Among the compounds containing a closed conjugated system of π-electrons, aromatic compounds are of interest. Despite the high degree of unsaturation, aromatic compounds are resistant to the action of oxidants and temperature, they are more prone to enter into substitution reactions rather than addition. These compounds have increased thermodynamic stability compared to conjugated open chain systems. It is also known that certain cyclic compounds tend to convert to aromatic compounds under favorable conditions.

Aromatic compounds primarily include benzene and substances similar to it. But they can also have a significantly different structure. A closed chain can not only consist of 12 C (carbocycles), but also contain heteroatoms (heterocycles). A single closed system of π-electrons can be formed through both π, π- and p, π-conjugation. The set of characteristic properties of conjugate systems was united by the concept aromaticity. In 1865 F.A.Kekule proposed to describe benzene using two structures between which the benzene molecule oscillates. But the individual structure of Kekulé cannot explain the symmetry and characteristic reactivity of benzene. Benzene is a flat regular hexagon with 120 ° bond angles. All 12 C - 12 C bonds are equivalent, their length is 0.139 nm, that is, it is intermediate between the lengths of single and double bonds. All 12 С are in sp 2 -hybridization, and all σ-bonds С-С and С-Н are in the same plane .

Each 12 C in a benzene molecule has one unhybridized p-orbital. Six of these orbitals are located perpendicular to the flat σ-skeleton and parallel to each other. When they overlap, a single π-electron cloud is formed, that is, circular conjugation occurs. π-Electron density is evenly distributed throughout the cyclic system.

Determination of the heats of combustion or hydrogenation of cyclic compounds and comparison of experimental values \u200b\u200bwith those calculated on the assumption that the compound contains only isolated double bonds is one of the proofs of aromaticity. When cyclohexene is hydrogenated to cyclohexane, 120 kJ / mol of heat is released.

If benzene is represented by the Kekule structure with three double bonds, then the heat of hydrogenation of benzene should be three times higher than the heat of hydrogenation of cyclohexene:

The experimentally determined value is much less. Consequently, benzene is less energetic than the hypothetical cyclohexatriene. 151 kJ / mol - empirical conjugation energy (delocalization energy). For benzene, the conjugation energy is an order of magnitude higher than for butadiene-1,3. To break the aromatic system of benzene, you need to spend an amount of energy equal to this value.

Aromaticity criteria.Based on theoretical calculations and experimental study of cyclic conjugated systems, it was found that a compound is aromatic if it has:

flat cyclic σ-skeleton;

conjugated closed π-electron system, covering all atoms of the cycle and containing 4p + 2π-electrons, where p \u003d 0, 1, 2, 3, etc. - hückel's rule.

Aromaticity criteria make it possible to distinguish conjugated aromatic systems from all others. Benzene contains a sextet of π-electrons and corresponds to the Hückel rule for p = 1.

Condensed aroma systems.Hückel's rule was formulated for planar monocyclic systems. But it can also be applied to flat condensed systems in which there are no atoms common to more than two cycles. These systems include multinuclear aromatic hydrocarbons - naphthalene, anthracene, phenanthrene:

In these compounds, all carbon atoms are in the sp 2 -hybridization state, the cyclic σ-skeleton is flat, the π-electron cloud covers all carbon atoms of the cycles, the number of π-electrons obeys the Hückel rule. In condensed arenes, the electron density is not completely even, and they are less thermodynamically stable.

Many aromatic polycyclic hydrocarbons have carcinogenic properties and are being intensively studied in connection with the problems of the occurrence and prevention of cancer. Several carcinogenic aromatic compounds are found in tobacco smoke.

Non-benzoic aromatic compounds.There are cyclic conjugated systems that do not contain six-membered rings, but meet the criteria for aromaticity and possess aromatic properties. Hückel's rule does not restrict aromaticity to neutral particles only. Aromatic can be carbanions and carbocations.

