Isomerism was first discovered by J. Liebig in 1823, who found that the silver salts of fulminant and isocyanic acids: Ag-O-N=C and Ag-N=C=O have the same composition, but different properties. The term "isomerism" in 1830 introduced

According to the modern definition, two compounds of the same composition are considered isomers if their molecules cannot be combined in space so that they completely coincide. The combination, as a rule, is done mentally; in complex cases, spatial models or calculation methods are used.

There are several causes of isomerism.

In complex cases, the question of whether two compounds are isomers is decided by using various rotations around valence bonds (simple bonds allow this, which to a certain extent corresponds to their physical properties). After the movement of individual fragments of the molecule (without breaking the bonds), one molecule is superimposed on another (Fig.

One of the types of stereoisomerism is cis-trans-isomerism (cis

Physical and chemical properties

Optical isomerism arises not only in the case of an asymmetric atom, it is also realized in some framework molecules in the presence of a certain number of different substituents. For example, the frame hydrocarbon adamantane, which has four different substituents (Fig. 17), can have an optical isomer, while the entire molecule plays the role of an asymmetric center, which becomes obvious if the frame of adamantane is mentally contracted into a point. Similarly, the siloxane, which has a cubic structure (Fig. 17), also becomes optically active in the case of four different substituents:

When synthesizing optically active stereoisomers, a mixture of dextrorotatory and levorotatory compounds is usually obtained. The separation of isomers is carried out by reacting a mixture of isomers with reagents (often of natural origin) containing an asymmetric reaction center. Some living organisms, including bacteria, preferentially metabolize left-handed isomers.

Currently, processes (called asymmetric synthesis) have been developed that make it possible to purposefully obtain a specific optical isomer.

There are reactions that make it possible to convert an optical isomer into its antipode (

1. Structural isomerism.

2. Conformational isomerism.

3. Geometric isomerism.

4. Optical isomerism.

Isomers are substances that have the same composition and molecular weight, but different physical and chemical properties. Differences in the properties of isomers are due to differences in their chemical or spatial structure. In this regard, there are two types of isomerism.

isomerism

structural

spatial

carbon skeleton

Configuration

conformational

The position of the functional

Optical

Interclass

Geometric

1. Structural isomerism

Structural isomers differ in chemical structure, i.e. the nature and sequence of bonds between atoms in a molecule. Structural isomers are isolated in pure form. They exist as individual, stable substances, their mutual transformation requires high energy - about 350 - 400 kJ / mol. Only structural isomers, tautomers, are in dynamic equilibrium. Tautomerism is a common phenomenon in organic chemistry. It is possible with the transfer of a mobile hydrogen atom in a molecule (carbonyl compounds, amines, heterocycles, etc.), intramolecular interactions (carbohydrates).

All structural isomers are presented in the form of structural formulas and named according to the IUPAC nomenclature. For example, the composition of C 4 H 8 O corresponds to structural isomers:

a)with different carbon skeleton

unbranched C-chain - CH 3 -CH 2 -CH 2 -CH \u003d O (butanal, aldehyde) and

branched C-chain -

(2-methylpropanal, aldehyde) or

cycle - (cyclobutanol, cyclic alcohol);

b)with a different position of the functional group

butanone-2, ketone;

in)with different composition of the functional group

3-butenol-2, unsaturated alcohol;

G)metamerism

The heteroatom of the functional group may be included in the carbon skeleton (cycle or chain). One of the possible isomers of this type of isomerism is CH 3 -O-CH 2 -CH \u003d CH 2 (3-methoxypropene-1, simple ether);

e)tautomerism (keto-enol)

enol form keto form

The tautomers are in dynamic equilibrium, while the more stable form, the keto form, predominates in the mixture.

For aromatic compounds, structural isomerism is considered only for the side chain.

