Which compounds are characterized by geometric isomerism. isomerism
Spatial isomers (stereoisomers) have the same qualitative and quantitative composition and the same order of binding atoms (chemical structure), but different spatial arrangement of atoms in the 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.
If there are 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 on opposite sides - tirsh / c-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 hands (from the Greek chair - hand), i.e. are mirror images that do not match when trying to combine them in space.
Configuration includes optical and geometric isomerism.
OPTICAL ISOMERIA
In 1815, J. Biot discovered the existence of optical activity for organic compounds. It was found that some organic compounds have the ability to rotate the plane of polarization of polarized light. Substances that have this ability are called optically active.
If a beam of ordinary light, in which, as is known, electromagnetic oscillations propagate in different planes perpendicular to the direction of its propagation, is passed through a Nicol prism, then the outgoing light will be plane-polarized. In such a beam, electromagnetic oscillations occur only in one plane. This plane is called the plane of polarization (Fig. 3.2).
When a polarized light beam passes through an optically active substance, the polarization plane rotates by a certain angle α to the right or left. If the substance deviates the plane of polarization to the right (when observed towards the beam), it is called right-handed, if to the left - left-handed. The right rotation is indicated by the sign (+), the left - by the sign (-).
Rice. 3.2. Scheme of the formation of polarized light and rotation of the plane of polarization by an optically active substance
Optical activity is measured using instruments called polarimeters.
The phenomenon of optical activity is common among organic substances, especially among natural ones (hydroxy- and amino acids, proteins, carbohydrates, alkaloids).
The optical activity of most organic compounds is due to their structure.
One of the reasons for the appearance of optical activity of organic molecules is the presence in their structure of sp 3 -hybridized carbon atom associated with four different substituents. Such a carbon atom is called chiral or asymmetric. Often a more general name is used for it - a chiral center. In structural formulas, an asymmetric carbon atom is usually denoted by an asterisk - C *:
Compounds containing one asymmetric carbon atom exist as two isomers, related to each other as an object to its mirror image. Such isomers are called enantiomers.
Rice. 3.3. Models of Enantiomeric Molecules of Bromiochloromethane
Stereochemical formulas can be used to depict the spatial structure of optical isomers on a plane. For example, the enantiomers of butanol-2, depicted using stereochemical formulas, are as follows:
However, stereochemical formulas are not always convenient for describing the spatial structure of molecules. Therefore, optical isomers are most often depicted on a plane using Fisher's projection formulas. For example, this is how the enantiomers of 2-bromobutane look like, depicted using the Fischer projection.
Enantiomers are very similar to each other, but nevertheless not identical. They have the same composition and sequence of binding atoms in a molecule, but differ from each other in their relative location in space, i.e., configuration. It is easy to verify that these molecules are different when trying to superimpose their models on each other.
The property of molecules not to be aligned with their mirror image is called chirality (from the Greek, cheir - hand), and molecules are also called chiral. A good example is the left and right hands, which are mirror images of each other, but at the same time they cannot be combined. Molecules that are compatible with their mirror image are called achiral.
The chirality of molecules is a prerequisite for the manifestation of optical activity by a substance.
How to determine if a molecule is chiral? The chirality of a molecule can be easily detected by constructing a model of the molecule and a model of its mirror image, followed by their superposition. If the models do not match, the molecule is chiral; if they match, it is achiral. The same conclusion can be made on the basis of the stereochemical formulas of molecules by the presence of or and the absence of symmetry elements, since the reason for the optical activity of organic compounds is their asymmetric structure. Since the molecule is a three-dimensional formation, its structure can be considered from the point of view of the symmetry of geometric shapes. The main elements of symmetry are the plane, center and axis of symmetry. If there is no plane of symmetry in the molecule, then such a molecule is chiral.
Enantiomers have the same physical and chemical properties (boiling point, melting point, solubility, electrical conductivity and other constants will be the same), rotate the polarization plane of the polarized beam by the same angle, but there are differences.
Enantiomers differ in the sign of rotation, one rotates the plane of polarization of the polarized beam to the left, the other to the right; they react at different rates with other chiral compounds, and there is also a difference in physiological action. For example, the drug levomycin is a broad-spectrum antibiotic. If its efficiency is taken as 100, then the dextrorotatory form will be only 2% of the efficiency of the levorotatory form.
If the molecule has one asymmetric atom, then it exists in the form of two isomers, but if the molecule has several asymmetric carbon atoms, then the number of possible isomers increases. The number of optical isomers is determined by the formula:
where N is the number of isomers; n is the number of asymmetric carbon atoms.
So, if there are two asymmetric carbon atoms in the molecule, the number of isomers is 2 2 \u003d 4, three - 2 3 \u003d 8, four - 2 4 \u003d 16, etc.
For example, bromomalic acid, which contains two asymmetric carbon atoms, exists as four stereoisomers (I–IV).
Stereomers I and II, as well as III and IV, relate to each other as an object and its mirror image and are enantiomers.
