Arenas(aromatic hydrocarbons) - compounds whose molecules contain one or more benzene rings - cyclic groups of carbon atoms with a specific nature of bonds.
Benzene - molecular formula C 6 H 6. It was first proposed by A. Kekule:
Arena structure.
All 6 carbon atoms are in sp 2-hybridization. Each carbon atom forms 2 σ -bonds with two neighboring carbon atoms and one hydrogen atom, which are in the same plane. The angles are 120°. Those. All carbon atoms lie in the same plane and form a hexagon. Each atom has a non-hybrid R-the habitation on which the unpaired electron is located. This orbital is perpendicular to the plane, and therefore π -the electron cloud is “smeared” over all carbon atoms:
All connections are equal. Conjugation energy is the amount of energy that must be expended to destroy an aromatic system.
This is what determines the specific properties of benzene - the manifestation of aromaticity. This phenomenon was discovered by Hückel, and is called Hückel's rule.
Arene isomerism.
Arenas can be divided into 2 groups:
- benzene derivatives:
- condensed arenas:
The general formula of arenes is WITHnH 2 n -6 .
Arenes are characterized by structural isomerism, which is explained by the mutual arrangement of substituents in the ring. If there are 2 substituents in the ring, then they can be in 3 different positions - ortho (o-), meta (m-), para (p-):
If one proton is “taken away” from benzene, a radical is formed - C 6 H 5, which is called the aryl radical. Protozoa:
Arenes are called the word “benzene”, indicating the substituents in the ring and their positions:
Physical properties of arenas.
The first members of the series are colorless liquids with a characteristic odor. They are highly soluble in organic solvents, but insoluble in water. Benzene is toxic, but has a pleasant smell. Causes headaches and dizziness; inhalation of large quantities of vapor can cause loss of consciousness. Irritating to mucous membranes and eyes.
Getting arenas.
1. From aliphatic hydrocarbons using the “aromatization” of saturated hydrocarbons that make up the oil. When passed over platinum or chromium oxide, dihydrocyclization occurs:
2. Dehydrogenation of cycloalkanes:
3. From acetylene (trimerization) when passing over hot coal at 600°C:
4. Friedel-Crafts reaction in the presence of aluminum chloride:
5. Fusion of salts of aromatic acids with alkali:
Chemical properties of arenes.
Arene substitution reactions.
The arene core has a mobile π - a system that is affected by electrophilic reagents. Arenes are characterized by electrophilic substitution, which can be represented as follows:
An electrophilic particle is attracted to π -ring system, then a strong bond is formed between the reagent X and one of the carbon atoms, in which case the unity of the ring is disrupted. To restore aromaticity, a proton is emitted and 2 electrons S-N pass into the π-system of the ring.
1. Halogenation occurs in the presence of catalysts - anhydrous chlorides and bromides of aluminum and iron:
2. Nitration of arenes. Benzene reacts very slowly with concentrated nitric acid when heated. But if you add sulfuric acid, the reaction proceeds very easily:
3. Sulfonation occurs under the influence of 100% sulfuric acid - oleum:
4. Alkylation with alkenes. As a result, chain elongation occurs; the reaction occurs in the presence of a catalyst - aluminum chloride.
Cyclic conjugated systems are of great interest as a group of compounds with increased thermodynamic stability compared to conjugated open systems. These compounds also have other special properties, the totality of which is united by the general concept aromaticity. These include the ability of such formally unsaturated compounds to undergo substitution rather than addition reactions, resistance to oxidizing agents and temperature.
Typical representatives of aromatic systems are arenes and their derivatives. 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.
INTRODUCTION
Aromatic compounds (arenes) are a very interesting group of organic substances. They attract the attention of researchers with their unusual structure and properties, multifaceted transformations, and wide possibilities for practical application.
Arenes are noticeably distinguished from all other classes of organic substances by their high stability and the concept of “aromaticity,” which unites cyclic planar electronic systems, is one of the key ones in organic chemistry, characterizing not only geometry, but also the electronic structure, paths and mechanisms of transformations.
The flat cyclic highly symmetrical structures of benzene, naphthalene and other similar compounds at first glance exclude the possibility of stereoisomerism in this series. However, the helical molecules of various helicenes, which do not contain a single tetrahedral carbon atom, can be separated into enantiomers (as, for example, hexagelicenes 1 And 2 , characterized by unusually high optical activity).
