Structural and geometric isomerism.

Alkenes, ethylene hydrocarbons or olefins (oil-forming) are hydrocarbons whose molecules contain at least two carbon atoms connected to each other by two bonds. These atoms are in the state of sp 2 hybridization.

Alkenes form a homologous series with the general formula C n H 2n.

The first member of the homologous series is ethylene having the molecular formula C 2 H 4 and structural formula CH 2 \u003d CH 2. Due to the peculiarity of sp 2 hybridization, the ethylene molecule has a planar structure. The presence of the π-bond eliminates the possibility of free rotation around the carbon-carbon bond. Therefore, the bonds of carbon atoms spent on connecting with other atoms or groups are rigidly located in the same plane at an angle of 120 0 to each other. The rigid structure of the double bond system in alkene molecules causes certain features in their structure.

The structure of alkene molecules suggests the existence of three types of isomerism:

1. Isomerism of the carbon skeleton in radicals with more than two carbon atoms.

2. Isomerism of the position of the double bond. For example:

3. Geometric or cis –, trance-isomerism

Geometric isomers are spatial or stereoisomers that differ in the position of the substituents relative to the double bond. Due to the lack of the possibility of rotation around the double bond, substituents can be located either on one side of the double bond or on opposite sides. For example:

Nomenclature, E, Z-nomenclature.

There are also three nomenclatures for alkenes: trivial, rational, and systematic.

Trivial names:

According to the rational nomenclature, alkane is considered as a derivative of ethylene. Moreover, if the substituents are attached to different carbon atoms of the double bond, then the olefin is called symmetrical and is denoted by the symbol " sim-”, if the substituents are attached to one carbon atom of the double bond, then the olefin is called unsymmetrical and is denoted by the symbol “ not simm-". For example:

The names of olefins according to the systematic nomenclature are formed from the name of an alkane having a similar structure, replacing the suffix "an" with "en". The main chain is the longest chain containing a double bond. The numbering of carbon atoms starts from the end of the chain, which is closer to the double bond. For example:

Choose the longest (main) chain containing a double bond;

Decide on the seniority of groups;

Number the main chain, giving the double bond the smallest of the locant numbers;

List prefixes;

Compile the full name of the connection.

For example:

When naming, the radical –CH=CH is called “vinyl”.

Two nomenclatures are used to designate geometric isomers:

cis-, trance- and E-, Z-

In accordance with cis-, trance- nomenclature geometric isomers in which substituents are located on one side of the double bond are called with cis-isomers.

Geometric isomers in which substituents are located on opposite sides of the double bond are called trance-isomers.

If hydrocarbon radicals act as substituents, then radicals with a longer carbon chain have an advantage in determining the alkene configuration (the configuration is determined relative to the radical with a larger chain). For example:

Often cis-, trance- the nomenclature does not allow unambiguous determination of geometric isomers. More perfect in this regard is the E-, Z- nomenclature.

E-isomers are those geometric isomers in which the senior substituents on the carbon atoms of the double bond are on opposite sides of the double bond (from German word"entgegen" - on the contrary).

Z-isomers are those geometric isomers in which the senior substituents on the carbon atoms of the double bond are on the same side of the double bond (from the German word "zusamen" - together).

The designation E- and Z- are placed before the name of the compound according to the IUPAC nomenclature and enclosed in brackets (designation cis- and trance- not enclosed in parentheses). For example:

The seniority of the substituents is determined by the atomic number of the element whose atom is bonded to the carbon atom of the double bond, and with the same element, by the atomic numbers of the elements following the chain of the substituent. A number of deputies in ascending order of precedence:

Ways to get.

industrial ways.

1. The first four members of the olefin series are produced commercially by cracking petroleum distillates.

2. Some olefins, such as 1-butene and 2-butene, as well as normal and isomeric pentenes, are obtained by dehydrogenation of the corresponding saturated hydrocarbons. The process is carried out using a heterogeneous catalyst based on chromium trioxide and at temperatures up to 450 0 C:

laboratory methods.

The most common laboratory methods for obtaining olefins are the dehydration of alcohols (the elimination of water from alcohols) and the dehydrohalogenation of halogenated alkanes (the elimination of hydrogen halides from haloalkanes). Both of these reactions obey the Zaitsev rule:

During the dehydration of alcohols and dehydrohalogenation of haloalkanes, the proton is split off mainly from the least hydrogenated (having a smaller number of hydrogen atoms) carbon atom (1875).

This direction of flow of these elimination reactions is explained by the increased thermodynamic stability of the resulting olefin. The more substituents, the more opportunities for superconjugation. The higher the degree of delocalization of electrons in the π-bond. Accordingly, the thermodynamic stability is higher. Stereoselectivity is determined by greater stability trance-isomer.

1. Dehydration of alcohols (elimination).

The splitting of water from alcohol is carried out in the gas and liquid phases. In both cases, the reaction is carried out at high temperature in the presence of a dewatering agent. Sulfuric or phosphoric acid is used in the liquid phase, and phosphorus (V) oxide, alumina, thorium oxide or aluminum salts are used in the gas phase. For example:

The elimination mechanism in the liquid phase includes two stages. In the first stage, an ester is formed from an acid and an alcohol, and in the second stage, the decomposition of the ester leads to the formation of an olefin:

2. Dehydrohalogenation of haloalkanes.

The cleavage of hydrogen halides from haloalkanes is carried out using an alcoholic solution of caustic potassium (KOH), less often NaOH is used:

3. Dehalogenation of vicinal dihaloalkanes.

Olefins are obtained by elimination of halogens from dihalogen derivatives with halogen atoms at adjacent (or vicinal) carbon atoms. Elimination is carried out in an alcohol or acetic acid solution by the action of zinc dust:

4. Hydrogenation of acetylenic hydrocarbons and alkadienes.

In some cases, during the synthesis, it is easier to obtain an acetylenic hydrocarbon than an alkene. Acetylene hydrocarbons are relatively easily converted into alkenes by partial hydrogenation. Hydrogen does not add to the π-electron system without a catalyst. In the case of obtaining alkenes from alkynes, two variants of the catalytic reaction are used: in the gas phase on hydrogenation catalysts (platinum, palladium, nickel) poisoned with lead (PbO) and in the liquid phase with sodium in liquid ammonia. In this case, alkenes of various configurations are formed:

Hydrogenation of 1,3-dienes leads to the formation of a mixture of alkenes isomeric in the position of the double bond:

physical properties.

Under normal conditions, the first four members of the homologous series of ethylene hydrocarbons are gases. Olefins with the number of carbon atoms from 5 to 17 - liquid. Next come the solids.

Straight chain olefins boil at a higher temperature than their branched chain isomers. Terminal olefins (terminal double bond) boil at a lower temperature than their intrachain isomers. trance-Isomers melt at a higher temperature than cis-isomers. cis-Isomers usually boil at a higher temperature than trance-isomers.

The density of olefins is less than unity, but greater than the density of the corresponding paraffins. In the homologous series, the density increases.

The solubility of olefins in water is low, but higher than that of paraffins.

Chemical properties.