2. Spatial isomerism (stereoisomerism)

Spatial isomers have the same chemical structure, differ in the spatial arrangement of atoms in the molecule. This difference creates a difference in physical and chemical properties. Spatial isomers are depicted as various projections or stereochemical formulas. The branch of chemistry that studies the spatial structure and its influence on the physical and chemical properties of compounds, on the direction and rate of their reactions, is called stereochemistry.

a)Conformational (rotational) isomerism

Without changing either bond angles or bond lengths, one can imagine a multitude of geometric shapes (conformations) of a molecule that differ from each other by the mutual rotation of carbon tetrahedra around the σ-C-C bond connecting them. As a result of such rotation, rotational isomers (conformers) arise. The energy of different conformers is not the same, but the energy barrier separating different conformational isomers is small for most organic compounds. Therefore, under normal conditions, as a rule, it is impossible to fix molecules in one strictly defined conformation. Usually, several conformational isomers coexist in equilibrium.

The image methods and the nomenclature of isomers can be considered using the example of the ethane molecule. For it, one can foresee the existence of two conformations that differ as much as possible in energy, which can be represented as perspective projections(1) ("sawhorses") or projections Newman(2):

hindered conformation eclipsed conformation

In a perspective projection (1), the C-C connection must be imagined as going into the distance; the carbon atom standing on the left is close to the observer, standing on the right is removed from it.

In the Newman projection (2), the molecule is viewed along the C-C bond. Three lines diverging at an angle of 120 o from the center of the circle indicate the bonds of the carbon atom closest to the observer; the lines "protruding" from behind the circle are the bonds of the remote carbon atom.

The conformation shown on the right is called obscured . This name is reminiscent of the fact that the hydrogen atoms of both CH 3 groups are opposite each other. The shielded conformation has an increased internal energy and is therefore unfavorable. The conformation shown on the left is called inhibited , implying that the free rotation around the C-C bond "slows down" in this position, i.e. the molecule exists predominantly in this conformation.

The minimum energy required for complete rotation of a molecule around a particular bond is called the rotational barrier for that bond. The rotational barrier in a molecule like ethane can be expressed in terms of the change in the potential energy of the molecule as a function of the change in the dihedral (torsion - τ) angle of the system. The energy profile of rotation around the C-C bond in ethane is shown in Figure 1. The rotational barrier separating the two forms of ethane is about 3 kcal/mol (12.6 kJ/mol). The minima of the potential energy curve correspond to hindered conformations, the maxima correspond to obscured ones. Since at room temperature the energy of some collisions of molecules can reach 20 kcal / mol (about 80 kJ / mol), this barrier of 12.6 kJ / mol is easily overcome and rotation in ethane is considered as free. In a mixture of all possible conformations, hindered conformations predominate.

Fig.1. Potential energy diagram of ethane conformations.

For more complex molecules, the number of possible conformations increases. Yes, for n-butane can already be depicted in six conformations that arise when turning around the central bond C 2 - C 3 and differ in the mutual arrangement of CH 3 groups. The various eclipsed and hindered conformations of butane differ in energy. Hindered conformations are energetically more favorable.

The energy profile of rotation around the C 2 -C 3 bond in butane is shown in Figure 2.

Fig.2. Potential energy diagram of n-butane conformations.

For a molecule with a long carbon chain, the number of conformational forms increases.

The molecules of alicyclic compounds are characterized by different conformational forms of the ring (for example, for cyclohexane armchair, bath, twist-forms).

So, conformations are various spatial forms of a molecule that has a certain configuration. Conformers are stereoisomeric structures that correspond to energy minima on the potential energy diagram, are in mobile equilibrium and are capable of interconversion by rotation around simple σ-bonds.

If the barrier of such transformations becomes high enough, then stereoisomeric forms can be separated (an example is optically active biphenyls). In such cases, one speaks no longer of conformers, but of actually existing stereoisomers.

b)geometric isomerism

Geometric isomers arise as a result of the absence in the molecule:

1. rotation of carbon atoms relative to each other - a consequence of the rigidity of the C=C double bond or cyclic structure;

2. two identical groups at one carbon atom of a double bond or cycle.

Geometric isomers, unlike conformers, can be isolated in pure form and exist as individual, stable substances. For their mutual transformation, a higher energy is required - about 125-170 kJ / mol (30-40 kcal / mol).