One of the striking features of the behavior of arenes in various reactions is that they are capable of undergoing skeletal isomerization. It has been established that valence isomers of benzene and other arenes [for example, Dewar benzene (3), Ladenburg benzene (4), Hückel benzene (5)], easily obtained from various precursors, including from arenes, participate in many thermal and photochemical transformations of the latter. The lower thermodynamic stability of valence isomers 3 - 5 compared to benzene often predetermines their transition to benzene.
Aromatic compounds are not only the classic arenes and hetarenes. These also include structurally qualitatively new substances - fullerenes, first described in 1985. One of the representatives of this group of three-dimensional compounds is fullerene C 60.
After the isolation of benzene in its individual state (M. Faraday, 1825) until the moment when a structural formula was proposed for it (A. Kekule, 1865), considerable time passed. During these years, very important discoveries were made regarding the behavior of both benzene and many other arenes in various reactions. It is possible to note the reactions found N. Zinin(conversion of nitrobenzene to aniline, J.prakt.Chem. 1842, Bd. 27, S. 140 ), G. Kolbe(synthesis of salicylic acid from phenol according to Kolbe-Schmitt, Ann. 1860, Bd. 113, S. 125) , which, along with many others, form the basis for the technological production of various functional derivatives of arenes at the present time.
Offered to your attention Issue 3 a series "Methodological materials for the general course of organic chemistry" includes tasks and exercises on general problems of arene chemistry: arene nomenclature, aromaticity of carbo- and heterocycles, patterns of electrophilic substitution reactions of arenes. The release material has been used for many years at the Faculty of Chemistry of Moscow State University for independent work of third-year students and in tests. The Methodological Commission of the Department of Organic Chemistry recommends the proposed manual for publication.
SECTION "A"
Nomenclature of aromatic compounds.
Aromaticity of carbo- and heterocycles.
General principles of electrophilic substitution reactions of arenes
(nitration, halogenation, sulfonation)
1.
Write the structural formulas of the following compounds:
P-bromotoluene,
O-chloroaniline,
2,4-dinitrochlorobenzene,
m-diethynylbenzene,
O-di- rubs butylbenzene,
2,4,6-tribromoanisole,
P-chlorophenol,
m-nitrotoluene,
O-bromochlorobenzene,
P-chlorostyrene,
cumene,
4-(N,N-dimethylamino)benzaldehyde,
2,4,6-tribromobenzoic acid,
3,5-dichlorotoluene,
2,4,6-trinitrotoluene,
2-phenylpentane,
1,3,5-triphenylbenzene,
1,1,2-triphenylcyclopropane,
4,4"-dinitrobiphenyl,
benzyl chloride,
a,a"-dibromodibenzyl ketone,
benzyl alcohol,
1,3-diphenylpropane,
9,10-dibromoanthracene,
1-chloro-3-methyl-1-phenylbutane,
2-phenyl-2-propanol,
P-xylene,
m-cresol,
2,4,6,-tribromophenol,
1,5-diaminonaphthalene,
4-methyl-1-naphthol,
8-methyl-1-naphthol
2. Name the following connections:
3. Which of the following compounds can be classified as aromatic, non-aromatic, or anti-aromatic?
4. Among the compounds proposed below, indicate examples of the coordinated and inconsistent influence of groups that control the entry of a third substituent into the benzene ring under conditions of electrophilic substitution reactions.
5. A comparison of the dipole moments of benzophenone and diphenylcyclopropenone indicates greater polarity of the cyclic ketone compared to aryl, diaryl and cycloalkyl ketones. Suggest an explanation for the high polarity of the cyclopropenone derivative.
6 .The basicity of N,N-dimethylaniline is 2 times higher than the basicity of aniline. At the same time, when moving from 2,4,6-trinitroaniline to N,N-dimethyl-2,4,6-trinitroaniline, the basicity of the latter increases 40,000 times. Why does the introduction of two methyl groups into the amino group of 2,4,6-trinitroaniline so dramatically increase the basicity of the polynitro derivative of aniline?
7. Propose the structures of compounds that can be used to synthesize the following benzene and naphthalene derivatives using electrophilic substitution reactions.