The main structural element that determines Chemical properties olefins, is a double bond, including one σ- and one π-bond. The carbon atoms of the double bond are in a state of sp 2 hybridization. A comparison of static factors, in particular bond length and energy, shows that a double bond is shorter and stronger than a single bond:

The energy of the double bond is 607.1 kJ/mol, which is more than the energy of the single bond - 349.6 kJ/mol. However, two single bonds exceed the energy of one double bond by 92.1 kJ/mol. Therefore, a double bond easily transforms into two ordinary σ-bonds by adding two atoms or atomic groups at the place of the double bond.

From this it follows that addition reactions are most characteristic of olefins. But some types of olefins are characterized by substitution reactions. Hydrogen is most easily replaced at the α-carbon atom with respect to the double bond. The so-called allyl position. The radical formed during the homolytic bond cleavage is able to interact with the electrons of the π-bond, which ensures its high stability and, accordingly, high reactivity.

Since the π bond is a cloud of negative charge located above and below the plane of the molecule, olefins should be prone to interact with particles that carry a positive charge. Reagents that carry a positive charge are electrophiles.

5.1. electrophilic addition

Electrophilic addition (Ad E) is an addition reaction in which an electrophile is the attacking particle in the rate-limiting step.

The mechanism of electrophilic addition includes three stages.

For example, the addition of hydrogen bromide to ethylene to form ethyl bromide in carbon tetrachloride:

Mechanism:

1. At the first stage, the so-called π-complex is formed:

A feature of the π-complex is that the carbon atoms of the double bond are in a state of sp 2 hybridization.

2. Formation of an intermediate carbocation. This stage is slow (rate-limiting):

At this stage, one of the carbon atoms of the double bond goes into the state of sp 3 hybridization. The other remains in the state of sp 2 hybridization and acquires a vacant p-orbital.

3. In the third stage, the bromide ion formed in the second stage quickly attaches to the carbocation:

A similar mechanism can be given for the reaction of electrophilic addition of bromine to ethylene with the formation of 1,2-dibromoethane in carbon tetrachloride.

1. Formation of a π-complex:

2. Formation of a cyclic bromonium ion:

The cyclic bromonium ion is more stable than the open ethyl cation. The reason for this stability is that in the cyclic bromonium ion, all atoms have eight electrons on the outer electronic level. While in the ethyl cation, the positively charged carbon atom has only six electrons. The formation of the bromonium ion is associated with heterolytic cleavage of the Br-Br bond and elimination of the bromide ion.

3. Addition of a bromide ion to a cyclic bromonium ion:

Since one side of the original alkene is shielded in the bromonium ion by a positively charged bromine atom, the bromide ion can attack the bromine ion only from the opposite side. In this case, the three-membered ring opens, and the bromide ion forms covalent bond with a carbon atom. The addition product is vicinal dibromide.

The proof of the presented mechanism, which provides for the attack of the bromonium ion by the bromide ion from the rear, is the formation trance-1,2-dibromocyclohexane according to the reaction of cyclohexene with bromine:

Markovnikov's rule.

The interaction of hydrogen halides with asymmetric alkenes by the mechanism of electrophilic addition leads to the formation of products of a strictly defined structure. So, according to the reaction of 2-methyl-2-butene with hydrogen bromide, 2-bromo-2-methylbutane is predominantly formed:

The structure of the resulting product in the case of an electrophilic addition reaction to unsymmetrical alkenes obeys the Markovnikov rule:

When a hydrogen halide is added to an unsymmetrical alkene, the proton of the reagent is predominantly attached to the most hydrogenated (having a larger number of hydrogen atoms) carbon atom (1869).

The explanation for this direction of the reaction is that the carbocations formed at the second stage of the electrophilic addition mechanism form a stability series similar to the stability series of radicals:

Methyl cation<первичный <вторичный <третичный.

In accordance with the stability series, the product of addition of a halide ion to a tertiary carbon atom will be more preferable than addition to a secondary one.

According to the mechanism of electrophilic addition, in accordance with the Markovnikov rule, the following are added to olefins:

hydrogen halides; halogens, water, hypohalogenic acids:

In the case of the addition of hypohalogenic acids, the halogen ion (except for fluorine) acts as an electrophilic particle, since the electronegativity of chlorine, bromine and iodine is less than that of oxygen.

radical reactions.

radical connection.

The addition of halogens to the double bond can proceed both by the ionic (attack by the electrophilic particle) and by the radical mechanism.

With radical addition, halogen atoms, formed as a result of the decay of molecules under the action of light quanta, are attached to the most accessible of the carbon atoms with the formation of the most stable of possible radicals:

The radical (1) is more easily formed and more stable. In this radical, the unpaired electron is conjugated with five CH bonds. For radical (2), conjugation with only one C-H bond is possible. The primary carbon atom is more accessible to the attacking particle than the secondary one. Radical (1) then reacts with a halogen molecule to form a product and generate a new bromine radical, which ensures the chain growth of the radical mechanism:

In the presented mechanism, the attacking particle is the bromine radical. If bromine radicals are generated under the conditions of addition of hydrogen halides, then at the first stage an attack by bromine will also occur, since the bromine radical is more stable than the hydrogen radical. The addition of hydrogen bromide to unsymmetrical alkenes according to Karash is based on this principle - against Markovnikov's rule. The stage of chain initiation in this case is provided by the introduction of peroxides, which, when writing the reaction equation, is indicated by the symbol “ROOR” above the arrow (the formula for carbon tetrachloride means that the reaction proceeds according to the ionic mechanism, in accordance with the Markovnikov rule):

This fact is explained by the mechanism of the reaction. Since peroxide easily decomposes into two oxide radicals, which is the stage of chain initiation, further chain growth is associated with the formation of a bromine radical (or atom):

In the next step, the bromine radical is attached to the olefin. In this case, the formation of two radicals is possible:

Of the two possible radicals (1) and (2), the first one is more stable and is formed faster. Therefore, the first radical promotes further chain growth:

The reaction proceeds as a radical chain process at low temperatures (-80 0 С)

radical substitution.

The interaction of ethylene homologues with halogens (chlorine, bromine) at high temperatures, above 400 0 C, leads only to the replacement of the hydrogen atom in the allyl position by a halogen and is called allyl substitution. The double bond is preserved in the final product:

The reaction proceeds as a chain process radical substitution (S R). High temperature promotes the homolysis of chlorine molecules and the formation of radicals.

Hydrogenation.

Alkenes do not directly add molecular hydrogen. This reaction can only be carried out in the presence of heterogeneous catalysts, for example, platinum, palladium, nickel, or homogeneous, for example, a complex rhodium salt. Usually in laboratories and in industry, heterogeneous catalysts are used to add hydrogen to a double bond:

Thermodynamically, this reaction is very favorable:

Because hydrogenation using a heterogeneous catalyst, it is necessary to adsorb the olefin on the catalyst surface at the double bond. Accordingly, olefins are hydrogenated the easier, the fewer substituents in the double bond - Lebedev's rule.

Oxidation.

There are two main directions (types) in the oxidation of olefins:

1. with the preservation of the carbon skeleton - these are epoxidation and hydroxylation;

2. with a break in the double carbon - carbon bond - this is ozonolysis and the exhaustive oxidation of alkenes.

Depending on the type, various oxidizers are used.