The neutral cyclopentadiene molecule is not aromatic, (one 12 C in sp 3 -hybridization and does not have r- AO, the cycle is not flat). Hydrogen atoms of the methylene group are highly mobile. When cyclopentadiene is acted upon with sodium in tetrahydrofuran or sodium hydride in 1,2-dimethoxyethane, a proton is removed and a cyclopentadienide ion is formed:

After the C-H bond is broken, 12 C has two electrons. Now all carbon atoms are in the sp 2 -hybrid state, the molecule has a flat cyclic σ-skeleton and a single closed conjugated system containing six π-electrons on five p-orbitals. This meets all the criteria for aromaticity. To reflect the uniform distribution of "-" charge, the cyclopentadienide ion is depicted as a structure with a circle and a minus sign in a circle:

Cyclopentadienide ion is a π-excess system that acts as a donor of electron density in relation to atoms or molecules with vacant orbitals. It forms metallocenes (ferrocene) with metal ions. In ferrocene, an iron ion is located at an equal distance between two parallel planes of cyclopentadienide ions - a "sandwich structure". A ferrocene derivative - ferroceron - stimulates the processes of hematopoiesis and is used for anemia.

Cycloheptatriene is a cyclic system containing seven 12 C and 6 π electrons. And in this case 12 С of the methylene group is in the state of sp 3 -hybridization and does not have a p-orbital. When hydrogen is cleaved from the methylene group in the form of a hydride ion, a cycloheptatrienyl cation (tropylium cation) is formed:

In the tropylium cation, the seventh p-orbital is vacant and overlaps with neighboring p-orbitals to form a single conjugated system. It meets the criteria for aromaticity. The positive charge is evenly distributed throughout the system. The seven-membered ring lies in one plane, the C-C distances are equal to 0.140 nm. The seven-membered aromatic system of tropolone is widespread in nature. Some tropolone derivatives are natural antibiotics - fungicides.

Azulene is another example of non-benzoic aromatic compounds. It is a hydrocarbon containing condensed seven-membered and five-membered rings. Each of 10 12 C is in a state of sp 2 -hybridization. A single conjugated system contains 10 π-electrons. Azulene is aromatic and has a high stabilization energy (180 kJ / mol). Unlike other aromatic hydrocarbons, azulene (I) has a dipole moment. The presence of a dipole moment suggests that a significant contribution to the structure of azulene is made by structure (II), in which one ring is a cyclopentadienide ion, and the other is an aromatic tropylium cation:

Electronic configuration an atom is a numerical representation of its electron orbitals. Electronic orbitals are regions of various shapes located around an atomic nucleus in which an electron is mathematically likely to be found. Electronic configuration helps to quickly and easily tell the reader how many electron orbitals an atom has, as well as determine the number of electrons in each orbital. After reading this article, you will have mastered the method of generating electronic configurations.

Steps

Distribution of electrons using the periodic system of D. I. Mendeleev

Find the atomic number of your atom. Each atom has a certain number of electrons associated with it. Find the symbol for your atom in the periodic table. An atomic number is a positive integer starting at 1 (for hydrogen) and increasing by one for each subsequent atom. An atomic number is the number of protons in an atom, and therefore it is also the number of electrons in an atom with zero charge.

Determine the charge of an atom. Neutral atoms will have the same number of electrons as shown in the periodic table. However, charged atoms will have more or fewer electrons, depending on the amount of their charge. If you are working with a charged atom, add or subtract electrons as follows: add one electron for every negative charge and subtract one for every positive one.

• For example, a sodium atom with a charge of -1 will have an extra electron in addition to its base atomic number 11. In other words, the atom will have 12 electrons in total.
• If we are talking about a sodium atom with a charge of +1, one electron must be subtracted from the base atomic number 11. Thus, the atom will have 10 electrons.

1. Remember the basic list of orbitals. As the number of electrons increases, they fill various sublevels of the electron shell of the atom in a specific sequence. Each sublevel of the electron shell, when filled, contains an even number of electrons. The following sublevels are available:

   Understand the electronic configuration record. Electronic configurations are recorded to clearly reflect the number of electrons in each orbital. Orbitals are written sequentially, with the number of atoms in each orbital being superscript to the right of the orbital name. The completed electronic configuration takes the form of a sequence of sub-level designations and superscripts.

   • For example, the simplest electronic configuration: 1s 2 2s 2 2p 6. This configuration shows that there are two electrons at the 1s sublevel, two electrons at the 2s sublevel, and six electrons at the 2p sublevel. 2 + 2 + 6 \u003d 10 electrons in total. This is the electronic configuration of a neutral neon atom (neon atomic number is 10).

