Properties of structural isomers. What are structural isomers?

Lectures for students of the pediatric faculty

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. For the first time, the phenomenon of isomerism was 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 - the existence of compounds with the same qualitative and quantitative composition, but different structure or their 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 bond positions

Functional group positions

Configuration

Conform-

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 rapidly increases the number of structural isomers. So, for hexane (C 6 H 14) it is 5, for nonane (C 9 H 20) - 35.

Carbon atoms differ in their position 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 alkane molecules contain only primary and secondary carbon atoms, while branched-chain alkane molecules 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 - methyl propyl ether,

H 5 C 2 - O - C 2 H 5 - diethyl ether

Interclass isomerism. With the same qualitative and quantitative composition of molecules, the structure of substances is different.

For example: aldehydes are isomeric to ketones:

Alkynes - alkadienam

H 2 C \u003d CH - CH \u003d CH 2 butadiene -1.3 HC \u003d C - CH 2 - CH 3 - butin-1

Structural isomerism also determines the diversity of hydrocarbon radicals. Radical isomerism begins with propane, for which two radicals are possible. If a hydrogen atom is taken away from the primary carbon atom, then the radical propyl (n-propyl) is obtained. If the hydrogen atom is taken away from the secondary carbon atom, then the radical isopropyl is obtained

-

isopropyl

CH 2 - CH 2 - CH 3 - propyl

Spatial 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 oscillatory-rotational motion, and this motion greatly changes the mutual arrangement of atoms in the molecule. At this time, chemical bonds are stretched and bond angles change, and thus various configurations and conformations of molecules arise.

Therefore, spatial isomers are divided into two types: conformational and configurational.

Configurations are the arrangement of atoms in space without taking into account the differences that arise as a result of rotation around single bonds. These isomers exist in various 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 certain 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, since the minimum angle of rotation is considered to be an angle equal to 60 °, which is called torsion angle.

Distinguish between eclipsed and hindered conformations.

Shielded conformation arises if identical substituents are located at a minimum distance from each other and mutual repulsive forces arise between them, and the molecule must have a large energy reserve in order to maintain this conformation. This conformation is energetically unfavorable.

hindered conformation - occurs when identical substituents are as far apart as possible and the molecule has a minimum energy reserve. This conformation is energetically favorable.

P The first compound known to have conformational isomers is ethane. Its structure in space is represented by a perspective formula or Newman's formula:

With 2 H 6

obscured inhibited

conformation conformation

Newman's projection formulas.

The carbon atom closest to us is indicated by a dot in the center of the circle, the circle depicts a distant carbon atom. The three bonds of each atom are depicted as lines radiating from the center of the circle - for the nearest carbon atom and small - for the remote 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 data, long chains of saturated hydrocarbons have zigzag and pincer conformations. 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-like conformation.

Conformations of cyclic compounds

Cyclic compounds 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 the normal one. The stress caused by the deviation of bond angles between carbon atoms in a cycle from a normal value is called corner or Bayer.

For example, in cyclohexane, the carbon atoms are in the sp 3 - hybrid state and, accordingly, the bond angle should be equal to 109 about 28 /. If the carbon atoms were in the same plane, then in a planar cycle the internal bond angles would be equal to 120 o, and all hydrogen atoms would be in a eclipsed conformation. But cyclohexane cannot be planar due to the presence of strong angular and torsional stresses. It has less strained non-planar conformations due to partial rotation around ϭ-bonds, among which conformations are more stable armchairs and baths.

The chair conformation is the most energetically favorable, since it does not contain eclipsed positions of hydrogen and carbon atoms. The arrangement of H atoms for all C atoms is the same as in the hindered conformation of ethane. In this conformation, all hydrogen atoms are open and available for reactions.

The bath conformation is less energetically favorable, since in 2 pairs of C atoms (C-2 and C-3), (C-5 and C-6) lying at the base, the H atoms are in a eclipsed conformation, therefore this conformation has a large supply of energy and unstable.

