Isomerism example. Isomerism. Types of isomerism. Structural isomerism, geometric, optical. See what “structural isomerism” is in other dictionaries

During the lesson, you will get a general idea of the types of isomerism and learn what an isomer is. Learn about the types of isomerism in organic chemistry: structural and spatial (stereoisomerism). Using the structural formulas of 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 - ethyl alcohol and dimethyl ether. Rice. 1.

Ethyl alcohol, a liquid that reacts with metallic sodium 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 have 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 the Greek words “isos” - “equal” and “meros” - “part”, “share”).

Types of isomerism

There are different types of isomerism.

Structural isomerism is associated with different orders 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 directions: 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.

Bibliography

1. 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. Chemistry. Grade 10. 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. Grade 11. 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?

The properties of organic substances depend both on their composition and on the order of connection of atoms in the molecule. Isomers – These are substances that have the same composition and molar mass, but different physical and chemical properties. Distinguish structural And spatial isomerism.

Structural isomerism- the result of differences in chemical structure. According to Butlerov's theory, in molecules atoms are connected to each other in a certain sequence, this is called chemical structure. The atoms or groups of atoms that form a molecule mutually influence each other, and the properties of the molecule depend on its structure. Highlight:

1. Isomerism of the carbon skeleton (butane - isobutane).

2. Isomerism of functional groups or multiple bonds (1-propanol - 2-propanol).

3. Interclass isomerism (ethanol CH 3 -CH 2 -OH - dimethyl ether CH 3 -O-CH 3).

Spatial isomerism(stereoisomerism) - arises as a result of differences in the spatial configuration of molecules that have the same chemical structure.

1. Optical isomerism ( enantiomerism) - pairs of optical antipodes, i.e. substances characterized by opposite rotations of the plane of polarization of light, which are complete a reflection of each other. At the same time, their other physical and chemical properties are usually identical (with the exception of reactions with optically active substances).

2. Diasteamerism is a spatial isomerism that does not form a pair of optical isomers.

A special form of spatial isomerism is called cis-trans isomerism(geometric). It arises due to a change in the relative position of the substituents relative to the double bond or ring. Characteristic, for example, of diene hydrocarbons, when conjugated dienes rotate around the s-bond (cis- and trans-1,3-butadiene). IN cis-isomers, the substituents are located on one side of the plane of the double bond or ring, in trance-isomers – different.

They also highlight tautomerism- rapid, spontaneous interconversion of several forms belonging to different classes of molecules into each other. For example, keto-enol tautomerism - interconversion of ketones (aldehydes) and enols (CH 3 -CHO - CH 2 =CH-OH), ring-chain Carbohydrate tautomerism - the presence of two forms of carbohydrate structure - cyclic and chain.

Electronic structure of organic compounds. Hybrid orbitals. Formation and characteristics of sigma and pi bonds.

The properties of organic compounds are determined by: the chemical, electronic and spatial structure of the molecules; type of chemical bonds; the nature and electronic structure of atoms; the type of atomic orbitals and the nature of their interaction.

The electron has a dual nature, exhibiting properties of both a particle and a wave. The movement of an electron obeys the laws of quantum mechanics.

Electronic clouds(electron orbitals) are the regions with the highest probability of an electron being present. They differ in shape, size, orientation in space. For example, in a hydrogen atom, a single electron, when moving, forms a negatively charged cloud of spherical (spherical) shape. Such electrons are called s electrons. p-electrons form an orbital in the form of a three-dimensional figure eight. s-, p-, d-, f- and other orbitals differ in energy, shape, size and spatial orientation.

For organic substances, the most typical location of valence electrons is in the s- and p-atomic orbitals.

Hybridization- this is the interaction of atomic orbitals of different types, but similar in energy, with the formation of the same number of hybrid orbitals. For a carbon atom, the resulting hybrid orbitals will have the same shape and energy.

Hybridization of a carbon atom is accompanied by its excitement and the transfer of one electron from s to an empty p orbital, allowing four bonds to form.

sp3 hybridization formed by one s and three p orbitals. As a result, the axes of the hybrid orbitals are directed to the vertices of a regular tetrahedron, the angle between them is 109°28". Such orbitals can form four -communications- covalent single bonds formed with maximum overlap of atomic orbitals along one axis. (methane CH 4)

sp2 hybridization is formed due to one s and two p orbitals, forming three hybrid orbitals located in the same plane at an angle of 120°. These orbitals form three -bonds. Third R-the orbital remains unhybridized and is oriented perpendicular to the plane of location of the hybrid orbitals, participating in the formation -communications- bonds formed by lateral overlap of unhybridized p-orbitals on both sides of the axis connecting the nuclei of atoms. (ethylene C 2 H 4)

sp hybridization is formed due to one s and one p orbitals, forming two hybrid orbitals located on the same axis, i.e. at an angle of 180°. These orbitals form two -bonds. Two p-orbitals remain unhybridized and, located in mutually perpendicular planes, participate in the formation of two -bonds. (acetylene C 2 H 2)

Valence isomerism is 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 (interclass isomerism)

It differs in the nature of the functional group; for example, ethanol (CH 3 -CH 2 -OH) and dimethyl ether (CH 3 -O-CH 3).