2. Remember the order of the orbitals. Keep in mind that the electron orbitals are numbered in ascending order of the electron shell number, but in ascending order of energy. For example, a filled 4s 2 orbital is less energetic (or less mobile) than a partially filled or filled 3d 10, so the 4s orbital is recorded first. Once you know the order of the orbitals, you can easily fill them in according to the number of electrons in the atom. The order of filling the orbitals is as follows: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p.

   • The electronic configuration of an atom in which all orbitals are filled will have the following form: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 6 6s 2 4f 14 5d 10 6p 6 7s 2 5f 14 6d 10 7p 6
   • Note that the above entry, when all orbitals are filled, is the electronic configuration of the element Uuo (ununoctium) 118, the highest numbered atom in the periodic table. Therefore, this electronic configuration contains all the currently known electronic sublevels of a neutral charged atom.

3. Fill in the orbitals according to the number of electrons in your atom. For example, if we want to write down the electronic configuration of a neutral calcium atom, we must start by looking for its atomic number in the periodic table. Its atomic number is 20, so we will write the configuration of an atom with 20 electrons according to the above order.

   • Fill in the orbitals in the order above until you reach the twentieth electron. The first 1s orbital will have two electrons, the 2s orbitals will also have two, 2p - six, 3s - two, 3p - 6, and 4s - 2 (2 + 2 + 6 +2 + 6 + 2 \u003d 20 .) In other words, the electronic configuration of calcium is: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2.
   • Note that the orbitals are in ascending order of energy. For example, when you are ready to go to the 4th energy level, then first write down the 4s orbital, and then 3d. After the fourth energy level, you go to the fifth, where the same order is repeated. This only happens after the third energy level.

4. Use the periodic table as a visual clue. You've probably already noticed that the shape of the periodic table corresponds to the order of electronic sublevels in electronic configurations. For example, the atoms in the second column from the left always end in "s 2", while the atoms on the right edge of the thin middle section always end in "d 10", and so on. Use the periodic table as a visual guide to writing configurations — as the order in which you add to orbitals corresponds to your position in the table. See below:

   • In particular, the two leftmost columns contain atoms whose electronic configurations end in s-orbitals, the right block of the table contains atoms whose configurations end in p-orbitals, and in the lower part, atoms end in f-orbitals.
   • For example, when you write down the electronic configuration of chlorine, think like this: "This atom is located in the third row (or" period ") of the periodic table. It is also located in the fifth group of the p orbital block of the periodic system. Therefore, its electronic configuration will end in. ..3p 5
   • Please note: the elements in the region of the d and f orbitals of the table are characterized by energy levels that do not correspond to the period in which they are located. For example, the first row of the block of elements with d-orbitals corresponds to 3d orbitals, although it is located in the 4th period, and the first row of elements with f-orbitals corresponds to the 4f orbital, despite the fact that it is in the 6th period.

5. Learn shorthand for writing long electronic configurations. The atoms on the right edge of the periodic table are called noble gases. These elements are chemically very stable. To shorten the process of writing long electronic configurations, simply write in square brackets the chemical symbol of the nearest noble gas with fewer electrons than your atom, and then continue writing the electronic configuration of subsequent orbital levels. See below:

   • To understand this concept, it is helpful to write an example configuration. Let's write the configuration for zinc (atomic number 30) using noble gas abbreviation. The complete configuration of zinc looks like this: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10. However, we see that 1s 2 2s 2 2p 6 3s 2 3p 6 is the electronic configuration of argon, a noble gas. Just replace part of the electronic configuration of zinc with the chemical symbol argon in square brackets (.)
   • So, the electronic configuration of zinc, written in an abbreviated form, is: 4s 2 3d 10.
   • Note that if you are writing the electronic configuration of a noble gas, say argon, you cannot write it! One should use the reduction of the noble gas facing this element; for argon it will be neon ().

   Using the periodic table ADOMAH

   1. Learn the ADOMAH periodic table. This method of recording the electronic configuration does not require memorizing, however, it requires a revised periodic table, since in the traditional periodic table, starting from the fourth period, the period number does not correspond to the electron shell. Find the ADOMAH Periodic Table - a special type of periodic table developed by scientist Valery Zimmerman. It is easy to find it with a short internet search.

