Spatial isomers (stereoisomers) have the same qualitative and quantitative composition and the same order of binding atoms (chemical structure), but different spatial arrangement atoms in a molecule.
There are two types of spatial isomerism: optical And geometric.
Optical isomerism
In optical isomerism, different fragments of molecules are located differently relative to some atom, i.e. have different configuration. For example:
Such molecules are not identical, they relate to each other as an object and its mirror image and are called enantiomers.
Enantiomers have the properties of chirality. The simplest case of chirality is due to the presence in the molecule center of chirality(chiral center), which can be an atom containing four different substituents. Such an atom has no symmetry elements. In this regard, it is also called asymmetric.
To establish whether a molecule is chiral, it is necessary to build its model, the model of its mirror image (Fig. 3.1 , but) and find out if they are combined in space. If they are not combined, the molecule is chiral (Fig. 3.1, b), if they are combined, it is achiral.
Rice. 3.1.
All chemical properties of enantiomers are identical. Their physical properties are the same, with the exception of optical activity: one form rotates the plane of polarization of light to the left, the other - to the same angle to the right.
A mixture of equal amounts of optical antipodes behaves like an individual chemical compound, devoid of optical activity and very different in physical properties from each of the antipodes. Such a substance is called racemic mixture, or racemate.
In all chemical transformations in which new asymmetric carbon atoms are formed, racemates are always obtained. There are special techniques for separating racemates into optically active antipodes.
In the case of the presence of several asymmetric atoms in a molecule, a situation is possible when spatial isomers are not optical antipodes. For example:
Spatial isomers that are not enantiomers of each other are called diastereomers.
A special case of diastereomers - geometric (cis-trais-) isomers.
geometric isomerism
Geometric (cis-trans-) isomerism characteristic of compounds containing double bonds (C=C, C=N, etc.), as well as non-aromatic cyclic compounds, and is due to the impossibility of free rotation of atoms around a double bond or in a cycle. Substituents in geometric isomers can be located on one side of the plane of the double bond or cycle - ^wc-position, or different sides- tirsh / s-position (Fig. 3.2).
Rice. 3.2. Dis-isomer (a) andtrance-isomer(b)
Geometric isomers usually differ significantly in physical properties (boiling and melting points, solubility, dipole moments, thermodynamic stability, etc.)
- The term "chirality" means that two objects are in such a relationship to each other as the left and right hand(from the Greek chair - hand), i.e. are mirror images that do not match when trying to combine them in space.
Organic chemistry- a branch of chemistry in which carbon compounds are studied, their structure, properties, interconversions.
The very name of the discipline - "organic chemistry" - arose quite a long time ago. The reason for this lies in the fact that most of the carbon compounds encountered by researchers at the initial stage of formation chemical science were of plant or animal origin. However, as an exception, individual carbon compounds are classified as inorganic. So, for example, carbon oxides, carbonic acid, carbonates, hydrocarbonates, hydrogen cyanide and some others are considered to be inorganic substances.
There are currently less than 30 million known organic matter and this list is constantly updated. Such a huge number organic compounds is associated primarily with the following specific properties of carbon:
1) carbon atoms can be connected to each other in chains of arbitrary length;
2) not only sequential (linear) connection of carbon atoms to each other is possible, but also branched and even cyclic;
3) possible different types bonds between carbon atoms, namely single, double and triple. In this case, the valence of carbon in organic compounds is always equal to four.
In addition, a wide variety of organic compounds is also facilitated by the fact that carbon atoms are able to form bonds with atoms of many other chemical elements, for example, hydrogen, oxygen, nitrogen, phosphorus, sulfur, halogens. Hydrogen, oxygen and nitrogen are the most common.
It should be noted that for quite a long time organic chemistry represented a “dark forest” for scientists. For some time, the theory of vitalism was even popular in science, according to which organic substances cannot be obtained in an “artificial” way, i.e. outside of living matter. However, the theory of vitalism did not last very long, in view of the fact that one by one substances were discovered, the synthesis of which is possible outside living organisms.
