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Geometric isomers. Geometric isomerism

Ligand isomerism

Ligand isomerism is divided into bond isomerism (which is determined by different types of coordination of the same ligand) and isomerism proper of the ligand.

Examples of bond isomerism can be the existence of nitro- and nitrite-complexes of cobalt(III) of the composition K 3 and K 3 , in which the coordination of the NO 2 - ligand is carried out, respectively, through a nitrogen atom or an oxygen atom. Another example is the coordination of the thiocyanate ion NCS - through a nitrogen atom or through a sulfur atom, with the formation of thiocyanato-N- or thiocyanato-S-complexes.

In addition, ligands of complex structure (for example, amino acids) can themselves form isomers, the coordination of which leads to the formation of complexes of the same composition with different properties.

Geometric isomerism caused by unequal placement of ligands in the inner sphere relative to each other. A necessary condition for geometric isomerism is the presence of at least two different ligands in the inner coordination sphere. Geometric isomerism manifests itself predominantly in complex compounds that have an octahedral structure, the structure of a flat square or a square pyramid.

Complex compounds with tetrahedral, triangular and linear structures do not have geometric isomers, since the locations of ligands of two different types around the central atom are equivalent.

Complexes with a flat square structure, in the presence of two different ligands L ′ and L ′′, can already have two isomers (cis- and trans-):

An example of a complex compound having cis- and trans-isomers is dichlorodiammineplatinum(II):

Note that a complex compound of a composition with a flat square structure cannot have isomers: the position of the ligand L′′ is equally probable in any corner of the square. When two different ligands appear, then the existence of two isomers (cis- and trans-), differing in properties, is already possible. Thus, cis-dichlorodiammineplatinum (II) is orange-yellow crystals, highly soluble in water, and trans-dichlorodiammineplatinum(II) is pale yellow crystals, the solubility of which in water is slightly lower than that of the cis isomer.

As the number of different ligands in the inner sphere increases, the number of geometric isomers increases. For nitro(hydroxylamine)ammine(pyridine)platinum(II)Cl chloride, all three isomers were obtained:

Octahedral complexes can have many isomers. If in a complex compound of this kind all six ligands are the same () or only one is different from all the others (), then there is no possibility of different arrangements of the ligands in relation to each other. For example, in octahedral compounds, any position of the L′′ ligand in relation to the other five L′ ligands will be equivalent and therefore there should be no isomers here:

Appearance two L′′'s ligands octahedral complex compounds will lead to the possibility of the existence two geometric isomers. In this case, two different ways of arranging the L′′ ligands relative to each other appear. For example, the dihydroxotetraammine cobalt(III) + cation has two isomers:

When trying to find some other relative arrangement of the H 3 N and OH - ligands, which would differ from those indicated above, we will always end up with the structure of one of the ones already given.

As the number of ligands with different chemical compositions in the complex increases, the number of geometric isomers increases rapidly. Compounds of the type will have four isomers, and for compounds of the type containing six different ligands, the number of geometric isomers reaches 15. Such complex compounds have not yet been well studied.

Geometric isomers differ significantly in physical and chemical properties, such as color, solubility, density, crystal structure, etc.

Geometric isomers arise if the free rotation of atoms in a molecule is limited due to the presence of a double bond. An example of such a pair of isomers is maleic (12.23) and fumaric (12.24) acids (cis- and trans-, respectively).

The geometric isomers are very similar in chemical structure, but they are not mirror images of each other and do not rotate the plane of polarization of light. As a rule, cis- and trans-isomers differ significantly in physical properties. For example, maleic acid (12.23) melts at 130 °C, its pKa value is 1.9, it is very soluble in cold water (79 g per 100 ml); the constants of its geometric isomer - fumaric acid (12.24) are respectively 287 °C, 3.0 and 0.7 g per 100 ml. It is not surprising that geometric isomers have different biological properties and therefore it is very important when studying the chemical formula of a new compound to take into account all the possibilities for the existence of this type of isomerism.

