spatial

carbon skeleton

Configuration

Conformational

Functional position

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 through 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 C 4 H 8 O corresponds to structural isomers:

A)with different carbon skeleton

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

branched C-chain -

(2-methylpropanal, aldehyde) or

cycle - (cyclobutanol, cyclic alcohol);

b)with different position of the functional group

butanone-2, ketone;

V)with different composition of the functional group

3-butenol-2, unsaturated alcohol;

G)metamerism

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

d)tautomerism (keto-enol)

enol form keto form

Tautomers are in dynamic equilibrium, with the more stable form, the keto form, predominating 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 and differ in the spatial arrangement of atoms in the molecule. This difference creates a difference in physical and chemical properties. Spatial isomers are depicted in the form of 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 many geometric shapes (conformations) of the molecule, differing from each other in the mutual rotation of carbon tetrahedra around the σ-C-C bond connecting them. As a result of this rotation, rotary 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 ordinary conditions, as a rule, it is impossible to fix molecules in one strictly defined conformation. Typically, several conformational isomers easily transform into each other coexist in equilibrium.

The methods of depiction and the nomenclature of isomers can be considered using the example of the ethane molecule. For it, we can foresee the existence of two conformations that differ maximally in energy, which can be depicted in the form perspective projections(1) (“sawmill goats”) or projections Newman(2):

inhibited conformation eclipsed conformation

In perspective projection (1) the C-C connection must be imagined going into the distance; The carbon atom on the left is close to the observer, and the carbon atom on the right is farther away from him.

In the Newman projection (2), the molecule is viewed along the C-C bond. Three lines diverging at an angle of 120° from the center of the circle indicate the bonds of the carbon atom closest to the observer; the lines “poking out” from behind the circle are the bonds of the distant carbon atom.

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

The minimum energy required to completely rotate a molecule around a particular bond is called the rotation barrier for that bond. The rotation 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 rotation 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 inhibited conformations, and the maxima correspond to occluded conformations. Since at room temperature the energy of some molecular collisions 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 free. In a mixture of all possible conformations, inhibited 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 rotating around the central C 2 - C 3 bond and differing in the mutual arrangement of CH 3 groups. The different eclipsed and inhibited conformations of butane differ in energy. Inhibited 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 molecule of alicyclic compounds is characterized by different conformational forms of the cycle (for example, for cyclohexane armchair, bath, twist-forms).

So, conformations are different 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 to such transformations becomes high enough, then stereoisomeric forms can be separated (for example, optically active biphenyls). In such cases, we no longer talk about conformers, but about actually existing stereoisomers.

b)Geometric isomerism

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

1. rotation of carbon atoms relative to each other is 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 ring.

Geometric isomers, unlike conformers, can be isolated in pure form and exist as individual, stable substances. For their mutual transformation, 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 identical substituents lie on the same side of the plane of the π bond or ring, trance- forms are geometric isomers in which identical 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

melting temperature

138.9 0 C - 105.6 0 C

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 it is used Z,E-nomenclature (Kanna, Ingold, Prelog nomenclature - KIP, nomenclature of seniority of deputies). In connection

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

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

To indicate the isomer configuration, indicate the arrangement of senior substituents at a double bond (or ring) is Z- (from the German Zusammen - together) or E- (from the German Entgegen - opposite).

In the Z,E system, substituents with a large atomic number are considered senior. If the atoms directly bonded to the unsaturated carbon atoms are the same, then move on to the “second layer”, if necessary - to the “third layer”, etc.

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

Geometric isomers are widespread 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, in the composition of fats - cis-oleic, linoleic, linolenic acids.

V)Optical isomerism

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

Chiral substances are capable of rotating the plane of polarization of light. This phenomenon is called optical activity, and the corresponding substances are 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 thing - the sign of rotation of the plane of polarization: one of the optical antipodes deflects the plane of polarization 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 a molecule). This is the name given to the carbon atom in sp 3 - hybridization and associated with four different substituents. Two tetrahedral arrangements of substituents around an asymmetric atom are possible. In this case, 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 .

Optical isomers are depicted in the form of projection formulas by E. Fischer. They are obtained by projecting a molecule with an asymmetric carbon atom. In this case, the asymmetric carbon atom itself on the plane is designated by a dot, and on the horizontal line are indicated by the symbols of the substituents protruding in front of the plane of the drawing. The vertical line (dashed or solid) indicates substituents that are removed beyond the plane of the drawing. Below 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 of (+) and (-) alanine - CH 3 - * CH(NH 2)-COOH are presented as follows:

A mixture with the same content of enantiomers is called a racemate. The racemate does not have optical activity and is characterized by physical properties different from the enantiomers.

Rules for transforming projection formulas.

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

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

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

4. A 90° rotation in the drawing plane 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 drawing plane.

Optical activity is possessed by organic compounds in whose molecules other atoms, such as silicon, phosphorus, nitrogen, and sulfur, are chiral centers.

Compounds with several asymmetric carbon atoms exist in the form 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 Fischer 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 the 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 an optical isomer, it is necessary to arrange the substituents in the tetrahedron (stereochemical formula) around the asymmetric carbon atom in such a way that the youngest substituent (usually hydrogen) has the direction “away from the observer.” If the transition of the three remaining substituents from senior to middle and junior in seniority occurs clockwise, this is an R-isomer (the decrease in seniority coincides with the movement of the hand when writing the upper part of the letter R). If the transition occurs counterclockwise, it is S - isomer (declining precedence coincides with the movement of the hand when writing the top of the letter S).

To determine the R- or S-configuration of an optical isomer using 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 decrease in seniority of the remaining three substituents clockwise corresponds to the R-configuration, and 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, almond), terpene hydrocarbons, terpene alcohols and ketones, steroids, alkaloids, etc.

b) splitting of racemates;

c) asymmetric synthesis;

d) biochemical production of optically active substances.

DO YOU KNOW THAT

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

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 concept 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 this, which 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).

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 of organic compounds and their structural features

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. Since complete chaos reigned in the field of structure of both organic and inorganic substances at that time, they did not attach much importance to the discovery.

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 of 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