Examples of structural isomers. Isomerism

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

Provisions functional group

Configuration

Conforma-

Structural isomerism.

Structural isomers are isomers that have the same qualitative and quantitative composition, but differ in chemical structure.

Structural isomerism causes diversity 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 - cut

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 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 if identical substituents are located at a minimum distance from each other and mutual repulsion forces arise between them, and the molecule must have large supply 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.

P 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 oscillations in the 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.

P 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.

For example: Wine house

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.

During the lesson you will receive general idea about the types of isomerism, learn what an isomer is. Learn about the types of isomerism in organic chemistry: structural and spatial (stereoisomerism). By using structural formulas substances, consider the subtypes of structural isomerism (skeletal and positional isomerism), learn about the types of spatial isomerism: geometric and optical.

Topic: Introduction to organic chemistry

Lesson: Isomerism. Types of isomerism. Structural isomerism, geometric, optical

The types of formulas describing organic substances that we examined earlier show that several different structural formulas can correspond to one molecular formula.

For example, the molecular formula C 2H 6O correspond two substances with different structural formulas - ethanol and dimethyl ether. Rice. 1.

Ethyl alcohol, a liquid that reacts with sodium metal to release hydrogen, boils at +78.5 0 C. Under the same conditions, dimethyl ether, a gas that does not react with sodium, boils at -23 0 C.

These substances differ in their structure - different substances corresponds to the same molecular formula.

Rice. 1. Interclass isomerism

The phenomenon of the existence of substances that have the same composition, but different structures and therefore different properties is called isomerism (from Greek words“izos” - “equal” and “meros” - “part”, “share”).

Types of isomerism

There are different types of isomerism.

Structural isomerism is associated with a different order of joining of atoms in a molecule.

Ethanol and dimethyl ether are structural isomers. Since they belong to different classes of organic compounds, this type of structural isomerism is called also interclass . Rice. 1.

Structural isomers can also exist within the same class of compounds, for example, the formula C 5 H 12 corresponds to three different hydrocarbons. This carbon skeleton isomerism. Rice. 2.

Rice. 2 Examples of substances - structural isomers

There are structural isomers with the same carbon skeleton, which differ in the position of multiple bonds (double and triple) or atoms replacing hydrogen. This type of structural isomerism is called positional isomerism.

Rice. 3. Structural position isomerism

In molecules containing only single bonds, almost free rotation of molecular fragments around the bonds is possible at room temperature, and, for example, all images of the formulas of 1,2-dichloroethane are equivalent. Rice. 4

Rice. 4. Position of chlorine atoms around a single bond

If rotation is hindered, for example, in a cyclic molecule or with a double bond, then geometric or cis-trans isomerism. In cis-isomers, the substituents are located on one side of the plane of the ring or double bond, in trans-isomers - on opposite sides.

Cis-trans isomers exist when they are bonded to a carbon atom. two different deputy Rice. 5.

Rice. 5. Cis and trans isomers

Another type of isomerism arises due to the fact that a carbon atom with four single bonds forms a spatial structure with its substituents - a tetrahedron. If a molecule has at least one carbon atom bonded to four different substituents, optical isomerism. Such molecules do not match their mirror image. This property is called chirality - from the Greek Withhier- "hand". Rice. 6. Optical isomerism is characteristic of many molecules that make up living organisms.

Rice. 6. Examples of optical isomers

Optical isomerism is also called enantiomerism (from Greek enantios- “opposite” and meros- “part”), and optical isomers - enantiomers . Enantiomers are optically active; they rotate the plane of polarization of light by the same angle, but in opposite sides: d- , or (+)-isomer, - to the right, l- , or (-)-isomer, - to the left. A mixture of equal amounts of enantiomers called racemate, is optically inactive and is indicated by the symbol d,l- or (±).

Summing up the lesson

During the lesson, you received a general understanding of the types of isomerism and what an isomer is. We learned about the types of isomerism in organic chemistry: structural and spatial (stereoisomerism). Using the structural formulas of substances, we examined the subtypes of structural isomerism (skeletal and positional isomerism), and became acquainted with the types of spatial isomerism: geometric and optical.

