How are isomers different? Theory of the structure of organic compounds: homology and isomerism
Lecture No. 5
Topic: “Isomerism and its types”
Class type: combined
Purpose: 1. To reveal the main position of the theory of structure on the phenomenon of isomerism. Give a general idea of the types of isomerism. Show the main directions of development of the theory of structure using stereoisomerism as an example.
2. continue to develop the ability to construct formulas of isomers, name substances using formulas.
3. cultivate a cognitive attitude towards learning
Equipment: models of Stewart-Brigleb molecules, colored plasticine, matches, a pair of gloves, caraway seeds, mint chewing gum, three test tubes.
Lesson plan
Greeting, roll call
Core knowledge survey
Learning new material:
Theory of structure and the phenomenon of isomerism;
Types of isomerism;
Consolidation
Progress of the lesson
2. Survey of basic knowledge: frontal
Explain by what criteria organic compounds are classified using a diagram.
Name the main classes organic compounds, feature of their structure
Complete exercises No. 1 and 2 §6. One student at the blackboard, the rest in notebooks
3. Learning new material: Theory of structure and the phenomenon of isomerism
Remember the definition of isomerism and isomers. Explain the reason for their existence.
The phenomenon of isomerism (from the Greek isos - different and meros - share, part) was discovered in 1823 by J. Liebig and F. Wöhler using the example of salts of two inorganic acids: cyanic and explosive. NOSE = N cyan; N-O-N = C rattlesnake
In 1830, J. Dumas extended the concept of isomerism to organic compounds. The term “Isomer” appeared a year later, and was suggested by J. Bercellius. Because in the field of structure, both organic and inorganic substances At that time, complete chaos reigned, and the discovery was not given much importance.
A scientific explanation for the phenomenon of isomerism was given by A.M. Butlerov within the framework of the theory of structure, while neither the theory of types nor the theory of radicals revealed the essence of this phenomenon. A.M. Butlerov saw the reason for isomerism in the fact that the atoms in the molecules of isomers are connected in different orders. The theory of structure made it possible to predict the number of possible isomers and their structure, which was brilliantly confirmed in practice by A.M. Butlerov himself and his followers.
Types of isomerism: give an example of isomers and suggest a sign by which isomers could be classified?(obviously, the structure of the isomer molecules will be the basis). I explain the material using the diagram:
There are two types of isomerism: structural and spatial (stereoisomerism). Structural isomers are those that have different bonding orders between the atoms in the molecule. Spatial isomers have the same substituents on each carbon atom, but differ in their relative location in space.
Structural isomerism is of three types: interclass isomerism associated with the structure of the carbon skeleton, and isomerism of the position of a functional group or multiple bond.
Interclass isomers contain different functional groups and belong to different classes organic compounds, and therefore the physical and chemical properties of interclass isomers differ significantly.
The isomerism of the carbon skeleton is already familiar to you; the physical properties are different, but the chemical properties are similar, because these substances belong to the same class.
Isomerism of the position of a functional group or the position of multiple bonds. The physical properties of such isomers are different, but the chemical properties are similar
Geometric isomerism: have different physical constants but similar chemical properties
Optical isomers are mirror images of each other; like two palms, they cannot be brought together so that they coincide.
4. Consolidation: recognize isomers, determine the type of isomerism in substances whose formula: perform exercise 3§ 7
Lectures for students of the Faculty of Pediatrics
Lecture2
Topic: Spatial structure of organic compounds
Target: acquaintance with the types of structural and spatial isomerism of organic compounds.
Plan:
Classification of isomerism.
Structural isomerism.
Spatial isomerism
Optical isomerism
The first attempts to understand the structure of organic molecules date back to the beginning of the 19th century. The phenomenon of isomerism was first discovered by J. Berzelius, and A. M. Butlerov in 1861 proposed a theory of the chemical structure of organic compounds, which explained the phenomenon of isomerism.
