Just as in inorganic chemistry the fundamental theoretical basis is the Periodic law and the Periodic system of chemical elements of D. I. Mendeleev, so in organic chemistry the leading scientific basis is the theory of the structure of organic compounds of Butlerov-Kekule-Cooper.
Like any other scientific theory, the theory of the structure of organic compounds was the result of a generalization of the richest factual material accumulated by organic chemistry, which took shape as a science at the beginning of the 19th century. More and more new carbon compounds were discovered, the number of which increased like an avalanche (Table 1).
Table 1
Number of organic compounds known in different years
To explain this variety of organic compounds, scientists of the early XIX century. could not. Even more questions were raised by the phenomenon of isomerism.
For example, ethyl alcohol and dimethyl ether are isomers: these substances have the same composition C 2 H 6 O, but a different structure, that is, a different order of connection of atoms in molecules, and therefore different properties.
F. Wöhler, already known to you, in one of his letters to J. J. Berzelius, described organic chemistry as follows: “Organic chemistry can now drive anyone crazy. It seems to me a dense forest, full of amazing things, a boundless thicket from which you can’t get out, where you don’t dare to penetrate ... "
The development of chemistry was greatly influenced by the work of the English scientist E. Frankland, who, relying on the ideas of atomism, introduced the concept of valency (1853).
In the hydrogen molecule H 2, one covalent chemical bond H-H is formed, i.e., hydrogen is monovalent. The valence of a chemical element can be expressed by the number of hydrogen atoms that one atom of a chemical element attaches to itself or replaces. For example, sulfur in hydrogen sulfide and oxygen in water are divalent: H 2 S, or H-S-H, H 2 O, or H-O-H, and nitrogen in ammonia is trivalent:
In organic chemistry, the concept of "valency" is analogous to the concept of "oxidation state", which you are used to working with in the course of inorganic chemistry in elementary school. However, they are not the same. For example, in a nitrogen molecule N 2, the oxidation state of nitrogen is zero, and the valence is three:
In hydrogen peroxide H 2 O 2, the oxidation state of oxygen is -1, and the valency is two:
In the ammonium ion NH + 4, the oxidation state of nitrogen is -3, and the valency is four:
Usually, in relation to ionic compounds (sodium chloride NaCl and many other inorganic substances with an ionic bond), the term “valency” of atoms is not used, but their oxidation state is considered. Therefore, in inorganic chemistry, where most substances have a non-molecular structure, it is preferable to use the concept of "oxidation state", and in organic chemistry, where most compounds have a molecular structure, as a rule, use the concept of "valence".
The theory of chemical structure is the result of a generalization of the ideas of outstanding organic scientists from three European countries: the German F. Kekule, the Englishman A. Cooper and the Russian A. Butlerov.
In 1857, F. Kekule classified carbon as a tetravalent element, and in 1858, together with A. Cooper, he noted that carbon atoms can combine with each other in various chains: linear, branched and closed (cyclic).
The works of F. Kekule and A. Cooper served as the basis for the development of a scientific theory explaining the phenomenon of isomerism, the relationship between the composition, structure and properties of molecules of organic compounds. Such a theory was created by the Russian scientist A. M. Butlerov. It was his inquisitive mind that “dared to penetrate” the “dense forest” of organic chemistry and begin the transformation of this “limitless thicket” into a regular park filled with sunlight with a system of paths and alleys. The main ideas of this theory were first expressed by A. M. Butlerov in 1861 at the congress of German naturalists and doctors in Speyer.
Briefly formulate the main provisions and consequences of the Butlerov-Kekule-Cooper theory of the structure of organic compounds as follows.
1. The atoms in the molecules of substances are connected in a certain sequence according to their valency. Carbon in organic compounds is always tetravalent, and its atoms are able to combine with each other, forming various chains (linear, branched and cyclic).
Organic compounds can be arranged in series of substances similar in composition, structure and properties - homologous series.
- Butlerov Alexander Mikhailovich (1828-1886), Russian chemist, professor at Kazan University (1857-1868), from 1869 to 1885 - professor at St. Petersburg University. Academician of the St. Petersburg Academy of Sciences (since 1874). Creator of the theory of the chemical structure of organic compounds (1861). Predicted and studied the isomerism of many organic compounds. Synthesized many substances.
For example, methane CH 4 is the ancestor of the homologous series of saturated hydrocarbons (alkanes). Its closest homologue is ethane C 2 H 6, or CH 3 -CH 3. The next two members of the homologous series of methane are propane C 3 H 8, or CH 3 -CH 2 -CH 3, and butane C 4 H 10, or CH 3 -CH 2 -CH 2 -CH 3, etc.
It is easy to see that for homologous series one can derive a general formula for the series. So, for alkanes, this general formula is C n H 2n + 2.
2. The properties of substances depend not only on their qualitative and quantitative composition, but also on the structure of their molecules.
This position of the theory of the structure of organic compounds explains the phenomenon of isomerism. Obviously, for butane C 4 H 10, in addition to the linear structure molecule CH 3 -CH 2 -CH 2 -CH 3, a branched structure is also possible:
This is a completely new substance with its own individual properties, different from those of linear butane.
Butane, in the molecule of which the atoms are arranged in the form of a linear chain, is called normal butane (n-butane), and butane, the chain of carbon atoms of which is branched, is called isobutane.
There are two main types of isomerism - structural and spatial.
In accordance with the accepted classification, three types of structural isomerism are distinguished.
Isomerism of the carbon skeleton. Compounds differ in the order of carbon-carbon bonds, for example, n-butane and isobutane considered. It is this type of isomerism that is characteristic of alkanes.
Isomerism of the position of a multiple bond (C=C, C=C) or a functional group (i.e., a group of atoms that determine whether a compound belongs to a particular class of organic compounds), for example:
Interclass isomerism. Isomers of this type of isomerism belong to different classes of organic compounds, for example, ethyl alcohol (the class of saturated monohydric alcohols) and dimethyl ether (the class of ethers) discussed above.
There are two types of spatial isomerism: geometric and optical.
Geometric isomerism is characteristic, first of all, for compounds with a double carbon-carbon bond, since the molecule has a planar structure at the site of such a bond (Fig. 6).
Rice. 6.
Model of the ethylene molecule
For example, for butene-2, if the same groups of atoms at the carbon atoms in the double bond are on the same side of the C=C bond plane, then the molecule is a cisisomer, if on opposite sides it is a transisomer.