Epoxidation

Epoxidation is the formation of an epoxide, a three-membered cyclic ether. With atmospheric oxygen in the presence of a silver catalyst, ethylene is epoxidized to ethylene oxide:

The remaining olefins are epoxidized by the action of peroxycarboxylic acids or simply peracids (the Prilezhaev reaction). Peroxycarboxylic acids contain an "O-O" peroxide structure that donates one oxygen atom to the double bond:

Hydroxylation

A dilute (5-10%) solution of potassium permanganate (Wagner reaction) with olefins form cis- glycols or cis-1,2-diol:

Similar information.

CHAPTER 7. STEREOCHEMICAL BASIS OF THE STRUCTURE OF ORGANIC COMPOUNDS

CHAPTER 7. STEREOCHEMICAL BASIS OF THE STRUCTURE OF ORGANIC COMPOUNDS

Stereochemistry (from the Greek. stereos- spatial) is "chemistry in three dimensions". Most molecules are three-dimensional (threedimentional, abbreviated as 3D). Structural formulas reflect the two-dimensional (2D) structure of the molecule, which includes the number, type, and sequence of binding atoms. Recall that compounds having the same composition but different chemical structure are called structural isomers (see 1.1). A broader concept of the structure of a molecule (sometimes figuratively called molecular architecture), along with the concept of chemical structure, includes stereochemical components - configuration and conformation, reflecting the spatial structure, i.e., the three-dimensionality of the molecule. Molecules that have the same chemical structure may differ in spatial structure, i.e., exist in the form of spatial isomers - stereoisomers.

The spatial structure of molecules is the mutual arrangement of atoms and atomic groups in three-dimensional space.

Stereoisomers - compounds in the molecules of which there is the same sequence of chemical bonds of atoms, but a different arrangement of these atoms relative to each other in space.

In turn, stereoisomers can be configuration and conformational isomers, i.e. vary accordingly configuration and conformation.

7.1. Configuration

A configuration is the arrangement of atoms in space without taking into account the differences that arise due to rotation around single bonds.

Configurational isomers can transform into each other by breaking one and forming other chemical bonds and can exist separately as individual compounds. They are divided into two main types - enantiomers and diastereomers.

7.1.1. enantiomers

Enantiomers are stereoisomers that relate to each other as an object and an incompatible mirror image.

Only enantiomers exist as enantiomers. chiral molecules.

Chirality is the property of an object to be incompatible with its mirror image. Chiral (from the Greek. cheir- hand), or asymmetric, the objects are the left and right hand, as well as gloves, boots, etc. These paired objects represent an object and its mirror image (Fig. 7.1, a). Such items cannot be completely combined with each other.

At the same time, there are many objects around us that are compatible with their mirror image, that is, they are achiral(symmetrical), such as plates, spoons, glasses, etc. Achiral objects have at least one symmetry plane, which divides the object into two mirror-identical parts (see Fig. 7.1, b).

Similar relationships are also observed in the world of molecules, i.e. molecules are divided into chiral and achiral. Achiral molecules have planes of symmetry, chiral ones do not.

Chiral molecules have one or more centers of chirality. In organic compounds, the center of chirality is most often asymmetric carbon atom.

Rice. 7.1.Reflection in the mirror of a chiral object (a) and a plane of symmetry cutting the achiral object (b)

Asymmetric is a carbon atom bonded to four different atoms or groups.

When depicting the stereochemical formula of a molecule, the symbol "C" of the asymmetric carbon atom is usually omitted.

To determine whether a molecule is chiral or achiral, it is not necessary to represent it with a stereochemical formula, it is enough to carefully consider all the carbon atoms in it. If there is at least one carbon atom with four different substituents, then this carbon atom is asymmetric and the molecule, with rare exceptions (see 7.1.3), is chiral. So, of the two alcohols - propanol-2 and butanol-2 - the first is achiral (two CH 3 groups at the C-2 atom), and the second is chiral, since in its molecule at the C-2 atom all four substituents are different ( H, OH, CH 3 and C 2 H 5). An asymmetric carbon atom is sometimes marked with an asterisk (C*).

Therefore, the butanol-2 molecule is able to exist as a pair of enantiomers that do not combine in space (Fig. 7.2).

Rice. 7.2.Enantiomers of chiral molecules of butanol-2 do not combine

Properties of enantiomers. Enantiomers have the same chemical and physical properties (melting and boiling points, density, solubility, etc.), but exhibit different optical activity, i.e., the ability to deflect the plane of polarized light*.

When such light passes through a solution of one of the enantiomers, the plane of polarization deviates to the left, the other - to the right by the same angle α. The value of the angle α reduced to standard conditions is the constant of the optically active substance and is called specific rotation[α]. Left rotation is denoted by a minus sign (-), right rotation is indicated by a plus sign (+), and enantiomers are called left and right rotation, respectively.

Other names of enantiomers are associated with the manifestation of optical activity - optical isomers or optical antipodes.

Each chiral compound can also have a third, optically inactive form - racemate. For crystalline substances, this is usually not just a mechanical mixture of crystals of two enantiomers, but a new molecular structure formed by the enantiomers. Racemates are optically inactive because the left rotation of one enantiomer is compensated by the right rotation of an equal amount of the other. In this case, a plus-minus sign (?) is sometimes placed before the name of the connection.

7.1.2. Relative and absolute configurations

Fisher projection formulas. Stereochemical formulas can be used to depict configurational isomers on a plane. However, it is more convenient to use simpler Fisher projection formulas(easier - Fisher projections). Let us consider their construction using lactic (2-hydroxypropanoic) acid as an example.

The tetrahedral model of one of the enantiomers (Fig. 7.3) is placed in space so that the chain of carbon atoms is in a vertical position, and the carboxyl group is on top. Bonds with non-carbon substituents (H and OH) at the chiral center should

* See tutorial for details Remizov A.N., Maksina A.G., Potapenko A.Ya. Medical and biological physics. 4th ed., revised. and additional - M.: Bustard, 2003.- S. 365-375.

Rice. 7.3.Construction of the Fischer projection formula of (+)-lactic acid

us to be directed towards the observer. After that, the model is projected onto a plane. In this case, the symbol of the asymmetric atom is omitted; it is understood as the point of intersection of the vertical and horizontal lines.

The tetrahedral model of a chiral molecule before projection can be placed in space in different ways, not only as shown in Fig. 7.3. It is only necessary that the links that form a horizontal line on the projection be directed towards the observer, and the vertical links - beyond the plane of the picture.

The projections obtained in this way can, with the help of simple transformations, be brought to a standard form in which the carbon chain is located vertically, and the senior group (in lactic acid this is COOH) is on top. Transformations allow two operations:

In the projection formula, it is allowed to interchange any two substituents at the same chiral center an even number of times (two permutations are enough);

The projection formula can be rotated in the plane of the figure by 180? (which is equivalent to two permutations), but not by 90?.

D.L-Configuration designation system. At the beginning of the twentieth century. a classification system for enantiomers was proposed for relatively simple (in terms of stereoisomerism) molecules, such as α-amino acids, α-hydroxy acids, and the like. Behind configuration standard glyceraldehyde was taken. Its levorotatory enantiomer was arbitrarily formula (I) is assigned. This configuration of the carbon atom was designated by the letter l (from lat. laevus- left). The dextrorotatory enantiomer was accordingly assigned the formula (II), and the configuration was denoted by the letter d (from Lat. dexter- right).