Isomerism is the existence of compounds with the same qualitative and quantitative composition, but different structure or location in space, and the substances themselves are called isomers.
Classification of isomers
Structural
(different order of connection of atoms)
Stereoisomerism
(different arrangement of atoms in space)
Multiple connection provisions
Functional Group Provisions
Configuration
Conforma-
Structural isomerism.
Structural isomers are isomers that have the same qualitative and quantitative composition, but differ in chemical structure.
Structural isomerism determines the diversity of organic compounds, in particular alkanes. With an increase in the number of carbon atoms in molecules alkanes, the number of structural isomers rapidly increases. So, for hexane (C 6 H 14) it is 5, for nonane (C 9 H 20) - 35.
Carbon atoms vary in location in the chain. The carbon atom at the beginning of the chain is bonded to one carbon atom and is called primary. A carbon atom bonded to two carbon atoms – secondary, with three – tertiary, with four – quaternary. Straight-chain alkanes contain only primary and secondary carbon atoms, while branched-chain alkanes contain both tertiary and quaternary carbon atoms.
Types of structural isomerism.
Metamers– compounds belonging to the same class of compounds, but having different radicals:
H 3 C – O – C 3 H 7 – methylpropyl ether,
H 5 C 2 – O – C 2 H 5 – diethyl ether
Interclass isomerism. Despite the same qualitative and quantitative composition of molecules, the structure of substances is different.
For example: aldehydes are isomeric to ketones:
Alkynes – alkadienes
H 2 C = CH – CH = CH 2 butadiene -1.3 HC = C - CH 2 – CH 3 – butine-1
Structural isomerism also determines the diversity of hydrocarbon radicals. The isomerism of radicals begins with propane, for which two radicals are possible. If a hydrogen atom is subtracted from the primary carbon atom, the radical propyl (n-propyl) is obtained. If a hydrogen atom is subtracted from a secondary carbon atom, the radical isopropyl is obtained.
-
isopropyl
CH 2 – CH 2 – CH 3 - cutSpatial isomerism (stereoisomerism)
This is the existence of isomers that have the same composition and order of connection of atoms, but differ in the nature of the arrangement of atoms or groups of atoms in space relative to each other.
This type of isomerism was described by L. Pasteur (1848), J. Van't Hoff, Le Bel (1874).
In real conditions, the molecule itself and its individual parts (atoms, groups of atoms) are in a state of vibrational-rotational motion, and this movement greatly changes the relative arrangement of atoms in the molecule. At this time, chemical bonds are stretched and bond angles change, and thus different configurations and conformations of molecules arise.
Therefore, spatial isomers are divided into two types: conformational and configurational.
Configurations are the order in which atoms are arranged in space without taking into account the differences that result from rotation around single bonds. These isomers exist in different conformations.
Conformations are very unstable dynamic forms of the same molecule that arise as a result of the rotation of atoms or groups of atoms around single bonds, as a result of which the atoms occupy different spatial positions. Each conformation of a molecule is characterized by a specific configuration.
The b-bond allows rotation around it, so one molecule can have many conformations. Of the many conformations, only six are taken into account, because The minimum angle of rotation is considered to be an angle equal to 60°, which is called torsion angle.
There are: eclipsed and inhibited conformations.
Eclipsed conformation occurs when identical substituents are located at a minimum distance from each other and mutual repulsion forces arise between them, and the molecule must have a large supply of energy to maintain this conformation. This conformation is energetically unfavorable.
Inhibited conformation – occurs when identical substituents are as far apart as possible from each other and the molecule has a minimum energy reserve. This conformation is energetically favorable.
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The first compound for which the existence of conformational isomers is known is ethane. Its structure in space is depicted by the perspective formula or Newman's formula:
WITH 2 N 6
obscured inhibited
conformation conformation
Newman's projection formulas.