There are cis-trans-(Z,E) isomers; cis- forms are geometric isomers in which the same substituents lie on one side of the plane of the π-bond or cycle, trance- forms are called geometric isomers, in which the same substituents lie on opposite sides of the plane of the π-bond or ring.

The simplest example is the isomers of butene-2, which exists in the form of cis-, trans-geometric isomers:

cis-butene-2 ​​trans-butene-2

the melting temperature

138.9 0 С - 105.6 0 С

boiling temperature

3.72 0 С 1.00 0 С

density

1,2 - dichlorocyclopropane exists in the form of cis-, trans-isomers:

cis-1,2-dichlorocyclopropane trans-1,2-dichlorocyclopropane

In more complex cases, apply Z,E-nomenclature (the nomenclature of Kann, Ingold, Prelog - KIP, the nomenclature of seniority of deputies). In conjunction

1-bromo -2-methyl-1-chlorobutene-1 (Br) (CI) C \u003d C (CH 3) - CH 2 -CH 3 all substituents at carbon atoms with a double bond are different; therefore, this compound exists in the form of Z-, E- geometric isomers:

Е-1-bromo-2-methyl-1-chlorobutene-1 Z-1-bromo-2-methyl-1-chlorobutene-1.

To indicate the configuration of an isomer, indicate the location of senior substituents in a double bond (or cycle) - Z- (from the German Zusammen - together) or E- (from the German Entgegen - opposite).

In the Z,E-system, substituents with a higher atomic number are considered senior. If the atoms directly bonded to unsaturated carbon atoms are the same, then they go to the "second layer", if necessary, to the "third layer", etc.

In the first projection, the older groups are opposite each other relative to the double bond, so this is the E isomer. In the second projection, the older groups are on the same side of the double bond (together), so this is the Z-isomer.

Geometric isomers are widely distributed in nature. For example, natural polymers rubber (cis-isomer) and gutta-percha (trans-isomer), natural fumaric (trans-butenedioic acid) and synthetic maleic (cis-butenedioic acid) acids, fats contain cis-oleic, linoleic, linolenic acids.

in)Optical isomerism

Molecules of organic compounds can be chiral and achiral. Chirality (from the Greek cheir - hand) - the incompatibility of a molecule with its mirror image.

Chiral substances are able to rotate the plane of polarization of light. This phenomenon is called optical activity, and the corresponding substances - optically active. Optically active substances occur in pairs optical antipodes- isomers, the physical and chemical properties of which are the same under normal conditions, with the exception of one - the sign of rotation of the polarization plane: one of the optical antipodes deflects the polarization plane to the right (+, dextrorotatory isomer), the other - to the left (-, levorotatory). The configuration of optical antipodes can be determined experimentally using a device - a polarimeter.

Optical isomerism appears when the molecule contains asymmetric carbon atom(there are other reasons for the chirality of the molecule). This is the name of the carbon atom in sp 3 - hybridization and associated with four different substituents. Two tetrahedral arrangements of substituents around an asymmetric atom are possible. At the same time, two spatial forms cannot be combined by any rotation; one of them is a mirror image of the other:

Both mirror forms form a pair of optical antipodes or enantiomers .

Depict optical isomers in the form of E. Fisher projection formulas. They are obtained by projecting a molecule with an asymmetric carbon atom. In this case, the asymmetric carbon atom itself on the plane is indicated by a dot, the symbols of substituents protruding in front of the plane of the figure are indicated on the horizontal line. The vertical line (dashed or solid) indicates the substituents that are removed from the plane of the figure. The following are different ways to write the projection formula corresponding to the left model in the previous figure:

In projection, the main carbon chain is depicted vertically; the main function, if it is at the end of the chain, is indicated at the top of the projection. For example, the stereochemical and projection formulas (+) and (-) of alanine - CH 3 - * CH (NH 2) -COOH are as follows:

A mixture with the same content of enantiomers is called a racemate. The racemate has no optical activity and is characterized by physical properties different from the enantiomers.

Rules for transforming projection formulas.