Researchers were perplexed by the fact that many organic substances have the same qualitative and quantitative composition, but often have completely different physical and chemical properties. For example, dimethyl ether and ethanol have exactly the same elemental composition, however, under normal conditions, dimethyl ether is a gas, and ethyl alcohol is a liquid. In addition, dimethyl ether does not react with sodium, but ethyl alcohol interacts with it, releasing hydrogen gas.
Researchers of the 19th century put forward many assumptions about how organic substances are nevertheless arranged. Significantly important assumptions were put forward by the German scientist F.A. Kekule, who was the first to put forward the idea that atoms of different chemical elements have specific valence values, and carbon atoms in organic compounds are tetravalent and can combine with each other, forming chains. Later, starting from the assumptions of Kekule, the Russian scientist Alexander Mikhailovich Butlerov developed a theory of the structure of organic compounds, which has not lost its relevance in our time. Consider the main provisions of this theory:
1) all atoms in the molecules of organic substances are connected to each other in a certain sequence in accordance with their valency. Carbon atoms have a constant valency of four and can form chains of various structures with each other;
2) the physical and chemical properties of any organic substance depend not only on the composition of its molecules, but also on the order in which the atoms in this molecule are connected to each other;
3) individual atoms, as well as groups of atoms in a molecule, influence each other. This mutual influence is reflected in the physical and chemical properties ah connections;
4) by examining the physical and chemical properties of an organic compound, its structure can be established. The opposite is also true - knowing the structure of the molecule of a substance, you can predict its properties.
In the same way as periodic law D.I. Mendelev became the scientific foundation not organic chemistry, theory of the structure of organic substances A.M. Butlerova actually became the starting point in the development of organic chemistry as a science. It should be noted that after the creation of Butler's theory of structure, organic chemistry began its development at a very rapid pace.
Isomerism and homology
According to the second position of Butlerov's theory, the properties of organic substances depend not only on the qualitative and quantitative composition of the molecules, but also on the order in which the atoms in these molecules are connected to each other.
In this regard, such a phenomenon as isomerism is widespread among organic substances.
Isomerism is a phenomenon when different substances have exactly the same molecular composition, i.e. the same molecular formula.
Very often, isomers differ greatly in physical and chemical properties. For example:
Types of isomerism
Structural isomerism
a) Isomerism of the carbon skeleton
b) Position isomerism:
multiple bond
deputies:
functional groups:
c) Interclass isomerism:
Interclass isomerism occurs when compounds that are isomers belong to different classes of organic compounds.
Spatial isomerism
Spatial isomerism is a phenomenon when different substances with the same order of attachment of atoms to each other differ from each other by a fixed-different position of atoms or groups of atoms in space.
There are two types of spatial isomerism - geometric and optical. There are no assignments for optical isomerism at the Unified State Examination, so we will consider only the geometric one.
If there is a double C=C bond or a cycle in the molecule of any compound, sometimes in such cases the phenomenon of geometric or cis-trans-isomerism.
For example, this type of isomerism is possible for butene-2. Its meaning lies in the fact that the double bond between carbon atoms actually has a planar structure, and substituents at these carbon atoms can be fixedly located either above or below this plane:
When the same substituents are on the same side of the plane, they say that this cis-isomer, and when different - trance-isomer.
On in the form of structural formulas cis- And trance-isomers (for example, butene-2) are depicted as follows:
Note that geometric isomerism is impossible if at least one carbon atom in the double bond has two identical substituents. For example, cis-trans- isomerism is impossible for propene:
propene has no cis-trans-isomers, since at one of the carbon atoms in the double bond there are two identical "substituents" (hydrogen atoms) As you can see from the illustration above, if we swap the methyl radical and the hydrogen atom located at the second carbon atom on opposite sides of the plane, we get the same molecule, which we just looked at from the other side.
The influence of atoms and groups of atoms on each other in the molecules of organic compounds
The concept of a chemical structure as a sequence of atoms connected to each other was significantly expanded with the advent of the electronic theory. From the standpoint of this theory, it is possible to explain how atoms and groups of atoms in a molecule influence each other.