Stereoisomers 1 and III, 1 and IV, as well as II and HI, H and IV are not mirror images of each other, they differ in configuration at one of the asymmetric carbon atoms. Such stereoisomers are called diastereomers. Unlike enantiomers, diastereomers have different physical and chemical properties.
For compounds containing two chiral carbon atoms bonded to the same substituents, the total number of stereoisomers is reduced to three. For example, tartaric acid must exist as four stereoisomers (2 2 = 4), but only three are known. This is due to the appearance in one of the stereoisomers of such an element as a plane of symmetry.
Stereomers 1 and II are enantiomers. Stereoisomer III (meso form) is optically inactive. The mesotartaric acid molecule is achiral. Each enantiomer of tartaric acid, relative to the meso form, is a diastereomer.
Nomenclature of optical isomers
In the nomenclature, along with the name of the compound, the configuration and direction of rotation of the plane of polarized light are also indicated. The latter is denoted by a (+) sign for a dextrorotatory isomer or a (-) sign for a levorotatory isomer.
To designate the configuration of optical isomers, there are D, L- and R,S stereochemical systems.
D,L-configuration designation system. Determining the absolute configuration of molecules has proven to be a challenging task for chemists. For the first time, this was possible only in 1951 by the method of X-ray diffraction analysis. Until that time, the configuration of optical isomers was established by comparison with a specially selected standard substance. This configuration is called relative. In 1906, the Russian scientist M.A. Rozanov proposed glyceraldehyde as a standard for establishing the relative configuration,
For the dextrorotatory isomer, the Fischer formula was chosen, in which the hydroxyl group at the chiral carbon atom is on the right, and for the left-handed isomer, on the left. The configuration of the dextrorotatory isomer is denoted by the letter D and the left-handed isomer by L.
Using glyceraldehyde as a comparison standard, a D,L-system for the stereochemical classification of chiral compounds was developed, i.e., assigning compounds to the D- or L-stereochemical series, respectively
The D,L system is mainly used in the series of polyhydric alcohols, hydroxy-, amino acids and carbohydrates:
For compounds with several asymmetric carbon atoms, such as α-pyroxy acids, α-amino acids, tartaric acids, the configuration is conditionally determined by the upper asymmetric carbon atom (by hydroxy acid key), while in the carbohydrate molecule the configuration is (conditionally) determined by the lower asymmetric a carbon atom.
R,S-configuration designation system. D,L- the system turned out to be practically unacceptable for compounds that bear little resemblance to glyceraldehyde. Therefore, R Kahn, K. Ingold and V. Prelog proposed R,S- a system for designating the absolute configuration of optical isomers. R,S- the system is based on determining the seniority of substituents at the chiral center.
The seniority of the substituents is determined by the values of the atomic numbers of the elements. The higher the atomic number, the older the substituent. For example, in the bromiodochloromethane molecule, the seniority of the substituents decreases in the series:
After establishing the seniority of the substituents, the model of the molecule is oriented so that the substituent with the lowest serial number is directed in the direction opposite to the observer's eye. If the seniority of the other three substituents decreases clockwise, then the molecule has a configuration denoted by the letter R (otlat, rectus - right), and if the seniority of the substituents decreases counterclockwise, the configuration is denoted by the letter S (lat. sinister - left). For example, for the bromiodochloromethane molecule:
Fig 3.4. Determination of the configuration by the R,S-system for the bromiodochloromethane molecule
Consider the definition of the seniority of substituents and configuration for more complex molecules using lactic acid as an example (Fig. 3.4). Already in the first layer (8 O, b C, 1 H, 6 C) it becomes clear that the senior substituent is the OH group, and the junior one is hydrogen. To determine the seniority of the other two substituents CH^ and COOH with the same atomic number (6 C) in the first layer, it is necessary to consider the second layer. The sum of the atomic numbers of the second layer of the CH 3 group \u003d 1 + 1 + 1 \u003d 3, and the COOH group \u003d 8 + 8 * 2 \u003d 24. This means that the COOH group is older than the -CH 3 group. decreases in the series: OH > COOH > CH 3 > H
Rice. 3.5. Determination of the configuration by R,S-system for lactic acid
Racemates. A mixture of equal amounts of enantiomers is optically inactive and is called a racemic mixture (racemate). Racemates differ from individual enantiomers in physical properties, they can have different melting points, solubility; differ in their spectral characteristics.
In practice, more often one has to deal not with individual enantiomers, but with racemates, which are formed as a result of chemical reactions that occur with the formation of chiral molecules.
Three methods are used to separate racemates into enantiomers:
1. Mechanical method. As a result of the crystallization of some optically active compounds, two forms of crystals can be formed that are similar to each other as an object and its mirror image. They can be separated under the microscope with a preparative needle (mechanically).
2. The biochemical method is based on the fact that certain types of microorganisms prefer one of the enantiomeric forms and eat it, the second remains and can be easily isolated.
