-dihydroanthracene. Its further hydrogenation leads to perhydroanthracene.
9,10 -dishdroantrapene
2. ±ssh,u]ii ashpeschemp. halogenation of anthracene will occur
melanism of attachment-cleavage. For example:
Chloranthracene
Nitration of anthracene with nitric acid in acetic acid at room temperature leads to the formation of 9-nitro-anthracene. As a result of the sulfonation of anthracene with sulfuric
12. Multinuclear arenas with condensed benzene rings
when heated, a- and p-isomers are formed, which are more stable than the y-isomer.
a-anthracene-p-anthracene-sulfonic acid sulfonic acid
h. oxidation reaction. Anthracene is very easily oxidized with concentrated nitric acid or a chromium mixture to anth-raquinone.
Anthraquinone is a yellow crystalline substance that has the properties of quinones.
PHENANTHRENE
The structural isomer of anthracene is phenanthrene.
In anthracene, the three benzene rings are connected in a linear fashion, while in phenanthrene they are connected angularly, that is, at an angle.
Multi-core arenas
an increase in the number of aromatic rings in polynuclear
The compound increases the number of monosubstituted isomers: for example, two products (a, p) are possible for naphthalene, three for anthracene, and five for phenanthrene.
the numbers indicate the positions in which substituents can be located.
Receipt. Get phenanthrene mainly from coal tar.
Physical properties of phenanthrene. Phenantrene is a solid crystalline substance with ^ = 101 °C, ^bp = 340.2 °C. Soluble in ether, benzene, difficult - in alcohol. Solutions have blue fluorescence.
lymic properties. Phenantrene, like anthracene, has a weaker aromatic character than naphthalene, and even more so than benzene. The electron density in its molecule is distributed unevenly, the aromaticity of the middle benzene ring is so broken here that the bond between 9 and 10 carbon atoms takes on the character of a double bond. Substitution reactions proceed according to the mechanism of addition-cleavage.
hack, first bromine is added at positions 9 and 10, followed by cleavage of IVg and the formation of 9-bromophenanthrene:
12. Multinuclear arenas with condensed benzene rings
9,10-dibromo-9,i-di1hydrophenanthrene can be isolated in the free state.
When phenanthrene is oxidized, phenanthrenquinone is formed. With further oxidation, the C9-C10 bond is broken, and diphenic acid is formed:
9,10-phenanthrenquinone diphenic acid
When phenanthrene is reduced, a product is formed, which is called perhydrophenanthrene.
perhydrophenanthrene
The condensed system of perhydrophenanthrene and cyclopentane is called, or sterane.
cyclopentanperhydrophenanthrene
This structure underlies steroids.
Lecture 16
POLYCYCLIC AROMATIC HYDROCARBONS
Lecture outline.
1. Polycyclic aromatic hydrocarbons with isolated rings
1.1 Biphenyl group
1.2. Polyphenylmethanes
2. Condensed benzenoid hydrocarbons
2.1 Naphthalene
2.2. Anthracene, phenanthrene
1. Polycyclic aromatic hydrocarbons with isolated rings
There are two groups of polycyclic aromatic hydrocarbons (arenes) with several benzene rings.
1. Hydrocarbons with isolated rings. These include biphenyl and di- and triphenylmethanes.
2. Hydrocarbons with condensed rings or benzoid hydrocarbons. These include naphthalene, anthracene, and phenanthrene.
1.1. Biphenyl group
Definition: Aromatic compounds in which two (or more) rings (rings) are connected to each other by a single bond are called polycyclic aromatic hydrocarbons with isolated rings.
The simplest aromatic hydrocarbon compound with isolated rings is biphenyl. The positions of the substituents in the biphenyl formula are indicated by numbers. In one ring, the numbers are not marked: 1, 2 ..... In the second ring, the numbers are marked with a stroke 1, 2, etc.:
Scheme 1.
Biphenyl is a crystalline substance with T pl. 70 0 C, T b.p. 254 0 C, has a wide application due to thermal and chemical resistance. It is used in industry as a high-temperature coolant. In industry, biphenyl is produced by pyrolysis of benzene:
Scheme 2.
The laboratory method of obtaining is the action of sodium or copper on iodobenzene
Scheme 3.