The carbon atom closest to us is designated by a dot in the center of the circle, the circle represents the distant carbon atom. The three bonds of each atom are depicted as lines diverging from the center of the circle - for the nearest carbon atom and small ones - for the distant carbon atom.
In long carbon chains, rotation is possible around several C–C bonds. Therefore, the entire chain can take on a variety of geometric shapes. According to X-ray diffraction data, long chains of saturated hydrocarbons have a zigzag and claw-shaped conformation. For example: palmitic (C 15 H 31 COOH) and stearic (C 17 H 35 COOH) acids in zigzag conformations are part of the lipids of cell membranes, and monosaccharide molecules in solution take on a claw-shaped conformation.
Conformations of cyclic compounds
Cyclic connections are characterized by angular stress associated with the presence of a closed cycle.
If we consider the cycles to be flat, then for many of them the bond angles will deviate significantly from normal. The stress caused by the deviation of bond angles between carbon atoms in the ring from the normal value is called corner or Bayer's
For example, in cyclohexane the carbon atoms are in the sp 3 hybrid state and, accordingly, the bond angle should be equal to 109 o 28 /. If the carbon atoms lay in the same plane, then in the planar ring the internal bond angles would be equal to 120°, and all the hydrogen atoms would be in an eclipsed conformation. But cyclohexane cannot be flat due to the presence of strong angular and torsional stresses. It develops less stressed non-planar conformations due to partial rotation around ϭ-bonds, among which the conformations are more stable armchairs And baths.
The chair conformation is the most energetically favorable, since it does not have occluded positions of hydrogen and carbon atoms. The arrangement of the H atoms of all C atoms is the same as in the inhibited conformation of ethane. In this conformation, all hydrogen atoms are open and available for reactions.
The bath conformation is less energetically favorable, since 2 pairs of C atoms (C-2 and C-3), (C-5 and C-6) lying at the base have H atoms in an eclipsed conformation, therefore this conformation has large reserve of energy and unstable.
C 6 H 12 cyclohexane
The “chair” shape is more energetically beneficial than the “bathtub”.
Optical isomerism.
At the end of the 19th century, it was discovered that many organic compounds are capable of rotating the plane of a polarized beam left and right. That is, a light beam incident on a molecule interacts with its electron shells, and polarization of the electrons occurs, which leads to a change in the direction of vibrations in electric field. If a substance rotates the plane of vibration clockwise, it is called dextrorotatory(+) if counterclockwise – left-handed(-). These substances were called optical isomers. Optically active isomers contain an asymmetric carbon atom (chiral) - this is an atom containing four different substituents. The second important condition is the absence of all types of symmetry (axis, plane). These include many hydroxy and amino acids
Studies have shown that such compounds differ in the order of arrangement of substituents on carbon atoms in sp 3 hybridization.
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the simplest compound is lactic acid (2-hydroxypropanoic acid)
Stereoisomers, the molecules of which are related to each other as an object and an incompatible mirror image or as a left and right hand are called enantiomers(optical isomers, mirror isomers, antipodes, and the phenomenon is called enantiomerism. All chemical and physical properties of enantiomers are the same, except for two: rotation of the plane of polarized light (in a polarimeter device) and biological activity.
The absolute configuration of molecules is determined by complex physicochemical methods.
The relative configuration of optically active compounds is determined by comparison with a glyceraldehyde standard. Optically active substances having the configuration of dextrorotatory or levorotatory glyceraldehyde (M. Rozanov, 1906) are called substances of the D- and L-series. An equal mixture of dextro- and levorotary isomers of one compound is called a racemate and is optically inactive.
Research has shown that the sign of the rotation of light cannot be associated with the belonging of a substance to the D- and L-series; it is determined only experimentally in instruments - polarimeters. For example, L-lactic acid has a rotation angle of +3.8 o, D-lactic acid - 3.8 o.
Enantiomers are depicted using Fischer's formulas.
The carbon chain is represented by a vertical line.