Optical isomerism is possessed, for example, by substances whose molecules have an asymmetric, or chiral, carbon atom bonded to four various deputies. Optical isomers are mirror images of each other, like two palms, and are not compatible. (Now, obviously, the second name of this type of isomerism has become clear to you: Greek chiros - hand - a sample of an asymmetric figure.) For example, in the form of two optical isomers, there is 2-hydroxypropanoic (lactic) acid containing one asymmetric carbon atom.
Chiral molecules have isomeric pairs, in which the isomer molecules are related to one another in their spatial organization in the same way as an object and its mirror image are related to each other. A pair of such isomers always has the same chemical and physical properties, with the exception of optical activity: if one isomer rotates the plane of polarized light clockwise, then the other necessarily counterclockwise. The first isomer is called dextrorotatory, and the second is called levorotatory.
The importance of optical isomerism in the organization of life on our planet is very great, since optical isomers can differ significantly both in their biological activity and in compatibility with other natural compounds.
3. The atoms in the molecules of substances influence each other. You will consider the mutual influence of atoms in the molecules of organic compounds in the further study of the course.
The modern theory of the structure of organic compounds is based not only on the chemical, but also on the electronic and spatial structure of substances, which is considered in detail at the profile level of the study of chemistry.
Several types of chemical formulas are widely used in organic chemistry.
The molecular formula reflects the qualitative composition of the compound, that is, it shows the number of atoms of each of the chemical elements that form the molecule of the substance. For example, the molecular formula of propane is C 3 H 8 .
The structural formula reflects the order of connection of atoms in a molecule according to valency. The structural formula of propane is:
Often there is no need to depict in detail the chemical bonds between carbon and hydrogen atoms, therefore, in most cases, abbreviated structural formulas are used. For propane, such a formula is written as follows: CH 3 -CH 2 -CH 3.
The structure of molecules of organic compounds is reflected using various models. The best known are volumetric (scale) and ball-and-stick models (Fig. 7).
Rice. 7.
Models of the ethane molecule:
1 - ball-and-stick; 2 - scale
New words and concepts
- Isomerism, isomers.
- Valence.
- Chemical structure.
- Theory of the structure of organic compounds.
- Homological series and homological difference.
- Formulas molecular and structural.
- Models of molecules: volumetric (scale) and spherical.
Questions and tasks
- What is valency? How is it different from oxidation state? Give examples of substances in which the values of the oxidation state and valence of atoms are numerically the same and different,
- Determine the valency and oxidation state of atoms in substances whose formulas are Cl 2, CO 2, C 2 H 6, C 2 H 4.
- What is isomerism; isomers?
- What is homology; homologues?
- How, using knowledge of isomerism and homology, to explain the diversity of carbon compounds?
- What is meant by the chemical structure of molecules of organic compounds? Formulate the position of the theory of structure, which explains the difference in the properties of isomers. Formulate the position of the theory of structure, which explains the diversity of organic compounds.
- What contribution did each of the scientists - the founders of the theory of chemical structure - make to this theory? Why did the contribution of the Russian chemist play a leading role in the formation of this theory?
- It is possible that there are three isomers of the composition C 5 H 12. Write down their full and abbreviated structural formulas,
- According to the model of the substance molecule presented at the end of the paragraph (see Fig. 7), make up its molecular and abbreviated structural formulas.
- Calculate the mass fraction of carbon in the molecules of the first four members of the homologous series of alkanes.
The basis for the creation of the theory of the chemical structure of organic compounds A.M. Butlerov was the atomic and molecular theory (works by A. Avagadro and S. Cannizzaro). It would be wrong to assume that before its creation the world knew nothing about organic substances and no attempts were made to substantiate the structure of organic compounds. By 1861 (the year A.M. Butlerov created the theory of the chemical structure of organic compounds), the number of known organic compounds reached hundreds of thousands, and the separation of organic chemistry as an independent science occurred as early as 1807 (J. Berzelius).
Background of the theory of the structure of organic compounds
A wide study of organic compounds began in the 18th century with the work of A. Lavoisier, who showed that substances obtained from living organisms consist of several elements - carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorus. The introduction of the terms "radical" and "isomerism" was of great importance, as well as the formation of the theory of radicals (L. Giton de Morvo, A. Lavoisier, J. Liebig, J. Dumas, J. Berzelius), success in the synthesis of organic compounds (urea, aniline, acetic acid, fats, sugar-like substances, etc.).
The term "chemical structure", as well as the foundations of the classical theory of chemical structure, were first published by A.M. Butlerov on September 19, 1861 in his report at the Congress of German Naturalists and Physicians in Speyer.
The main provisions of the theory of the structure of organic compounds A.M. Butlerov
1. The atoms that form the molecule of an organic substance are interconnected in a certain order, and one or more valences from each atom are spent on bonding with each other. There are no free valences.
Butlerov called the sequence of connection of atoms "chemical structure". Graphically, the bonds between atoms are indicated by a line or a dot (Fig. 1).
Rice. 1. Chemical structure of the methane molecule: A - structural formula, B - electronic formula
2. The properties of organic compounds depend on the chemical structure of the molecules, i.e. the properties of organic compounds depend on the order in which the atoms are connected in the molecule. By studying the properties, you can depict the substance.
Consider an example: a substance has the gross formula C 2 H 6 O. It is known that when this substance interacts with sodium, hydrogen is released, and when an acid acts on it, water is formed.
C 2 H 6 O + Na = C 2 H 5 ONa + H 2
C 2 H 6 O + HCl \u003d C 2 H 5 Cl + H 2 O
This substance can correspond to two structural formulas:
CH 3 -O-CH 3 - acetone (dimethyl ketone) and CH 3 -CH 2 -OH - ethyl alcohol (ethanol),
based on the chemical properties characteristic of this substance, we conclude that it is ethanol.
Isomers are substances that have the same qualitative and quantitative composition, but different chemical structure. There are several types of isomerism: structural (linear, branched, carbon skeleton), geometric (cis- and trans-isomerism, characteristic of compounds with a multiple double bond (Fig. 2)), optical (mirror), stereo (spatial, characteristic of substances , capable of being located in space in different ways (Fig. 3)).