1. Formulas can be rotated in the plane of the drawing by 180 o without changing their stereochemical meaning:

2. Two (or any even number) permutations of substituents on one asymmetric atom do not change the stereochemical meaning of the formula:

3. One (or any odd number) permutation of substituents at the asymmetric center leads to the optical antipode formula:

4. Turning in the plane of the drawing by 90 turns the formula into an antipode.

5. Rotation of any three substituents clockwise or counterclockwise does not change the stereochemical meaning of the formula:

6. Projection formulas cannot be derived from the plane of the drawing.

Organic compounds have optical activity, in the molecules of which other atoms are also chiral centers, for example, silicon, phosphorus, nitrogen, and sulfur.

Compounds with multiple asymmetric carbons exist as diastereomers , i.e. spatial isomers that do not constitute optical antipodes with each other.

Diastereomers differ from each other not only in optical rotation, but also in all other physical constants: they have different melting and boiling points, different solubilities, etc.

The number of spatial isomers is determined by the Fisher formula N=2 n , where n is the number of asymmetric carbon atoms. The number of stereoisomers may decrease due to partial symmetry appearing in some structures. Optically inactive diastereomers are called meso-forms.

Nomenclature of optical isomers:

a) D-, L- nomenclature

To determine the D- or L-series of an isomer, the configuration (position of the OH group at the asymmetric carbon atom) is compared with the configurations of enantiomers of glyceraldehyde (glycerol key):

L-glyceraldehyde D-glyceraldehyde

The use of D-, L-nomenclature is currently limited to three classes of optically active substances: carbohydrates, amino acids and hydroxy acids.

b) R -, S-nomenclature (nomenclature of Kahn, Ingold and Prelog)

To determine the R (right) - or S (left) - configuration of the optical isomer, it is necessary to arrange the substituents in the tetrahedron (stereochemical formula) around the asymmetric carbon atom so that the lowest substituent (usually hydrogen) has the direction "from the observer". If the transition of the other three substituents from senior to middle and junior in seniority occurs clockwise, this is the R-isomer (the fall in seniority coincides with the movement of the hand when writing the upper part of the letter R). If the transition occurs counterclockwise - this is S - isomer (the fall in seniority coincides with the movement of the hand when writing the upper part of the letter S).

To determine the R- or S-configuration of the optical isomer by the projection formula, it is necessary to arrange the substituents by an even number of permutations so that the youngest of them is at the bottom of the projection. The fall in the seniority of the remaining three substituents clockwise corresponds to the R-configuration, counterclockwise - to the S-configuration.

Optical isomers are obtained by the following methods:

a) isolation from natural materials containing optically active compounds, such as proteins and amino acids, carbohydrates, many hydroxy acids (tartaric, malic, mandelic), terpene hydrocarbons, terpene alcohols and ketones, steroids, alkaloids, etc.

b) cleavage of racemates;

c) asymmetric synthesis;

d) biochemical production of optically active substances.

DO YOU KNOW THAT

The phenomenon of isomerism (from Greek - isos - different and meros - share, part) was opened in 1823. J. Liebig and F. Wöhler on the example of salts of two inorganic acids: cyanic H-O-C≡N and fulminant H-O-N= C.

In 1830, J. Dumas extended the concept of isomerism to organic compounds.

In 1831 the term "isomer" for organic compounds was proposed by J. Berzelius.

Stereoisomers of natural compounds are characterized by different biological activities (amino acids, carbohydrates, alkaloids, hormones, pheromones, medicinal substances of natural origin, etc.).

Another example was tartaric and tartaric acids, after the study of which J. Berzelius introduced the term isomerism and suggested that the differences arise from the "different distribution of simple atoms in a complex atom" (i.e., a molecule). The true explanation of isomerism was received only in the 2nd half of the 19th century. based on the theory of the chemical structure of A. M. Butlerov (structural isomerism) and the stereochemical theory of J. G. van't Hoff (spatial isomerism).