      • In the periodic table of ADOMAH, horizontal rows represent groups of elements such as halogens, inert gases, alkali metals, alkaline earth metals, etc. The vertical columns correspond to electronic levels, and the so-called "cascades" (diagonal lines connecting the s, p, d and f blocks) correspond to periods.
      • Helium is moved to hydrogen as both of these elements have a 1s orbital. Period blocks (s, p, d and f) are shown on the right side, and level numbers are shown at the bottom. Elements are presented in boxes numbered 1 through 120. These numbers are common atomic numbers that represent the total number of electrons in a neutral atom.

   2. Find your atom in the ADOMAH table. To record the electronic configuration of an element, find its symbol in the periodic table ADOMAH and cross out all elements with a higher atomic number. For example, if you need to write down the electronic configuration of erbium (68), cross out all elements from 69 to 120.

      • Note the numbers 1 through 8 at the bottom of the table. These are electronic level numbers, or column numbers. Ignore columns that contain only crossed out items. For erbium, the columns numbered 1,2,3,4,5 and 6 remain.

   3. Count the orbital sublevels to your element. Looking at the block symbols shown to the right of the table (s, p, d, and f) and the column numbers shown at the bottom, ignore the diagonal lines between the blocks and break the columns into column blocks, listing them in order from bottom to top. Again, ignore the boxes with all the elements crossed out. Write down the column blocks, starting with the column number followed by the block symbol, thus: 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 6s (for erbium).

      • Note: The above electronic configuration Er is written in ascending order of the electronic sublevel number. It can also be written in the order of filling the orbitals. To do this, follow the cascades from bottom to top, and not along the columns when you write the column blocks: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 6 6s 2 4f 12.

   4. Count the electrons for each electronic sublevel. Count the elements in each block-column that were not crossed out, attaching one electron from each element, and write their number next to the block symbol for each block-column as follows: 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 4d 10 4f 12 5s 2 5p 6 6s 2. In our example, this is the electronic configuration of erbium.

   5. Consider incorrect electronic configurations. There are eighteen typical exceptions related to the electronic configurations of atoms in the lowest energy state, also called the ground energy state. They do not obey the general rule only in the last two or three positions occupied by electrons. In this case, the actual electronic configuration assumes that the electrons are in a state with a lower energy in comparison with the standard configuration of the atom. Exception atoms include:

      • Cr(..., 3d5, 4s1); Cu(..., 3d10, 4s1); Nb(..., 4d4, 5s1); Mo(..., 4d5, 5s1); Ru(..., 4d7, 5s1); Rh(..., 4d8, 5s1); Pd(..., 4d10, 5s0); Ag(..., 4d10, 5s1); La(..., 5d1, 6s2); Ce(..., 4f1, 5d1, 6s2); Gd(..., 4f7, 5d1, 6s2); Au(..., 5d10, 6s1); Ac(..., 6d1, 7s2); Th(..., 6d2, 7s2); Pa(..., 5f2, 6d1, 7s2); U(..., 5f3, 6d1, 7s2); Np(..., 5f4, 6d1, 7s2) and Cm(..., 5f7, 6d1, 7s2).
   • To find the atomic number of an atom when written in electronic configuration form, simply add up all the numbers that follow the letters (s, p, d, and f). This only works for neutral atoms, if you are dealing with an ion, then nothing will work - you have to add or subtract the number of extra or lost electrons.
   • The number following the letter is a superscript, do not make a mistake in the check.
   • There is no "stability of a half-filled" sublevel. This is a simplification. Any stability that is related to the "half filled" sublevels is due to the fact that each orbital is occupied by one electron, so the repulsion between the electrons is minimized.
   • Each atom tends to a stable state, and the most stable configurations have filled sublevels s and p (s2 and p6). Noble gases have such a configuration, therefore they rarely enter into reactions and are located on the right in the periodic table. Therefore, if the configuration ends at 3p 4, then it needs two electrons to reach a stable state (to lose six, including electrons of the s-sublevel, it will take more energy, so four is easier to lose). And if the configuration ends in 4d 3, then it needs to lose three electrons to reach a stable state. In addition, half-filled sublevels (s1, p3, d5 ..) are more stable than, for example, p4 or p2; however, s2 and p6 will be even more stable.
   • When you are dealing with an ion, it means that the number of protons is not equal to the number of electrons. In this case, the charge of an atom will be shown at the top right (as a rule) of the chemical symbol. Therefore, an antimony atom with a charge of +2 has the electronic configuration 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 1. Note that 5p 3 has changed to 5p 1. Be careful when the configuration of a neutral atom ends up at sublevels other than s and p. When you pick up electrons, you can only pick them up from the valence orbitals (s and p orbitals). Therefore, if the configuration ends at 4s 2 3d 7 and the atom gets a charge of +2, then the configuration will end at 4s 0 3d 7. Please note that 3d 7 not changes, instead of losing s-orbital electrons.
   • There are conditions when the electron is forced to "go to a higher energy level." When a sublevel lacks one electron to half or full filling, take one electron from the nearest s or p-sublevel and move it to the sublevel that needs an electron.
   • There are two options for recording an electronic configuration. They can be written in ascending order of energy level numbers or in the order of filling of electron orbitals, as was shown above for erbium.
   • You can also write down the electronic configuration of an element by writing down only the valence configuration, which is the last s and p sublevels. Thus, the valence configuration of antimony will have the form 5s 2 5p 3.
   • Jonah is not the same. It's much more difficult with them. Skip two levels and follow the same pattern depending on where you started and how large the number of electrons is.