3. Chemical method. The chemical method is based on the conversion of enantiomers with the help of optically active reagents into diastereomers, which already differ from each other in physical properties. Diastereomers are much easier to separate.
For example, a racemic mixture of two acids (A + B) should be separated. To do this, an optically active base (C) is added to the mixture. The reaction between the racemic form and the optically active base is
AC and BC are diastereomers. They have different solubility and the method of successive crystallization can isolate two diastereomers separately.
But since AC and BC are formed by a weak organic acid and base, mineral acids are used to decompose them.
In this way, the pure enantiomers A and B are obtained.
GEOMETRIC Isomerism
The reason for the occurrence of geometric isomerism is the lack of free rotation around the σ-bond. This type of isomerism is typical for compounds containing a double bond and for compounds of the alicyclic series.
Geometric isomers are substances that have the same molecular formula, the same sequence of binding atoms in molecules, but differ from each other in a different arrangement of atoms or atomic groups in space relative to the plane of the double bond or the plane of the cycle.
The reason for the emergence of this type of isomerism is the impossibility of free rotation around the double bond or σ-bonds that form the cycle.
For example, butene-2 CH 3 -CH=CH-CH 3 can exist in the form of 2 isomers, which differ in the arrangement of methyl groups in space relative to the plane of the double bond.
or 1,2-dimethylcyclopropane exists as two isomers that differ in the arrangement of methyl groups in space relative to the ring plane:
The cis-trans system is used to designate the configuration of geometric isomers. If the same substituents are located on the same side of the plane of the double bond or cycle, the configuration is designated cis-. if on different sides - trans-.
For compounds that have different substituents at the carbon atoms with a double bond, the Z,E notation is used.
The Z,E-system is more general. It is applicable to geometric isomers with any set of substituents. This system is based on the seniority of substituents, which is determined separately for each carbon atom. If the senior substituents from each pair are located on one side of the double bond, the configuration is denoted by the letter Z (from German zusammen - together), if on opposite sides - by the letter £ (from German entgegen - opposite).
So for 1-bromo-1-chloropropene, two isomers are possible:
The senior substituent on one carbon atom is a methyl group (substituents 1 H and 6 CH 3). and the other has a bromine atom (substituents 17 Cl and 35 Br). In isomer 1, the senior substituents are located on one side of the double bond plane, it is assigned the Z-configuration, and isomer II is the E-configuration (senior substituents are located on opposite sides of the double bond plane).
Geometric isomers have different physical properties (melting and boiling points, solubility, etc.), spectral characteristics and chemical properties. Such a difference in properties makes it quite easy to establish their configuration using physical and chemical methods.
Recall that isomers are compounds that have the same molecular formula, but a different arrangement of atoms.
Among the various types of isomers, two main ones can be distinguished. Structural isomers, as we have already indicated above, have the same molecular formula, but their atoms are linked to each other in a different sequence. Stereoisomers also have the same molecular formula and even the same atomic binding sequence, but differ in the different spatial arrangement of their atoms. Stereoisomers, in turn, are divided into two categories: geometric and optical isomers.
Before proceeding to consider stereoisomerism of each type, it should be noted that isomers, both structural and stereo, are compounds that can be separated from each other and that have different physical and/or chemical properties.
Stereoisomerism is found not only in organic but also inorganic compounds. For example, a compound exhibits geometric isomerism, while an octahedral complex ion exhibits optical isomerism.
geometric isomerism
Geometric isomers, as their name indicates, differ in the geometric arrangement of their atoms. An example is 1,2-dibromoethene-dibromoethylene). It has a molecular formula and a structural formula A molecule with a structural formula can have two different geometric arrangements of atoms. Compounds with such arrangements of atoms are called cis- and trans-isomers. These isomers differ markedly in physical properties. For example, they have distinctly different melting points and
The word "cis" means "on the same side" and "trans" means "on the opposite side." This type of isomerism is sometimes called cis-trans isomerism. A double bond in a molecule holds the atoms in one of these two positions. Rotation around the double bond is impossible. Let us compare 1,2-dibromoethylene with 1,2-dibromoethane. In the latter compound, free rotation around a simple carbon-carbon bond is possible, so the molecules shown below are not isomers:
These two arrangements are equivalent to the staggered and eclipsed conformations discussed above. A molecule can be in a checkerboard conformation at one point in time, and in a eclipsed conformation at the next point in time. Therefore, it is impossible to isolate molecules in any one conformation. Since the spatial arrangement of atoms is not fixed in any way, and since it is impossible to isolate molecules with the same conformation, it is impossible to consider different conformations as isomers.
In conclusion, it should be noted that a compound with a molecular formula may exist as one of two structural isomers. One of them has a structural formula which, as shown above, can in turn correspond to two geometric isomers. Another structural isomer has the structural formula This structure has no geometric isomers. Formulas are just two different ways of writing the same structure. They correspond to the same connection.