C 6 H 12 cyclohexane

The shape of the "armchair" is more energetically favorable than the "bath".

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 to the left and to the right. That is, a light beam incident on a molecule interacts with its electron shells, and the electrons are polarized, which leads to a change in the direction of oscillations in the electric field. If a substance rotates the plane of oscillation clockwise, it is called dextrorotatory(+) if counterclockwise - levorotatory(-). 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 (axes, planes). These include many hydroxy and amino acids

Studies have shown that such compounds differ in the order of substituents at carbon atoms in sp 3 hybridization.

P the simplest compound is lactic acid (2-hydroxypropanoic)

Stereoisomers whose molecules relate 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 enantiomers. All chemical and physical properties of enantiomers are the same, except for two: the rotation of the plane of polarized light (in the 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 right and left isomers of one compound is called a racemate and is optically inactive.

Studies have 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 devices - polarimeters. For example, L-milk acid has an angle of rotation of +3.8 o, D-milk acid - 3.8 o.

Enantiomers are depicted using Fisher's formulas.

The carbon chain is shown as a vertical line.

The highest functional group is placed at the top, the youngest is placed below.

An asymmetric carbon atom is represented by a horizontal line with substituents at the ends.

The number of isomers is determined by the formula 2 n , n is the number of asymmetric carbon atoms.

L-row D-row

Among the enantiomers, there may be symmetrical molecules that do not have optical activity, and are called mesoisomers.

For example: Wine list

D - (+) - row L - (-) - row

Mezovinnaya to - that

Racemate - grape acid

Optical isomers that are not mirror isomers, differing in the configuration of several, but not all, asymmetric C atoms, which have 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 carboxylic to-t, unsaturated dicarboxylic to-t. For example:

Cis-butene-2 ​​Trans-butene-2

The biological activity of organic things is related to their structure. For example:

Cis-butenedioic acid, Trans-butenedioic acid,

maleic acid - fumaric acid - non-toxic,

very toxic contained in the body

All natural unsaturated higher carboxylic acids are cis-isomers.

In this article we will talk about structural isomers, their structural features and types of isomerism. We will analyze in detail the very phenomenon of isomerism, and examples of their use in life will also be provided.

The phenomenon of isomerism

Isomerism is a special phenomenon that predetermines the existence of chem. compounds, those same isomers, substances with identical compositions of atoms and molecular weights, which differ only in the atomic arrangement in space or in their structure, which leads to a change and the acquisition of different, new properties by them. Structural isomers are substances formed as a result of such a change in the position of their atoms in space, which will be discussed in more detail below.

Speaking of isomerism, it is worth remembering the existence of such a process as isomerization, which is the process of transition of one isomer to another as a result of a chemical reaction. transformations.

Types of isomerism

Valence isomerism is a type of isomer structure in which the transfer of the isomers themselves (one to another) is possible as a result of the redistribution of valence bonds.

Positional isomerism is a type of substance with an identical carbon skeleton but a different position of the functional groups. A striking example is the 2- and 4-acids of chlorobutane.

Interclass isomerism hides its difference between isomers in the nature of functional groups.

Metamerism is the distribution of the position of carbon atoms between a certain number of carbon radicals, the heteroatom of the molecule serves as a separator. This type of isomerism is typical for amines, thioalcohols, and ethers, both simple and complex.

The isomerism of the carbon skeleton is the difference in the position of carbon atoms, or rather their order. For example: phenanthrene and anthracene have the general formula C14H10, but a different type of redistribution of valency bonds.

Structural isomers

Structural isomers are substances that have a similar formula of the structure of a substance, but differ in the formula of the molecule. Structural isomers are those that are identical to each other in quantitative and qualitative compositions, but the order of atomic binding (chemical structure) has differences.

Structural isomers are classified according to the type of isometric structure, the types of which are given above in the paragraph on types of isomerism.