References

1. Rudzitis G.E. Chemistry. Basics general chemistry. 10th grade: textbook for educational institutions: basic level / G. E. Rudzitis, F.G. Feldman. - 14th edition. - M.: Education, 2012.

2. Chemistry. 10th grade. Profile level: academic. for general education institutions/ V.V. Eremin, N.E. Kuzmenko, V.V. Lunin et al. - M.: Bustard, 2008. - 463 p.

3. Chemistry. 11th grade. Profile level: academic. for general education institutions/ V.V. Eremin, N.E. Kuzmenko, V.V. Lunin et al. - M.: Bustard, 2010. - 462 p.

4. Khomchenko G.P., Khomchenko I.G. Collection of problems in chemistry for those entering universities. - 4th ed. - M.: RIA "New Wave": Publisher Umerenkov, 2012. - 278 p.

Homework

1. Nos. 1,2 (p.39) Rudzitis G.E. Chemistry. Fundamentals of general chemistry. 10th grade: textbook for general education institutions: basic level / G. E. Rudzitis, F.G. Feldman. - 14th edition. - M.: Education, 2012.

2. Why is the number of isomers in hydrocarbons of the ethylene series greater than that of saturated hydrocarbons?

3. Which hydrocarbons have spatial isomers?

Structural isomers- these are compounds that have the same molecular formula, but differing from each other in the order of bonding of atoms in the molecule.

Structural isomerism is subdivided into isomerism carbon chain, positional isomerism and functional group isomerism.

Carbon chain isomerism. It is caused by different sequences of bonding of atoms that form the carbon skeleton of the molecule. For example, for an alkane of composition C 4 H 10, two isomers can be written;

For organic compounds with a cyclic structure, chain isomerism can be caused by the size of the cycle.

Position isomerism due to different positions of functional groups, substituents or multiple bonds in the molecule.

Isomerism of functional groups is due to the presence of functional groups of different nature in isomers of the same composition.

SPATIAL ISOMERISM (STEREOISOMERISM)

Spatial isomers- these are compounds that have the same molecular formula, the same order of bonding of atoms in the molecule, but differ from each other in the arrangement of atoms in space.

Spatial isomers are also called stereo isomers and (from the Greek stereos - spatial).

Spatial isomerism is divided into configurational and conformational.

But before moving on to consider these types of stereoisomerism, let us dwell on ways to depict the spatial structure of molecules of organic compounds.

To depict the spatial structure of molecules, their configuration or conformation, molecular models and special stereoformulas are used.

Molecular models - a visual representation of organic and inorganic compounds, allowing one to judge relative position atoms that make up a molecule.

Three main types of models are most often used: ball-and-stick (Kekule-van't-Hoff models), skeletal (Dryding-ga models) and hemispherical (Stewart-Brigleb models). Models allow us to judge not only the relative position of atoms in a molecule, but they are convenient and to consider bond angles and the possibility of rotation around simple bonds. The Dryding models also take into account interatomic distances, while the Stewart–Brigleb models also reflect the volumes of atoms. The figure below shows models of ethane and ethylene molecules.

Rice. 3.1. Models of ethane (left) and ethylene (right) molecules; a – ball-and-rod; b – Dryding; V – hemispherical (Stuart–Briegleb)

Stereoformulas. To depict the spatial structure of a molecule on a plane, stereochemical and perspective formulas are most often used, as well as projection formulas Newman.

IN stereochemical formulas chemical bonds located in the plane of the drawing are represented by a regular line; connections located above the plane - with a thick wedge or a bold line, and those located above the plane - with a dashed wedge or a dotted line:

Promising formulas describe the spatial structure on a plane, taking into account consideration of the molecule along one of the carbon-carbon bonds. By appearance they resemble sawmill goats:

When building Newman's projection formulas the molecule is viewed in the direction of one C–C bond in such a way that the atoms forming this bond obscure each other. Of the selected pair, the carbon atom closest to the observer is represented by a dot, and the farthest one is represented by a circle. The chemical bonds of the nearest carbon atom with other atoms are represented by lines originating from the center of the circle, and the farthest from the circle:

There are Fischer projection formulas, which are usually used to depict the spatial structure of optical isomers on a plane.