Structural isomerism

Structural isomerism is the result of differences in chemical structure. This type includes:

Isomerism of the hydrocarbon chain (carbon skeleton)

Isomerism of the carbon skeleton, due to the different bonding order of carbon atoms. The simplest example is butane CH 3 -CH 2 -CH 2 -CH 3 and isobutane (CH 3) 3 CH. Dr. examples: anthracene and phenanthrene (formulas I and II, respectively), cyclobutane and methylcyclopropane (III and IV).

Valence isomerism

Valence isomerism (a special type of structural isomerism), in which isomers can be converted into each other only by redistributing bonds. For example, the valence isomers of benzene (V) are bicyclohexa-2,5-diene (VI, "Dewar's benzene"), prisman (VII, "Ladenburg's benzene"), benzvalene (VIII).

Functional group isomerism

It differs in the nature of the functional group. Example: Ethanol (CH 3 -CH 2 -OH) and Dimethyl ether (CH 3 -O-CH 3)

position isomerism

A type of structural isomerism characterized by a difference in the position of the same functional groups or double bonds with the same carbon skeleton. Example: 2-chlorobutanoic acid and 4-chlorobutanoic acid.

Spatial isomerism (stereoisomerism)

Enantiomerism (optical isomerism)

Spatial isomerism (stereoisomerism) arises as a result of differences in the spatial configuration of molecules that have the same chemical structure. This type of isomer is subdivided into enantiomers(optical isomerism) and diastereomerism.

Enantiomers (optical isomers, mirror isomers) are pairs of optical antipodes of substances characterized by opposite in sign and equal in magnitude rotations of the plane of polarization of light with the identity of all other physical and chemical properties (with the exception of reactions with other optically active substances and physical properties in a chiral medium ). A necessary and sufficient reason for the appearance of optical antipodes is the assignment of a molecule and one of the following point symmetry groups C n, D n, T, O, I (Chirality). Most often we are talking about an asymmetric carbon atom, that is, an atom associated with four different substituents, for example:

Other atoms can also be asymmetric, such as silicon, nitrogen, phosphorus, and sulfur atoms. The presence of an asymmetric atom is not the only reason for enantiomers. So, derivatives of adamantane (IX), ferrocene (X), 1,3-diphenylallene (XI), 6,6"-dinitro-2,2"-diphenic acid (XII) have optical antipodes. The reason for the optical activity of the latter compound is atropisomerism, that is, spatial isomerism caused by the lack of rotation around a single bond. Enantiomerism also appears in the helical conformations of proteins, nucleic acids, hexahelycene(XIII).

(R)-, (S)- nomenclature of optical isomers (naming rule)

The four groups attached to the asymmetric carbon atom C abcd are assigned different seniority corresponding to the sequence: a>b>c>d. In the simplest case, seniority is established by the serial number of the atom attached to the asymmetric carbon atom: Br(35), Cl(17), S(16), O(8), N(7), C(6), H(1) .

For example, in bromochloroacetic acid:

The seniority of the substituents at the asymmetric carbon atom is as follows: Br(a), Cl(b), C of the COOH group (c), H(d).

In butanol-2, oxygen is the senior substituent (a), hydrogen is the junior substituent (d):

It is required to resolve the issue of substituents CH 3 and CH 2 CH 3 . In this case, the seniority is determined by the serial number or numbers of other atoms in the group. The primacy remains with the ethyl group, since in it the first C atom is bonded to another C(6) atom and to other H(1) atoms, while in the methyl group carbon is bonded to three H atoms with the atomic number 1. In more complex cases continue to compare all the atoms until they reach atoms with different serial numbers. If there are double or triple bonds, then the atoms attached to them are considered to be two and three atoms, respectively. Thus, the -COH group is considered as C (O, O, H), and the -COOH group is considered as C (O, O, OH); the carboxyl group is older than the aldehyde group, since it contains three atoms with a serial number of 8.