A good example to illustrate the difference in chemical properties between geometric isomers is the geometric isomers of butenedioic acid (Fig. 17.26). Its isomer, which has the trivial name "maleic acid", is characterized by a melting point of 139-140 ° C. When heated to 160 ° C or to 100 ° C under reduced pressure, maleic acid loses water and turns, with a low yield, into maleic anhydride (Fig. 17.26, a). trans-isomer of butenedioic acid, which has the trivial name "fumaric acid", is characterized by a melting point of 287 ° C. When heated to 290 ° C, fumaric acid sublimates. Upon further heating to a temperature of 300 ° C, it rearranges to form the -isomer (maleic acid) and a small amount of maleic anhydride (Fig. 17.26, b). The mechanism of this reaction involves breaking the α-bond between two carbon atoms. This is followed by rotation around the -bond (Fig. 17.26, c) until a new -bond is formed.
Rice. 17.26. Geometric isomerism.
Optical isomerism
If in an organic compound four different atoms or groups are attached to any carbon atom, such an atom is called asymmetric or chiral. A molecule containing one or more asymmetric carbon atoms is usually, though not always, also asymmetric (chiral). An example of a compound with one asymmetric carbon atom is 2-hydroxypropa-oic (lactic) acid. Its central carbon atom is asymmetric because four different atoms or groups are bonded to it. Such a molecule cannot be combined with its mirror image by any rotations in space (Fig. 17.27). Therefore, two molecules that are mirror images of each other are isomers. They are called enantiomers.
Enantiomers can exist in isolation from each other or as a mixture. A mixture containing an equimolar amount (equal number of moles) of each of the two enantiomers is called a racemic mixture. Splitting the racemic mixture in two
Rice. 17.27. 2-Hydroxypropanoic (lactic) acid.
pure enantiomer is called cleavage. The crystals of the two enantiomers are mirror images of each other.
Enantiomers differ only in optical activity, all other physical and chemical properties are the same.
Chirality
If you look at your left hand in the mirror, it looks exactly like this. like a right hand. Thus, the left and right hands of a person are mirror images of each other. Now imagine mentally that you have swapped the left and right hands. At the same time, no matter how hard you try to turn them - up with your palm or down - the left hand will never be identical to the right and, conversely, the right - to the left. Thus, although the right hand is a mirror image of the left hand, one of them cannot be combined with the other. This is the property called chirality. This concept extends to all equivalent, but left-handed and right-handed objects, such as legs. The word "chirality" comes from the Greek word for hand.
Chirality is due to the lack of symmetry, i.e. asymmetry. Any pair of objects that are mirror images of each other, but cannot be aligned by any rotation, is a pair of asymmetrical objects.
What is optical activity? A beam of ordinary light is a stream of electromagnetic waves, the oscillations of which occur in all directions at right angles to the direction of propagation of the light beam. On fig. 17.28,a a beam of light is schematically depicted, in which oscillations occur in four different directions, i.e. in four different planes. The right side of the figure also shows the cross section of these planes. Light that oscillates in only one plane is called plane polarized. Such a light is shown schematically in Fig. 17.28, b.
A compound capable of rotating the plane-polarized light passing through it in such a way that after this the light oscillations occur in another plane is called optically active. For a compound to be optically active, it must be composed of asymmetric molecules (or ions). Everything
Rice. 17.28. Unpolarized (a) and plane polarized (b) light.
compounds containing one asymmetric carbon atom exhibit optical activity.
All enantiomers have this property. Therefore, sometimes they are also called optical isomers. If one enantiomer rotates the plane of polarization of light clockwise, then the other enantiomer necessarily rotates it counterclockwise. Substances that rotate the plane of polarization of light clockwise are called dextrorotatory (Fig. 17.29). The dextrorotatory enantiomer is denoted by the symbol Substances that rotate plane polarized light counterclockwise are called levorotatory. The left-handed enantiomer is denoted by the symbol
Optical isomerism is of great importance in biochemistry. For example, all amino acids from which proteins are synthesized have optical activity, with the exception of the simplest amino acid of the first member of the amino acid series, which does not contain an asymmetric carbon atom. On fig. 17.30 amino acids are shown, the systematic name (according to the IUPAC nomenclature) of which is 2-aminopropanoic acid, and the trivial name is alanine. Only this amino acid is found in nature (it is shown on the left in the figure). If we replace the group in alanine with an arbitrary group R, then it is easy to see that all other naturally occurring amino acids have the same configuration as y. However, the sign of rotation can be either depending on the specific nature of the R group. Many carbohydrates also have optical activity. Let's take glucose as an example.
Rice. 17.29. optical rotation.
Rice. 17.30. 2-Aminopropanoic acid (alanine).
We have already noted that a pair of enantiomers always has the same chemical and physical properties, with the exception of optical activity. However, the chemical activity of each compound from a pair of enantiomers can be completely different in reactions with other optically active compounds. Such stereospecificity characterizes many biochemical reactions. This is especially true for enzymes.