Rice. 2. An example of geometric isomerism
3. The chemical properties of organic compounds are also influenced by other atoms present in the molecule. Such groups of atoms are called functional groups, due to the fact that their presence in the molecule of a substance gives it special chemical properties. For example: -OH (hydroxo group), -SH (thio group), -CO (carbonyl group), -COOH (carboxyl group). Moreover, the chemical properties of organic matter depend to a lesser extent on the hydrocarbon skeleton than on the functional group. It is the functional groups that provide the variety of organic compounds, due to which they are classified (alcohols, aldehydes, carboxylic acids, etc. The functional groups sometimes include carbon-carbon bonds (multiple double and triple). If there are several identical functional groups, then it is called homopolyfunctional (CH 2 (OH) -CH (OH) -CH 2 (OH) - glycerol), if several, but different - heteropolyfunctional (NH 2 -CH (R) -COOH - amino acids).
Fig.3. An example of stereoisomerism: a - cyclohexane, "chair" form, b - cyclohexane, "bath" form
4. The valency of carbon in organic compounds is always four.
The largest event in the development of organic chemistry was the creation in 1961 by the great Russian scientist A.M. Butlerov theory of the chemical structure of organic compounds.
Before A.M. Butlerov, it was considered impossible to know the structure of the molecule, that is, the order of the chemical bond between atoms. Many scientists even denied the reality of atoms and molecules.
A.M. Butlerov refuted this opinion. He started from the right materialistic and philosophical ideas about the reality of the existence of atoms and molecules, about the possibility of knowing the chemical bond of atoms in a molecule. He showed that the structure of a molecule can be established empirically by studying the chemical transformations of a substance. Conversely, knowing the structure of the molecule, one can derive the chemical properties of the compound.
The theory of chemical structure explains the diversity of organic compounds. It is due to the ability of tetravalent carbon to form carbon chains and rings, combine with atoms of other elements and the presence of isomerism in the chemical structure of organic compounds. This theory laid the scientific foundations of organic chemistry and explained its most important regularities. The basic principles of his theory A.M. Butlerov stated in the report "On the theory of chemical structure".
The main provisions of the theory of structure are as follows:
1) in molecules, atoms are connected to each other in a certain sequence in accordance with their valency. The bonding order of atoms is called chemical structure;
2) the properties of a substance depend not only on which atoms and in what quantity are part of its molecule, but also on the order in which they are interconnected, that is, on the chemical structure of the molecule;
3) atoms or groups of atoms that formed a molecule mutually influence each other.
In the theory of chemical structure, much attention is paid to the mutual influence of atoms and groups of atoms in a molecule.
Chemical formulas that show the order in which atoms are combined in molecules are called structural formulas or building formulas.
The value of the theory of chemical structure of A.M. Butlerov:
1) is an essential part of the theoretical foundation of organic chemistry;
2) in importance it can be compared with the Periodic system of elements of D.I. Mendeleev;
3) it made it possible to systematize a huge amount of practical material;
4) made it possible to predict in advance the existence of new substances, as well as indicate ways to obtain them.
The theory of chemical structure serves as the guiding basis in all research in organic chemistry.
12 Phenols, hydroxy derivatives aromatic compounds containing one or more hydroxyl groups (–OH) attached to the carbon atoms of the aromatic nucleus. By the number of OH groups, monoatomic phosphorus is distinguished, for example, oxybenzene C 6 H 5 OH, usually called simply phenol, oxytoluenes CH 3 C 6 H 4 OH - the so-called cresols, oxynaphthalenes - naphthols, diatomic, for example dioxybenzenes C 6 H 4 (OH) 2 ( hydroquinone, pyrocatechin, resorcinol), polyatomic, for example pyrogallol, phloroglucinol. F. - colorless crystals with a characteristic odor, less often liquids; well soluble in organic solvents (alcohol, ether, oenzol). Possessing acidic properties, F. form salt-like products - phenolates: ArOH + NaOH (ArONa + H 2 O (Ar is an aromatic radical). Alkylation and acylation of phenolates leads to F. esters - simple ArOR and complex ArOCOR (R - organic radical). Esters can be obtained by the direct interaction of phosphorus with carboxylic acids, their anhydrides, and acid chlorides.When phenols are heated with CO 2, phenolic acids are formed, for example salicylic acid. Unlike alcohols, the hydroxyl group F. with great difficulty is replaced by a halogen. Electrophilic substitution in the F.'s core (halogenation, nitration, sulfonation, alkylation, etc.) is carried out much more easily than in unsubstituted aromatic hydrocarbons; replacement groups are sent to ortho- And pair-positions to the OH group (see. Rule orientations). Catalytic hydrogenation of F. leads to alicyclic alcohols, for example, C 6 H 5 OH is reduced to cyclohexanol. F. is also characterized by condensation reactions, for example, with aldehydes and ketones, which is used in industry to obtain phenol- and resorcinol-formaldehyde resins, diphenylolpropane, and other important products.
F. is obtained, for example, by hydrolysis of the corresponding halogen derivatives, alkaline melting of arylsulfonic acids ArSO 2 OH, isolated from coal tar, brown coal tar, etc. F. is an important raw material in the production of various polymers, adhesives, paints and varnishes, dyes, and drugs ( phenolphthalein, salicylic acid, salol), surfactants and fragrances. Some F. are used as antiseptics and antioxidants (for example, polymers, lubricating oils). For the qualitative identification of F., solutions of ferric chloride are used, which form colored products with F.. F. toxic (see. Wastewater.).
13 Alkanes
general characteristics
Hydrocarbons are the simplest organic compounds, consisting of two elements: carbon and hydrogen. Limit hydrocarbons, or alkanes (international name), are compounds whose composition is expressed by the general formula C n H 2n + 2, where n is the number of carbon atoms. In molecules of saturated hydrocarbons, carbon atoms are interconnected by a simple (single) bond, and all other valences are saturated with hydrogen atoms. Alkanes are also called saturated hydrocarbons or paraffins (The term "paraffins" means "having a low affinity").
The first member of the homologous series of alkanes is methane CH 4 . The ending -an is typical for the names of saturated hydrocarbons. This is followed by ethane C 2 H 6, propane C 3 H 8, butane C 4 H 10. Starting from the fifth hydrocarbon, the name is formed from the Greek numeral, indicating the number of carbon atoms in the molecule, and the ending -an. These are C 5 H 12 pentane, C 6 H 14 hexane, C 7 H 16 heptane, C 8 H 18 octane, C 9 H 20 nonane, C 10 H 22 decane, etc.