The reaction proceeds especially smoothly in the presence of electron-withdrawing substituents in the aryl halides, which increase the mobility of the halogen in the nucleus:
The most important derivative of biphenyl is the diamine benzidine. It is usually obtained by reducing nitrobenzene to hydrazobenzene and isomerizing the latter under the influence of acids:
Scheme 5.
Benzidine is the starting material for the production of many substantive (direct) dyes. The presence of two amino groups that can be diazotized makes it possible to obtain bis-azo dyes with a deep color. An example of a dye derived from benzidine is the Congo red indicator:
Scheme 6.
In the crystalline state, both benzene rings of biphenyl lie in the same plane. In solution and in the gaseous state, the angle between the planes of the rings is 45 0 . The exit of benzene rings from the plane is explained by the spatial interaction of hydrogen atoms in positions 2, 2 and 6, 6:
Scheme 7.
If there are large substituents in the ortho positions, then rotation about the C-C bond becomes difficult. If the substituents are not the same, then the corresponding derivatives can be separated into optical isomers. This form of spatial isomerism is called rotary. optical isomerism or atropisomerism.
Scheme 8.
Biphenyl participates much more actively than benzene in electrophilic aromatic substitution reactions. Bromination of biphenyl with an equimolar amount of bromine leads to the formation of 4-bromobiphenyl. An excess of bromine leads to the formation of 4,4`-dibromobiphenyl:
Scheme 9.
Biphenyl nitration reactions, Friedel-Crafts acylation, and other electrophilic aromatic substitution reactions proceed similarly.
1.2. Polyphenylmethanes
Definition:
Aromatic compounds in which from two to four benzene rings are connected to one carbon atom in the state of sp 3 hybridization.
The founder of the homologous series of polyphenylmethane is toluene, the following compound is diphenylmethane:
Scheme 10.
Di- and triphenylmethane are produced using benzene by the Friedel-Crafts reaction by two methods:
1. From methylene chloride and chloroform:
Scheme 11.
2. From benzyl chloride and benzylidene chloride:
Scheme 12..
Diphenylmethane is a crystalline substance with T pl. 26-27 0 C, has the smell of orange.
When diphenylmethane is oxidized, benzophenone is formed:
Scheme 13.
Triphenylmethane is a crystalline substance with T pl. 92.5 0 C. With benzene gives a crystalline molecular compound T pl. 78 0 C. Triphenylmethane is easily oxidized to triphenylcarbinol. The hydrogen atom in its molecule is easily replaced by metals and halogens. In turn, triphenylcarbinol under the action of hydrogen chloride triphenylchloromethane. Triphenylchloromethane upon reduction forms triphenylmethane, and upon hydrolysis, triphenylcarbinol:
Scheme 14..
The structure of triphenylmethane forms the basis of the so-called dyes of the triphenylmethane series. Aminotriphenylmethanes are colorless substances, they are called leuco compounds (from the Greek leukos - white, colorless). When oxidized in an acidic medium, they form colored salts. In these salts, the color carrier (chromophore) is a conjugated ion with a positive charge distributed between the carbon and nitrogen atoms. The most prominent representative of this group is malachite green. It is obtained by the Friedel-Crafts reaction:
Scheme 15.
During the oxidation of the leuco compound, a system of conjugated bonds is formed through the benzene ring between the nitrogen atom and the carbon of the triphenylmethane system, which has passed into the state of sp 2 hybridization. Such a structure is called quinoid. The presence of a quinoid structure ensures the appearance of a deep intense color.
The widely used indicator phenolphthalein belongs to the group of triphenylmethane dyes. Obtained by the reaction of phenol and phthalic anhydride (phthalic anhydride) in the presence of sulfuric acid:
Scheme 16.
2. Condensed benzenoid hydrocarbons
Hydrocarbons containing two or more benzene rings sharing two carbon atoms are called fused benzenoid hydrocarbons.
2.1. Naphthalene
The simplest of the condensed benzoic hydrocarbons is naphthalene:
Scheme 17.