The senior functional group is placed at the top, the junior functional group at the bottom.
An asymmetric carbon atom is represented by a horizontal line, at the ends of which there are substituents.
The number of isomers is determined by the formula 2 n, n is the number of asymmetric carbon atoms.
L-row D-row
Among enantiomers there may be symmetrical molecules that do not have optical activity, and are called mesoisomers.
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For example: Wine house |
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D – (+) – row L – (–) – row |
Mezovinnaya k-ta |
Racemate – grape juice
Optical isomers that are not mirror isomers, differing in the configuration of several, but not all asymmetric C atoms, having different physical and chemical properties, are called - di-A-stereoisomers.
-Diastereomers (geometric isomers) are stereomers that have a bond in the molecule. They are found in alkenes, unsaturated higher carbon kits, unsaturated dicarbonate compounds. For example:
Cis-butene-2 Trans-butene-2
The biological activity of organic substances is related to their structure. For example:
Cis-butenediic acid, Trans-butenediic acid,
maleic acid - fumaric acid - non-toxic,
very toxic found in the body
All natural unsaturated higher carbon compounds are cis-isomers.
Introduction
Isomerism ( Greek isos - identical, meros - part) is one of the most important concepts in chemistry, mainly in organic. Substances may have the same composition and molecular weight, but different structures and compounds containing the same elements in the same quantity, but differing in the spatial arrangement of atoms or groups of atoms, are called isomers. Isomerism is one of the reasons that organic compounds are so numerous and varied.
History of the discovery of isomerism
Isomerism was first discovered by J. Liebig in 1823, who established that the silver salts of fulminate 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 a different order. The idea of isomerism was finally formed after A. M. Butlerov created the theory of chemical structure (1860s). Isomerism received a true explanation only in the 2nd half of the 19th century. based on the theory of chemical structure of A.M. Butlerov (structural isomerism) and stereochemical teachings of Ya.G. Van't Hoff (spatial isomerism). Based on this theory, he proposed that there should be four different butanols (Fig. 1). By the time the theory was created, only one butanol was known (CH 3) 2 CHCH 2 OH, obtained from plant materials
Fig.1. Various positions of the OH group in the butanol molecule.
The subsequent synthesis of all butanol isomers and determination of their properties became 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. 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, structural is divided into:
isomerism of the carbon chain (carbon skeleton)
valence isomerism
functional group isomerism
positional isomerism.
Spatial isomerism (stereoisomerism) is divided into:
diastereomerism (cis, trans - isomerism)
enantiomerism (optical isomerism).
Structural isomerism
As a rule, it is caused by differences in the structure of the hydrocarbon skeleton or 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) have no 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.
As the number of carbon atoms in a hydrocarbon molecule increases, the number of possible isomers increases dramatically. For heptane C 7 H 16 there are nine isomers, for hydrocarbon C 14 H 30 - 1885 isomers, for hydrocarbon C 20 H 42 - over 366,000. In complex cases, the question of whether two compounds are isomers is solved using various turns around valence bonds (simple bonds allow that in to a certain extent corresponds to their physical properties). After moving individual fragments of the molecule (without breaking the bonds), one molecule is superimposed on another. If two molecules are completely identical, 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).
>> Chemistry: Isomerism and its types
There are two types of isomerism: structural and spatial (stereoisomerism). Structural isomers differ from each other by the order of bonds of atoms in the molecule, stereo-isomers - by the arrangement of atoms in space with the same order of bonds between them.
The following types of structural isomerism are distinguished: isomerism of the carbon skeleton, positional isomerism, isomerism of various classes of organic compounds (interclass isomerism).
Structural isomerism
Isomerism of the carbon skeleton is due to the different bond order between the carbon atoms forming the skeleton of the molecule. As has already been shown, the molecular formula C4H10 corresponds to two hydrocarbons: n-butane and isobutane. For the C5H12 hydrocarbon, three isomers are possible: pentane, iso-pentane and neopentane.