Note that in the standard projection formula l -glyceraldehyde group OH is on the left, and at d -glyceraldehyde - on the right.

Assignment to d- or l - a number of other structurally related optically active compounds is produced by comparing the configuration of their asymmetric atom with the configuration d- or l -glyceraldehyde. For example, in one of the enantiomers of lactic acid (I) in the projection formula, the OH group is on the left, as in l -glyceraldehyde, so the enantiomer (I) is referred to as l -row. For the same reasons, the enantiomer (II) is assigned to d -row. Thus, from a comparison of the Fisher projections, we determine relative configuration.

It should be noted that l -glyceraldehyde has a left rotation, and l -lactic acid - right (and this is not an isolated case). Moreover, the same substance can be both left-handed and right-handed, depending on the determination conditions (different solvents, temperature).

The sign of the rotation of the plane of polarized light is not related to belonging to d- or l -stereochemical series.

The practical determination of the relative configuration of optically active compounds is carried out using chemical reactions: either the test substance is converted into glyceraldehyde (or another substance with a known relative configuration), or, conversely, from d- or l -glyceraldehyde, the test substance is obtained. Of course, in the course of all these reactions, the configuration of the asymmetric carbon atom should not change.

Arbitrary assignment of conditional configurations to left- and right-handed glyceraldehyde was a forced step. At that time, the absolute configuration was not known for any chiral compound. The establishment of the absolute configuration became possible only thanks to the development of physicochemical methods, especially X-ray diffraction analysis, with the help of which in 1951 the absolute configuration of a chiral molecule was determined for the first time - it was a salt of (+)-tartaric acid. After that, it became clear that the absolute configuration of d- and l-glyceraldehydes is indeed the same as was originally attributed to them.

d,l-System is currently used for α-amino acids, hydroxy acids and (with some additions) for carbohydrates

(see 11.1.1).

R,S-Configuration designation system. The d,L-System is of very limited use, since it is often impossible to assign the configuration of any compound to glyceraldehyde. The universal system for designating the configuration of centers of chirality is the R,S-system (from lat. rectus- straight, sinister- left). It is based on sequence rule, based on the seniority of the substituents associated with the center of chirality.

The seniority of the substituents is determined by the atomic number of the element directly associated with the center of chirality - the larger it is, the older the substituent.

Thus, the OH group is older than NH 2, which, in turn, is older than any alkyl group and even COOH, since in the latter a carbon atom is bonded to the asymmetric center. If the atomic numbers turn out to be the same, the group in which the atom following the carbon has a higher serial number is considered the eldest, and if this atom (usually oxygen) is double bonded, it is counted twice. As a result, the following groups are arranged in descending order of precedence: -COOH > -CH=O > -CH 2 OH.

To determine the configuration, the tetrahedral model of the compound is placed in space so that the smallest substituent (in most cases, this is a hydrogen atom) is the farthest from the observer. If the seniority of the other three substituents decreases clockwise, then the R-configuration is assigned to the center of chirality (Fig. 7.4, a), if counterclockwise -S- configuration (see Fig. 7.4, b), as seen by the driver behind the wheel (see Fig. 7.4, in).

Rice. 7.4.Determination of the configuration of enantiomers of lactic acid by R,S- system (explanation in text)

Fisher projections can be used to designate a configuration according to the RS-system. To do this, the projection is transformed so that the junior deputy is located on one of the vertical links, which corresponds to its position behind the plane of the drawing. If, after the projection transformation, the seniority of the remaining three substituents decreases clockwise, then the asymmetric atom has the R-configuration, and vice versa. The use of this method is shown on the example of l-lactic acid (numbers indicate the seniority of the groups).

There is an easier way to determine the R- or S-configuration according to the Fisher projection, in which the junior substituent (usually an H atom) is located on one of horizontal connections. In this case, the above permutations are not carried out, but the seniority of the substituents is immediately determined. However, since the H atom is “out of place” (which is equivalent to the opposite configuration), a drop in precedence will now mean not an R-configuration, but an S-configuration. This method is shown on the example of l-malic acid.

This method is especially convenient for molecules containing several chiral centers, when permutations would be required to determine the configuration of each of them.

There is no correlation between the d,l and RS systems: these are two different approaches to designating the configuration of chiral centers. If in the d,L-system, compounds similar in configuration form stereochemical series, then in the RS-system, chiral centers in compounds, for example, of the l-series, can have both R- and S-configurations.

7.1.3. diastereomerism

Diastereomers are called stereoisomers that are not related to each other, like an object and an incompatible mirror image, that is, not being enantiomers.

The most important groups of diastereomers are σ-diastereomers and π-diastereomers.

σ -Diastereomers. Many biologically important substances contain more than one center of chirality in the molecule. In this case, the number of configurational isomers increases, which is defined as 2 n , where n is the number of centers of chirality. For example, in the presence of two asymmetric atoms, the compound can exist as four stereoisomers (2 2 = 4) that make up two pairs of enantiomers.

2-Amino-3-hydroxybutanoic acid has two centers of chirality (C-2 and C-3 atoms) and therefore must exist as four configurational isomers, one of which is a natural amino acid.

Structures (I) and (II), corresponding to l- and d-threonine, as well as (III) and (IV), corresponding to l- and d-allotreonine (from the Greek. alios- another), relate to each other, as an object and an incompatible mirror image, i.e. they are pairs of enantiomers. Comparison of structures (I) and (III), (I) and (IV), (II) and (III), (II) and (IV) shows that in these pairs of compounds, one asymmetric center has the same configuration, while the other is the opposite. These pairs of stereoisomers are diastereomers. Such isomers are called σ-diastereomers, since the substituents in them are linked to the center of chirality by σ-bonds.

Amino acids and hydroxy acids with two centers of chirality are classified as d- or l -series according to the configuration of the asymmetric atom with the smallest number.

Diastereomers, unlike enantiomers, differ in physical and chemical properties. For example, l-threonine, which is part of proteins, and l-allotreonine have different values ​​of specific rotation (as shown above).

Meso compounds. Sometimes a molecule contains two or more asymmetric centers, but the molecule as a whole remains symmetrical. An example of such compounds is one of the stereoisomers of tartaric (2,3-dihydroxybutanedioic) acid.

Theoretically, this acid, which has two centers of chirality, could exist in the form of four stereoisomers (I)-(IV).

Structures (I) and (II) correspond to the enantiomers of the d- and l-series (the assignment was made according to the "upper" center of chirality). It might seem that structures (III) and (IV) also correspond to a pair of enantiomers. In fact, these are formulas of the same compound - optically inactive mesotartaric acid. It is easy to verify the identity of formulas (III) and (IV) by turning formula (IV) by 180? without taking it out of the plane. Despite the two centers of chirality, the mesotartaric acid molecule as a whole is achiral, since it has a symmetry plane passing through the middle of the C-2-C-3 bond. With respect to d- and l-tartaric acids, mesotartaric acid is a diastereomer.

Thus, there are three (not four) stereoisomers of tartaric acids, not counting the racemic form.