Positions 1,4,5 and 8 are designated "α", positions 2, 3,6,7 are designated "β". Accordingly, for naphthalene, the existence of two monosubstituted ones, which are called 1 (α)- and 2 (β)-derivatives, and ten disubstituted isomers is possible, for example:
Scheme 18.
Ways to get.
The bulk of naphthalene is obtained from coal tar.
In laboratory conditions, naphthalene can be obtained by passing benzene and acetylene vapors over charcoal:
Scheme 19.
Dehydrocyclization over platinum of benzene homologues with a side chain of four or more carbon atoms:
Scheme 20.
By the reaction of the diene synthesis of 1,3-butadiene with P-benzoquinone:
Scheme 21.
A convenient laboratory method for obtaining naphthalene and its derivatives is a method based on the Friedel-Crafts reaction:
Scheme 22.
Naphthalene is a crystalline substance with T pl. 80 0 C, characterized by high volatility.
Naphthalene enters into electrophilic substitution reactions more easily than benzene. In this case, the first substituent almost always becomes in the α-position, since in this case an energetically more favorable σ-complex is formed than with substitution in the β-position. In the first case, the σ-complex is stabilized by the redistribution of electron density without disturbing the aromaticity of the second ring; in the second case, such stabilization is not possible:
Scheme 23.
A number of electrophilic substitution reactions in naphthalene:
Scheme 24.
Entry of an electrophilic agent into the β-position is less common. As a rule, this occurs in specific conditions. In particular, the sulfonation of naphthalene at 60 0 C proceeds as a kinetically controlled process, with the predominant formation of 1-naphthalenesulfonic acid. Sulfonation of naphthalene at 160 0 C proceeds as a thermodynamically controlled process and leads to the formation of 2-naphthalenesulfonic acid:
Scheme 25.
The place of entry of the second substituent into the naphthalene system is determined by:
1. orientational influence of an existing substituent;
2. Differences in the reactivity of the α and β positions.
In this case, the following conditions are met:
1. If one of the naphthalene rings has a substituent of the first kind, then the new substituent enters the same ring. A substituent of the first kind in the 1(α)-position sends the second substituent, mainly to 4( pair)-position. Isomer with a second substituent in 2( ortho)-position is formed in small quantities, for example:
Scheme 26.
The electron-withdrawing substituents located in the naphthalene molecule direct the attack to another ring in the 5th and 8th positions:
Scheme 27.
Scheme 28.
Oxidation of naphthalene with atmospheric oxygen using vanadium pentoxide as a catalyst leads to the formation of phthalic anhydride:
Scheme 29.
Naphthalene can be reduced by the action of various reducing agents with the addition of 1, 2 or 5 moles of hydrogen:
Scheme 30.
2.2. Anthracene, phenanthrene
By building up another ring from naphthalene, two isomeric hydrocarbons can be obtained - anthracene and phenanthrene:
Scheme 31..
Positions 1, 4, 5 and 8 are designated "α", positions 2, 3, 6 and 7 are designated "β", positions 9 and 10 are designated "γ" or "meso" - the middle position.
Ways to get.
The bulk of anthracene is obtained from coal tar.
Under laboratory conditions, anthracene is obtained by the Friedel-Crafts reaction from benzene or with tetrabromoethane:
Scheme 32.
or by reaction with phthalic anhydride:
Scheme 33.
As a result of the first stage of the reaction, anthraquinone is obtained, which is easily reduced to anthracene, for example, with sodium borohydride.The Fittig reaction is also used, according to which the anthracene molecule is obtained from two molecules ortho- bromobenzyl bromide:
Scheme 34.
Properties:
Anthracene is a crystalline substance with T pl. 213 0 C. All three benzene rings of anthracene lie in the same plane.
Anthracene easily adds hydrogen, bromine and maleic anhydride to positions 9 and 10:
Scheme 35.
The bromine addition product easily loses hydrogen bromide to form 9-bromoanthracene.
Under the action of oxidizing agents, anthracene is easily oxidized to anthraquinone:
Scheme 36.
Phenantrene, as well as anthracene, is a constituent of coal tar.
Just like anthracene, phenanthrene adds hydrogen and bromine to the 9 and 10 positions:
Scheme 37.
Under the action of oxidizing agents, phenanthrene is easily oxidized to phenanthrenquinone, which is further oxidized to 2,2`-bifenic acid:
Scheme 36.