The structural formula of an isomer of a substance has a wide range of modifications. Some examples of isomerism are substances such as butanoic acid, 2-methylpropanoic acid, methyl propionate, dioxane, ethyl acetate, isopropyl formate have the same composition of all three types of atoms in the composition of the substance, but differ in the position of atoms in the compound itself.

Another striking example of isomerism is the existence of pentane, neopentane, and isopentane.

Names of isomers

As mentioned earlier, structural isomers are substances that have a similar formula of the structure of the substance, but differ in the formula of the molecule. Such compounds have a classification that corresponds to the features of their properties, the structure and position of atoms in the isomer molecule, differences in the number of functional groups, valence bonds, the presence of atoms of a certain element in a substance, etc. The names of the structural isomers are obtained in various ways. Let us consider this using the example of 3-methylbutanol 1 as a representative of alcohols.

In the case of alcohols, when obtaining the name of alcohols, everything begins with the choice of the carbon chain, which is dominant, numbering is carried out, the purpose of which is to assign the smallest possible number to the OH group, taking into account the order. The name itself begins to be composed of a substituent in the carbon chain, then the name of the main chain follows, and after that the suffix -ol is added, and the number indicates the carbon atom associated with the OH group.

And the Greek μέρος - share, part), a phenomenon consisting in the existence of chemical compounds that are identical in composition with the same molecular weight, but differ in structure. Such compounds are called isomers. Structural differences cause different mutual influence of atoms in molecules and predetermine different physical and chemical properties of isomers. Isomerism is extremely common in organic chemistry and is one of the main reasons for the diversity and abundance of organic compounds. In inorganic chemistry, isomerism occurs mainly for complex compounds.

The term "isomerism" was introduced by J. Berzelius in 1830, completing the controversy between J. Liebig and F. Wöhler on the existence of two substances that differ sharply in properties and have the same AgCNO composition - silver cyanate and fulminate, and based on the results of research tartaric and tartaric acids. The essence of isomerism was later explained on the basis of the theory of chemical structure.

There are two main types of isomerism: structural and spatial (stereoisomerism). Structural isomers differ in the order of bonds of atoms in a molecule, that is, in their chemical structure. Stereoisomers (spatial isomers) with the same order of bonds of atoms in a molecule differ in the mutual arrangement of atoms in space.

Structural isomerism is subdivided into carbon skeleton isomerism (skeletal isomerism), position isomerism (positional isomerism), metamerism and other types. The isomerism of the carbon skeleton is due to the different order of bonds of the carbon atoms that form the skeleton of the molecule. To specify the structural features of isomers, skeletal isomerism is subdivided into carbon chain isomerism, ring isomerism, and side chain isomerism. For example, carbon chain isomerism is characteristic of alkanes starting from the fourth member of the C 4 H 10 homologous series, which has two structural isomers: n-butane CH 3 -CH 2 -CH 2 -CH 3 and isobutane (2-methylpropane) CH 3 -CH (CH 3)-CH 3. The fifth member of the C 5 H 12 alkane series has three isomers: CH 3 -CH 2 -CH 2 -CH 2 -CH 3 - n-pentane, CH 3 -CH (CH 3) -CH 2 -CH 3 - isopentane (2- methylbutane) and neopentane (2,2-dimethylpropane) CH 3 -C (CH 3) 2 -CH 3. As the chain lengthens, the number of possible isomers increases rapidly. So, for alkanes of the composition C 10 H 22, 75 structural isomers are possible, for C 13 H 28 - 802 isomers, for C 20 H 42 - more than 366 thousand isomers. Alicyclic compounds are characterized by ring isomerism and side chain isomerism. For example, among the skeletal isomers (formulas I-IV), methylcyclopentane (I), cyclohexane (II) and propylcyclopropane (III) are cyclic isomers, while propylcyclopropane (III) and isopropylcyclopropane (IV) are side chain isomers. Differences in the properties of skeletal isomers are manifested in the difference in their boiling points (isomers with a normal carbon chain boil at a higher temperature than isomers with a branched chain), density, and other n-Alkanes, for example, in contrast to branched isomers, they have lower detonation resistance ( see article Octane number), form complexes with urea (clathrates).