8. Write the structures of the nitration products of the following benzene derivatives and indicate the conditions under which the orientation you indicated is realized:
9. It is known that nitration of toluene leads to a mixture O-, m- And P-nitrotoluenes, in which O- And P-isomers add up to 95%. In contrast to toluene, , , -trifluorotoluene under similar conditions preferentially forms 3-nitro- , , -trifluorotoluene. Suggest an explanation for the observed orientation in the nitration reaction of trifluorotoluene.
10.
The easy reaction of cyclooctatetraene (COT) in an ethereal solution with alkali metals ends with the formation of salts of cyclooctatetraenyl dianions having a planar structure. Express your thoughts on:
a) high activity of COT in such transformations,
b) changes in the geometry of the eight-membered ring upon transition from a neutral COT molecule to a dianion salt.
11.
Each of the three isomers ( 1
, 2
And 3
) dibromobenzene was placed in separate flasks. Based on the facts below, determine their structure.
a) nitration of the compound 1
(mp 87 °C) leads to only one nitrodibromobenzene,
b) connections 2
And 3
are liquids
c) nitration of the compound 2
gives 2 isomeric nitrodibromobenzene,
d) during nitration of a compound 3
3 nitrodibromobenzene obtained .
12.
Among the following substituents on the aromatic ring, indicate
12.1. ortho, pair- orientations,
12.2. meta- orientations,
12.3. activating substituents in electrophilic substitution reactions, deactivating substituents in electrophilic substitution reactions.
NH 3 +, -NMe 2, CH 3 C(O)-, -SO 3 H, -C? N, -NO 2 , -NMe 3 + , -C(O)H, Alk-, -NHC(O)CH 3 , -OH, -OCH 3 , -OC(O)CH 3 , -NH 2 , -Cl , -Br, -I, -C(O)NH 2, -C(O)OCH 3, CH 3 CH=CH-, CF 3 -, C 6 H 5 -, -CH 2 NO 2.
Explain your references.
13. Treatment of N,N-dimethylaniline with a nitrating mixture (HNO 3 + H 2 SO 4, 5-10 °C) and then with aqueous ammonia resulted in a yield of about 60% m-nitro-N,N-dimethylaniline. Give a scheme of the transformation and explain the reason for the observed place of entry of the nitro group into the aromatic ring.
14. Based on consideration of all possible resonance structures of the aromatic compound phenanthrene, explain why the C(9)-C(10) bond is more similar to a C=C double bond than other carbon-carbon bonds in the molecule.
15.
What products do you expect in the transformations below:
15.1. phenetol + Br 2 (Fe)
15.2. benzaldehyde + Br 2 (Fe)
15.3. acetanilide + (HNO 3 + HSO 4)
15.4. cumene + (HNO 3 + HSO 4)
15.5. ethyl benzoate + (HNO 3 + HSO 4)
15.6. deuterobenzene + (H 2 O + H 2 SO 4)
16. Give resonance structures for the carbocationic intermediate proposed in the reaction of electrophilic substitution of naphthalene at C(1); consider only structures that retain aromaticity in the unsubstituted ring. Explain why substitution at C(1) is preferable to substitution at C(2).(answer)
17. Among the following pyrazoles 1-4 identify aromatic and non-aromatic compounds. Motivate the assignments you make.
18. To a solution of 4-nitroaniline (0.32 mol) in 400 ml of acetic acid, add a solution of bromine (0.64 mol) in 240 ml of acetic acid with stirring and a bath temperature of 65 o C. After stirring at the same temperature for 1 hour, the reaction mixture is cooled and poured into a mixture of 1 liter of water with 500 g of ice. After washing the resulting precipitate with water, drying at 100 o C and recrystallization from ethylene glycol monomethyl ether, a substance is obtained in a 96% yield in the form of greenish-yellow prisms with mp. 201-202 o C, in the IR spectrum of which absorption bands were found at 3490, 3380, 1600, 1510 cm -1. Write the reaction equation, name the resulting compound, and assign the given absorption bands. (answer)
Name the original connection 1 . Give structures and also name compounds 2 -4 , formed during the above transformations.
To establish the structures of compounds, use the data from the IR spectra and PMR spectra given in Table.
| Compound | IR spectra (n, cm -1) | PMR spectra, d, ppm. |
| 2 | 3300, 1665, 1610, 1555, 1515, 1325, 825 |
CDCl 3 / (CD 3) 2 SO, 9.3 (s - wide, 1H), |
| 3 | 3380, 3360, 1720, 1520, 1345 |
CDCl 3 10.2 (s - wide, 1H), |
| 4 | 3340, 3275, 1645, 1605, 1520, 1245 |
CDCl 3 7.85 (d, J=1.5 Hz, 1H), |
20.