Position isomerism

A type of structural isomerism characterized by differences in the positions of identical functional groups or multiple bonds on the same carbon skeleton. Example: 2-chlorobutanoic acid and 4-chlorobutanoic acid.

Metamerism

This type of isomerism is divided into enantiomerism(optical isomerism) and diastereomerism.

Enantiomerism (optical isomerism)

Enantiomers (optical isomers, mirror isomers) are pairs of optical antipodes - substances characterized by opposite sign and equal rotations 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 that the molecule belongs to one of the following point symmetry groups: C n, D n, T, O or I (chirality). Most often we are talking about an asymmetric carbon atom, that is, an atom connected to four different substituents.

Other atoms can also be asymmetric, for example atoms of silicon, nitrogen, phosphorus, 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 last connection is atropisomerism, that is, spatial isomerism caused by the lack of rotation around a single bond. Enantiomerism also appears in the helical conformations of proteins, nucleic acids, and in hexagelicene (XIII).

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-diastereomerism

σ-diastereomers differ from each other in the configuration of some of the chiral elements they contain. Thus, diastereomers are (+)-tartaric acid and meso-tartaric acid, D-glucose and D-mannose, for example:

π-diastereomerism (geometric isomerism)

π-diastereomers, 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,

Theory A.M. Butlerov

1. Atoms in molecules are connected to each other in a certain sequence by chemical bonds in accordance with 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 the reactivity of the molecule.

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

Organic compounds have a number of characteristic features that 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 of covalent bonds in organic substances. The ionic type of bond is realized only in salts of organic acids, for example, CH3COONa.

Homologous series- this is an endless series of organic compounds that have a similar structure and, therefore, similar chemical properties and differ 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– compounds of the same qualitative 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- substances with the same molecular structure, but different chemical structures and properties.

Types of isomerism

I. Structural - lies in the different sequence of connections of atoms in the chain of a molecule:

1) Chain isomerism

It should be noted that the carbon atoms in a branched chain differ in the type of connection with other carbon atoms. Thus, a carbon atom bonded to only one other carbon atom is called primary, with two other carbon atoms - secondary, with three - tertiary, with four - quaternary.

2) Position isomerism

3) Interclass isomerism

4) Tautomerism

Tautomerism(from the Greek ταύτίς - the same and μέρος - measure) - the phenomenon of reversible isomerism, in which two or more isomers easily transform into each other. In this case, a tautomeric equilibrium is established, and the substance simultaneously contains molecules of all isomers in a certain ratio. Most often, tautomerization involves the movement of hydrogen atoms from one atom in a molecule to another and back again in the same compound.

II. Spatial (stereo) - due to different positions of atoms or groups relative to a double bond or ring, excluding the free rotation of connected carbon atoms

1. Geometric (cis -, trans - isomerism)

If a carbon atom in a molecule is bonded to four different atoms or atomic groups, for example:

then the existence of two compounds with the same structural formula, but differing in spatial structure, is possible. The molecules of such compounds relate to each other as an object and its mirror image and are spatial isomers.

This type of isomerism is called optical; isomers are called optical isomers or optical antipodes:

Molecules of optical isomers are incompatible in space (like left and right hands); they lack a plane of symmetry.
Thus,

optical isomers are called spatial isomers, the molecules of which are related to each other as an object and an incompatible mirror image.

Optical isomers of amino acids

3. Conformational isomerism

It should be noted that atoms and groups of atoms connected to each other by a σ bond constantly rotate relative to the bond axis, occupying different positions in space relative to each other.

Molecules that have the same structure and differ in the spatial arrangement of atoms as a result of rotation around C-C bonds are called conformers.

To depict conformational isomers, it is convenient to use formulas - Newman projections:

The phenomenon of conformational isomerism can also be considered using the example of cycloalkanes. Thus, cyclohexane is characterized by the following conformers:

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 C2H6O correspond two substances with different structural formulas - ethyl alcohol and dimethyl ether. Rice. 1.

Ethyl alcohol is a liquid that reacts with sodium metal to release hydrogen and boils at +78.50C. Under the same conditions, dimethyl ether, a gas that does not react with sodium, boils at -230C.

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

Rice. 1. Interclass isomerism

Types of isomerism

There are different types of isomerism.

Structural isomerism is associated with different orders 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 C5H12 corresponds to three different hydrocarbons. This carbon skeleton isomerism. Rice. 2.