In the homologous series, a gradual change in the physical properties of hydrocarbons is observed: the boiling and melting points increase, and the density increases. Under normal conditions (temperature ~ 22 ° C), the first four members of the series (methane, ethane, propane, butane) are gases, C 5 H 12 to C 16 H 34 are liquids, and C 17 H 36 are solids.
Alkanes, starting from the fourth member of the series (butane), have isomers.
All alkanes are saturated with hydrogen to the limit (maximum). Their carbon atoms are in a state of sp 3 hybridization, which means they have simple (single) bonds.
Nomenclature
The names of the first ten members of the series of saturated hydrocarbons have already been given. To emphasize that an alkane has an unbranched carbon chain, the word normal (n-) is often added to the name, for example:
CH 3 -CH 2 -CH 2 -CH 3 CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3
n-butane n-heptane
(normal butane) (normal heptane)
When a hydrogen atom is detached from an alkane molecule, one-valve particles are formed, called hydrocarbon radicals (abbreviated as R). The names of monovalent radicals are derived from the names of the corresponding hydrocarbons with the ending –an replaced by -yl. Here are the relevant examples:
Radicals are formed not only by organic but also by inorganic compounds. So, if we take away the hydroxyl group OH from nitric acid, then we get a monovalent radical - NO 2, called a nitro group, etc.
When two hydrogen atoms are removed from a hydrocarbon molecule, divalent radicals are obtained. Their names are also derived from the names of the corresponding saturated hydrocarbons with the ending -an replaced by -ylidene (if the hydrogen atoms are detached from one carbon atom) or -ylene (if the hydrogen atoms are detached from two adjacent carbon atoms). The CH 2 = radical is called methylene.
The names of radicals are used in the nomenclature of many derivatives of hydrocarbons. For example: CH 3 I - methyl iodide, C 4 H 9 Cl - butyl chloride, CH 2 Cl 2 - methylene chloride, C 2 H 4 Br 2 - ethylene bromide (if bromine atoms are bonded to different carbon atoms) or ethylidene bromide (if bromine atoms are bonded to one carbon atom).
Two nomenclatures are widely used for the name of isomers: the old - rational and modern - substitution, which is also called systematic or international (proposed by the International Union of Pure and Applied Chemistry IUPAC).
According to the rational nomenclature, hydrocarbons are considered as derivatives of methane, in which one or more hydrogen atoms are replaced by radicals. If the same radicals are repeated several times in the formula, then they are indicated by Greek numerals: di - two, three - three, tetra - four, penta - five, hexa - six, etc. For example:
Rational nomenclature is convenient for not very complex connections.
According to substitutional nomenclature, the name is based on one carbon chain, and all other fragments of the molecule are considered as substituents. In this case, the longest chain of carbon atoms is chosen, and the chain atoms are numbered from the end closest to the hydrocarbon radical. Then they name: 1) the number of the carbon atom to which the radical is associated (starting with the simplest radical); 2) hydrocarbon, which corresponds to a long chain. If the formula contains several identical radicals, then before their name indicate the number in words (di-, tri-, tetra-, etc.), and the numbers of the radicals are separated by commas. Here is how hexane isomers should be named according to this nomenclature:
Here's a more complex example:
Both substitutional and rational nomenclature are used not only for hydrocarbons, but also for other classes of organic compounds. For some organic compounds, historically established (empirical) or so-called trivial names are used (formic acid, sulfuric ether, urea, etc.).
When writing the formulas of isomers, it is easy to notice that the carbon atoms occupy an unequal position in them. A carbon atom that is connected to only one carbon atom in the chain is called primary, with two - secondary, with three - tertiary, with four - Quaternary. So, for example, in the last example, carbon atoms 1 and 7 are primary, 4 and 6 are secondary, 2 and 3 are tertiary, 5 is quaternary. The properties of hydrogen atoms, other atoms, and functional groups depend on which carbon atom they are associated with: primary, secondary, or tertiary. This must always be taken into account.
Receipt. Properties.
physical properties. Under normal conditions, the first four members of the homologous series of alkanes (C 1 - C 4) are gases. Normal alkanes from pentane to heptadecane (C 5 - C 17) are liquids, starting from C 18 and above are solids. As the number of carbon atoms in the chain increases, i.e. with an increase in the relative molecular weight, the boiling and melting points of alkanes increase. With the same number of carbon atoms in a molecule, branched alkanes have lower boiling points than normal alkanes.
Alkanes are practically insoluble in water, since their molecules are low-polar and do not interact with water molecules, they dissolve well in non-polar organic solvents such as benzene, carbon tetrachloride, etc. Liquid alkanes mix easily with each other.
The main natural sources of alkanes are oil and natural gas. Various oil fractions contain alkanes from C 5 H 12 to C 30 H 62 . Natural gas consists of methane (95%) with an admixture of ethane and propane.
Of the synthetic methods for obtaining alkanes, the following can be distinguished:
1. Obtaining from unsaturated hydrocarbons. The interaction of alkenes or alkynes with hydrogen ("hydrogenation") occurs in the presence of metal catalysts (Ni, Pd) at
heating:
CH s -C≡CH + 2H 2 → CH 3 -CH 2 -CH 3.
2. Obtaining from halogenated. When monohalogenated alkanes are heated with sodium metal, alkanes with twice the number of carbon atoms are obtained (Wurtz reaction):
C 2 H 5 Br + 2Na + Br-C 2 H 5 → C 2 H 5 -C 2 H 5 + 2NaBr.
A similar reaction is not carried out with two different halogen-substituted alkanes, since this produces a mixture of three different alkanes
3. Obtaining from salts of carboxylic acids. When anhydrous salts of carboxylic acids are fused with alkalis, alkanes are obtained containing one less carbon atom compared to the carbon chain of the original carboxylic acids:
4. Obtaining methane. In an electric arc burning in a hydrogen atmosphere, a significant amount of methane is formed:
C + 2H 2 → CH 4.
The same reaction occurs when carbon is heated in a hydrogen atmosphere to 400–500°C at elevated pressure in the presence of a catalyst.