Theory A.M. Butlerov

1. Atoms in molecules are connected to each other in a certain sequence chemical bonds according to their valence. The order in which atoms bond is called their chemical structure. Carbon in all organic compounds is tetravalent.

2. The properties of substances are determined not only by the qualitative and quantitative composition of molecules, but also by their structure.

3. Atoms or groups of atoms mutually influence each other, which determines reactivity molecules.

4. The structure of molecules can be established based on the study of their chemical properties.

Organic compounds have a number of characteristic features, which distinguish them from inorganic ones. Almost all of them (with rare exceptions) are flammable; Most organic compounds do not dissociate into ions, which is due to the nature covalent bond V organic matter. The ionic type of bond is realized only in salts of organic acids, for example, CH3COONa.

Homologous series is an endless series of organic compounds that have similar structure and therefore similar chemical properties and differing from each other by any number of CH2– groups (homologous difference).

Even before the creation of the theory of structure, substances with the same elemental composition, but with different properties, were known. Such substances were called isomers, and this phenomenon itself was called isomerism.

The basis of isomerism, as shown by A.M. Butlerov, lies the difference in the structure of molecules consisting of the same set of atoms.

Isomerism- this is the phenomenon of the existence of compounds that have the same qualitative and quantitative composition, but different structures and, therefore, different properties.

There are 2 types of isomerism: structural isomerism and spatial isomerism.

Structural isomerism

Structural isomers– connections of the same quality and quantitative composition, differing in the order of bonding of atoms, i.e. chemical structure.

Spatial isomerism

Spatial isomers(stereoisomers) with the same composition and the same chemical structure differ in the spatial arrangement of atoms in the molecule.
Spatial isomers are optical and cis-trans isomers (geometric).

Cis-trans isomerism

lies in the possibility of placing substituents on one or opposite sides of the plane of a double bond or non-aromatic ring. B cis isomers the substituents are on one side of the plane of the ring or double bond, in trans isomers- in different ways.

In the butene-2 molecule CH3–CH=CH–CH3, CH3 groups can be located either on one side of the double bond - in the cis isomer, or on opposite sides - in the trans isomer.

Optical isomerism

Appears when a carbon has four different substituents.
If you swap any two of them, you get another spatial isomer of the same composition. The physicochemical properties of such isomers differ significantly. Compounds of this type are distinguished by their ability to rotate the plane of polarized light transmitted through a solution of such compounds by a certain amount. In this case, one isomer rotates the plane of polarized light in one direction, and its isomer rotates in the opposite direction. Due to such optical effects, this type of isomerism is called optical isomerism.

Isomers, isomerism

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

The phenomenon of the existence of isomers is called isomerism

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

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

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

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

Another definition of isomers can be given as follows:

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

Types of isomerism

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

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

Structural isomerism is divided into:

1.Isomerism of the carbon skeleton

2.Positional isomerism

(multiple bond, functional group, substituent)

3.Interclass isomerism

CH 3 -CH 2 -NO 2

nitroethane

HOOC-CH 2 -NH 2 aminoacetic acid (glycine)

Position isomerism

multiple connection

CH 2 = CH-CH = CH 2

CH 3 -CH= C= CH 2

functional group

CH 3 -CHON -CH 3

CH 2 OH -CH 2 -CH 3

Deputy

CH 3 -CHCI -CH 3

CH 2 CI -CH 2 -CH 3

Structural isomerism

Isomerism of the position of a multiple (double) bond:

Butene-1 and butene-2

Carbon skeleton isomerism:

Cyclobutane and methylcyclopropane

Interclass isomerism:

Butene and cyclobutane

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