Starting from benzene using suitable reagents, obtain:
20.1. m-chlornitrobenzene,
20.2. 1-phenyl-1-propanol,
20.3. m-nitrobenzamide,
20.4. isopropyl ether m-bromobenzoic acid,
20.5. 1-bromo-2-phenylethane,
20.6. methyl benzyl ketone,
20.7. ethylphenyl ketone,
20.8. n-propylbenzene (without using the Friedel-Crafts reaction),
20.9. 1,2-diphenylethylene oxide
(answer)
21. Starting from toluene using suitable reagents, without resorting to alkylation and acylation reactions of the aromatic ring, obtain:
21.1. 4-(P-tolyl)butanol-1,
21.2. P-toluylaldehyde.
21.3. 4-deuterotoluene (answer)
22. Establish the structure of the aromatic hydrocarbon C 9 H 12, when treated with bromine in the presence of bromine iron, a single bromine derivative is formed. (answer)
23. Suggest reagents and conditions for carrying out the transformations below:
(answer)
24. Which starting material is better to use for the one-step synthesis of 3-bromo-5-nitrobenzoic acid: 3-bromobenzoic acid or 3-nitrobenzoic acid? Explain. (answer)
25. Give the conditions for the transformations below.
26. When processing 4-isopropyltoluene ( P-cymene) acetyl nitrate in acetic anhydride at 0°C along with 4-isopropyl-2-nitrotoluene (I)(~40%) and a small amount of 4-isopropyl-3-nitrotoluene (II) 2 more products were obtained: C 12 H 17 NO 4 (III)(~40%) and C 7 H 7 NO 2 (IV)(~10%).Connection (III), which is a mixture cis-, trance-isomers, easily converted into a compound (I) under the action of sulfuric acid. Suggest structures and possible schemes for the mechanisms of formation of compounds (III) And (IV). (
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— (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. 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.
Let us recall that all organic compounds are divided into two large groups:
- open chain compounds (aliphatic) And
- cyclic compounds.
Cyclic compounds are characterized by the presence of so-called cycles in their molecules.
A cycle is a closed chain, that is, a chain that, starting at a certain vertex, ends at the same vertex.
Cyclic compounds, in turn, are divided into:
- Carbocyclic compounds - alicyclic compounds,
- aromatic compounds.
Carbocyclic compounds- these are compounds in the molecules of which there are cycles consisting only of carbon atoms.
In addition to bonding with each other, carbon atoms are also bonded with other atoms (hydrogen, oxygen, etc.), but the cycle itself is made up of carbon atoms. This circumstance is reflected in their name (Carboneum in Latin - carbon).
These are cyclic compounds, in the cycles of which, in addition to carbon atoms, there are atoms of other elements (oxygen, nitrogen, sulfur, etc.). And this is also reflected in their name (from the Greek ετερος - “different”, “different”).
In the figure above (right), Pyridine is shown as an example of a heterocyclic compound.
Carbocyclic compounds
Carbocyclic compounds are divided into alicyclic and aromatic.
Alicyclic compounds are one of two subtypes of carbocyclic compounds.
They are called so because their chemical properties are closest to aliphatic compounds, although they are ring-shaped in structure.
They differ in the number of carbon atoms in the cycle and, depending on the nature of the bond between these atoms, can be saturated or unsaturated.
In the molecules of saturated cyclic hydrocarbons, the carbon atoms are connected by simple bonds, as in the molecules of saturated hydrocarbons with an open chain, which makes them similar in properties to the latter.
Examples of such compounds are cycloparaffins:
The names of cyclic compounds are constructed similarly to the names of compounds of the fatty (aliphatic) series with the addition of the prefix “cyclo”.
The second subtype of carbocyclic compounds are aromatic compounds.
Aromatic series covers all carbocyclic compounds in the molecules of which there is a specific grouping of atoms - benzene ring. This grouping of atoms determines certain physical and chemical properties of aromatic compounds.
The simplest of them are benzene C 6 H 6 and its homologues, for example, toluene(methylbenzene) C 6 H 5 -CH 3, ethylbenzene C 6 H 5 -CH 2 CH 3. General formula of these compounds C n H 2n-2.