Demonstration material for the lecture
Scheme 1. Structural formula of biphenyl and the order of designation of the position of substituents in the biphenyl molecule.
Scheme 2.
Scheme for the synthesis of biphenyl by pyrolysis of benzene.
Scheme 3. Scheme for the synthesis of biphenyl from iodobenzene.
Scheme 4.
Scheme for the synthesis of biphenyl according to the Ullmann reaction.
Scheme 5. Scheme for the synthesis of benzidine.
Scheme 6. Congo indicator is red.
Scheme 7. Scheme of steric interactions of hydrogen atoms in ortho- and ortho-provisions.
Scheme 8. Rotational optical isomers.
Scheme 9. Scheme of the electrophilic substitution reaction.
The following compound is diphenylmethane: 
Scheme 10. Polyphenylmethanes.
Scheme 11. Scheme for the synthesis of di- and triphenylmethane, methylene chloride and chloroform.
Scheme 12. Scheme for the synthesis of di- and triphenylmethane from benzyl chloride and benzylidene chloride.
Scheme 13. Scheme of diphenylmethane oxidation.
Scheme 14. Reactions involving derivatives of triphenylmethane.
Scheme 15. Scheme for the synthesis of malachite green dye.
Scheme 16. Scheme for the synthesis of the indicator phenolphthalein.
Scheme 18. Naphthalene derivatives.
Ways to get.
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 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 the saturated hydrocarbon containing the same number of carbon atoms - hexane (C 6 H 14), then we can see that benzene contains eight fewer 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 it structural formula as cyclohexantriene-1,3,5.
Thus, the molecule corresponding to the 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, numerous experimental data have shown that benzene enters into addition reactions only under harsh conditions(at high temperatures and lighting), resistant to oxidation. The most characteristic of it are the substitution reactions, therefore, benzene is closer in character to saturated hydrocarbons.
In an attempt to explain these inconsistencies, many scholars have proposed various options benzene structures. 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 these studies and calculations, it was concluded that all six carbon atoms are in the state of sp 2 hybridization and lie in the same plane. The unhybridized p-orbitals of carbon atoms that make up double bonds (Kekule's 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 π-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 cyclohexantriene-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:
The measured bond lengths confirm this assumption. 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 ties(0.154 nm) and longer doubles (0.132 nm).
There are also compounds whose molecules contain several cyclic structures, for example:
Isomerism and nomenclature of aromatic hydrocarbons
For benzene homologues the isomerism of the position of several substituents is characteristic. The simplest homologue of benzene is 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 starting from senior deputy to junior:
If the substituents are the same, then numbering is carried out according to the shortest path: for example, substance:
called 1,3-dimethylbenzene, not 1,5-dimethylbenzene.
According to the old nomenclature, positions 2 and 6 are called ortho positions, 4 - para-, 3 and 5 - meta positions.
Physical properties of aromatic hydrocarbons
Benzene and its simplest homologues under normal conditions - highly toxic liquids with a characteristic unpleasant odour. They are poorly soluble in water, but well - in organic solvents.
Chemical properties of aromatic hydrocarbons
substitution reactions. Aromatic hydrocarbons enter into substitution reactions.
1. Bromination. When reacting with bromine in the presence of a catalyst, iron (III) bromide, 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:
Reduction of nitrobenzene is obtained aniline- 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 and its derivatives are formed.
1. Hydrogenation. The catalytic hydrogenation of benzene proceeds at more high temperature than the hydrogenation of alkenes:
2. Chlorination. The reaction proceeds under illumination with ultraviolet light and is a free radical:
Chemical properties of aromatic hydrocarbons - compendium
Benzene homologues
The composition of their molecules corresponds to the formula CnH2n-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 as o-) - substituents are located at neighboring carbon atoms, meta-(m-) - through one carbon atom and pair-(p-) - substituents against each other.
The first members of the homologous series of benzene are liquids with a specific odor. They are lighter than water. Are good solvents. Benzene homologues enter into substitution reactions:
bromination:
nitration:
Toluene is oxidized by permanganate when heated:
Reference material for passing the test:
Mendeleev table
Solubility table