Cis and trans isomers can be easily separated by crystallization or chromatography. There is no general method for converting one isomer to another, but heating usually produces the most stable isomer, while exposure to light produces a less stable one. Human vision depends on the conversion of the 11-cis isomer of retinal to the 11-trans form under the influence of light. As soon as the exciting beam of light disappears,
This carotenoid pigment again goes into the cis form, thereby interrupting the impulse going to the brain. Cis and trans isomers also exist on the flat cyclopentane ring, which is like a large double bond. Although the cyclohexane ring is not flat at all, it is nevertheless flat enough to form cis and trans isomers. Thus, both cis (12.25) and trans (12.26) forms of diaminocyclohexane exist and are available. The same molecule can form both geometric and optical isomers. For example, the trans isomer (12.26) can be separated into (S,S) (12.27) and (R.R) (12.28) chiral isomers. However, the cis isomer cannot be divided into chiral forms, since it has a plane of symmetry. The benzene ring has no geometric isomers because each carbon atom of the ring has only one substituent.

Cis-Trans-TpaHC-(S S) TpaHC-(R.R)

(12.25) (12.26) (12.27) (12.28)

Stereoisomers of 1,2-diaminocinlogensane

It can sometimes be difficult to select two of the four substituents on a double bond to determine the cis or trans configuration. The sequence rule dictates that substituents with the heaviest atoms be selected, with the cis form designated by the letter Z (from the German word zusammen) and the transform by the letter E (entgegen). Sometimes in the names of compounds in which geometric isomerism can occur many times, the substituent with the smallest number (according to the numbering rule) is designated by the letter g, and the designations c-(cis) and t-(trans) in front of other substituents indicate their position in relation to to Mr. Deputy.

Similar to indole-3-ylacetic acid (4.82), which stimulates the growth of plant cells, other carboxylic acids can act, the carboxyl group of which is at an angle to the plane of the aromatic ring. Geometric isomerism limits the possibility of such an arrangement of two substituents, therefore, of the cinnamic acids, only the cis isomer is active. In 2-phenylcyclopropane-1-carboxylic and 1,2,3,4-tetrahydronaphthalidene-acetic acids, only cis-isomers are also active. Molecular models show that the ring and carboxyl group in the trans-isomer (inactive) of these substances lie in the same plane, while in the cis-form (active) they are non-coplanar. Veidstra was the first to point out this connection between noncoplanarity and growth-promoting activity. Non-coplanarity can also arise due to steric hindrance. Thus, benzoic acid has a planar shape and is not active, while 2,6-dichlorobenzoic and 8-methyl-1-naphthoic acids are non-planar and biologically active.

In auxin analogs, the carboxyl group can be replaced by other electron-withdrawing groups (-CN, -N02, -SO3H), while the biological activity is only slightly reduced. On the relationship between structure and action in this series, see Koepfli, Thimann, Went (1938) and Veidstra

Geometric isomerism of steroids deserves special consideration. Formula (12.29) shows the general structure of this group of natural saturated compounds (the numbering of carbon atoms and the letter designations of the four cycles are shown). In natural steroids, the B and C rings are in a trans junction, with both of them anchored in a chair conformation. In cardiac glycosides, the junction of cycles C and D has a cis configuration, but in animal hormones, sterols and bile acids it is a trans junction. In most biologically active steroids, rings A and B are in a trans-junction (“5a” series, previously called “allo”). Each of the rings in the steroid molecule forms folds, which is clearly visible in the side projection of formula (12.30).

The designation "5a" indicates that the hydrogen atom at position 5 is below the general plane of the rings. All substituents located below this plane are designated by the symbol “a”, and above - by the symbol shown in formulas (12.34) and (12.35).

tion of substituents in space and the shape of the cycle. For example, cyclohexane can exist in three conformations: chair (12.36), bathtub (12.37), and twisted (or half-chair) (12.38).