When using the R,S system, there are no difficulties in describing the stereochemistry of compounds with several chiral centers. To do this, determine the configuration of each center according to the R,S-system and indicate it (in brackets with the corresponding locants) before the full name. Thus, d-tartaric acid will receive the systematic name (2R,3R)-2,3-dihydroxybutanedioic acid, and mesotartaric acid will have stereochemical symbols (2R,3S)-.

Like mesotartaric acid, there is a mesoform of the α-amino acid cystine. With two centers of chirality, the number of stereoisomers of cystine is three due to the fact that the molecule is internally symmetrical.

π -Diastereomers. These include configurational isomers containing a π-bond. This type of isomerism is typical, in particular, for alkenes. With respect to the π-bond plane, the same substituents on two carbon atoms can be located one at a time (cis) or at different (trance) sides. In this regard, there are stereoisomers known as cis- and trance-isomers, as shown in the case of cis- and trans-butenes (see 3.2.2). π-Diastereomers are the simplest unsaturated dicarboxylic acids - maleic and fumaric.

Maleic acid is thermodynamically less stable cis-isomer compared to trance-isomer - fumaric acid. Under the action of certain substances or ultraviolet rays, an equilibrium is established between both acids; when heated (~150 ?C), it is shifted towards a more stable trance-isomer.

7.2. Conformations

Around a simple C-C bond, free rotation is possible, as a result of which the molecule can take on various shapes in space. This can be seen in the stereochemical formulas of ethane (I) and (II), where the CH groups marked in color 3 located differently relative to another CH group 3.

Rotation of one CH group 3 relative to the other occurs without breaking the configuration - only the relative position in space of hydrogen atoms changes.

The geometric shapes of the molecule, passing into each other by rotation around σ-bonds, are called conformations.

Regarding this conformational isomers are stereoisomers, the difference between which is caused by the rotation of individual sections of the molecule around σ-bonds.

Conformational isomers usually cannot be isolated in an individual state. The transition of different conformations of the molecule into each other occurs without breaking bonds.

7.2.1. Conformations of acyclic compounds

The simplest compound with a C-C bond is ethane; consider two of its many conformations. In one of them (Fig. 7.5, a) the distance between the hydrogen atoms of two CH groups 3 the smallest, so the C-H bonds that are opposite each other repel each other. This leads to an increase in the energy of the molecule and, consequently, to a lower stability of this conformation. When looking along the C-C bond, it is seen that the three C-H bonds at each carbon atom “overshadow” each other in pairs. This conformation is called obscured.

Rice. 7.5.obscured (a, b) and inhibited (in, G) ethane conformations

In another conformation of ethane, which occurs upon rotation of one of the CH groups 3 at 60? (see Fig. 7.5, c), the hydrogen atoms of the two methyl groups are as far apart as possible. In this case, the repulsion of the electrons of the C-H bonds will be minimal, and the energy of such a conformation will also be minimal. This more stable conformation is called inhibited. The difference in the energy of both conformations is small and amounts to ~12 kJ/mol; it defines the so-called energy barrier of rotation.

Newman's projection formulas. These formulas (more simply, Newman projections) are used to depict conformations on a plane. To construct a projection, the molecule is viewed from the side of one of the carbon atoms along its bond with the neighboring carbon atom, around which rotation takes place. When projecting, three bonds from the carbon atom closest to the observer to hydrogen atoms (or, in the general case, to other substituents) are arranged in the form of a three-beam star with angles of 120?. The (invisible) carbon atom removed from the observer is depicted as a circle, from which it is also at an angle of 120? three connections go. Newman projections also give a visual representation of the eclipsed (see Fig. 7.5, b) and hindered (see Fig. 7.5, d) conformations.

Under normal conditions, ethane conformations easily transform into each other, and one can speak of a statistical set of different conformations that differ insignificantly in energy. It is impossible to single out even a more stable conformation in an individual form.

In more complex molecules, the replacement of hydrogen atoms at neighboring carbon atoms with other atoms or groups leads to their mutual repulsion, which affects the increase in potential energy. So, in the butane molecule, the eclipsed conformation will be the least favorable, and the hindered conformation with the most distant CH 3 groups will be the most advantageous. The difference between the energies of these conformations is ~25 kJ/mol.

As the carbon chain lengthens in alkanes, the number of conformations rapidly increases as a result of the expansion of the possibilities of rotation around each C-C bond, so the long carbon chains of alkanes can take many different forms, for example, zigzag (I), irregular (II) and pincer (III ).

A zigzag conformation is preferred, in which all C-C bonds in the Newman projection form an angle of 180°, as in the staggered conformation of butane. For example, fragments of long-chain palmitic C 15 H 31 COOH and stearic C 17 H 35 COOH acids in a zigzag conformation (Fig. 7.6) are part of the lipids of cell membranes.

Rice. 7.6.Skeletal formula (a) and molecular model (b) of stearic acid

In the pincer conformation (III), carbon atoms that are distant from each other in other conformations approach each other. If functional groups, such as X and Y, are at a sufficiently close distance, capable of reacting with each other, then as a result of an intramolecular reaction this will lead to the formation of a cyclic product. Such reactions are quite widespread, which is associated with the advantage of the formation of thermodynamically stable five- and six-membered rings.

7.2.2. Conformations of six-membered rings

The cyclohexane molecule is not a flat hexagon, since with a flat structure the bond angles between carbon atoms would be 120°, i.e., they would significantly deviate from the normal bond angle of 109.5°, and all hydrogen atoms would be in an unfavorable eclipsed position. This would lead to cycle instability. In fact, the six-membered cycle is the most stable of all cycles.

The various conformations of cyclohexane result from partial rotation around σ bonds between carbon atoms. Of several non-planar conformations, the most energetically favorable is the conformation armchairs(Fig. 7.7), since in it all the bond angles between the C-C bonds are equal to ~ 110?, and the hydrogen atoms at neighboring carbon atoms do not obscure each other.

In a non-planar molecule, one can only conditionally speak of the arrangement of hydrogen atoms "above and below the plane." Instead, other terms are used: bonds directed along the vertical axis of symmetry of the cycle (in Fig. 7.7, a shown in color), called axial(a), and bonds oriented from the cycle (as if along the equator, by analogy with the globe) are called equatorial(e).

If there is a substituent in the ring, the conformation with the equatorial position of the substituent is more favorable, such as, for example, conformation (I) of methylcyclohexane (Fig. 7.8).

The reason for the lower stability of conformation (II) with the axial arrangement of the methyl group is 1,3-diaxial repulsion CH groups 3 and H atoms in positions 3 and 5. In this

Rice. 7.7.Cyclohexane in chair conformation:

a- skeletal formula; b- ball-and-stick model

Rice. 7.8.Cycle inversion of a methylcyclohexane molecule (not all hydrogens shown)

case, the cycle is subjected to the so-called inversions, adopting a more stable conformation. The repulsion is especially strong in cyclohexane derivatives having positions 1 and 3 of the bulk groups.

In nature, there are many derivatives of the cyclohexane series, among which six-hydric alcohols play an important role - inositols. Due to the presence of asymmetric centers in their molecules, inositols exist in the form of several stereoisomers, of which the most common is myoinositis. The myoinositol molecule has a stable chair conformation in which five of the six OH groups are in equatorial positions.