Electronic configuration of the atom is a formula showing the arrangement of electrons in an atom by levels and sublevels. After studying the article, you will find out where and how electrons are located, get acquainted with quantum numbers and be able to construct the electronic configuration of an atom by its number, at the end of the article there is a table of elements.

Why study the electronic configuration of elements?

Atoms as a constructor: there are a certain number of parts, they differ from each other, but two parts of the same type are exactly the same. But this constructor is much more interesting than the plastic one and here's why. The configuration changes depending on who is nearby. For example, oxygen next to hydrogen can turn into water, next to sodium into gas, and being next to iron completely turns it into rust. To answer the question why this is happening and to predict the behavior of an atom next to another, it is necessary to study the electronic configuration, which will be discussed below.

How many electrons are there in an atom?

An atom consists of a nucleus and electrons revolving around it, the nucleus consists of protons and neutrons. In a neutral state, each atom has the same number of electrons as the number of protons in its nucleus. The number of protons was designated by the ordinal number of the element, for example, sulfur, has 16 protons - the 16th element of the periodic system. Gold has 79 protons - 79th element of the periodic table. Accordingly, in sulfur in the neutral state there are 16 electrons, and in gold there are 79 electrons.

Where to find an electron?

Observing the behavior of the electron, certain regularities were derived, they are described by quantum numbers, there are four of them:

• Principal Quantum Number
• Orbital quantum number
• Magnetic quantum number
• Spin quantum number

Orbital

Further, instead of the word orbit, we will use the term "orbital", the orbital is the wave function of the electron, roughly it is the area in which the electron spends 90% of the time.

N - level
L - shell
M l - orbital number
M s - the first or second electron in the orbital

Orbital quantum number l

As a result of the study of the electron cloud, it was found that depending on the energy level, the cloud takes four basic forms: a ball, dumbbells and the other two, more complex. In order of increasing energy, these shapes are called s-, p-, d- and f-shells. Each of these shells can contain 1 (s), 3 (p), 5 (d) and 7 (f) orbitals. The orbital quantum number is the shell that the orbitals are on. The orbital quantum number for s, p, d and f-orbitals takes the values \u200b\u200b0,1,2 or 3, respectively.

On the s-shell, one orbital (L \u003d 0) - two electrons
The p-shell has three orbitals (L \u003d 1) - six electrons
The d-shell has five orbitals (L \u003d 2) - ten electrons
The f-shell has seven orbitals (L \u003d 3) - fourteen electrons

Magnetic quantum number m l

There are three orbitals on the p-shell, they are denoted by numbers from -L to + L, that is, for the p-shell (L \u003d 1) there are orbitals "-1", "0" and "1". The magnetic quantum number is denoted by the letter m l.

Inside the shell, it is easier for electrons to be located in different orbitals, so the first electrons fill one for each orbital, and then a pair of it is added to each.

Consider a d-shell:
d-shell corresponds to the value L \u003d 2, that is, five orbitals (-2, -1,0,1 and 2), the first five electrons fill the shell taking the values \u200b\u200bM l \u003d -2, M l \u003d -1, M l \u003d 0 , M l \u003d 1, M l \u003d 2.

