The largest event in the development of organic chemistry was the creation in 1961 by the great Russian scientist A.M. Butlerov's 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 proceeded from correct 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, which depict the order of connection of atoms in molecules, are called structural formulas or structure 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.
5. Isomerism. The electronic structure of atoms of elements of small periods. Chemical bond
The properties of organic substances depend not only on their composition, but also on the order of connection of atoms in a molecule.
Isomers are substances that have the same composition and the same molar mass, but different molecular structure, and therefore have different properties.
Scientific significance of the theory of chemical structure:
1) deepens ideas about the substance;
2) indicates the way to the knowledge of the internal structure of molecules;
3) makes it possible to understand the facts accumulated in chemistry; predict the existence of new substances and find ways to synthesize them.
All this theory greatly contributed to the further development of organic chemistry and the chemical industry.
The German scientist A. Kekule expressed the idea of connecting carbon atoms to each other in a chain.
The doctrine of the electronic structure of atoms.
Features of the doctrine of the electronic structure of atoms: 1) made it possible to understand the nature of the chemical bond of atoms; 2) find out the essence of the mutual influence of atoms.
The state of electrons in atoms and the structure of electron shells.
Electron clouds are areas of the greatest probability of an electron being present, which differ in their shape, size, and orientation in space.
In the atom hydrogen a single electron during its movement forms a negatively charged cloud of a spherical (spherical) shape.
S-electrons are electrons that form a spherical cloud.
The hydrogen atom has one s-electron.
In the atom helium are two s-electrons.
Features of the helium atom: 1) clouds of the same spherical shape; 2) the highest density is equally removed from the core; 3) electron clouds are combined; 4) form a common two-electron cloud.
Features of the lithium atom: 1) has two electronic layers; 2) has a spherical cloud, but is much larger than the inner two-electron cloud; 3) the electron of the second layer is weaker attracted to the nucleus than the first two; 4) is easily captured by other atoms in redox reactions; 5) has an s-electron.
Features of the beryllium atom: 1) the fourth electron is an s-electron; 2) the spherical cloud coincides with the cloud of the third electron; 3) there are two paired s-electrons in the inner layer and two paired s-electrons in the outer.
The more electron clouds overlap when atoms connect, the more energy is released and the stronger chemical bond.
Alexander Mikhailovich Butlerov was born on September 3 (15), 1828 in the city of Chistopol, Kazan province, into the family of a landowner, a retired officer. He received his first education in a private boarding school, then studied at the gymnasium and the Kazan Imperial University. From 1849 he taught, in 1857 he became an ordinary professor of chemistry at the same university. Twice he was its rector. In 1851 he defended his master's thesis "On the oxidation of organic compounds", and in 1854 at Moscow University - his doctoral dissertation "On essential oils". From 1868 he was an ordinary professor of chemistry at St. Petersburg University, from 1874 - an ordinary academician of the St. Petersburg Academy of Sciences. In addition to chemistry, Butlerov paid attention to the practical issues of agriculture, horticulture, beekeeping, and under his leadership tea cultivation began in the Caucasus. He died in the village of Butlerovka, Kazan province, on August 5 (17), 1886.
Before Butlerov, a considerable number of attempts were made to create a theory of the chemical structure of organic compounds. This issue was addressed more than once by the most eminent chemists of that time, whose work was partially used by the Russian scientist for his theory of structure. For example, the German chemist August Kekule concluded that carbon can form four bonds with other atoms. Moreover, he believed that several formulas could exist for the same compound, but he always added that, depending on the chemical transformation, this formula could be different. Kekule believed that formulas do not reflect the order in which atoms are connected in a molecule. Another prominent German scientist, Adolf Kolbe, considered it fundamentally impossible to elucidate the chemical structure of molecules.