In laboratory conditions, methane is often obtained from aluminum carbide:
Al 4 C 3 + 12H 2 O \u003d ZSN 4 + 4Al (OH) 3.
Chemical properties. Under normal conditions, alkanes are chemically inert. They are resistant to the action of many reagents: they do not interact with concentrated sulfuric and nitric acids, with concentrated and molten alkalis, they are not oxidized by strong oxidizing agents - potassium permanganate KMnO 4, etc.
The chemical stability of alkanes is explained by the high strength of C-C and C-H s-bonds, as well as their non-polarity. Nonpolar C-C and C-H bonds in alkanes are not prone to ionic cleavage, but are capable of cleaving homolytically under the action of active free radicals. Therefore, alkanes are characterized by radical reactions, as a result of which compounds are obtained where hydrogen atoms are replaced by other atoms or groups of atoms. Therefore, alkanes enter into reactions proceeding according to the mechanism of radical substitution, denoted by the symbol S R (from English, substitution radicalic). According to this mechanism, hydrogen atoms are most easily replaced at tertiary, then at secondary and primary carbon atoms.
1. Halogenation. When alkanes interact with halogens (chlorine and bromine) under the action of UV radiation or high temperature, a mixture of products from mono- to polyhalogen-substituted alkanes is formed. The general scheme of this reaction is shown using methane as an example:
b) Chain growth. The chlorine radical takes away a hydrogen atom from the alkane molecule:
Cl + CH 4 → HCl + CH 3
In this case, an alkyl radical is formed, which takes away the chlorine atom from the chlorine molecule:
CH 3 + Cl 2 → CH 3 Cl + Cl
These reactions are repeated until chain termination occurs in one of the following reactions:
Cl + Cl → Cl 2, CH 3 + CH 3 → C 2 H 6, CH 3 + Cl → CH 3 Cl
Overall reaction equation:
In radical reactions (halogenation, nitration), first of all, hydrogen atoms are mixed at the tertiary, then at the secondary and primary carbon atoms. This is explained by the fact that the bond of the tertiary carbon atom with hydrogen is most easily broken homolytically (bond energy 376 kJ/mol), then the secondary one (390 kJ/mol) and only then the primary one (415 kJ/mol).
3. Isomerization. Normal alkanes can be converted to branched-chain alkanes under certain conditions:
4. Cracking is a hemolytic rupture of C-C bonds, which occurs when heated and under the action of catalysts.
When higher alkanes are cracked, alkenes and lower alkanes are formed; when methane and ethane are cracked, acetylene is formed:
C 8 H 18 → C 4 H 10 + C 4 H 8,
2CH 4 → C 2 H 2 + ZH 2,
C 2 H 6 → C 2 H 2 + 2H 2.
These reactions are of great industrial importance. In this way, high-boiling oil fractions (fuel oil) are converted into gasoline, kerosene and other valuable products.
5. Oxidation. With the mild oxidation of methane with atmospheric oxygen in the presence of various catalysts, methyl alcohol, formaldehyde, and formic acid can be obtained:
 |
Soft catalytic oxidation of butane with atmospheric oxygen is one of the industrial methods for producing acetic acid:
t°
2C 4 H 10 + 5O 2 → 4CH 3 COOH + 2H 2 O.
cat
In air, alkanes burn to CO 2 and H 2 O:
C n H 2n + 2 + (Zn + 1) / 2O 2 \u003d nCO 2 + (n + 1) H 2 O.
Alkenes
Alkenes (otherwise olefins or ethylene hydrocarbons) are acyclic unsaturated hydrocarbons containing one double bond between carbon atoms, forming a homologous series with the general formula CnH2n. Carbon atoms in a double bond are in a state of sp² hybridization.
The simplest alkene is ethene (C2H4). According to the IUPAC nomenclature, the names of alkenes are formed from the names of the corresponding alkanes by replacing the suffix "-an" with "-en"; the position of the double bond is indicated by an Arabic numeral.
homologous series
Alkenes with more than three carbon atoms have isomers. Alkenes are characterized by isomerism of the carbon skeleton, double bond positions, interclass and geometric.
| ethene | C2H4 |
| propene | C3H6 |
| n-butene | C4H8 |
| n-pentene | C5H10 |
| n-hexene | C6H12 |
| n-heptene | C7H14 |
| n-octene | C8H16 |
| n-nonene | C9H18 |
| n-decene | C10H20 |
Physical Properties
Melting and boiling points increase with molecular weight and length of the main carbon chain.
Under normal conditions, alkenes from C2H4 to C4H8 are gases; from C5H10 to C17H34 - liquids, after C18H36 - solids. Alkenes are insoluble in water, but readily soluble in organic solvents.
Chemical properties
Alkenes are chemically active. Their chemical properties are determined by the presence of a double bond.
Ozonolysis: The alkene is oxidized to aldehydes (in the case of monosubstituted vicinal carbons), ketones (in the case of disubstituted vicinal carbons), or a mixture of aldehyde and ketone (in the case of a tri-substituted alkene on the double bond):
R1–CH=CH–R2 + O3 → R1–C(H)=O + R2C(H)=O + H2O
R1–C(R2)=C(R3)–R4+ O3 → R1–C(R2)=O + R3–C(R4)=O + H2O
R1–C(R2)=CH–R3+ O3 → R1–C(R2)=O + R3–C(H)=O + H2O
Ozonolysis under severe conditions - the alkene is oxidized to an acid:
R"–CH=CH–R" + O3 → R"–COOH + R"–COOH + H2O
Double bond attachment:
CH2=CH2 +Br2 → CH2Br-CH2Br
Oxidation with peracids:
CH2=CH2 + CH3COOOOH →
or
CH2=CH2 + HCOOH → HOCH2CH2OH
Lecture 15
Theory of the structure of organic substances. Main classes of organic compounds.
Organic chemistry - the science that studies organic matter. Otherwise, it can be defined as chemistry of carbon compounds. The latter occupies a special place in the periodic system of D.I. Mendeleev in terms of the variety of compounds, of which about 15 million are known, while the number of inorganic compounds is five hundred thousand. Organic substances have been known to mankind for a long time as sugar, vegetable and animal fats, coloring, fragrant and medicinal substances. Gradually, people learned to process these substances to obtain a variety of valuable organic products: wine, vinegar, soap, etc. Advances in organic chemistry are based on achievements in the field of chemistry of proteins, nucleic acids, vitamins, etc. Organic chemistry is of great importance for the development of medicine, since the vast majority of drugs are organic compounds not only of natural origin, but also obtained mainly by synthesis. Exceptional value wandered macromolecular organic compounds (synthetic resins, plastics, fibers, synthetic rubbers, dyes, herbicides, insecticides, fungicides, defoliants…). The importance of organic chemistry for the production of food and industrial goods is enormous.