Position isomerism is due to the different positions of functional groups, substituents, or multiple bonds. For example, position isomers are 1-propanol CH 3 -CH 2 -CH 2 OH and 2-propanol CH 3 -CH (OH) -CH 3, 1-butene CH 2 \u003d CH-CH 2 -CH 3 and 2-butene CH 3 -CH=CH-CH 3 . Changing the position of the functional group may lead to a change in the class of the compound. For example, the position isomers acetone CH 3 -C(O)-CH 3 and propanal CH 3 -CH 2 -CHO refer to ketones and aldehydes, respectively. Structural isomers with different functional groups differ greatly in chemical properties.

Metamerism is due to the different positions of the heteroatom (O, N, S) in the chain. For example, metamers are methyl propyl ether CH 3 O-CH 2 -CH 2 -CH 3 and diethyl ether CH 3 -CH 2 -O-CH 2 -CH 3, diethylamine CH 3 -CH 2 -NH-CH 2 -CH 3 and CH 3 -NH-CH 2 -CH 2 -CH 3 - methylpropylamine.

Often, differences in isomers are determined by several structural features. For example, methylisopropyl ketone (3-methyl-2-butanone) CH 3 -C (O) -CH (CH 3) 2 and valeric aldehyde (pentanal) CH 3 -CH 2 -CH 2 -CH 2 -CHO differ from each other as the structure of the carbon skeleton, and the position of the functional group.

A special type of structural isomerism is tautomerism (equilibrium dynamic isomerism). In this case, isomers that differ in functional groups easily pass into each other until an equilibrium is reached, at which the substance simultaneously contains tautomer molecules in a certain ratio.

Spatial isomerism is subdivided into geometric (cis, trans and syn, anti-isomerism, or E, Z-isomerism) and optical (enantiomerism). Geometric isomerism is characteristic of compounds containing double bonds or non-aromatic rings, which are structurally rigid fragments of molecules. For cis-isomers, two substituents are located on the same side of the plane of the double bond or cycle, for trans-isomers - on opposite sides. For example, geometric isomers are cis-2-butene (formula V) and trans-2-butene (VI), cis-1,2-dichlorocyclopropane (VII) and trans-1,2-dichlorocyclopropane (VIII).

Characteristic differences between the cis-trans isomers are the lower melting point of the cis-isomers, significantly better solubility in water, and a pronounced dipole moment. Trans isomers are usually more stable. See, for example, the article Maleic and fumaric acids.

The geometric isomerism observed for compounds with double bonds C=N (oximes) and N=N (azo-, azoxy compounds) is often called syn, anti-isomerism. For example, geometric isomers are anti-benzaldoxime (formula IX) and syn-benzaldoxime (X); syn-azobenzene (XI) and anti-azobenzene (XII).

In the general case, the Ε,Z-nomenclature is used. For Z-isomers, senior substituents (having a higher atomic number) are located on one side of the double bond or cycle, for E-isomers - on opposite sides. For example, geometric isomers are (Z)-1-bromo1-iodo-2-chloroethylene (formula XIII) and (E)-1-bromo-1-iodine-2-chloroethylene (XIV).

Optical isomerism is characteristic of compounds whose molecules have elements of chirality, such as an asymmetric (chiral) carbon atom bonded to four different substituents. It was first discovered by L. Pasteur in 1848 using the example of tartaric acids and explained by J. H. van't Hoff and J. A. Le Bel in 1874 based on the concept of the tetrahedral configuration of carbon atoms in saturated compounds. Molecules containing an asymmetric carbon atom can be represented as two optical isomers that cannot be combined in space (i.e., they relate to each other like an object to its mirror image). Such mirror isomers, which differ only in the opposite arrangement of the same substituents at the chiral center, are called enantiomers (from the Greek έναντίος - opposite and μέρος - part). For example, lactic acid enantiomers (XV and XVI) can be represented in 3D or as Fisher formulas (see Chemical Nomenclature).