In D-glyceraldehyde, the OH(a) group is the highest, followed by CHO(b), CH 2 OH(c) and H(d):

The next step is to determine whether the arrangement of the groups is right, R (lat. rectus), or left, S (lat. sinister). Moving on to the corresponding model, it is oriented so that the minor group (d) in the perspective formula is at the bottom, and then viewed from above along the axis passing through the shaded face of the tetrahedron and group (d). In the D-glyceraldehyde group

located in the direction of right rotation, and therefore, it has an R-configuration:

(R)-glyceraldehyde

In contrast to the D,L nomenclature, the designations for (R)- and (S)-isomers are enclosed in brackets.

diastereomerism

σ-diastereomerism

Any combination of spatial isomers that do not form a pair of optical antipodes is considered diastereomeric. There are σ and π-diastereomers. σ-diasteriomers differ from each other in the configuration of some of the chirality elements they contain. So, diasteriomers are (+)-tartaric acid and meso-tartaric acid, D-glucose and D-mannose, for example:

For some types of diastereomerism, special designations have been introduced, for example, threo- and erythro-isomers are a diastereomerism with two asymmetric carbon atoms and spaces, the arrangement of substituents at these atoms, reminiscent of the corresponding threose (related substituents are on opposite sides in Fisher's projection formulas) and erythrose ( deputies - on one side):

Erythro isomers whose asymmetric atoms are bonded to the same substituents are called meso forms. They, unlike the other σ-diastereomers, are optically inactive due to the intramolecular compensation of the contributions to the rotation of the light polarization plane of two identical asymmetric centers of the opposite configuration. Pairs of diastereomers that differ in the configuration of one of several asymmetric atoms are called epimers, for example:

The term "anomers" refers to a pair of diastereomeric monosaccharides differing in the configuration of the glycosidic atom in the cyclic form, for example, α-D- and β-D-glucose are anomeric.

π-diastereomerism (geometric isomerism)

π-diasteriomers, also called geometric isomers, differ from each other in the different spatial arrangement of substituents relative to the plane of the double bond (most often C=C and C=N) or the ring. These include, for example, maleic and fumaric acids (formulas XIV and XV, respectively), (E)- and (Z)-benzaldoximes (XVI and XVII), cis- and trans-1,2-dimethylcyclopentanes (XVIII and XIX).

conformers. Tautomers

The phenomenon is inextricably linked with the temperature conditions of its observation. So, for example, chlorocyclohexane at room temperature exists in the form of an equilibrium mixture of two conformers - with the equatorial and axial orientations of the chlorine atom:

However, at minus 150 °C, an individual a-form can be isolated, which behaves under these conditions as a stable isomer.

On the other hand, compounds that are isomers under normal conditions may turn out to be tautomers in equilibrium with increasing temperature. For example, 1-bromopropane and 2-bromopropane are structural isomers, however, as the temperature rises to 250 °C, an equilibrium is established between them, which is characteristic of tautomers.

Isomers that transform into each other at temperatures below room temperature can be considered as non-rigid molecules.

The existence of conformers is sometimes referred to as "rotational isomerism". Among dienes, s-cis- and s-trans isomers are distinguished, which, in essence, are conformers resulting from rotation around a simple (s-single) bond:

Isomerism is also characteristic of coordination compounds. So, compounds that differ in the way of coordination of ligands (ionization isomerism) are isomeric, for example, are isomeric:

SO 4 - and + Br -

Here, in essence, there is an analogy with the structural isomerism of organic compounds.

Chemical transformations, as a result of which structural isomers are converted into each other, is called isomerization. Such processes are important in industry. So, for example, isomerization of normal alkanes into isoalkanes is carried out to increase the octane number of motor fuels; pentane isomerized to isopentane for subsequent dehydrogenation to isoprene. Intramolecular rearrangements are also isomerizations, of which, for example, the conversion of cyclohexanone oxime to caprolactam, a raw material for the production of capron, is of great importance.

Isomers, isomerism

Isomers- these are substances that have the same qualitative and quantitative composition, but a different structure and, therefore, different properties

The phenomenon of the existence of isomers is called isomerism

For example, a substance of composition C 4 H 10 has two isomeric compounds.

The physical properties of butane and isobutane are different: isobutane has lower melting and boiling points than n.butane.