Butlerov first expressed his main ideas about the structure of organic compounds in 1861 in the report “On the chemical structure of matter”, which he presented to the participants of the Congress of German Naturalists and Physicians in Speyer. In his theory, he incorporated the ideas of Kekule about valency (the number of bonds for a particular atom) and the Scottish chemist Archibald Cooper that carbon atoms could form chains. The fundamental difference between Butlerov's theory and others was the position on the chemical (and not mechanical) structure of molecules - the method by which atoms bonded to each other, forming a molecule. At the same time, each atom established a bond in accordance with the “chemical force” belonging specifically to it. In his theory, the scientist made a clear distinction between a free atom and an atom that has joined with another (it passes into a new form, and as a result of mutual influence, the connected atoms, depending on the structural environment, have different chemical functions). The Russian chemist was convinced that the formulas not only represent molecules schematically, but also reflect their real structure. Moreover, each molecule has a certain structure, which changes only in the course of chemical transformations. It followed from the provisions of the theory (subsequently it was confirmed experimentally) that the chemical properties of an organic compound are determined by its structure. This statement is especially important, since it made it possible to explain and predict the chemical transformations of substances. There is also an inverse relationship: the structural formula can be used to judge the chemical and physical properties of a substance. In addition, the scientist drew attention to the fact that the reactivity of compounds is explained by the energy with which atoms bind.
With the help of the created theory, Butlerov was able to explain isomerism. Isomers are compounds in which the number and "quality" of atoms are the same, but at the same time they have different chemical properties, and hence a different structure. The theory made it possible to explain well-known cases of isomerism in an accessible way. Butlerov believed that it was possible to determine the spatial arrangement of atoms in a molecule. His predictions were later confirmed, which gave impetus to the development of a new branch of organic chemistry - stereochemistry. It should be noted that the scientist was the first to discover and explain the phenomenon of dynamic isomerism. Its meaning lies in the fact that two or more isomers under certain conditions can easily pass into each other. Generally speaking, it was isomerism that became a serious test for the theory of chemical structure and was brilliantly explained by it.
The irrefutable propositions formulated by Butlerov very soon brought universal recognition to the theory. The correctness of the ideas put forward was confirmed by the experiments of the scientist and his followers. In their process, they proved the hypothesis of isomerism: Butlerov synthesized one of the four butyl alcohols predicted by the theory, deciphered its structure. In accordance with the rules of isomerism, which directly followed from the theory, the possibility of the existence of four valeric acids was also expressed. Later they were received.
These are just a few facts in a chain of discoveries: the chemical theory of the structure of organic compounds had an amazing predictive ability.
In a relatively short period, a large number of new organic substances and their isomers were discovered, synthesized and studied. As a result, Butlerov's theory gave impetus to the rapid development of chemical science, including synthetic organic chemistry. Thus, Butlerov's numerous syntheses are the main products of entire industries.
The theory of chemical structure continued to develop, which brought many revolutionary ideas to organic chemistry at that time. For example, Kekule put forward an assumption about the cyclic structure of benzene and the movement of its double bonds in a molecule, about the special properties of compounds with conjugated bonds, and much more. Moreover, the mentioned theory made organic chemistry more visual - it became possible to draw the formulas of molecules.
And this, in turn, marked the beginning of the classification of organic compounds. It was the use of structural formulas that helped to determine the ways of synthesis of new substances, to establish the structure of complex compounds, that is, it led to the active development of chemical science and its branches. For example, Butlerov began to conduct serious studies of the polymerization process. In Russia, this undertaking was continued by his students, which eventually made it possible to discover an industrial method for producing synthetic rubber.
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
Theory of the structure of organic compounds: homology and isomerism (structural and spatial). Mutual influence of atoms in molecules
Theory of the chemical structure of organic compounds A. M. Butlerova
Just as for inorganic chemistry the basis of development is the Periodic law and the Periodic system of chemical elements of D. I. Mendeleev, for organic chemistry the theory of the structure of organic compounds of A. M. Butlerov became fundamental.
The main postulate of Butlerov's theory is the provision on chemical structure of matter, which is understood as the order, the sequence of mutual connection of atoms into molecules, i.e. chemical bond.
The chemical structure is understood as the order of connection of atoms of chemical elements in a molecule according to their valency.
This order can be displayed using structural formulas in which the valencies of atoms are indicated by dashes: one dash corresponds to the unit of valence of an atom of a chemical element. For example, for the organic substance methane, which has the molecular formula $CH_4$, the structural formula looks like this:
The main provisions of the theory of A. M. Butlerov
- The atoms in the molecules of organic substances are connected to each other according to their valency. Carbon in organic compounds is always tetravalent, and its atoms are able to combine with each other, forming various chains.