As the number of carbon atoms in a molecule increases, the number of isomers increases rapidly. For hydrocarbon C10H22 there are already 75 of them, and for hydrocarbon C20H44 - 366,319.
Positional isomerism is due to different positions of the multiple bond, substituent, and functional group with the same carbon skeleton of the molecule:
Isomerism of different classes of organic compounds (interclass isomerism) is due to different positions and combinations of atoms in the molecules of substances that have the same molecular formula, but belong to different classes. Thus, the molecular formula C6B12 corresponds to the unsaturated hydrocarbon hexene-1 and cyclic cyclohexane: 
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 positional isomers.
Spatial isomerism
Spatial isomerism is divided into two types: geometric and optical.
Geometric isomerism is characteristic of compounds containing double bonds and cyclic compounds. Since free rotation of atoms around a double bond or in a ring is impossible, the substituents can be located either on the same side of the plane of the double bond or ring (cis position) or on opposite sides (trans position). The designations cis and trans usually refer 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 a 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 the molecule α-aminopropionic acid (α-alanine) CH3CH(KH2)COOH.
As you can see, the a-alanine molecule cannot coincide with its mirror image no matter how it moves. 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 occur under the action of enzymes - biological catalysts. The molecules of these substances must fit the molecules of the compounds on which they act, like a key to a lock; therefore, the spatial structure, the relative arrangement of sections of the molecules and other spatial factors play a role in the course of these reactions great importance. Such reactions are called stereoselective.
Most natural compounds are individual enantiomers, and their biological effects (ranging from taste and smell to medicinal effect) differs sharply from the properties of their optical antipodes obtained in the laboratory. A similar difference in biological activity has great value, since it underlies most important property all living organisms - metabolism.
What types of isomerism do you know?
How does structural isomerism differ from spatial isomerism?
Which of the proposed connections are:
a) isomers;
b) homologues? 
Give all substances names.
4. Is geometric (cis-, trans) isomerism possible for: a) alkanes; b) alkenes; c) alkynes; d) cycloalkanes?
Explain, give examples. 
Another example was tartaric and grape acids, after studying which J. Berzelius introduced the term ISOMERIA and suggested that the differences arise from "the different distribution of simple atoms in a complex atom" (i.e., a molecule). Isomerism received a true explanation 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 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 order of bonding 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 through the redistribution of bonds. For example, the valence isomers of benzene (V) are bicyclohexa-2,5-diene (VI, “Dewar benzene”), prismane (VII, “Ladenburg benzene”), and 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)
Positional isomerism
A type of structural isomerism characterized by differences in the positions of identical functional groups or double bonds on 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 having the same chemical structure. This type of isomers is divided into enantiomerism(optical isomerism) and diastereomerism.
Enantiomers (optical isomers, mirror isomers) are pairs of optical antipodes of substances characterized by opposite sign and equal rotation of the plane of polarization of light with the identity of all other physical and chemical properties(except for reactions with other optically active substances and physical properties in a chiral environment). A necessary and sufficient reason for the appearance of optical antipodes is the assignment of the molecule to one of the following point groups of symmetry C n, D n, T, O, I (Chirality). More often we're talking about about an asymmetric carbon atom, that is, about an atom connected to four different substituents, for example:
Other atoms can also be asymmetric, for example, atoms of silicon, nitrogen, phosphorus, and sulfur. The presence of an asymmetric atom is not the only reason for enantiomerism. Thus, the derivatives of adamantane (IX), ferrocene (X), 1,3-diphenylallene (XI), and 6,6"-dinitro-2,2"-diphenic acid (XII) have optical antipodes. The reason for the optical activity of the latter compound is atropoisomerism, that is, spatial isomerism caused by the absence of rotation around a simple bond. Enantiomerism also appears in helical conformations of proteins, nucleic acids, and hexagelicene (XIII).