Ball-and-stick model of the butane molecule
Ball-and-stick model of the isobutane molecule

The chemical properties of these isomers differ slightly, because they have the same qualitative composition and the nature of the bond between the atoms in the molecule.

In another way, the definition of isomers can be given as follows:

Isomers - substances that have the same molecular formula but different structural formulas.

Types of isomerism

Depending on the nature of the differences in the structure of isomers, there are structural and spatial isomerism.

Structural isomers- compounds of the same qualitative and quantitative composition, differing in the order of binding atoms, i.e. chemical structure.

Structural isomerism

Spatial isomers (stereoisomers) with the same composition and the same chemical structure, they differ in the spatial arrangement of atoms in the molecule

Optical isomers are also called mirror or chiral (like left and right hand)

Introduction

Isomerism ( Greek isos - the same, meros - part) - one of the most important concepts in chemistry, mainly in organic. Substances can have the same composition and molecular weight, but different structures and compounds that contain the same elements in the same amount, but differ in the spatial arrangement of atoms or groups of atoms, are called isomers. Isomerism is one of the reasons why organic compounds are so numerous and varied.

The history of the discovery of isomerism

Isomerism was first discovered by J. Liebig in 1823, who found that the silver salts of fulminant and isocyanic acids: Ag-O-N=C and Ag-N=C=O have the same composition, but different properties. The term "isomerism" was introduced in 1830 by I. Berzelius, who suggested that differences in the properties of compounds of the same composition arise due to the fact that the atoms in the molecule are arranged in an unequal order. Ideas about isomerism were finally formed after the creation of the theory of chemical structure by A. M. Butlerov (1860s). The true explanation of isomerism was received only in the 2nd half of the 19th century. based on the theory of chemical structure of A.M. Butlerov (structural isomerism) and the stereochemical doctrine of Ya.G. Van't Hoff (spatial isomerism). Based on the provisions of this theory, he suggested that there must be four different butanols (Fig. 1). By the time the theory was created, only one butanol (CH 3) 2 CHCH 2 OH, obtained from plant materials, was known

Fig.1. Different positions of the OH group - in the butanol molecule.

The subsequent synthesis of all isomers of butanol and the determination of their properties became a convincing confirmation of the theory.

According to the modern definition, two compounds of the same composition are considered isomers if their molecules cannot be combined in space so that they completely coincide. The combination, as a rule, is done mentally; in complex cases, spatial models or calculation methods are used.

Types of isomerism

In isomerism, two main types can be distinguished: structural isomerism and spatial isomerism, or, as it is also called, stereoisomerism.

In turn, the structural is divided into:

isomerism of the carbon chain (carbon skeleton)

valence isomerism

functional group isomerism

position isomerism.

Spatial isomerism (stereoisomerism) is divided into:

diastereomerism (cis, trans - isomerism)

enantiomerism (optical isomerism).

Structural isomerism

It is caused, as a rule, by differences in the structure of the hydrocarbon skeleton or by an unequal arrangement of functional groups or multiple bonds.

Isomerism of the hydrocarbon skeleton

Saturated hydrocarbons containing from one to three carbon atoms (methane, ethane, propane) do not have isomers. For a compound with four carbon atoms C 4 H 10 (butane), two isomers are possible, for pentane C 5 H 12 - three isomers, for hexane C 6 H 14 - five (Fig. 2):

Fig.2.

With an increase in the number of carbon atoms in a hydrocarbon molecule, the number of possible isomers increases dramatically. For C 7 H 16 heptane, there are nine isomers, for C 14 H 30 hydrocarbon - 1885 isomers, for C 20 H 42 hydrocarbon - over 366,000. In complex cases, the question of whether two compounds are isomers is solved using various rotations around valence bonds (simple bonds allow this, which to a certain extent corresponds to their physical properties). After the individual fragments of the molecule are moved (without breaking bonds), one molecule is superimposed on another. If two molecules are exactly the same, then these are not isomers, but the same compound. Isomers that differ in skeletal structure usually have different physical properties (melting point, boiling point, etc.), which makes it possible to separate one from the other. This type of isomerism also exists in aromatic hydrocarbons (Fig. 4).