Enantiomers have different biological activities; they are also characterized by optical activity - the ability to act on plane-polarized light (rotate the plane of polarization). Enantiomers rotate the plane of polarization by the same angle but in the opposite direction, which is why they are called optical antipodes.

For a long time, the configuration of enantiomers was determined relative to the configuration of a known standard, which was the enantiomers of glyceraldehyde (D, L-steric series). More universal is the R, S-nomenclature (proposed by R. Kahn, K. Ingold and V. Prelog), which establishes the absolute configuration of spatial isomers. In accordance with the rules of R, S nomenclature, lactic acid enantiomers (XV, XVI) are respectively (R)-lactic and (S)-lactic acids. There are no rules for translating the D, L-nomenclature into the R, S-system, since these nomenclatures use different principles. No connection has been established between the absolute configuration and optical rotation parameters.

For compounds having n chiral centers in a molecule, the number of possible stereoisomers is 2 ". However, at n ≥ 2, there are stereoisomers that differ from each other in part of the chirality elements they contain. Such stereoisomers that are not enantiomers are called diastereomers (from the Greek δια ... - through, between, stereo... and μέρος - part). and XX are enantiomers, the remaining pairs (XVII and XIX, XVII and XX, XVIII and XIX, XVIII and XX) are diastereomers.

With the appearance of additional symmetry elements (plane, axis, or center of symmetry), the total number of stereoisomers, as well as the number of optically active forms, may decrease. For example, tartaric acids have three stereoisomers, of which two are optically active: D-tartaric acid, or (2R,3R)-tartaric acid (formula XXI), and L-tartaric acid, or (2S,3S)-tartaric acid (XXII ), which are enantiomers. Their diastereomer - mesotartaric acid, or (2R,3S)-tartaric acid (formula XXIII, or identical configuration XXIV), due to the presence of a symmetry plane (indicated by a dotted line) is optically inactive - is the so-called intramolecular racemate.

The process of interconversion of enantiomers is called racemization. A mixture of equal amounts of optical antipodes - a racemic mixture, or racemate, does not have optical activity. Stereoisomerism is given great attention in the study of natural compounds and the synthesis of biologically active substances. Substances of natural origin containing elements of chirality have a certain stereoconfiguration, as well as optical activity. When a chiral center is formed under the conditions of chemical synthesis (with the exception of asymmetric synthesis), a racemate is formed; isolation of enantiomers requires the use of complex methods for separating the racemate into optically active components.

As a result of the internal rotation of molecules, conformational isomers, or conformers, arise that differ in the degree of rotation of molecular fragments about one or more simple bonds. In some cases, individual conformers, sometimes also called rotational isomers, can be isolated. Conformational analysis is used to study the formation, differences in properties and reactivity of conformers.

Isomers can be converted into each other by isomerization reactions.

Lit .: Potapov V. M. Stereochemistry. 2nd ed. M., 1988; Traven VF Organic chemistry. M., 2004. T. 1.

The content of the article

isomerism(gr. isos - the same, meros - part) is 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.

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). 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.

Rice. 1. Isomers of butanol

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.

There are several causes of 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):

Rice. 2. Isomers of the simplest hydrocarbons

With an increase in the number of carbon atoms in a hydrocarbon molecule, 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 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 bonds), one molecule is superimposed on another (Fig. 3). 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. Isomerism of this type also exists in aromatic hydrocarbons (Fig. 4):

Rice. 4. Aromatic isomers

Position isomerism.

Another type of structural isomerism - position isomerism occurs when functional groups, individual heteroatoms or multiple bonds are located in different places of the hydrocarbon skeleton. Structural isomers can belong to different classes of organic compounds, so they can differ not only in physical but also in chemical properties. On fig. 5 shows three isomers for the compound C 3 H 8 O, two of them are alcohols, and the third is an ether

Rice. 5. Position isomers

Often, differences in the structure of position isomers are so obvious that it is not even necessary to mentally combine them in space, for example, isomers of butene or dichlorobenzene (Fig. 6):

Rice. 6. Isomers of butene and dichlorobenzene

Sometimes structural isomers combine features of hydrocarbon skeleton isomerism and positional isomerism (Fig. 7).