Distinguish two possible ways influence of some parts of the molecule on others:
1) Inductive effect
2) Mesomeric effect
Inductive effect
To demonstrate this phenomenon, let us take, for example, a molecule of 1-chloropropane (CH 3 CH 2 CH 2 Cl). The bond between carbon and chlorine is polar because chlorine has a much higher electronegativity than carbon. As a result of the displacement of the electron density from the carbon atom to the chlorine atom, a partial positive charge (δ+) is formed on the carbon atom, and a partial negative charge (δ-) is formed on the chlorine atom:
The shift of electron density from one atom to another is often indicated by an arrow pointing towards the more electronegative atom:
However, it is interesting that, in addition to the shift in electron density from the first carbon atom to the chlorine atom, there is also a shift, but to a somewhat lesser extent, from the second carbon atom to the first, and also from the third to the second:
Such a shift of the electron density along the chain of σ-bonds is called the inductive effect ( I). This effect fades with distance from the influencing group and practically does not manifest itself after 3 σ-bonds.
In the case when an atom or group of atoms has a greater electronegativity compared to carbon atoms, such substituents are said to have a negative inductive effect (- I). Thus, in the example discussed above, the chlorine atom has a negative inductive effect. In addition to chlorine, the following substituents have a negative inductive effect:
–F, –Cl, –Br, –I, –OH, –NH 2 , –CN, –NO 2 , –COH, –COOH
If the electronegativity of an atom or group of atoms is less than the electronegativity of a carbon atom, there is actually a transfer of electron density from such substituents to carbon atoms. In this case, the substituent is said to have a positive inductive effect (+ I) (is electron-donating).
So, substituents with + I-effect are saturated hydrocarbon radicals. At the same time, the expression I-effect increases with elongation of the hydrocarbon radical:
–CH 3 , –C 2 H 5 , –C 3 H 7 , –C 4 H 9
It should be noted that carbon atoms in different valence states also have different electronegativity. Sp carbon atoms have a higher electronegativity than sp 2 carbon atoms, which in turn are more electronegative than sp 3 carbon atoms.
Mesomeric effect (M), or conjugation effect, is the influence of a substituent transmitted through a system of conjugated π-bonds.
The sign of the mesomeric effect is determined by the same principle as the sign of the inductive effect. If a substituent increases the electron density in the conjugated system, it has a positive mesomeric effect (+ M) and is electron-donating. Double carbon-carbon bonds, substituents containing an unshared electron pair: -NH 2, -OH, halogens have a positive mesomeric effect.
Negative mesomeric effect (– M) have substituents that pull the electron density away from the conjugated system, while the electron density in the system decreases.
The following groups have a negative mesomeric effect:
–NO 2 , –COOH, –SO 3 H, -COH, >C=O
Due to the redistribution of the electron density due to the mesomeric and inductive effects in the molecule, partial positive or negative charges appear on some atoms, which is reflected in the chemical properties of the substance.
Graphically, the mesomeric effect is shown by a curved arrow that starts at the center of the electron density and ends where the electron density shifts. So, for example, in the vinyl chloride molecule, the mesomeric effect occurs when the lone electron pair of the chlorine atom is conjugated with the electrons of the π-bond between carbon atoms. Thus, as a result of this, a partial positive charge appears on the chlorine atom, and the mobile π-electron cloud, under the influence of an electron pair, shifts towards the extreme carbon atom, on which a partial negative charge arises as a result:
If a molecule contains alternating single and double bonds, then the molecule is said to contain a conjugated π-electron system. An interesting property of such a system is that the mesomeric effect does not decay in it.
During the course 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). Using the structural formulas of substances, consider the subspecies of structural isomerism (skeletal and positional isomerism), learn about the varieties of spatial isomerism: geometric and optical.
Topic: Introduction to organic chemistry
Lesson: Isomerism. Types of isomerism. Structural isomerism, geometric, optical
The types of formulas that we considered earlier, describing organic substances, 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. one.
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 corresponds to the same molecular formula.