- The properties of substances are determined not only by their qualitative and quantitative composition, but also by the order of connection of atoms in a molecule, i.e., by the chemical structure of the substance.
- The properties of organic compounds depend not only on the composition of the substance and the order of connection of atoms in its molecule, but also on the mutual influence of atoms and groups of atoms on each other.
The theory of the structure of organic compounds is a dynamic and developing doctrine. With the development of knowledge about the nature of the chemical bond, about the influence of the electronic structure of the molecules of organic substances, they began to use, in addition to empirical And structural, electronic formulas. In such formulas indicate the direction of displacement of electron pairs in the molecule.
Quantum chemistry and the chemistry of the structure of organic compounds confirmed the theory of the spatial direction of chemical bonds ( cis- And transisomerism), studied the energy characteristics of mutual transitions in isomers, made it possible to judge the mutual influence of atoms in the molecules of various substances, created the prerequisites for predicting the types of isomerism and the direction and mechanism of chemical reactions.
Organic substances have a number of features:
- All organic substances contain carbon and hydrogen, so when burned, they form carbon dioxide and water.
- Organic substances are complex and can have a huge molecular weight (proteins, fats, carbohydrates).
- Organic substances can be arranged in rows of homologues similar in composition, structure and properties.
- For organic substances, the characteristic is isomerism.
Isomerism and homology of organic substances
The properties of organic substances depend not only on their composition, but also on the order of connection of atoms in a molecule.
isomerism- this is the phenomenon of the existence of different substances - isomers with the same qualitative and quantitative composition, i.e. with the same molecular formula.
There are two types of isomerism: structural And spatial (stereoisomerism). Structural isomers differ from each other in the order of bonding of atoms in a molecule; stereoisomers - the arrangement of atoms in space with the same order of bonds between them.
The following types of structural isomerism are distinguished: carbon skeleton isomerism, position isomerism, isomerism of various classes of organic compounds (interclass isomerism).
Structural isomerism
Isomerism of the carbon skeleton due to the different bond order between the carbon atoms that form the skeleton of the molecule. As has already been shown, two hydrocarbons correspond to the molecular formula $C_4H_(10)$: n-butane and isobutane. Three isomers are possible for the hydrocarbon $С_5Н_(12)$: pentane, isopentane and neopentane:
$CH_3-CH_2-(CH_2)↙(pentane)-CH_2-CH_3$
With an increase in the number of carbon atoms in a molecule, the number of isomers increases rapidly. For the hydrocarbon $С_(10)Н_(22)$ there are already $75$, and for the hydrocarbon $С_(20)Н_(44)$ - $366 319$.
position isomerism due to the different position of the multiple bond, substituent, functional group with the same carbon skeleton of the molecule:
$CH_2=(CH-CH_2)↙(butene-1)-CH_3$ $CH_3-(CH=CH)↙(butene-2)-CH_3$
$(CH_3-CH_2-CH_2-OH)↙(n-propyl alcohol(1-propanol))$
Isomerism of various classes of organic compounds (interclass isomerism) due to the different position and combination of atoms in the molecules of substances that have the same molecular formula, but belong to different classes. Thus, the molecular formula $С_6Н_(12)$ corresponds to the unsaturated hydrocarbon hexene-1 and the cyclic hydrocarbon cyclohexane:
The isomers are a hydrocarbon related to alkynes - butyne-1 and a hydrocarbon with two double bonds in the butadiene-1,3 chain:
$CH≡C-(CH_2)↙(butyne-1)-CH_2$ $CH_2=(CH-CH)↙(butadiene-1,3)=CH_2$
Diethyl ether and butyl alcohol have the same molecular formula $C_4H_(10)O$:
$(CH_3CH_2OCH_2CH_3)↙(\text"diethyl ether")$ $(CH_3CH_2CH_2CH_2OH)↙(\text"n-butyl alcohol (butanol-1)")$
Structural isomers are aminoacetic acid and nitroethane, corresponding to the molecular formula $C_2H_5NO_2$:
Isomers of this type contain different functional groups and belong to different classes of substances. Therefore, they differ in physical and chemical properties much more than carbon skeleton isomers or position isomers.