Rice. 7. Combination of two types of structural isomerism

In questions of isomerism, theoretical considerations and experiment are interconnected. If considerations show that there can be no isomers, then experiments should show the same. If the calculations indicate a certain number of isomers, then they can be obtained as much, or less, but not more - not all theoretically calculated isomers can be obtained, since interatomic distances or bond angles in the proposed isomer may be out of range. For a substance containing six CH groups (for example, benzene), 6 isomers are theoretically possible (Fig. 8).

Rice. 8. Benzene isomers

The first five of the isomers shown exist (the second, third, fourth and fifth isomers were obtained almost 100 years after the structure of benzene was established). The last isomer will most likely never be obtained. Presented as a hexagon, it is the least likely, its deformations leading to structures in the form of an oblique prism, a three-beam star, an incomplete pyramid, and a double pyramid (an incomplete octahedron). Each of these options contains either very different C-C bonds, or strongly distorted bond angles (Fig. 9):

Chemical transformations, as a result of which structural isomers are converted into each other, is called isomerization.

stereoisomerism

arises due to the different arrangement of atoms in space with the same order of bonds between them.

One of the types of stereoisomerism is cis-trans-isomerism (cis - lat. one side, trans - lat. through, on opposite sides) is observed in compounds containing multiple bonds or flat cycles. Unlike a single bond, a multiple bond does not allow individual fragments of the molecule to rotate around it. In order to determine the type of isomer, a plane is mentally drawn through the double bond and then the way the substituents are placed relative to this plane is analyzed. If identical groups are on the same side of the plane, then this cis-isomer, if on opposite sides - trance-isomer:

Physical and chemical properties cis- and trance-isomers are sometimes noticeably different, in maleic acid the carboxyl groups -COOH are spatially close, they can react (Fig. 11), forming maleic anhydride (for fumaric acid, this reaction does not occur):

Rice. 11. Formation of maleic anhydride

In the case of planar cyclic molecules, it is not necessary to mentally draw a plane, since it is already set by the shape of the molecule, as, for example, in cyclic siloxanes (Fig. 12):

Rice. 12. Isomers of cyclosiloxane

In complex compounds of metals cis An isomer is a compound in which two identical groups, of those that surround the metal, are adjacent, in trance-isomer, they are separated by other groups (Fig. 13):

Rice. 13. Isomers of the cobalt complex

The second type of stereoisomerism - optical isomerism occurs when two isomers (in accordance with the definition formulated earlier, two molecules that are not compatible in space) are mirror images of each other. Molecules that can be represented as a single carbon atom with four different substituents have this property. The valences of the central carbon atom associated with four substituents are directed to the vertices of the mental tetrahedron - a regular tetrahedron ( cm. ORBITAL) and are rigidly fixed. Four different substituents are shown in Fig. 14 in the form of four balls with different colors:

Rice. 14. A carbon atom with four different substituents

To detect the possible formation of an optical isomer, it is necessary (Fig. 15) to reflect the molecule in the mirror, then the mirror image should be taken as a real molecule, placed under the original one so that their vertical axes coincide, and rotate the second molecule around the vertical axis so that the red ball the upper and lower molecules were located under each other. As a result, the position of only two balls, beige and red, coincides (marked with double arrows). If you rotate the lower molecule so that the blue balls are aligned, then again the position of only two balls will coincide - beige and blue (also marked with double arrows). Everything becomes obvious if these two molecules are mentally combined in space, putting one into the other, like a knife in a sheath, the red and green ball do not match:

For any mutual orientation in space of two such molecules, it is impossible to achieve complete coincidence when combined, according to the definition, these are isomers. It is important to note that if the central carbon atom has not four, but only three different substituents (that is, two of them are the same), then when such a molecule is reflected in the mirror, an optical isomer is not formed, since the molecule and its reflection can be combined in space (Fig. . sixteen):

In addition to carbon, other atoms can act as asymmetric centers, in which covalent bonds are directed to the corners of the tetrahedron, for example, silicon, tin, phosphorus.