Spin quantum number m s

Spin is the direction of rotation of an electron around its axis, there are two directions, so the spin quantum number has two values: +1/2 and -1/2. One energy sublevel can contain two electrons only with opposite spins. The spin quantum number is denoted by m s

Principal quantum number n

The main quantum number is the energy level, at the moment there are seven energy levels, each denoted by an Arabic number: 1,2,3, ... 7. The number of shells at each level is equal to the level number: at the first level, one shell, at the second two, etc.

Electron number

So, any electron can be described by four quantum numbers, the combination of these numbers is unique for each position of the electron, take the first electron, the lowest energy level is N \u003d 1, one shell is located at the first level, the first shell at any level has the shape of a ball (s -shell), i.e. L \u003d 0, the magnetic quantum number can take only one value, M l \u003d 0 and the spin will be +1/2. If we take the fifth electron (in whatever atom it is), then the main quantum numbers for it will be: N \u003d 2, L \u003d 1, M \u003d -1, spin 1/2.

CHAPTER 7. STEREOCHEMICAL BASIS OF THE STRUCTURE OF MOLECULES OF ORGANIC COMPOUNDS

Stereochemistry (from the Greek. stereos- spatial) is “chemistry in three dimensions”. Most molecules are threedimentional (3D for short). Structural formulas reflect the two-dimensional (2D) structure of a molecule, including the number, type and sequence of bonding of atoms. Recall that compounds that have the same composition but different chemical structures are called structural isomers (see 1.1). The 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 with the same chemical structure can 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 are compounds in the molecules of which there is the same sequence of chemical bonds of atoms, but different arrangement of these atoms relative to each other in space.

In turn, stereoisomers can be configurationand conformational isomers,i.e., differ accordingly configurationand conformation.

7.1. Configuration

Configuration is the order in which atoms are arranged in space without taking into account the differences arising from rotation around single bonds.

Configuration isomers can transform into each other by breaking some and forming other chemical bonds and can exist separately in the form of individual compounds. They are classified into two main types - enantiomersand diastereomers.

7.1.1. Enantiomerism

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

In the form of enantiomers, only chiralmolecules.

Chirality is the property of an object to be incompatible with its mirror image. Chiral (from the Greek. cheir- hand), or asymmetric, 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 plane of symmetry,which divides the object into two mirror-identical parts (see fig. 7.1, b).

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

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

Figure: 7.1.Reflection in a mirror of a chiral object (a) and a plane of symmetry cutting an 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 an asymmetric carbon atom is usually omitted.

To determine whether a molecule is chiral or achiral, there is no need 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, CH3 and C 2 H 5). An asymmetric carbon atom is sometimes marked with an asterisk (C *).

Consequently, the butanol-2 molecule is capable of existing as a pair of enantiomers that do not combine in space (Fig. 7.2).

Figure: 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,that is, the ability to deflect the plane of polarized light *.

When such light passes through a solution of one of the enantiomers, the polarization plane 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 indicated by a minus sign (-), right rotation is indicated by a plus sign (+), and enantiomers are called respectively left and dextrorotatory.

Other names for 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 enantiomers. Racemates are optically inactive, since 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 compound.

7.1.2. Relative and absolute configuration

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

The tetrahedral model of one of the enantiomers (Fig. 7.3) is placed in space so that the chain of carbon atoms is in the 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., Rev. and add. - M .: Bustard, 2003.- S. 365-375.

Figure: 7.3.Construction of Fisher's projection formula (+) - lactic acid

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

Before projection, the tetrahedral model of a chiral molecule can be positioned in space in different ways, not only as shown in Fig. 7.3. It is only necessary that the links forming a horizontal line on the projection should be directed towards the observer, and the vertical links - beyond the plane of the drawing.

The projections obtained in this way can, using simple transformations, be reduced to the standard form in which the carbon chain is located vertically, and the older group (in lactic acid it 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 may be sufficient);

It is allowed to rotate the projection formula in the plane of the drawing by 180? (which is equivalent to two permutations), but not 90 ?.

D.L-Configuration designation system. At the beginning of the twentieth century. a classification system for enantiomers has been proposed for relatively simple (in terms of stereoisomerism) molecules such as α-amino acids, α-hydroxy acids and the like. Per configuration standardglyceraldehyde was adopted. Its levorotatory enantiomer was arbitrarilyformula (I) is assigned. This configuration of the carbon atom was designated by the letter l (from lat. laevus- left). Formula (II) was assigned to the dextrorotatory enantiomer, and the configuration was designated by the letter d (from lat. dexter- right).