Rice. 1. Interclass isomerism
The phenomenon of the existence of substances having the same composition, but different structure and therefore different properties, is called isomerism (from the Greek words "isos" - "equal" and "meros" - "part", "share").
Types of isomerism
Exist different types isomerism.
Structural isomerism is associated with different order connection 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. one.
Structural isomers can also be within the same class of compounds, for example, the formula C 5 H 12 corresponds to three different hydrocarbons. This isomerism of the carbon skeleton. 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 that replace hydrogen. This kind of structural isomerism is called position isomerism.
Rice. 3. Structural position isomerism
In molecules containing only single bonds, almost free rotation of fragments of the molecule 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 difficult, for example, in a cyclic molecule or with a double bond, then geometric or cis-trans isomerism. In cis isomers, the substituents are on the same side of the ring plane or double bond, in trans isomers they are on opposite sides.
Cis-trans isomers exist when two different deputy. Rice. five.
Rice. 5. Cis and trans isomers
Another type of isomerism arises due to the fact that a carbon atom with four single bonds forms with its substituents a spatial structure - a tetrahedron. If a molecule has at least one carbon atom bonded to four different substituents, optical isomerism. Such molecules do not coincide with their mirror image. This property is called chirality - from the Greek fromhier- "hand". Rice. 6. Optical isomerism is characteristic of many molecules that make up living organisms.
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Rice. 6. Examples of optical isomers
Optical isomerism is also called enantiomers (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 is called racemate, is optically inactive and is denoted by the symbol d,l- or (±).
Summing up the lesson
During the lesson, you got a general idea of the types of isomerism, what is an isomer. Learned about the types of isomerism in organic chemistry: structural and spatial (stereoisomerism). With the help of the structural formulas of substances, we considered subspecies of structural isomerism (skeletal and positional isomerism), got acquainted with the varieties of spatial isomerism: geometric and optical.
Bibliography
1. Rudzitis G.E. Chemistry. Basics general chemistry. Grade 10: textbook for educational institutions: a basic level of/ G. E. Rudzitis, F.G. Feldman. - 14th edition. - M.: Education, 2012.
2. Chemistry. Grade 10. Profile level: textbook. for general education institutions / V.V. Eremin, N.E. Kuzmenko, V.V. Lunin and others - M.: Drofa, 2008. - 463 p.
3. Chemistry. Grade 11. Profile level: textbook. for general education institutions / V.V. Eremin, N.E. Kuzmenko, V.V. Lunin and others - M.: Drofa, 2010. - 462 p.
4. Khomchenko G.P., Khomchenko I.G. Collection of problems in chemistry for those entering the universities. - 4th ed. - M.: RIA "New Wave": Publisher Umerenkov, 2012. - 278 p.
Homework
1. No. 1,2 (p. 39) Rudzitis G.E. Chemistry. Fundamentals of General Chemistry. Grade 10: textbook for educational 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. What hydrocarbons have spatial isomers?
Structural isomers- These are compounds that have the same molecular formula, but differ from each other in the order of binding of atoms in the molecule.
Structural isomerism is subdivided into carbon chain isomerism, position isomerism, and functional group isomerism.
Isomerism of the carbon chain. Due to the different sequence of binding 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 due to the presence in the isomers of the same composition of functional groups of different nature.
SPATIAL ISOMERIA (STEREOISOMERIA)
Spatial isomers- these are compounds that have the same molecular formula, the same order of binding of atoms in a 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 subdivided into configurational and conformational.
But before proceeding to the consideration of these types of stereoisomerism, let us dwell on the ways of depicting the spatial structure of the 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 the molecules of organic and inorganic compounds, which makes it possible to judge relative position atoms that make up the molecule.
Three main types of models are most often used: ball-and-stick (Kekule-Vant-Hoff models), skeletal (Dryding-g models) and hemispherical (Stuart-Briegleb models). Models allow one to judge not only the mutual arrangement of atoms in a molecule, but they are convenient and for consideration bond angles and the ability to rotate around simple connections. Dryding models also take into account interatomic distances, while Stewart-Briegleb models also reflect the volumes of atoms. The figure below shows the models of ethane and ethylene molecules.