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:

Rice. 17. Optically active framework molecules

Variants are possible when the molecule does not contain an asymmetric center even in a latent form, but may itself be generally asymmetric, while optical isomers are also possible. For example, in a complex compound of beryllium, two cyclic fragments are located in mutually perpendicular planes; in this case, two different substituents are sufficient to obtain an optical isomer (Fig. 18). For the ferrocene molecule, which has the shape of a five-sided prism, three substituents are needed for the same purpose, the hydrogen atom in this case plays the role of one of the substituents (Fig. 18):

Rice. 18. Optical isomerism of asymmetric molecules

In most cases, the structural formula of a compound makes it possible to understand what exactly should be changed in it in order to make the substance optically active.

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 ( cm. WALDEN CONVERSATION).

Mikhail Levitsky

This publication is addressed to students in grades 10–11 and applicants taking an exam in chemistry in the form of the Unified State Examination. Training tasks will allow you to systematically prepare for the exam while passing the topic.
The workbook contains:
tasks of parts A, B and C on all topics of the exam;
answers to all questions.
The book will be useful to teachers of chemistry, as it makes it possible to effectively organize the preparation of students for a single exam directly in the classroom, in the process of studying all topics.

Examples.
Saturated hydrocarbons are characterized by reactions
1) substitution
2) connections
3) dehydrogenation
4) dehydration
5) isomerization
6) polymerization

Select features characteristic of structural isomers.
a) various chemical properties
B) similar chemical properties
B) different structure
D) the same structure
E) the same quantitative composition
G) various physical properties
3) the same physical properties

Select features characteristic of homologues.
A) the same physical properties
B) various physical properties
B) same and different chemical properties
D) the same quantitative composition
D) different quantitative composition
E) the same structure
G) similar structure
3) different structure

CONTENT
INTRODUCTION 3
TRAINING TASKS BY TOPICS. 10th CLASS 5
Topic 1. Basic provisions and directions of development of the theory of the chemical structure of organic substances A.M. Butlerov. Limit hydrocarbons 5
Topic 2. Unsaturated hydrocarbons 10
Topic 3. Aromatic hydrocarbons 14
Topic 4. Natural sources of hydrocarbons. Alcohols. Phenols 18
Topic 5. Aldehydes and carboxylic acids 23
Topic 6. Esters. Fats. Carbohydrates 28
Topic 7. Amines. Amino acids 33
Topic 8. Proteins. Amino acids. Macromolecular compounds 37
TRAINING TASKS BY TOPICS. 11th CLASS 41
Topic 1. Periodic law and periodic system of chemical elements D.I. Mendeleev. The structure of atoms 41
Topic 2. The structure of matter (types of chemical bonds, types of crystal lattices, oxidation states) 46
Topic 3. Variety of inorganic substances, their classes and properties. Allotropy 50
Topic 4. Electrolytic dissociation of salts, acids, alkalis. Ion exchange reactions. Salt hydrolysis 55
Topic 5. Types of chemical reactions. Redox reactions. The concept of the rate of a chemical reaction. Reversible reactions 59
Topic 6. Metals. Methods for obtaining metals. Electrolysis 64
Topic 7. Nonmetals 69
ANSWERS TO TRAINING TASKS BY TOPICS. 10th CLASS 73
ANSWERS TO TRAINING TASKS BY TOPICS. 11th CLASS 83
LITERATURE 94.

Free download e-book in a convenient format, watch and read:
Download the book USE 2013, Chemistry, Thematic training tasks, Sokolova I.A., 2012 - fileskachat.com, fast and free download.

Download pdf
Below you can buy this book at the best discounted price with delivery throughout Russia.