Polarimeter
The angle of rotation of the plane of polarization of light by any enantiomer is determined by its nature and, thus, is one of its characteristics. To measure this angle of rotation, a special device is used - a polarimeter (its diagram is shown in Fig. 17.31). This instrument typically uses a monochromatic light source such as a sodium lamp. Monochromatic light is characterized by a single wavelength, while ordinary white light is a mixture of all wavelengths within the visible range. Monochromatic light is not polarized. Therefore, it is first passed through a polarizer, which turns it into plane polarized light. Then, plane-polarized light is passed through a cuvette with a solution of the substance for which the optical rotation angle is measured. Light leaving the sample cell has a polarization plane rotated clockwise or counterclockwise by some angle, which is to be measured. The direction of rotation is determined with respect to the observer, and the angle of rotation is determined using a special analyzer, which is available in the polarimeter. The analyzer is a device that transmits only plane polarized light. First, it is set so as to transmit plane-polarized light that has exited the polarizer but has not been rotated by the sample. Plane polarized light, rotated by the sample, cannot pass through the analyzer, which is in its original position. The analyzer is then slowly rotated until it transmits as much light as possible through the sample cuvette. In this position, the plane of transmission of the analyzer coincides with the plane of polarization of the light that has passed through the sample (Fig. 17.31, b). The angle difference between the initial and final positions of the analyzer determines the angle of optical rotation of the test substance.
Rice. 17.31. Scheme of the device of the polarimeter.
Laboratory polarimeter.
Compounds containing two or more asymmetric carbon atoms
A compound containing two or more asymmetric carbon atoms may exist as three or more stereoisomers. As an example, consider 2,3-dihydroxybutanedioic acid (its trivial name is tartaric acid). This compound has two asymmetric carbon atoms, which are marked with asterisks in Fig. 17.32. 2,3-Dihydroxybutanedioic acid has three stereoisomers. Their Newman projections are shown in Fig. 17.32. All three isomers can exist in staggered, eclipsed, or intermediate, twisted, conformations. Two of the three isomers are completely asymmetric. They have neither a plane of symmetry nor a center of symmetry, no matter how one half is oriented in relation to the other. These two isomers are incompatible mirror images of one another. Therefore, they form a pair of enantiomers. On fig. 17.32 they are shown in a checkerboard conformation. Two asymmetric carbon atoms of one of these enantiomers rotate plane polarized light to the right. Therefore, this enantiomer is dextrorotatory and is denoted by the symbol (+). Asymmetric carbon atoms in the other enantiomer rotate plane polarized light to the left. Therefore, this enantiomer is left-handed and is denoted by the symbol (-).
The third stereoisomer also has two asymmetric carbon atoms, but in general its molecule is symmetrical. It has a plane of symmetry perpendicular to the bond line between the two central carbon atoms. This stereoisomer is shown in the eclipsed conformation in Fig. 17.32. Since its molecule is symmetrical, it is not optically active. One of its asymmetric carbon atoms rotates
Rice. 17.32. 2,3-Dihydroxybutanedioic (tartaric) acid.
plane polarized light to the right, and the other rotates it by the same angle to the left. The resulting effect is zero.
If any stereoisomer containing two or more asymmetric carbon atoms is optically inactive due to the presence of symmetry in its molecule, then it is considered to be internally compensated. A stereoisomer that has no optical activity (and therefore is not an enantiomer) is called a diastereoisomer. Thus, shown in Fig. The 17.32 isomer in the eclipsed conformation is a diastereoisomer.
A racemic mixture of two enantiomers is also optically inactive, since the dextrorotatory action of one enantiomer is compensated by the levorotatory action of the other enantiomer. The resulting rotation is zero. The racemic mixture is denoted by the symbol (+) and is considered externally compensated.
So let's do it again!
1. Methane has a tetrahedral molecular structure.
2. Ethylene has a planar molecular structure.
3. Acetylene has a linear molecular structure.
4. p-electrons in the cyclic structure of an aromatic compound are delocalized, forming an -electron cloud.
5. Conformations are different spatial arrangements of atoms in a molecule.
6. Chessboard and eclipsed conformations are the limiting types of conformations. Twisted conformations are intermediate cases between these two limiting conformations.
7. The cyclic structure of cyclohexane can be either chair conformation or bath conformation.
8. Stereoisomers have different spatial arrangement of their atoms.
9. Geometric isomers differ in the geometric arrangement of their atoms. The cis-trans isomerism is an example of this type of isomerism.
10. Optical isomers are molecules that are incompatible mirror images of each other. They are also called enapthiomers.
11. A racemic mixture contains equimolar amounts of each of a pair of enantiomers. Such a mixture is optically inactive.
12. A compound that rotates plane polarized light is called optically active.
13. The molecules of an optically active compound are asymmetric.
14. To measure the angle of rotation of plane polarized light by any enantiomer, a polarimeter is used.
During the lesson, you will get a general idea of 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
1. What is isomerism
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 is a liquid that reacts with sodium metal to release hydrogen, 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 correspond 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
There are different types of isomerism.