Fischer's system at one time made it possible to create a logical and consistent stereochemical systematics of a large number of natural compounds originating from amino acids and sugars. The relative configuration of the enantiomers in this system was determined by chemical correlation, i.e. by passing from this molecule to D- or L-glyceraldehyde through a sequence of chemical reactions that do not affect the asymmetric carbon atom (see section 8.5 for more details). However, if the molecule whose configuration was to be determined was very different in structure from glyceraldehyde, it would be very cumbersome to chemically correlate its configuration with that of glyceraldehyde. In addition, the assignment of a configuration to the D - or L - series was not always unambiguous. For example, D-glyceraldehyde can in principle be converted to glyceric acid, then by the action of diazomethane to methyl ester, and then by selective oxidation of the primary alcohol function and esterification with diazoethane to hydroxymalonic acid methyl ethyl ester (XXV). All these reactions do not affect the chiral center and therefore it can be said that the diester XXV belongs to the D - series.

If the first esterification is carried out with diazoethane, and the second with diazomethane, then the diester XXVI will be obtained, which, for the same reason, should also be attributed to the D-series. In fact, compounds XXV and XXVI are enantiomers; those. some belong to the D- and others to the L-series. Thus, the assignment depends on which of the ester groups, CO 2 Et or CO 2 Me, is considered "main".

These limitations of the Fisher system, as well as the fact that in 1951 an X-ray diffraction method for determining the true arrangement of groups around a chiral center appeared, led to the creation in 1966 of a new, more rigorous and consistent system for describing stereoisomers, known as R,S-nomenclature Cahn-Ingold-Prelog (KIP) or the rules of successive precedence. This system has now practically supplanted Fischer's D,L system (the latter, however, is still used for carbohydrates and amino acids). In the CIP system, special descriptors R- or S- are added to the usual chemical name, which strictly and unambiguously determine the absolute configuration.

Let us take a compound of the Xabcd type containing one asymmetric X center. 1>2>3>4. Substituents are considered by the observer from the side furthest from the youngest substituent (indicated by number 4). If in this case the direction of decreasing precedence 1  2  3 coincides with clockwise movement, then the configuration of this asymmetric center is denoted by the symbol R (from the Latin rectus - right) and if counterclockwise - by the symbol S (sinister - left).

Let us present several rules of successive precedence, which are sufficient for considering the vast majority of chiral compounds.

1) Preference for seniority is given to atoms with higher atomic numbers. If the numbers are the same (in the case of isotopes), then the atom with the highest atomic mass is considered to be the oldest. The youngest "deputy" is a lone electron pair. Thus, seniority increases in the series: lone pair< H < D < T < Li < B < C < N < O < F < Si < P

2) If two, three or all four identical atoms are directly connected to an asymmetric atom, the order is established by the atoms of the second belt, which are no longer connected to the chiral center, but to those atoms that had the same seniority. For example, in the XXVII molecule, seniority cannot be established by the first atom of the CH 2 OH and (CH 3) 2 CH groups, but preference is given to CH 2 OH, since the atomic number of oxygen is greater than that of carbon. The CH 2 OH group is older, despite the fact that only one oxygen atom is bonded to the carbon atom in it, and in the CH (CH 3) 2 group - two carbon atoms. If the second atoms in the group are the same, the order is determined by the atoms of the third belt, and so on.

If such a procedure did not lead to the construction of an unambiguous hierarchy, it is continued at ever increasing distances from the central atom, until, finally, differences are encountered and all four deputies still receive their seniority. At the same time, any preference acquired by one or another deputy at one of the stages of seniority agreement is considered final and is not subject to reassessment at subsequent stages. If branching points occur in the molecule, the precedence determination procedure should be continued along the molecular chain of the highest precedence. When establishing the seniority of one or another central atom, the number of other atoms of higher seniority associated with it is of decisive importance. For example, CCl 3 > CHCl 2 > CH 2 Cl.

3) It is formally assumed that the valence of all atoms, except hydrogen, is 4. If the true valence of an atom is less (for example, oxygen, nitrogen, sulfur), then this atom is considered to have 4-n (where n is the real valence) so-called phantom deputies, which are assigned a zero serial number and are given the last place in the list of substituents. Accordingly, groups with double and triple bonds are presented as if they were split into two or three single bonds. For example, when representing a C=C double bond, each atom is considered to be bonded to two carbon atoms, the second of these carbon atoms being considered to have three phantom substituents. As an example, consider the representations of the groups -CH=CH 2 , -CHO, -COOH, -CCH and -C 6 H 5 . These views look like this.

The first atoms in all these groups are bonded to (H,C,C), (H,O,O), (O,O,O), (C,C,C) and (C,C,C), respectively. This information is enough to put the COOH group in the first place (the oldest), the CHO group in the second, and the -CH \u003d CH 2 group in the last (fifth) place, since the presence of at least one oxygen atom is preferable to the presence of even three carbon atoms. To draw a conclusion about the relative seniority of the CCH and -C 6 H 5 groups, you need to go further along the chain. The C 6 H 5 group has two carbon atoms of the (C, C, C) type associated with (C, C, H), and the third atom is of the (O, O, O) type. The CCH group has only one grouping (C, C, H), but two groups (O, O, O). Therefore, C 6 H 5 is older than CCH, i.e. in order of precedence, the five indicated groups will occupy the row: COOH> CHO> C 6 H 5> C  CH> CH \u003d CH 2.

The seniority of the most frequently occurring substituents can be determined from Table. 8-2, in which the conditional number means greater seniority.

Table 8.2.

Seniority of some groups according to Kahn-Ingold-Prelog

	Conditional number		Conditional number
Allyl, CHSN \u003d CH 2		Mercapto, SH
Amino, NH 2		Methyl,  H 3
Ammonio, NH 3 +		Methylamino, NHCH 3
Acetyl, COCH 3		Methylsulfinyl, SOCH 3
Acetylamino, NHCOCH 3		Methylsulfinyloxy,OSOCH 3
Acetoxy, OCOCH 3		Methylsulfonyl, SO 2 CH 3
Benzyl, CH 2 C 6 H 5		Methylsulfonyloxy,OSO 2 CH 3
Benzyloxy, OCH 2 C 6 H 5		Methylthio,SCH 3
Benzoyl,  COC 6 H 5		Methoxy,OCH 3
Benzoylamino, NHCOC 6 H 5		Methylcarbonyl, COOCH 3
Benzoyloxy, OCOC 6 H 5		Neopentyl, CH 2 C (CH 3) 3
Benzoyloxycarbonyl-amino, NHCOOCH 2 C 6 H 5		Nitro, NO 2
Bromine, Br		Nitroso, NO
sec-Butyl, CH(CH 3)CH 3 CH 3		m-nitrophenyl,
n-Butyl, CH 2 CH 2 CH 2 CH 3		o-nitrophenyl,
tert-Butyl, C (CH 3) 3		p-nitrophenyl,
tert-Butoxycarbonyl, COOC (CH 3) 3		Pentyl, C 5 H 11
Vinyl, CH 2 = CH 2		Propenyl, CH=CHCH 3
Hydrogen, H		Propyl, CH 2 CH 2 CH 3
n-Hexyl, C 6 H 13		Propynyl, CCCH 3
Hydroxy, OH		Propargyl, CH 2 CCH
Glycosyloxy		Sulfo, SO 3 H
Dimethylamino, N (CH 3) 2		m-Tolyl,
2,4-Dinitrophenyl,		o-Tollil,
3,5-Dinitrophenyl,		p-Tolyl,
Diethylamino, N (C 2 H 5) 2		Trimethylammonio,
Isobutyl, CH 2 CH (CH 3) 2		Trityl, C (C 6 H 5) 3
Isopentyl, CH 2 CH 2 CH (CH 3) 2		Phenyl, C 6 H 5
Isopropenyl, CH (CH 3) \u003d CH 2		Phenilazo, N=NCC 6 H 5
Isopropyl, CH (CH 3) 2		Phenylamino, NHC 6 H 5
		Phenoxy, OC 6 H 5
Carboxyl, COOH		Formyl, CHO
2,6-Xylyl,		Formyloxy, OCHO
3,5-Xylyl,
		Chlorine, Cl
		Cyclohexyl, C 6 H 11
		Ethyl, CH 2 CH 3
		Ethylamino, NHC 2 H 5
		Ethynyl, CCH
		Ethoxy, OC 2 H 5
		Ethoxycarbonyl, COOC 2 H 5