Note that in the standard projection formulal -glyceric aldehyde, the OH group is on the left, and atd -glyceric aldehyde - 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 configurationd- or l -glyceric aldehyde. For example, in one of the enantiomers of lactic acid (I) in the projection formula, the OH group is on the left, as inl -glyceric aldehyde, therefore enantiomer (I) is referred tol -near. For the same reasons, enantiomer (II) is referred tod -near. Thus, from a comparison of the Fischer projections, we determine relativeconfiguration.

It should be noted thatl -glyceric aldehyde has a left-handed rotation, andl -lactic acid - right (and this is not an isolated case). Moreover, one and the same substance can be either levorotatory or dextrorotatory, depending on the conditions of determination (different solvents, temperature).

The sign of rotation of the plane of polarized light is not associated with belonging tod- 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, fromd- or l α-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.

The arbitrary assignment of conditional configurations to the left- and dextrorotatory 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 due to the development of physicochemical methods, especially X-ray diffraction analysis, with the help of which the absolute configuration was first determined in 1951, the chiral molecule was the salt of (+) - tartaric acid. After that, it became clear that the absolute configuration of d- and l-glycerolic aldehydes is indeed the same as 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. d, L-System has very limited application, since it is often impossible to correlate the configuration of any compound with glyceraldehyde. The universal designation system for the configuration of chirality centers is the R, S-system (from lat. rectus- straight, sinister- left). It is based on sequence rule,based on the seniority of the substitutes associated with the chiral center.

The seniority of substituents is determined by the atomic number of the element directly linked to the chiral center - the larger it is, the older the substituent.

So, 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 an asymmetric center. If the atomic numbers are the same, the highest is the group in which the atom next to the carbon has a higher ordinal number, and if this atom (usually oxygen) is double bonded, it is counted twice. As a result, the following groups are arranged in the order of decreasing seniority: -COOH\u003e -CH \u003d O\u003e -CH 2 OH.

To determine the configuration, the tetrahedral model of the compound is placed in space so that the lowest substituent (in most cases, this is a hydrogen atom) is farthest from the observer. If the precedence of the other three substituents decreases clockwise, then the R-configuration is attributed 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, at).

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

Fisher's projections can be used to indicate the configuration according to the RS system. For 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 transformation of the projection, the precedence of the other three substituents decreases clockwise, then the asymmetric atom has the R-configuration, and vice versa. The application of this method is shown by the example of l-lactic acid (the numbers indicate the seniority of the groups).

There is an easier way to determine the R- or S-configuration by the Fisher projection, in which the lowest substituent (usually the H atom) is located on one of the horizontalconnections. 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), the drop in seniority will now mean not the R-, but the S-configuration. This method is shown using l-malic acid as an example.

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. Whereas in the d, L-system, compounds with similar configurations form stereochemical rows, in the RS-system, chiral centers in compounds, for example, in the l-row, can have both the R- and S-configuration.

7.1.3. Diastereomerism

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

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

σ -Diastereomers.Many biologically important substances contain more than one chiral center in a molecule. In this case, the number of configuration isomers increases, which is defined as 2 n, where nis the number of chiral centers. For example, in the presence of two asymmetric atoms, the compound can exist as four stereoisomers (2 2 \u003d 4), constituting two pairs of enantiomers.

2-Amino-3-hydroxybutanoic acid has two centers of chirality (atoms C-2 and C-3) and, therefore, must exist as four configurational isomers, one of which is a naturally occurring 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- other), relate to each other as an object and an incompatible mirror image, i.e., they are pairs of enantiomers. When comparing structures (I) and (III), (I) and (IV), (II) and (III), (II) and (IV), it can be seen that in these pairs of compounds, one asymmetric center has the same configuration, while the other is the opposite. Such pairs of stereoisomers are diastereomers.Such isomers are called σ-diastereomers, since the substituents in them are linked to the chiral center by σ-bonds.

Amino acids and hydroxy acids with two centers of chirality are referred to asd- or l -series in the configuration of the asymmetric atom with the lowest number.