2. Interclass isomerism
Structural isomerism is associated with a different order of 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.
3. Isomerism in the carbon skeleton
Structural isomers can also be within the same class of compounds, for example, the formula C5H12 corresponds to three different hydrocarbons. This isomerism of the carbon skeleton. Rice. 2.
Rice. 2 Examples of substances - structural isomers
4. Position isomerism
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
5. Spatial 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
6. Optical isomerism
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 chier - "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 the 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 a 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. Fundamentals of General Chemistry. Grade 10: textbook for educational institutions: basic level / 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 .: Bustard, 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.: Bustard, 2010. - 462 p.
4. Khomchenko G. P., Khomchenko I. G. Collection of problems in chemistry for university students. - 4th ed. - M.: RIA "New Wave": Publisher Umerenkov, 2012. - 278 p.
1. Interneturok. ru.
2. Organic chemistry.
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?
Fix the material with the help of simulators
Trainer 1 Trainer 2 Trainer 3
1. Structural isomerism.
2. Conformational isomerism.
3. Geometric isomerism.
4. Optical isomerism.
Isomers are substances that have the same composition and molecular weight, but different physical and chemical properties. Differences in the properties of isomers are due to differences in their chemical or spatial structure. In this regard, there are two types of isomerism.
isomerism
structural
spatial
carbon skeleton
Configuration
conformational
The position of the functional
Optical
Interclass
Geometric
1. Structural isomerism
Structural isomers differ in chemical structure, i.e. the nature and sequence of bonds between atoms in a molecule. Structural isomers are isolated in pure form. They exist as individual, stable substances, their mutual transformation requires high energy - about 350 - 400 kJ / mol. Only structural isomers, tautomers, are in dynamic equilibrium. Tautomerism is a common phenomenon in organic chemistry. It is possible with the transfer of a mobile hydrogen atom in a molecule (carbonyl compounds, amines, heterocycles, etc.), intramolecular interactions (carbohydrates).
All structural isomers are presented in the form of structural formulas and named according to the IUPAC nomenclature. For example, the composition of C 4 H 8 O corresponds to structural isomers:
but)with different carbon skeleton
unbranched C-chain - CH 3 -CH 2 -CH 2 -CH \u003d O (butanal, aldehyde) and
branched C-chain -
(2-methylpropanal, aldehyde) or
cycle - 
(cyclobutanol, cyclic alcohol);
b)with a different position of the functional group
butanone-2, ketone;
in)with different composition of the functional group
3-butenol-2, unsaturated alcohol;
G)metamerism
The heteroatom of the functional group may be included in the carbon skeleton (cycle or chain). One of the possible isomers of this type of isomerism is CH 3 -O-CH 2 -CH \u003d CH 2 (3-methoxypropene-1, simple ether);
e)tautomerism (keto-enol)
enol form 
keto form
The tautomers are in dynamic equilibrium, while the more stable form, the keto form, predominates in the mixture.
For aromatic compounds, structural isomerism is considered only for the side chain.
2. Spatial isomerism (stereoisomerism)
Spatial isomers have the same chemical structure, differ in the spatial arrangement of atoms in the molecule. This difference creates a difference in physical and chemical properties. Spatial isomers are depicted as various projections or stereochemical formulas. The branch of chemistry that studies the spatial structure and its influence on the physical and chemical properties of compounds, on the direction and rate of their reactions, is called stereochemistry.
but)Conformational (rotational) isomerism
Without changing either bond angles or bond lengths, one can imagine a multitude of geometric shapes (conformations) of a molecule that differ from each other by the mutual rotation of carbon tetrahedra around the σ-C-C bond connecting them. As a result of such rotation, rotational isomers (conformers) arise. The energy of different conformers is not the same, but the energy barrier separating different conformational isomers is small for most organic compounds. Therefore, under normal conditions, as a rule, it is impossible to fix molecules in one strictly defined conformation. Usually, several conformational isomers coexist in equilibrium.
The image methods and the nomenclature of isomers can be considered using the example of the ethane molecule. For it, one can foresee the existence of two conformations that differ as much as possible in energy, which can be represented as perspective projections(1) ("sawhorses") or projections Newman(2):
hindered conformation eclipsed conformation
In a perspective projection (1), the C-C connection must be imagined as going into the distance; the carbon atom standing on the left is close to the observer, standing on the right is removed from it.
In the Newman projection (2), the molecule is viewed along the C-C bond. Three lines diverging at an angle of 120 o from the center of the circle indicate the bonds of the carbon atom closest to the observer; the lines "protruding" from behind the circle are the bonds of the remote carbon atom.
The conformation shown on the right is called obscured . This name is reminiscent of the fact that the hydrogen atoms of both CH 3 groups are opposite each other. The shielded conformation has an increased internal energy and is therefore unfavorable. The conformation shown on the left is called inhibited , implying that the free rotation around the C-C bond "slows down" in this position, i.e. the molecule exists predominantly in this conformation.