Spatial isomerism
Spatial isomerism divided into two types: geometric and optical. Geometric isomerism is characteristic of compounds containing double bonds and cyclic compounds. Since the free rotation of atoms around a double bond or in a cycle is impossible, substituents can be located either on one side of the plane of the double bond or cycle ( cis-position), or on opposite sides ( trance-position). Notation cis- And trance- usually referred to a pair of identical substituents:
Geometric isomers differ in physical and chemical properties.
Optical isomerism occurs when a molecule is incompatible with its image in the mirror. This is possible when the carbon atom in the molecule has four different substituents. This atom is called asymmetric. An example of such a molecule is $α$-aminopropionic acid ($α$-alanine) $CH_3CH(NH_2)COOH$.
The $α$-alanine molecule cannot coincide with its mirror image under any movement. Such spatial isomers are called mirror, optical antipodes, or enantiomers. All physical and almost all chemical properties of such isomers are identical.
The study of optical isomerism is necessary when considering many reactions occurring in the body. Most of these reactions are under the action of enzymes - biological catalysts. The molecules of these substances must approach the molecules of the compounds on which they act like a key to a lock; therefore, the spatial structure, the relative position of the molecular regions, and other spatial factors are of great importance for the course of these reactions. Such reactions are called stereoselective.
Most natural compounds are individual enantiomers, and their biological action differs sharply from the properties of their optical antipodes obtained in the laboratory. Such a difference in biological activity is of great importance, since it underlies the most important property of all living organisms - metabolism.
Homologous series is a number of substances arranged in ascending order of their relative molecular weights, similar in structure and chemical properties, where each term differs from the previous one by the homological difference $CH_2$. For example: $CH_4$ - methane, $C_2H_6$ - ethane, $C_3H_8$ - propane, $C_4H_(10)$ - butane, etc.
Types of bonds in molecules of organic substances. Hybridization of atomic orbitals of carbon. Radical. functional group.
Types of bonds in molecules of organic substances.
In organic compounds, carbon is always tetravalent. In the excited state, a pair of $2s^3$-electrons breaks in its atom and one of them passes to the p-orbital:
Such an atom has four unpaired electrons and can take part in the formation of four covalent bonds.
Based on the above electronic formula for the valence level of a carbon atom, one would expect that it contains one $s$-electron (spherical symmetric orbital) and three $p$-electrons having mutually perpendicular orbitals ($2p_x, 2p_y, 2p_z$- orbital). In reality, all four valence electrons of a carbon atom are completely equivalent and the angles between their orbitals are $109°28"$. In addition, calculations show that each of the four chemical bonds of carbon in a methane molecule ($CH_4$) is $s-$ by $25%$ and $p by $75%$ $-link, i.e. happens mixing$s-$ and $p-$ electron states. This phenomenon is called hybridization, and mixed orbitals hybrid.
A carbon atom in the $sp^3$-valence state has four orbitals, each of which contains one electron. In accordance with the theory of covalent bonds, it has the ability to form four covalent bonds with atoms of any monovalent elements ($CH_4, CHCl_3, CCl_4$) or with other carbon atoms. Such links are called $σ$-links. If a carbon atom has one $C-C$ bond, then it is called primary($Н_3С-CH_3$), if two - secondary($Н_3С-CH_2-CH_3$), if three - tertiary (
), and if four - Quaternary (
).
One of the characteristic features of carbon atoms is their ability to form chemical bonds by generalizing only $p$-electrons. Such bonds are called $π$-bonds. $π$-bonds in molecules of organic compounds are formed only in the presence of $σ$-bonds between atoms. So, in the ethylene molecule $H_2C=CH_2$ carbon atoms are linked by $σ-$ and one $π$-bond, in the acetylene molecule $HC=CH$ by one $σ-$ and two $π$-bonds. Chemical bonds formed with the participation of $π$-bonds are called multiples(in the ethylene molecule - double, in the acetylene molecule - triple), and compounds with multiple bonds - unsaturated.
Phenomenon$sp^3$-, $sp^2$- And$sp$ - hybridization of the carbon atom.