A characteristic feature of the structure of the benzene ring is three single and three double bonds alternating with each other. For ease of writing, the benzene ring is depicted in a simplified form as a hexagon, in which the symbols WITH And N, related to the ring, do not write:
Benzene monovalent radical C 6 H 5 -, formed by subtracting one hydrogen atom from any carbon atom of the benzene ring, is called phenyl.
Aromatic hydrocarbons with multiple bonds in side chains are known, for example styrene, as well as polynuclear hydrocarbons containing several benzene nuclei, for example naphthalene And anthracene:
Or simplified:
Preparation of aromatic compounds and their use.
Aromatic hydrocarbons are contained in coal tar, obtained by coking coal. Another important source of their production is oil from some fields.
Aromatic hydrocarbons are also produced by the catalytic aromatization of acyclic petroleum hydrocarbons.
Some aromatic compounds can be isolated from essential oils of plants. They are used to produce fragrant substances.
Aromatic hydrocarbons and their derivatives are widely used to produce plastics, synthetic dyes, medicinal and explosive substances, synthetic rubbers, and detergents.
Origin of name.
Benzene and all compounds containing a benzene nucleus were called aromatic (at the beginning of the 19th century), since the first representatives of this series studied were aromatic substances, or compounds isolated from natural aromatic substances. Now this series includes numerous compounds that do not have a pleasant odor, but have a complex of chemical properties called aromatic properties.
Features of the properties and structure of aromatic hydrocarbons.
The aromatic properties of benzene and its homologues, determined by the peculiarities of its structure, are expressed in the relative stability of the benzene ring, despite the unsaturated composition of benzene.
Thus, unlike unsaturated compounds with ethylene double bonds, benzene is resistant to oxidizing agents. For example, like saturated hydrocarbons, it does not discolor potassium permanganate. Addition reactions are not typical for benzene. On the contrary, it, like other aromatic compounds, is characterized by substitution reactions for hydrogen atoms in the benzene ring.
From the above it follows that the formula of benzene with alternating single and double bonds does not accurately express the nature of the bonds between the carbon atoms in the benzene ring.
In accordance with this formula, benzene must have three localized pi bonds, i.e. three pairs of pi electrons, each of which is fixed between two carbon atoms. If we designate these pi electrons as dots, then the structure can be represented by the diagram:
However, experience shows that in the benzene ring there are no ordinary double bonds alternating with single ones, and that all bonds between WITH-atoms are equivalent.
This equivalence is explained as follows.
Each of the carbon atoms in the benzene ring is in the state sp 2-hybridization and spends three valence electrons on the formation of sigma bonds with two neighboring carbon atoms and one hydrogen atom.
Moreover, all six carbon atoms and all sigma bonds S-S And S-N lie in the same plane:
The cloud of the fourth valence electron of each carbon atom (i.e. the cloud R-electron not involved in hybridization) has the shape of a three-dimensional figure eight (“dumbbell”) and is oriented perpendicular to the plane of the benzene ring.
Each of these R-electron clouds overlap above and below the plane of the ring with R-electron clouds of two neighboring carbon atoms.
Cloud density pi-electrons in benzene are evenly distributed between all bonds S-S. In other words, six pi-electrons are generalized by all carbon atoms of the ring and form a single ring cloud ( aromatic electronic sextet).
For this reason, in structural formulas, instead of the generally accepted symbol of a benzene ring with alternating double and single bonds, a hexagon with a circle inside is used:
Heterocyclic compounds are compounds with a closed chain, including not only carbon atoms, but also atoms of other elements.
Shown in the picture Pyridine can be considered as benzene, in which the group -SN replaced by a nitrogen atom.
– the most numerous class of compounds. These include many vitamins, pigments, antibiotics, most alkaloids, some amino acids, etc.
Elements that participate together with carbon atoms in the formation of a cycle are called heteroatoms. The most common and studied are heterocyclic compounds of oxygen, sulfur and nitrogen.
A heteromolecule can contain either one heteroatom or a larger number:
Heterocycles can contain three, four, five, six or more atoms. Similar to carbocyclic compounds, five- and six-membered heterocycles are the most stable.
The presence of a heteroatom leads to a violation of the uniform distribution of electron density in the cycle. This determines the ability of heterocyclic compounds to react with both electrophilic and nucleophilic reagents (i.e., to be both a donor and an acceptor of an electron pair), and also undergo ring cleavage relatively easily.