The minimum energy required for complete rotation of a molecule around a particular bond is called the rotational barrier for that bond. The rotational barrier in a molecule like ethane can be expressed in terms of the change in the potential energy of the molecule as a function of the change in the dihedral (torsion - τ) angle of the system. The energy profile of rotation around the C-C bond in ethane is shown in Figure 1. The rotational barrier separating the two forms of ethane is about 3 kcal/mol (12.6 kJ/mol). The minima of the potential energy curve correspond to hindered conformations, the maxima correspond to obscured ones. Since at room temperature the energy of some collisions of molecules can reach 20 kcal / mol (about 80 kJ / mol), this barrier of 12.6 kJ / mol is easily overcome and rotation in ethane is considered as free. In a mixture of all possible conformations, hindered conformations predominate.
Fig.1. Potential energy diagram of ethane conformations.
For more complex molecules, the number of possible conformations increases. Yes, for n-butane can already be depicted in six conformations that arise when turning around the central bond C 2 - C 3 and differ in the mutual arrangement of CH 3 groups. The various eclipsed and hindered conformations of butane differ in energy. Hindered conformations are energetically more favorable.
The energy profile of rotation around the C 2 -C 3 bond in butane is shown in Figure 2.
Fig.2. Potential energy diagram of n-butane conformations.
For a molecule with a long carbon chain, the number of conformational forms increases.
The molecules of alicyclic compounds are characterized by different conformational forms of the ring (for example, for cyclohexane armchair, bath, twist-forms).
So, conformations are various spatial forms of a molecule that has a certain configuration. Conformers are stereoisomeric structures that correspond to energy minima on the potential energy diagram, are in mobile equilibrium and are capable of interconversion by rotation around simple σ-bonds.
If the barrier of such transformations becomes high enough, then stereoisomeric forms can be separated (an example is optically active biphenyls). In such cases, one speaks no longer of conformers, but of actually existing stereoisomers.
b)geometric isomerism
Geometric isomers arise as a result of the absence in the molecule:
1. rotation of carbon atoms relative to each other - a consequence of the rigidity of the C=C double bond or cyclic structure;
2. two identical groups at one carbon atom of a double bond or cycle.
Geometric isomers, unlike conformers, can be isolated in pure form and exist as individual, stable substances. For their mutual transformation, a higher energy is required - about 125-170 kJ / mol (30-40 kcal / mol).
There are cis-trans-(Z,E) isomers; cis- forms are geometric isomers in which the same substituents lie on one side of the plane of the π-bond or cycle, trance- forms are called geometric isomers, in which the same substituents lie on opposite sides of the plane of the π-bond or ring.
The simplest example is the isomers of butene-2, which exists in the form of cis-, trans-geometric isomers:
cis-butene-2 trans-butene-2
melting temperature
138.9 0 С - 105.6 0 С
boiling temperature
3.72 0 С 1.00 0 С
density
1,2 - dichlorocyclopropane exists in the form of cis-, trans-isomers:
cis-1,2-dichlorocyclopropane trans-1,2-dichlorocyclopropane
In more complex cases, apply Z,E-nomenclature (the nomenclature of Kann, Ingold, Prelog - KIP, the nomenclature of seniority of deputies). In conjunction
1-bromo -2-methyl-1-chlorobutene-1 (Br) (CI) C \u003d C (CH 3) - CH 2 -CH 3 all substituents at carbon atoms with a double bond are different; therefore, this compound exists in the form of Z-, E- geometric isomers:
Е-1-bromo-2-methyl-1-chlorobutene-1 Z-1-bromo-2-methyl-1-chlorobutene-1.
To indicate the configuration of an isomer, indicate the location of senior substituents in a double bond (or cycle) - Z- (from the German Zusammen - together) or E- (from the German Entgegen - opposite).
In the Z,E-system, substituents with a higher atomic number are considered senior. If the atoms directly bonded to unsaturated carbon atoms are the same, then they go to the "second layer", if necessary, to the "third layer", etc.
In the first projection, the older groups are opposite each other relative to the double bond, so this is the E isomer. In the second projection, the older groups are on the same side of the double bond (together), so this is the Z-isomer.
Geometric isomers are widely distributed in nature. For example, natural polymers rubber (cis-isomer) and gutta-percha (trans-isomer), natural fumaric (trans-butenedioic acid) and synthetic maleic (cis-butenedioic acid) acids, fats contain cis-oleic, linoleic, linolenic acids.
in)Optical isomerism
Molecules of organic compounds can be chiral and achiral. Chirality (from the Greek cheir - hand) - the incompatibility of a molecule with its mirror image.
Chiral substances are able to rotate the plane of polarization of light. This phenomenon is called optical activity, and the corresponding substances - optically active. Optically active substances occur in pairs optical antipodes- isomers, the physical and chemical properties of which are the same under normal conditions, with the exception of one - the sign of rotation of the polarization plane: one of the optical antipodes deflects the polarization plane to the right (+, dextrorotatory isomer), the other - to the left (-, levorotatory). The configuration of optical antipodes can be determined experimentally using a device - a polarimeter.