During the formation of $π$-bonds, the hybrid state of the atomic orbitals of the carbon atom changes. Since the formation of $π$-bonds occurs due to p-electrons, then in molecules with a double bond, electrons will have $sp^2$ hybridization (there was $sp^3$, but one p-electron goes to $π$- orbital), and with a triple - $sp$-hybridization (two p-electrons moved to $π$-orbital). The nature of hybridization changes the direction of $σ$-bonds. If during $sp^3$ hybridization they form spatially branched structures ($a$), then during $sp^2$ hybridization all atoms lie in the same plane and the angles between $σ$ bonds are equal to $120°$(b) , and under $sp$-hybridization the molecule is linear (c):
In this case, the axes of the $π$-orbitals are perpendicular to the axis of the $σ$-bond.
Both $σ$- and $π$-bonds are covalent, which means that they must be characterized by length, energy, spatial orientation and polarity.
Characteristics of single and multiple bonds between C atoms.
Radical. functional group.
One of the features of organic compounds is that in chemical reactions their molecules exchange not individual atoms, but groups of atoms. If this group of atoms consists only of carbon and hydrogen atoms, then it is called hydrocarbon radical, but if it has atoms of other elements, then it is called functional group. So, for example, methyl ($CH_3$-) and ethyl ($C_2H_5$-) are hydrocarbon radicals, and the hydroxy group (-$OH$), aldehyde group ( 
), nitro group (-$NO_2$), etc. are functional groups of alcohols, aldehydes and nitrogen-containing compounds, respectively.
As a rule, the functional group determines the chemical properties of an organic compound and therefore is the basis of their classification.
Theory of A.M. Butlerov
1. Atoms in molecules are interconnected in a certain sequence by chemical bonds in accordance with their valency. The bonding order of atoms is called their chemical structure. Carbon in all organic compounds is tetravalent.
2. The properties of substances are determined not only by the qualitative and quantitative composition of molecules, but also by their structure.
3. Atoms or groups of atoms mutually influence each other, on which the reactivity of the molecule depends.
4. The structure of molecules can be established on the basis of the study of their chemical properties.
Organic compounds have a number of characteristic features that distinguish them from inorganic ones. Almost all of them (with rare exceptions) are combustible; most organic compounds do not dissociate into ions, which is due to the nature of the covalent bond in organic substances. The ionic type of bond is realized only in salts of organic acids, for example, CH3COONa.
homologous series- this is an infinite series of organic compounds that have a similar structure and, therefore, similar chemical properties and differ from each other by any number of CH2 groups (homologous difference).
Even before the creation of the theory of structure, substances of the same elemental composition, but with different properties, were known. Such substances were called isomers, and this phenomenon itself was called isomerism.
At the heart of isomerism, as shown by A.M. Butlerov, lies the difference in the structure of molecules consisting of the same set of atoms.
isomerism- this is the phenomenon of the existence of compounds that have the same qualitative and quantitative composition, but a different structure and, consequently, different properties.
There are 2 types of isomerism: structural isomerism and spatial isomerism.
Structural isomerism
Structural isomers- compounds of the same qualitative and quantitative composition, differing in the order of binding atoms, i.e. chemical structure.
Spatial isomerism
Spatial isomers(stereoisomers) with the same composition and the same chemical structure differ in the spatial arrangement of atoms in the molecule.
Spatial isomers are optical and cis-trans isomers (geometric).
Cis-trans isomerism
lies in the possibility of substituents being located on one or on opposite sides of the plane of the double bond or non-aromatic ring. cis isomers substituents are on the same side of the plane of the ring or double bond, in trans isomers- in different ways.
In the butene-2 CH3–CH=CH–CH3 molecule, CH3 groups can be located either on one side of the double bond, in the cis isomer, or on opposite sides, in the trans isomer.
Optical isomerism
Appears when carbon has four different substituents.
If any two of them are interchanged, another spatial isomer of the same composition is obtained. The physicochemical properties of such isomers differ significantly. Compounds of this type are distinguished by their ability to rotate the plane of polarized light passed through the solution of such compounds by a certain amount. In this case, one isomer rotates the plane of polarized light in one direction, and its isomer in the opposite direction. Due to such optical effects, this kind of isomerism is called optical isomerism. 