Modern organic chemistry has penetrated deeply into the chemical processes that occur during the storage and processing of food products: the processes of drying, rancidity and saponification of oils, fermentation, baking, pickling, obtaining drinks, in the production of dairy products, etc. The discovery and study of enzymes, perfumes and cosmetics also played an important role.
One of the reasons for the great variety of organic compounds is the peculiarity of their structure, which is manifested in the formation of covalent bonds and chains by carbon atoms, different in type and length. The number of bonded carbon atoms in them can reach tens of thousands, and the configuration of carbon chains can be linear or cyclic. In addition to carbon atoms, the chain can include oxygen, nitrogen, sulfur, phosphorus, arsenic, silicon, tin, lead, titanium, iron, etc.
The manifestation of these properties by carbon is associated with several reasons. It has been confirmed that the energies of the C–C and C–O bonds are comparable. Carbon has the ability to form three types of hybridization of orbitals: four sp 3 - hybrid orbitals, their orientation in space is tetrahedral and corresponds to simple covalent bonds; three hybrid sp 2 - orbitals located in the same plane, in combination with a non-hybrid orbital form double multiples connections (─С = С─); also with the help of sp - hybrid orbitals of linear orientation and non-hybrid orbitals between carbon atoms arise triple multiples bonds (─ C ≡ C ─). At the same time, these types of bonds form carbon atoms not only with each other, but also with other elements. Thus, the modern theory of the structure of matter explains not only a significant number of organic compounds, but also the influence of their chemical structure on properties.
It also fully confirms the fundamentals theories of chemical structure, developed by the great Russian scientist A.M. Butlerov. ITS main provisions:
1) in organic molecules, atoms are connected to each other in a certain order according to their valency, which determines the structure of the molecules;
2) the properties of organic compounds depend on the nature and number of their constituent atoms, as well as on the chemical structure of molecules;
3) each chemical formula corresponds to a certain number of possible isomer structures;
4) each organic compound has one formula and has certain properties;
5) in molecules there is a mutual influence of atoms on each other.
Classes of organic compounds
According to the theory, organic compounds are divided into two series - acyclic and cyclic compounds.
1. Acyclic compounds.(alkanes, alkenes) contain an open, open carbon chain - straight or branched:
N N N N N N
│ │ │ │ │ │ │
N─ S─S─S─S─ N N─S─S─S─N
│ │ │ │ │ │ │
N N N N N │ N
Normal butane isobutane (methyl propane)
2. a) Alicyclic compounds- compounds that have closed (cyclic) carbon chains in molecules:
cyclobutane cyclohexane
b) Aromatic compounds, in the molecules of which there is a benzene skeleton - a six-membered cycle with alternating single and double bonds (arenes):
c) Heterocyclic compounds- cyclic compounds containing, in addition to carbon atoms, nitrogen, sulfur, oxygen, phosphorus and some trace elements, which are called heteroatoms.
furan pyrrole pyridine
In each row, organic substances are divided into classes - hydrocarbons, alcohols, aldehydes, ketones, acids, esters, in accordance with the nature of the functional groups of their molecules.
There is also a classification according to the degree of saturation and functional groups. According to the degree of saturation, they distinguish:
1. Limit saturated There are only single bonds in the carbon skeleton.
─С─С─С─
2. Unsaturated unsaturated– there are multiple (=, ≡) bonds in the carbon skeleton.
─С=С─ ─С≡С─
3. aromatic– unlimiting cycles with ring conjugation of (4n + 2) π-electrons.
By functional groups
1. Alcohols R-CH 2 OH
2. Phenols 
3. Aldehydes R─COH Ketones R─C─R
4. Carboxylic acids R─COOH О
5. Esters R─COOR 1
Hydrogen type:
Such formulas are somewhat similar to modern ones. But supporters of the theory of types did not consider them to reflect the real structure of substances and wrote many different formulas for one compound, depending on the chemical reactions that they tried to write using these formulas. They considered the structure of molecules to be fundamentally unknowable, which harmed the development of science.
3. The introduction by J. Berzelius in 1830 of the term "isomerism" for the phenomenon of the existence of substances of the same composition with different properties.
4. Successes in the synthesis of organic compounds, as a result of which the doctrine of vitalism, that is, the "life force", under the influence of which organic substances are allegedly formed in the body of living beings, was dispelled:
In 1828, F. Wehler synthesized urea from an inorganic substance (ammonium cyanate);
In 1842, the Russian chemist N. N. Zinin received aniline;
In 1845, the German chemist A. Kolbe synthesized acetic acid;
In 1854, the French chemist M. Berthelot synthesized fats, and, finally,
In 1861, A. M. Butlerov himself synthesized a sugar-like substance.
5. In the middle of the XVIII century. chemistry becomes a more rigorous science. As a result of the work of E. Frankland and A. Kekule, the concept of the valence of atoms of chemical elements was established. Kekule developed the concept of tetravalence of carbon. Thanks to the works of Cannizzaro, the concepts of atomic and molecular masses became clearer, their meanings and methods of determination were refined.
In 1860, more than 140 leading chemists from different European countries gathered for an international congress in Karlsruhe. The congress became a very important event in the history of chemistry: the successes of science were summarized and conditions were prepared for a new stage in the development of organic chemistry - the emergence of the theory of the chemical structure of organic substances by A. M. Butlerov (1861), as well as for the fundamental discovery of D. I. Mendeleev - The Periodic Law and the System of Chemical Elements (1869).
In 1861, A. M. Butlerov spoke at the congress of doctors and naturalists in the city of Speyer with a report "On the chemical structure of bodies." In it, he outlined the foundations of his theory of the chemical structure of organic compounds. Under the chemical structure, the scientist understood the order of connection of atoms in molecules.
Personal qualities of A. M. Butlerov
A. M. Butlerov was distinguished by the encyclopedic nature of chemical knowledge, the ability to analyze and generalize facts, and to predict. He predicted the existence of an isomer of butane, and then received it, as well as the isomer of butylene - isobutylene.
