Which of them have geometric isomers. Spatial isomerism

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 depicted in the form 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 also 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.).

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.

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 as two isomers that 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:

To designate the configuration of geometric isomers, the cis-, trans-system is used. If the same substituents are located on the same side of the plane of the double bond or ring, the configuration is designated cis-. if on different sides - trans-.

CONFORMATIONAL ISOMERISM

Conformational (rotational) isomerism is due to the rotation of atoms or atomic groups around one or more simple σ-bonds. As a result of rotation around C-C bonds, molecules can have various spatial forms, which are called conformations.

For example, an ethane molecule, due to rotation around a carbon-carbon bond, can take on an infinite number of conformations. each of which is characterized by a certain value of potential energy. The two extreme conformations are called eclipsed and hindered.

In the eclipsed conformation of ethane, the hydrogen atoms of the methyl groups, when viewed along the carbon-carbon bond, are located one behind the other. In inhibited - the hydrogen atoms of one methyl group are as far as possible from the hydrogen atoms of the other. Between the eclipsed and hindered conformations, the molecule assumes many oblique conformations during rotation.

Each of the conformations of the ethane molecule is characterized by a different potential energy. The shielded conformation has the maximum energy, while the hindered conformation has the minimum.

The hindered conformation, in which the methyl groups (bulky substituents) are as far apart as possible, is called the anti-conformation. Another hindered conformation is called the gauche conformation.

The hindered gauche conformation has a somewhat higher potential energy (due to the methyl-methyl interaction) than the anti-conformation (there is no interaction between methyls in it at all).

Conformations with the lowest energy reserve are called conformers or conformational (rotational) isomers.

Thus, n-butane at 25°C exists approximately 70% as an anticonformer and 30% as a gauche conformer.

Unlike configurational isomers, conformers are converted into each other without breaking chemical bonds and cannot be separated. They are detected only by physicochemical methods.

The reason for the occurrence of geometric isomerism is the lack of free rotation around the st-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 st-bonds that form the cycle.

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

or t,2-dimtetylcytopropane exists as two isomers,

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

/to designate the configuration of geometric isomers using cis-, trans-system. If the same substituents

3. Isomerism of organic compounds. Spatial structure of molecules

ra ^ p ^ lizh ^ psh along one line from the flatness of the double ^ dajw or

cycle, the configuration is denoted as cis-, if on different sides, - trans-.

For compounds in which different substituents are found at carbon atoms with a double bond, the E^-system of denominations is used.

The EZ System is more general. Opa is applicable to geometric isomers with any set of substituents. The basis of this system is the seniority of substituents, which is determined separately for each carbon atom. If the senior substituents from each pair are located one stop from the double bond, the copy figure is denoted by the letter Z (from the letter zusammen - together), if they are located in different stops, by the letter E (from the letter entgegen - opposite).

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

senior! the substituent on one carbon atom is me-

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