The rules of successive precedence were deliberately designed to be as close as possible to Fisher's early taxonomy, since it turned out, fortunately, that D-glyceraldehyde did indeed have the configuration that was arbitrarily assigned to it at first. As a result, most of the D-centers and, very importantly, glyceraldehyde itself, have the (R)-configuration, and the L-stereoisomers usually belong to the (S)-series.

One exception is L-cysteine, which belongs to the (R)-series, as sulfur is preferred over oxygen by precedence rules. In the CIP system, the genetic relationship between molecules is not taken into account. This system can only be applied to connections with a known absolute configuration. If the configuration is unknown, then the connection must necessarily be characterized by the sign of its rotation.

The rules of successive precedence also apply to the description of geometric isomers of unsaturated compounds. Substituents at each end of a multiple bond must be considered separately when establishing precedence. If substituents with a higher seniority are located on the same side of the double bond, the compound is assigned the prefix Z - (from the German zusammen - together), and if on different sides, then the prefix E (entgegen - opposite). (Z, E) - The nomenclature of alkenes was discussed in Chapter 5. Below are examples of the assignment of structures using (Z, E) - designations.

The last example shows that the link with the Z-configuration has the priority right to be included in the main chain. (R,S) - Notation can also be used for compounds with axial chirality. To assign the configuration, a Newman projection is drawn onto a plane perpendicular to the chiral axis, and then an additional rule is applied according to which substituents at the end of the axis closest to the observer are considered to have a higher precedence than substituents at the far end of the axis. Then the configuration of the molecule is determined by the direction of bypassing the substituents clockwise or counterclockwise in the usual order of decreasing precedence from the first to the second and then to the third ligand. This is illustrated below for 1,3-allendicarboxylic and 2,2-iodiddiphenyl-6,6-dicarboxylic acids.

The rule of successive precedence has also been developed for planar and helical chiral molecules.

When depicting connections using Fisher projections, you can easily determine the configuration without building spatial models. The formula must be written so that the junior deputy is at the bottom; if, in this case, the remaining substituents are arranged clockwise in decreasing order of precedence, the compound is assigned to the (R) - series, and if counterclockwise, then to the (S) -series, for example:

If the lower group is not at the bottom, then you should swap it with the lower group, but remember that this reverses the configuration.

and V. Prelog in 1966.

The Kahn-Ingold-Prelog rules differ from other chemical nomenclatures, as they are focused on solving a specific problem - the description of the absolute configuration of stereoisomers.

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Nomenclature of enantiomers according to the Kahn-Ingold-Prelog system