Butlerov Alexander Mikhailovich (1828-1886)
Russian chemist, academician of the St. Petersburg Academy of Sciences (since 1874). Graduated from Kazan University (1849). He worked there (since 1857 - professor, in 1860 and 1863 - rector). Creator of the theory of the chemical structure of organic compounds, which underlies modern chemistry. Substantiated the idea of the mutual influence of atoms in a molecule. He predicted and explained the isomerism of many organic compounds. Wrote "Introduction to the complete study of organic chemistry" (1864) - the first manual in the history of science based on the theory of chemical structure. Chairman of the Department of Chemistry of the Russian Physical and Chemical Society (1878-1882).
A. M. Butlerov created the first school of organic chemists in Russia, from which brilliant scientists emerged: V. V. Markovnikov, D. P. Konovalov, A. E. Favorsky and others.
No wonder D. I. Mendeleev wrote: “A. M. Butlerov is one of the greatest Russian scientists, he is Russian both in terms of his scientific education and the originality of his works.”
The main provisions of the theory of the structure of chemical compounds
The theory of the chemical structure of organic compounds, put forward by A. M. Butlerov in the second half of the last century (1861), was confirmed by the work of many scientists, including Butlerov's students and himself. It turned out to be possible on its basis to explain many phenomena that until then had no interpretation: isomerism, homology, the manifestation of tetravalence by carbon atoms in organic substances. The theory also fulfilled its prognostic function: on its basis, scientists predicted the existence of still unknown compounds, described properties and discovered them.
So, in 1862-1864. A. M. Butlerov considered the isomerism of propyl, butyl and amyl alcohols, determined the number of possible isomers and derived the formulas of these substances. Their existence was later experimentally proven, and some of the isomers were synthesized by Butlerov himself.
During the XX century. the provisions of the theory of the chemical structure of chemical compounds were developed on the basis of new views that have spread in science: the theory of the structure of the atom, the theory of chemical bonding, ideas about the mechanisms of chemical reactions. At present, this theory has a universal character, that is, it is valid not only for organic substances, but also for inorganic ones.
First position. Atoms in molecules are connected in a certain order in accordance with their valency. Carbon in all organic and most inorganic compounds is tetravalent.
It is obvious that the last part of the first provision of the theory can be easily explained by the fact that carbon atoms in compounds are in an excited state: 
a) tetravalent carbon atoms can combine with each other, forming various chains:
open branched
- open unbranched
- closed
b) the order of connection of carbon atoms in molecules can be different and depends on the type of covalent chemical bond between carbon atoms - single or multiple (double and triple).
Second position. The properties of substances depend not only on their qualitative and quantitative composition, but also on the structure of their molecules.
This position explains the phenomenon of isomerism. Substances that have the same composition, but different chemical or spatial structure, and therefore different properties, are called isomers. The main types of isomerism:
Structural isomerism, in which substances differ in the order of bonding of atoms in molecules:
1) isomerism of the carbon skeleton 
3) isomerism of homologous series (interclass) 
Spatial isomerism, in which the molecules of substances differ not in the order of bonding of atoms, but in their position in space: cis-trans-isomerism (geometric).
This isomerism is typical for substances whose molecules have a planar structure: alkenes, cycloalkanes, etc.
Optical (mirror) isomerism also belongs to spatial isomerism.
The four single bonds around the carbon atom, as you already know, are arranged tetrahedrally. If a carbon atom is bonded to four different atoms or groups, then a different arrangement of these groups in space is possible, that is, two spatial isomeric forms.
Two mirror forms of the amino acid alanine (2-aminopropanoic acid) are shown in Figure 17.
Imagine that an alanine molecule is placed in front of a mirror. The -NH2 group is closer to the mirror, so it will be in front in the reflection, and the -COOH group will be in the background, etc. (see image on the right). Alanya exists in two spatial forms, which, when superimposed, do not combine with one another.
The universality of the second position of the theory of the structure of chemical compounds confirms the existence of inorganic isomers.
So, the first of the syntheses of organic substances - the synthesis of urea, carried out by Wehler (1828), showed that an inorganic substance - ammonium cyanate and an organic substance - urea are isomeric: 
If you replace the oxygen atom in urea with a sulfur atom, you get thiourea, which is isomeric to ammonium thiocyanate, a well-known reagent for Fe 3+ ions. Obviously, thiourea does not give this qualitative reaction.
Third position. The properties of substances depend on the mutual influence of atoms in molecules.
For example, in acetic acid, only one of the four hydrogen atoms reacts with alkali. Based on this, it can be assumed that only one hydrogen atom is bonded to oxygen:
On the other hand, from the structural formula of acetic acid, one can conclude that it contains one mobile hydrogen atom, that is, that it is monobasic.
To verify the universality of the position of the theory of structure on the dependence of the properties of substances on the mutual influence of atoms in molecules, which exists not only in organic, but also in inorganic compounds, we compare the properties of hydrogen atoms in hydrogen compounds of non-metals. They have a molecular structure and under normal conditions are gases or volatile liquids. Depending on the position of the non-metal in the Periodic system of D. I. Mendeleev, a pattern can be identified in the change in the properties of such compounds: 
Methane does not interact with water. The lack of basic properties of methane is explained by the saturation of the valence capabilities of the carbon atom.
Ammonia exhibits basic properties. Its molecule is capable of attaching a hydrogen ion to itself due to its attraction to the lone electron pair of the nitrogen atom (donor-acceptor bond formation mechanism).
In phosphine PH3, the basic properties are weakly expressed, which is associated with the radius of the phosphorus atom. It is much larger than the radius of the nitrogen atom, so the phosphorus atom attracts the hydrogen atom to itself more weakly.
In periods from left to right, the charges of the nuclei of atoms increase, the radii of atoms decrease, the repulsive force of the hydrogen atom with a partial positive charge G + increases, and therefore the acidic properties of hydrogen compounds of non-metals are enhanced. 
In the main subgroups, the atomic radii of elements increase from top to bottom, non-metal atoms with 5- attract hydrogen atoms with 5+ weaker, the strength of hydrogen compounds decreases, they easily dissociate, and therefore their acidic properties are enhanced.
The different ability of hydrogen compounds of non-metals to remove or add hydrogen cations in solutions is explained by the unequal effect that a non-metal atom has on hydrogen atoms.