(R)-, (S)- nomenclature of optical isomers (naming rule)
The four groups attached to the asymmetric carbon atom C abcd are assigned different precedence, corresponding to the sequence: a>b>c>d. In the simplest case, precedence 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 substituents at the asymmetric carbon atom is as follows: Br(a), Cl(b), C group COOH (c), H(d).
In butanol-2, oxygen is the senior substituent (a), hydrogen is the junior substituent (d):
It is necessary to resolve the issue of the substituents CH 3 and CH 2 CH 3 . In this case, seniority is determined by the atomic number or numbers of other atoms in the group. The primacy remains with the ethyl group, since in it the first C atom is connected to another C(6) atom and to other H(1) atoms, while in the methyl group the carbon is connected to three H atoms with serial number 1. In more complex cases They continue to compare all the atoms until they reach atoms with different serial numbers. If there are double or triple bonds, then the atoms located at them are counted as 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 because it contains three atoms with atomic number 8.
In D-glyceraldehyde, the eldest group is OH(a), followed by CHO(b), CH 2 OH(c) and H(d):
The next step is to determine whether the group arrangement is right-handed, R (lat. rectus), or left-handed, S (lat. sinister). Moving on to the corresponding model, it is oriented so that junior group(d) appears at the bottom in the perspective formula, and is then viewed from above along an axis passing through the shaded face of the tetrahedron and group (d). In D-glyceraldehyde group
are located in the direction of right rotation, and therefore it has an R-configuration:
(R)-glyceraldehyde
Unlike D, L nomenclature the designations of (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 chiral elements they contain. Thus, 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 - this is a diastereomerism with two asymmetric carbon atoms and spaces, the arrangement of substituents on these atoms, reminiscent of the corresponding threose (related substituents are on opposite sides in the Fischer projection formulas) and erythrose ( substituents - on one side):
Erythro-isomers, whose asymmetric atoms are linked to identical substituents, are called meso-forms. They, unlike other σ-diastereomers, are optically inactive due to intramolecular compensation of the contributions to the rotation of the plane of polarization of light from two identical asymmetric centers of opposite configurations. 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 that differ in the configuration of the glycosidic atom in the cyclic form, for example the α-D- and β-D-glucose anomerics.
π-diastereomerism (geometric isomerism)
π-diasteriomers, also called geometric isomers, differ from each other by different spatial arrangements of substituents relative to the plane of the double bond (most often C=C and C=N) or 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 temperature conditions his observations. For example, chlorocyclohexane at room temperature exists in the form of an equilibrium mixture of two conformers - with equatorial and axial orientation 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, connections, in normal conditions being isomers, with increasing temperature they can turn out to be tautomers in equilibrium. For example, 1-bromopropane and 2-bromopropane are structural isomers, but when the temperature increases to 250 °C, an equilibrium characteristic of tautomers is established between them.
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 are essentially conformers resulting from rotation around a simple (s-single) bond:
Isomerism is also characteristic of coordination compounds. Thus, compounds that differ in the method of coordination of ligands (ionization isomerism) are isomeric, for example, the following 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 are called isomerization. Such processes are important in industry. For example, isomerization of normal alkanes into isoalkanes is carried out to increase the octane number of motor fuels; pentane isomerizes to isopentane for subsequent dehydrogenation to isoprene. Isomerization also involves intramolecular rearrangements, of which great importance is, for example, the conversion of cyclohexanone oxime into caprolactam, the raw material for the production of caprone.
The process of interconversion of enantiomers is called racemization: it leads to the disappearance of optical activity as a result of the formation of an equimolar mixture of (-)- and (+)-forms, that is, the racemate. Interconversion of diastereomers leads to the formation of a mixture in which the thermodynamically more stable form predominates. In the case of π-diastereomers, it is usually the trans form. The interconversion of conformational isomers is called conformational equilibrium.