Optical isomerism appears when the molecule contains asymmetric carbon atom(there are other reasons for the chirality of the molecule). This is the name of the carbon atom in sp 3 - hybridization and associated with four different substituents. Two tetrahedral arrangements of substituents around an asymmetric atom are possible. At the same time, two spatial forms cannot be combined by any rotation; one of them is a mirror image of the other:
Both mirror forms form a pair of optical antipodes or enantiomers .
Depict optical isomers in the form of E. Fisher projection formulas. They are obtained by projecting a molecule with an asymmetric carbon atom. In this case, the asymmetric carbon atom itself on the plane is indicated by a dot, the symbols of substituents protruding in front of the plane of the figure are indicated on the horizontal line. The vertical line (dashed or solid) indicates the substituents that are removed from the plane of the figure. The following are different ways to write the projection formula corresponding to the left model in the previous figure:
In projection, the main carbon chain is depicted vertically; the main function, if it is at the end of the chain, is indicated at the top of the projection. For example, the stereochemical and projection formulas (+) and (-) of alanine - CH 3 - * CH (NH 2) -COOH are as follows:
A mixture with the same content of enantiomers is called a racemate. The racemate has no optical activity and is characterized by physical properties different from the enantiomers.
Rules for transforming projection formulas.
1. Formulas can be rotated in the plane of the drawing by 180 o without changing their stereochemical meaning:
2. Two (or any even number) permutations of substituents on one asymmetric atom do not change the stereochemical meaning of the formula:
3. One (or any odd number) permutation of substituents at the asymmetric center leads to the optical antipode formula:
4. Turning in the plane of the drawing by 90 turns the formula into an antipode.
5. Rotation of any three substituents clockwise or counterclockwise does not change the stereochemical meaning of the formula:
6. Projection formulas cannot be derived from the plane of the drawing.
Organic compounds have optical activity, in the molecules of which other atoms are chiral centers, for example, silicon, phosphorus, nitrogen, and sulfur.
Compounds with multiple asymmetric carbons exist as diastereomers , i.e. spatial isomers that do not constitute optical antipodes with each other.
Diastereomers differ from each other not only in optical rotation, but also in all other physical constants: they have different melting and boiling points, different solubilities, etc.
The number of spatial isomers is determined by the Fisher formula N=2 n , where n is the number of asymmetric carbon atoms. The number of stereoisomers may decrease due to partial symmetry appearing in some structures. Optically inactive diastereomers are called meso-forms.
Nomenclature of optical isomers:
a) D-, L- nomenclature
To determine the D- or L-series of the isomer, the configuration (the position of the OH group at the asymmetric carbon atom) is compared with the configurations of the enantiomers of glyceraldehyde (glycerol key):
L-glyceraldehyde D-glyceraldehyde
The use of D-, L-nomenclature is currently limited to three classes of optically active substances: carbohydrates, amino acids and hydroxy acids.
b) R -, S-nomenclature (nomenclature of Kahn, Ingold and Prelog)
To determine the R (right) - or S (left) - configuration of the optical isomer, it is necessary to arrange the substituents in the tetrahedron (stereochemical formula) around the asymmetric carbon atom so that the lowest substituent (usually hydrogen) has the direction "from the observer". If the transition of the other three substituents from senior to middle and junior in seniority occurs clockwise, this is the R-isomer (the fall in seniority coincides with the movement of the hand when writing the upper part of the letter R). If the transition occurs counterclockwise - this is S - isomer (the fall in seniority coincides with the movement of the hand when writing the upper part of the letter S).
To determine the R- or S-configuration of the optical isomer by the projection formula, it is necessary to arrange the substituents by an even number of permutations so that the youngest of them is at the bottom of the projection. The fall in the seniority of the remaining three substituents clockwise corresponds to the R-configuration, counterclockwise - to the S-configuration.
Optical isomers are obtained by the following methods:
a) isolation from natural materials containing optically active compounds, such as proteins and amino acids, carbohydrates, many hydroxy acids (tartaric, malic, mandelic), terpene hydrocarbons, terpene alcohols and ketones, steroids, alkaloids, etc.
b) cleavage of racemates;
c) asymmetric synthesis;
d) biochemical production of optically active substances.
DO YOU KNOW THAT
The phenomenon of isomerism (from Greek - isos - different and meros - share, part) was opened in 1823. J. Liebig and F. Wöhler on the example of salts of two inorganic acids: cyanic H-O-C≡N and fulminant H-O-N= C.
In 1830, J. Dumas extended the concept of isomerism to organic compounds.
In 1831 the term "isomer" for organic compounds was proposed by J. Berzelius.
Stereoisomers of natural compounds are characterized by different biological activity (amino acids, carbohydrates, alkaloids, hormones, pheromones, medicinal substances of natural origin, etc.).