The chair shape is the least stressed and therefore most preferable for the cyclohexane molecule, with each axial hydrogen atom removed by 0.25 nm from the other two axial hydrogen atoms located on the same side of the cycle. The twisted (or twist) shape occupies an intermediate position (between the chair and boat shapes), and the most intense is the bathtub shape. The latter, however, can be stabilized by two or more fused rings containing appropriate substituents. The decahydronaphthalene (decalin) molecule can exist in two stable forms, the structure of which was determined by electron diffraction, which showed that the trans form (melting point -30 ° C, boiling point 117 ° C at 100 mm Hg) consists of two trans-fused rings in the chair conformation, and the cis form (melting point -43 °C, boiling point 124 °C at 100 mmHg) consists of two cis-fused rings in the chair conformation. The cis form transforms into the trans form at elevated temperatures and in the presence of a catalyst. The decalin molecule is an example of geometric isomerism relative to the bridging carbon atoms, but each cycle remains conformationally mobile.

Of interest is the conformational analysis of the -CO-H- bond, which determines the structure of the peptides. In proteins, its conformation is always “extended” (trans), although in peptides containing proline, i.e., a tertiary amide group, the proportion of the equilibrium “occluded” (cis) conformation is quite large (up to 40%). In secondary formamides, there is an equilibrium mixture of “extended” and “occluded” conformers in a ratio of 8:92 in the case of N-methylformamide (12.39, 12.40). Their existence is established by the presence of two separate signals in the PMR spectra. With increasing volume of the substituent at the nitrogen atom, the proportion of the “extended” conformer increases, reaching 18% when replacing the methyl group with tert-butyl.

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.

References

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

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

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

Homework

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

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

3. Which hydrocarbons have spatial isomers?

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

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

Rice. 3. Structural position isomerism

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

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

Rice. 5. Cis and trans isomers

Rice. 6. Examples of optical isomers

Summing up the lesson

References

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

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

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

Homework

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

3. Which hydrocarbons have spatial isomers?

The reason for the occurrence of geometric isomerism is the absence of free rotation around the c-bond. This type of isomerism is characteristic of compounds containing a double bond and of alicyclic compounds.

Geometric isomers are substances that have the same molecular formula, the same sequence of bonding of atoms in molecules, but differ from each other in the different arrangement of atoms or atomic groups in space relative to the plane of the double bond or the ring plane.

The reason for the occurrence of this type of isomerism is the impossibility of free rotation around the double bond or st-bonds forming the cycle.

For example, butene-2 CH3-CH=CH-CH3 can exist in the form of two isomers, which differ in the location of the methyl groups in space relative to the plane of the double bond:

or t,2-dimthetylcyctopropane exists in the form of two isomers,

which differ in the arrangement of methyl groups in space relative to the ring plane:

/ to denote the configuration of geometric isomers, the cis-, trans-system is used. If the same substituents

3. Isomerism of organic compounds. Spatial structure of molecules

ra^p^lij^psh along one side from the flatness of the double ^dajw or

cycle, the configuration is designated cis-, if on opposite sides, trans-.

For compounds that have different substituents at the carbon atoms with a double bond, the E^-system of notations is used.

The EZ-System is more general. It is applicable to geometric isomers with any set of substituents. The basis of this system is the seniority of substituents, which is determined for each carbon atom separately. If the senior substituents from each pair are located along the same side of the double bond, the cop-figuration is denoted by the letter Z (from the letter zusammen - together), if on opposite sides, by the letter E (from the letter entgegen - opposite).

Thus, for 1-bromo-1-chloropropep, two isomers are possible:

seniors! the substituent on one carbon atom is me-

tyl group (1H and 6CH3 substituents), and the other has a bromine atom (17C1 and 35Br substituents). In isomer I, the senior substituents are located on one side of the double bond plane; it is assigned a Z-configuration, and isomer II is assigned an 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. This difference in properties makes it quite easy to establish their configuration using physical and chemical methods.

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