Name according to R/S-nomenclature (Kahn-Ingold-Prelog system), example 2

Cyclohexane conformations

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Now, based on what we already know, if we want to name this molecule, we first need to find the longest carbon chain. We have a two-carbon chain, and all the bonds are single, so we're dealing with ethane. Let's write it all down together. With one of the carbons we have, (let's call it the 1st carbon, which will be the 2nd carbon), we have bromine and fluorine. So we can call it 1-bromine, and we write bromine before fluorine because "b" comes before "f" in alphabetical order. 1-bromo-1-fluorine, and now we're dealing with ethane. We have a two-carbon chain with single bonds - fluoroethane. This is the name of the molecule. I just wanted to repeat the material of the previous videos in which we analyzed the organic nomenclature. Now we already know, based on several previous videos, that this is also a chiral carbon, and if we make it a mirror image, we will have an enantiomer for this molecule, and they will be enantiomers for each other. So what does the mirror image look like for 1-bromo-1-fluoroethane? Here we will have carbon. Let's paint with the same colors. We'll still have bromine upstairs. The methyl group that attaches to the carbon will now be on the left side, CH3. Fluorine, as before, will be behind carbon, and hydrogen will still stick out of the picture, but now to the right. This is hydrogen. As we remember, we called it 1-bromo-1-fluoroethane, and we will also call this molecule 1-bromo-1-fluoroethane, but these are two completely different molecules. Even though they are made up of the same molecules; they have the same molecular formula; the same device, that is, this carbon is connected to hydrogen, fluorine and bromine; and this carbon is connected with the same elements; this carbon is connected to carbon and three hydrogens; just like this one; both are stereoisomers. These are stereoisomers, and they are mirror images of each other, so they are also enantiomers. In fact, they, firstly, polarize light differently, and they have completely different chemical properties, both in the chemical and in the biological system. Therefore, it is not very good that we give the same names for them. in this form, we will focus on how to distinguish between them. So how do we label the differences between them? The naming system we'll be using here is called the Kahn-Ingold-Prelog rule, but it's a different Kahn, it's not me. It is spelled Kan, not Khan. The Kahn-Ingold-Prelog rule is a way of distinguishing between this enantiomer, which we now call 1-bromo-1-fluoroethane, and this enantiomer, 1-bromo-1-fluoroethane. It's pretty simple. The most difficult part is to imagine the rotation of the molecule in the desired direction and to figure out whether this molecule is left-handed or right-handed. Now we will understand this step by step. The first thing we do, according to the Cahn-Ingold-Prelog rule, is identify the chiral molecule. It's pretty obvious here. Here we have carbon. Focus on the left picture we started with. He is connected to 3 different groups. Now we need to sort the groups by atomic number. If we look here, out of bromine, hydrogen, fluorine, and carbon, which is directly bonded to that carbon, what is the largest atomic number? Here's bromine - let's mark it with a darker color. The number of bromine is 35, fluorine is 9, carbon is 6, and finally hydrogen is 1. That is, among them, bromine has the largest number. Let's assign it number 1. After it comes fluorine. This is #2. #3 is carbon. And hydrogen has the smallest number, so it will be number 4. Now we have numbered them, and the next step is to position the molecule so that the group with the smallest atomic number is behind the image. Position it behind the molecule. Hydrogen has the smallest number right now. Bromine has the largest, hydrogen has the smallest, so we need to place it behind the molecule. In the picture, he is now in front of her. And we need to place it behind the molecule, and this is the hardest part - to imagine it correctly. We remember that fluorine is at the back; this is the right side of the image; this part protrudes in front of the image. We need to make a rotation. You can imagine that we rotate the molecule in this direction and ... (let's draw again). Here we will have carbon. And since that's the direction of rotation, we've rotated it about 1/3 around ourselves, which is about 120 degrees. Now hydrogen is in place of fluorine. This is where the hydrogen is. Fluorine is now in place of this methyl group. Here is fluorine. The dotted line shows what is behind. And this is the front. And the methyl group is now instead of hydrogen. She now stands in front of the image. She will be on the left and outside. Here is the methyl group protruding in front of the image, outside and to the left. This is where our methyl group will be. All we did was just rotate the image 120 degrees. We made it go backwards, which is the first step after we've identified the chiral carbon and sorted the elements by their atomic number. Of course, bromine will still be at the top. Now that we have placed the molecule with the smallest atomic number back, let's try to look at the distribution of the other 3. We have 4 molecules. We're looking at the largest, It's bromine, No. 1. No. 2 is fluorine, No. 2, and then No. 3 is the methyl group. We have a carbon bonded to this carbon, here we have #3. And according to the Kahn-Ingold-Prelog rule, we literally have to go from #1 to #2 to #3? In this case, let's go in that direction. Going from #1 to #2 to #3, we follow clockwise. Let's ignore hydrogen for now. He just stays behind. The first step was to orient it backwards as the smallest molecule. And we are left with 3 big ones, and we have determined the direction in which we need to move from # 1 to 2 and # 3, right? In this case, the direction is clockwise. If we move clockwise, then our molecule is called right-handed, or we can use the Latin word for right, which sounds like rectus. Therefore, now we can call this molecule not just 1-bromo-1-fluoroethane, but add R, R - from the word rectus. You might think that this is from the English right (right), but we will see that S is used for the left side, from the word sinister, so the letter R is still from Latin. And this is our (R)-1-bromo-1-fluoroethane. Here it is. You can guess that this one should be the other way around, it should rotate counterclockwise. Let's do this quickly. The idea is the same. We know the largest element. This is bromine number 1. It is the largest in terms of atomic number. Fluorine is #2. Carbon is #3. Hydrogen is #4. What we need to do is put the hydrogen back, so we'll have to turn it back to where the fluorine is now. If we have to redraw this molecule, then here we have carbon. At the top, there will still be bromine. But we're going to move the hydrogen back, so the hydrogen is now where the fluorine was. Here is our hydrogen. The methyl group, the carbon with 3 hydrogens, will now move to where the hydrogen used to be. It will now protrude in front of the image since we've rotated it in that direction, and here's our methyl group here. And the fluorine now moves to where the methyl group was, and here we have fluorine. Now, using the Kahn-Ingold-Prelog rule, we determine that this is No. 1, This is No. 2, just by atomic number, this is No. 3. We go from No. 1 through No. 2 to No. 3. Right in this direction. Counterclock-wise. In other words, we go to the left, or we can use the Latin word that sounds like sinister. The Latin word sinister in the original means "left". In modern English, the word "sinister" means "sinister". But it has nothing to do with Latin. We will use it simply as a symbol for left. So, we have the left version of the molecule. We'll call this variant, This enantiomer 1-bromo-1-fluoroethane. Let's denote it S, S from the word sinister, that is, left, or counterclockwise: (S) -1-bromo-1-fluoroethane. Now we can distinguish these names. We know that these are two different configurations. And that's what the S and R stands for, and if we're going to make that out of it, we're going to have to literally unmerge and re-merge the different groups. That is, you have to actually break the ties. And in fact, you have to swap these groups in a certain way in order to get this enantiomer from this one. Because they have different configurations, and basically they are different molecules, stereoisomers, enantiomers. Any of these names suits them… Subtitles by the Amara.org community

Determination of precedence

In modern IUPAC stereochemical nomenclature, configurations of double bonds, stereocenters, and other chirality elements are assigned based on relative position substituents (ligands) at these elements. The rules of Kahn - Ingold - Prelog establish the seniority of deputies, according to the following mutually subordinate provisions.

1. An atom with a higher atomic number is older than an atom with a lower atomic number. Comparison of substituents is carried out on the atom that is directly connected to the stereocenter or double bond. The higher the atomic number of this atom, the older the substituent. If the first atom of the substituents is the same, the comparison is carried out by atoms that are two bonds away from the stereocenter (double bond) (the so-called atoms of the second layer). To do this, these atoms for each substituent are written out as a list in order of decreasing atomic number and these lists are compared line by line. The senior is the deputy in whose favor the first difference will be. If the seniority of the substituents cannot be determined by the atoms of the second layer, the comparison is carried out by the atoms of the third layer, and so on until the first difference.
2. An atom with a higher atomic mass is older than an atom with a lower atomic mass. This rule usually applies to isotopes, since they cannot be distinguished by their atomic number.
3. Sectionis- deputies older sectrans- deputies. This rule applies to substituents containing double bonds or planar four-coordinate fragments.
4. diastereomeric substituents with like(English like) designations older than diastereomeric substituents with dissimilar(eng. unlike) designations. The former include substituents with the designations RR, SS, MM, PP, sectionissectionis, sectranssectrans, Rsection, Ssectrans, Mseccis and RM, SP. The second group includes substituents with designations RS, MP, RP, SM, sectionsecsectrans, Rsectrans, Ssection, Pseccis and MSektrans.
5. Deputy with designation R or M older than the deputy with the designation S or P .

The rules are applied sequentially one after the other, if it is not possible to determine the precedence of the deputies using the previous one. The exact wording of Rules 4 and 5 is currently under discussion.

Examples of using

AT R/S- nomenclature

Assigning a Configuration to a Stereo Center R or S is carried out on the basis of the mutual arrangement of substituents (ligands) around the stereocenter. In this case, at the beginning, their seniority is determined according to the Cahn-Ingold-Prelog rules, then the three-dimensional image of the molecule is positioned so that the junior substituent is located behind the image plane, after which the direction of decreasing the seniority of the remaining substituents is determined. If the precedence decreases clockwise, then the stereocenter configuration is denoted R(lat. rectus - right). Otherwise, the configuration is denoted S(lat. sinister - left)

AT E/Z- nomenclature

In the nomenclature of top sides

Main article: Topness

The Kahn-Ingold-Prelog rules are also used to denote the sides of planar trigonal molecules, such as ketones. For example, the sides of acetone are identical because attacking the nucleophile from both sides of the planar molecule results in a single product. If the nucleophile attacks butanone-2, then the sides of butanone-2 are non-identical (enantiotopic), since enantiomeric products are formed when attacking different sides. If the ketone is chiral, then attachment to opposite sides will result in diastereomeric products, so these sides are called diastereotopic.

To designate the top sides use the notation re and si, which respectively reflect the direction of decreasing order of substituents at the trigonal carbon atom of the carbonyl  group. For example, in the illustration, the acetophenone molecule is seen from re-sides.

Notes

1. . Retrieved February 5, 2013. Archived from the original February 14, 2013.
2. Cahn R. S., Ingold C., Prelog V. Specification of Molecular Chirality // Angew. Chem. Int. Ed. - 1966. - Vol. 5, no. 4 . - P. 385–415. - DOI:10.1002/anie.196603851 .
3. Preferred IUPAC Names. Chapter 9 . Retrieved February 5, 2013.