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
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 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 C5H12 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 (±).
SOURCES
video source - http://www.youtube.com/watch?v=mGS8BUEvkpY
http://www.youtube.com/watch?t=7&v=XIikCzDD1YE
http://interneturok.ru/ru/school/chemistry/10-klass - abstract
presentation source - http://ppt4web.ru/khimija/tipy-izomerii.html
http://www.youtube.com/watch?t=2&v=ii30Pctj6Xs
http://www.youtube.com/watch?t=1&v=v1voBxeVmao
http://www.youtube.com/watch?t=2&v=a55MfdjCa5Q
http://www.youtube.com/watch?t=1&v=FtMA1IJtXCE
presentation source - http://mirhimii.ru/10class/174-izomeriya.html
The content of the article
ISOMERIA(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.
Isomerism was first discovered by J. Liebig in 1823, who established that silver salts of fulminate and isocyanic acids: Ag-O-N=C and Ag-N=C=O have the same composition, but different properties. The term “Isomerism” was introduced in 1830 by I. Berzelius, who suggested that differences in the properties of compounds of the same composition arise due to the fact that the atoms in the molecule are arranged in a different order. The idea of isomerism was finally formed after A.M. Butlerov created the theory of chemical structure (1860s). 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.
Rice. 1. Butanol isomers
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.
There are several reasons for 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):
Rice. 2. Isomers of the simplest hydrocarbons
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 the hydrocarbon C 14 H 30 there are 1885 isomers, for the hydrocarbon C 20 H 42 there are over 366,000.
In complex cases, the question of whether two compounds are isomers is resolved using various rotations around the 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 (Fig. 3). 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):
Rice. 4. Aromatic isomers
Positional isomerism.
Another type of structural isomerism, positional isomerism, occurs in cases where functional groups, individual heteroatoms or multiple bonds are located in different places in 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. In Fig. Figure 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, the differences in the structure of positional isomers are so obvious that it is not even necessary to mentally combine them in space, for example, the isomers of butene or dichlorobenzene (Fig. 6):
Rice. 6. Isomers of butene and dichlorobenzene
Sometimes structural isomers combine the characteristics of hydrocarbon skeleton isomerism and positional isomerism (Fig. 7).
Rice. 7. Combination of two types of structural isomerism
In matters of isomerism, theoretical considerations and experiment are interconnected. If considerations show that isomers cannot exist, then experiments should show the same. If calculations indicate a certain number of isomers, then the same number or less of them can be obtained, but not more - not all theoretically calculated isomers can be obtained, since interatomic distances or bond angles in the proposed isomer may be outside the permissible limits. 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 latter isomer will most likely never be obtained. Represented as a hexagon, it is the least likely to form, and its deformations result in structures in the form of a beveled prism, a three-pointed star, an incomplete pyramid, and a double pyramid (an incomplete octahedron). Each of these options contains either very different C-C bonds in size or highly distorted bond angles (Fig. 9):
Chemical transformations as a result of which structural isomers are converted into each other are called isomerization.
Stereoisomerism
arises due to the different arrangement of atoms in space with the same order of bonds between them.
One type of stereoisomerism is cis-trans isomerism (cis - lat. on one side, trans - lat. through, on different sides) is observed in compounds containing multiple bonds or planar 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 acid anhydride (this reaction does not occur for fumaric acid):
Rice. 11. Formation of maleic anhydride
In the case of flat cyclic molecules, it is not necessary to mentally draw a plane, since it is already given by the shape of the molecule, as, for example, in cyclic siloxanes (Fig. 12):
Rice. 12. Isomers of cyclosiloxane
In complex metal compounds cis-isomer is a compound in which two identical groups, from those surrounding the metal, are located nearby, in trance-isomer, they are separated by other groups (Fig. 13):
Rice. 13. Isomers of cobalt complex
The second type of stereoisomerism, optical isomerism, occurs in cases where two isomers (in accordance with the definition formulated earlier, two molecules that are not compatible in space) are mirror images of each other. This property is possessed by molecules that can be represented as a single carbon atom having four different substituents. The valencies of the central carbon atom bound to four substituents are directed towards the vertices of a mental tetrahedron - a regular tetrahedron ( cm. ORBITAL) and rigidly fixed. Four unequal substituents are shown in Fig. 14 in the form of four balls with different colors:
Rice. 14. 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 a mirror, then the mirror image should be taken as a real molecule, placed under the original one so that their vertical axes coincide, and the second molecule should be rotated 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 by double arrows). If you rotate the lower molecule so that the blue balls align, then the position of only two balls will again 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 coincide:
For any mutual orientation in space, two such molecules cannot 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 a mirror, an optical isomer is not formed, since the molecule and its reflection can be combined in space (Fig. . 16):
In addition to carbon, other atoms in which covalent bonds are directed towards the corners of the tetrahedron, for example, silicon, tin, phosphorus, can act as asymmetric centers.
Optical isomerism occurs 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 framework hydrocarbon adamantane, which has four different substituents (Fig. 17), can have an optical isomer, with the entire molecule playing the role of an asymmetric center, which becomes obvious if the adamantane framework is mentally contracted to a point. Similarly, siloxane, which has a cubic structure (Fig. 17), also becomes optically active in the case of four different substituents:
Rice. 17. Optically active scaffold molecules
Options are possible when the molecule does not contain an asymmetric center, even in a hidden form, but may itself be generally asymmetrical, and optical isomers are also possible. For example, in a beryllium complex compound, 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 a ferrocene molecule, which has the shape of a pentahedral 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 allows us to understand what exactly needs to be changed in it to make the substance optically active.
Syntheses of optically active stereoisomers usually produce a mixture of dextro- and levorotatory compounds. The separation of isomers is carried out by reacting a mixture of isomers with reagents (usually of natural origin) containing an asymmetric reaction center. Some living organisms, including bacteria, preferentially metabolize levorotatory isomers.
Processes (called asymmetric synthesis) have now been developed to specifically produce a specific optical isomer.
There are reactions that allow you to convert an optical isomer into its antipode ( cm. WALDEN CONVERSION).
Mikhail Levitsky
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
|
Spatial isomerism is divided into: |
|
|
|
Characteristic of substances containing double bonds or cyclic ones. |
Optical isomers are also called mirror or chiral (like left and right hand)
Isomers- substances with the same molecular composition, but different chemical structure 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 Greek ταύτίς - the same and μέρος - measure) is 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.
Example, tautomeric forms of glucose:
1. Linear form of glucose (aldehyde alcohol)
2. Rearrangement of atoms and transition to cyclic forms of glucose (alha and beta)
II. Spatial (stereo) - due to the different positions of atoms or groups relative to the double bond or ring, excluding the free rotation of the connected carbon atoms
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.
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 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 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 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 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?