The different influence of atoms in the molecules of hydroxides formed by elements of the same period also explains the change in their acid-base properties.
The basic properties of hydroxides decrease, while acid ones increase, as the degree of oxidation of the central atom increases, therefore, the energy of its bond with the oxygen atom (8-) and the repulsion of the hydrogen atom (8+) by it increase.
Sodium hydroxide NaOH. Since the radius of the hydrogen atom is very small, it attracts the oxygen atom to itself more strongly and the bond between hydrogen and oxygen atoms will be stronger than between sodium and oxygen atoms. Aluminum hydroxide Al(OH)3 exhibits amphoteric properties.
In perchloric acid HclO 4, the chlorine atom with a relatively large positive charge is more strongly bonded to the oxygen atom and repels the hydrogen atom with 6+ more strongly. Dissociation proceeds according to the acid type.
The main directions in the development of the theory of the structure of chemical compounds and its significance
At the time of A. M. Butlerov, empirical (molecular) and structural formulas were widely used in organic chemistry. The latter reflect the order of connection of atoms in a molecule according to their valency, which is indicated by dashes.
For ease of recording, abbreviated structural formulas are often used, in which only the bonds between carbon or carbon and oxygen atoms are indicated by dashes.
Abbreviated structural formulas
Then, with the development of knowledge about the nature of the chemical bond and the influence of the electronic structure of the molecules of organic substances on their properties, they began to use electronic formulas in which the covalent bond is conventionally denoted by two dots. In such formulas, the direction of displacement of electron pairs in a molecule is often shown.
It is the electronic structure of substances that explains the mesomeric and induction effects.
The inductive effect is the displacement of electron pairs of gamma bonds from one atom to another due to their different electronegativity. Denoted (->).
The induction effect of an atom (or a group of atoms) is negative (-/), if this atom has a high electronegativity (halogens, oxygen, nitrogen), attracts gamma bond electrons and acquires a partial negative charge. An atom (or group of atoms) has a positive inductive effect (+/) if it repels the electrons of the gamma bonds. This property is possessed by some limiting radicals C2H5). Remember Markovnikov's rule about how hydrogen and a halogen of a hydrogen halide are added to alkenes (propene) and you will understand that this rule is of a particular nature. Compare these two examples of reaction equations:
[[Theory_of_the_chemical_compounds_A._M._Butlerov| 
]]
In the molecules of individual substances, both induction and mesomeric effects are manifested simultaneously. In this case, they either reinforce each other (in aldehydes, carboxylic acids), or mutually weaken (in vinyl chloride).
The result of the mutual influence of atoms in molecules is the redistribution of electron density.
The idea of the spatial direction of chemical bonds was first expressed by the French chemist J. A. Le Bel and the Dutch chemist J. X. Van't Hoff in 1874. The scientists' assumptions were fully confirmed by quantum chemistry. The properties of substances are significantly affected by the spatial structure of their molecules. For example, we have already given the formulas for cis- and trans-isomers of butene-2, which differ in their properties (see Fig. 16).
The average bond energy that must be broken during the transition from one form to another is approximately 270 kJ / mol; there is not so much energy at room temperature. For the mutual transition of butene-2 forms from one to another, it is necessary to break one covalent bond and form another instead. In other words, this process is an example of a chemical reaction, and both forms of butene-2 considered are different chemical compounds.
You obviously remember that the most important problem in the synthesis of rubber was getting stereoregular rubber. It was necessary to create a polymer in which the structural units would be arranged in a strict order (natural rubber, for example, consists only of cis-units), because such an important property of rubber as its elasticity depends on this.
Modern organic chemistry distinguishes two main types of isomerism: structural (chain isomerism, isomerism of the position of multiple bonds, isomerism of homologous series, isomerism of the position of functional groups) and stereoisomerism (geometric, or cis-trans-isomerism, optical, or mirror, isomerism).
So, you were able to make sure that the second position of the theory of chemical structure, clearly formulated by A. M. Butlerov, was incomplete. From a modern standpoint, this provision requires additions:
the properties of substances depend not only on their qualitative and quantitative composition, but also on their:
chemical,
electronic,
Spatial structure.
The creation of the theory of the structure of substances played an important role in the development of organic chemistry. From a predominantly descriptive science, it turns into a creative, synthesizing science; it became possible to judge the mutual influence of atoms in the molecules of various substances (see Table 10). The theory of structure created the prerequisites for explaining and predicting various types of isomerism of organic molecules, as well as the directions and mechanisms of chemical reactions.
On the basis of this theory, organic chemists create substances that not only replace natural ones, but significantly surpass them in their properties. So, synthetic dyes are much better and cheaper than many natural ones, for example, alizarin and indigo known in antiquity. Synthetic rubbers are produced in large quantities with a wide variety of properties. Plastics and fibers are widely used, products from which are used in engineering, everyday life, medicine, and agriculture.
The value of the theory of chemical structure of A. M. Butlerov for organic chemistry can be compared with the value of the Periodic law and the Periodic system of chemical elements of D. I. Mendeleev for inorganic chemistry. It is not for nothing that both theories have so much in common in the ways of their formation, directions of development and general scientific significance. However, in the history of any other leading scientific theory (Ch. Darwin's theory, genetics, quantum theory, etc.) one can find such common stages.
1. Establish parallels between the two leading theories of chemistry - the Periodic Law and the Periodic Table of Chemical Elements by D. I. Mendeleev and the theory of the chemical structure of organic compounds by A. M. Butlerov on the following grounds: common in prerequisites, common in the directions of their development, common in prognostic roles.
2. What role did the theory of the structure of chemical compounds play in the formation of the Periodic Law?
3. What examples from inorganic chemistry confirm the universality of each of the provisions of the theory of the structure of chemical compounds?
4. Phosphorous acid H3PO3 refers to dibasic acids. Propose its structural formula and consider the mutual influence of atoms in the molecule of this acid.
5. Write the isomers having the composition С3Н8O. Name them according to the systematic nomenclature. Determine the types of isomerism.
6. The following formulas of crystalline hydrates of chromium(III) chloride are known: [Cr(H20)6]Cl3; [Cr(H20)5Cl]Cl2 H20; [Cr(H20)4 * C12]Cl 2H2O. What would you call this phenomenon?