Rice. 3.1. Models of ethane (left) and ethylene (right) molecules; a - ball-and-stick; b - Dryding; in – hemispherical (Stuart-Brigleb)
stereo formulas. To depict the spatial structure of a molecule on a plane, stereochemical and perspective formulas, as well as Newman projection formulas, are most often used.
IN stereochemical formulas chemical bonds located in the plane of the drawing are depicted as a regular line; connections located above the plane - a bold wedge or a bold line, and located pssh plane - a dashed wedge or a dashed line:
Promising Formulas describe the spatial structure on the plane, taking into account the consideration of the molecule along one of the carbon-carbon bonds. By appearance they resemble sawmill goats:
When building Newman 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. From the selected pair, the carbon atom closest to the observer is represented by a dot, and the farthest by a circle. The chemical bonds of the nearest carbon atom with other atoms are represented by lines originating from the point in the center of the circle, and the far one - from the circle:
There are Fisher projection formulas, which are usually used to depict the spatial structure of optical isomers on a plane.
CH3CH2CH2CH3 and isobutane (CH3)2CHCH3, ethyl alcohol CH3CH2OH and dimethyl ether CH3OCH3. A special case of structural isomerism is tautomerism.
. 2000 .
See what "STRUCTURAL ISOMERIA" is in other dictionaries:
STRUCTURAL ISOMERISM- one of the types (see) chem. compounds having the same composition and molecular weight, but differing in the structure of molecules, physical. and chem. properties. Eg. the formula C2H60 corresponds to two substances with different properties, ethyl alcohol (a) and dimethyl ... ... Great Polytechnic Encyclopedia
One of the types of isomerism chemical compounds. It is due to differences in the arrangement of atoms in a molecule. Examples: normal butane CH3CH2CH2CH3 and isobutane (CH3)2CHCH3, ethyl alcohol CH3CH2OH and dimethyl ether CH3OCH3. A special case… … encyclopedic Dictionary
One of the types of isomerism chem. connections. It is due to differences in the arrangement of atoms in a molecule. Examples: normal butane CH3CH2CH2CH3 and isobutane (CH3)2CHCH3, ethyl alcohol CH3CH2OH and dimethyl ether CH3OCH3. A special case of S. and. ... ... Natural science. encyclopedic Dictionary
The phenomenon consisting in the existence of molecules with the same mol. mass and composition, but differing in the structure or arrangement of atoms in the right and, therefore, in chem. and physical St. you. Such molecules are called i z o m e r a m i. There are two ... ... Physical Encyclopedia
- (from iso ... and Greek meros share part) of chemical compounds, a phenomenon consisting in the existence of isomers of compounds that are identical in composition and molecular weight, but differ in structure ( structural isomerism) or the arrangement of atoms in ... ... Big Encyclopedic Dictionary
Not to be confused with isomerism atomic nuclei. Isomerism (from other Greek ἴσος “equal”, and μέρος “share, part”) is a phenomenon consisting in the existence of chemical compounds (isomers) that are identical in composition and molecular weight, but ... ... Wikipedia
AND; well. Chem. The phenomenon that chemical compounds exist with the same composition And molecular weight, but differing in structure, physical and chemical properties. Practice isomerism. Structural, spatial, rotary ... ... encyclopedic Dictionary
- (from iso ... (see ISO ... (part of compound words)) and Greek meros share, part), the existence of molecules (see MOLECULE) that have the same molar mass and composition, but differ in structure or arrangement atoms in space, and therefore ... encyclopedic Dictionary
Structural chemistry section, a field of chemistry that studies the relationship of various physical and physicochemical properties various substances with their chemical structure and reactivity. Structural chemistry considers not only geometric ... ... Wikipedia
The structural formula is a kind chemical formula, which graphically describes the arrangement and bond order of atoms in a compound, expressed in a plane. Bonds in structural formulas are indicated by valence lines. Often ... ... Wikipedia