Diastereomers, unlike enantiomers, differ in physical and chemical properties. For example, l-threonine, which is part of proteins, and l-allotreonine have different specific rotation values \u200b\u200b(as shown above).

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

Theoretically, this acid, which has two chiral centers, could exist as four stereoisomers (I) - (IV).

Structures (I) and (II) correspond to the d- and l-series enantiomers (assignment is based on the “upper” center of chirality). It would appear that structures (III) and (IV) also correspond to a pair of enantiomers. In fact, these are the formulas of the same compound - optically inactive meso tartaric acid.It is easy to verify the identity of formulas (III) and (IV) by rotating formula (IV) by 180 ° without taking it out of the plane. Despite the two centers of chirality, the meso-tartaric acid molecule as a whole is achiral, since it has a plane of symmetry passing through the middle of the C-2-C-3 bond. In relation 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 is no difficulty 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. So, d-tartaric acid will receive the systematic name (2R, 3R) -2,3-dihydroxybutanedioic acid, and meso-tartaric acid will have stereochemical symbols (2R, 3S) -.

Like meso tartaric acid, there is a meso form of the α-amino acid cystine. With two centers of chirality, the number of cystine stereoisomers is equal to three due to the fact that the molecule is internally symmetric.

π -Diastereomers.These include configurational isomers containing a π-bond. This type of isomerism is characteristic, in particular, for alkenes. With respect to the plane of the π-bond, the same substituents on two carbon atoms can be located one (cis) or different (trance)sides. In this regard, there are stereoisomers known as cis-and trance-isomers, as shown for the example 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 influence of certain substances or ultraviolet rays, an equilibrium is established between the two acids; when heated (~ 150? C), it is shifted towards a more stable trance-isomer.

7.2. Conformations

Free rotation is possible around a simple C-C bond, as a result of which the molecule can take various forms in space. This can be seen in the stereochemical formulas of ethane (I) and (II), where the colored CH groups3 located differently relative to another CH group3.

Rotate one CH group3 relative to the other occurs without disturbing the configuration - only the mutual arrangement of hydrogen atoms in space changes.

The geometric shapes of a molecule that transform into each other by rotation around σ-bonds are called conformations.

According to this conformationalisomers are stereoisomers, the difference between which is caused by the rotation of individual parts 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 the bonds.

7.2.1. Conformation 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 hydrogen atoms of two CH groups3 the smallest, therefore, the CH bonds that are opposite each other are repelled. 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 can be seen that the three C-H bonds at each carbon atom in pairs "shield" each other. This conformation is called obscured.

Figure: 7.5.Shielded (a, b)and inhibited (in, d)ethane conformations

In another conformation of ethane, arising upon rotation of one of the CH3 at 60? (see Fig. 7.5, c), the hydrogen atoms of the two methyl groups are maximally distant from each other. In this case, the repulsion of electrons of C-H bonds will be minimal, 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; she defines the so-called energy barrier to rotation.

Newman's projection formulas. These formulas (easier - Newman projection) are used to represent conformations on a plane. To construct a projection, the molecule is considered from the side of one of the carbon atoms along its bond with the neighboring carbon atom, around which rotation occurs. 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-pointed star with angles of 120 °. The carbon atom removed from the observer (invisible) is depicted as a circle, from which it is also at an angle of 120? three connections depart. Newman's projections also give a visual representation of the eclipsed (see Fig. 7.5, b) and inhibited (see Fig. 7.5, d) conformations.

Under normal conditions, the conformations of ethane easily transform into each other, and we can speak of a statistical set of different conformations that differ slightly in energy. It is impossible to single out even more stable conformation individually.

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 an increase in potential energy. So, in the butane molecule, the eclipsed conformation will be the least advantageous, and the inhibited 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 in alkanes lengthens, the number of conformations rapidly increases as a result of the expansion of the possibilities of rotation around each C – C bond; therefore, long carbon chains of alkanes can take many different forms, for example, zigzag (I), irregular (II), and cheek-like (III ).

A zigzag conformation is preferred, in which all C – C bonds in the Newman projection form an angle of 180 °, as in the hindered 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.

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

In a claw-like conformation (III), carbon atoms that are distant from each other in other conformations converge. If functional groups, for example X and Y, capable of reacting with each other, appear at a sufficiently close distance, 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 advantageousness of the formation of thermodynamically stable five- and six-membered rings.

7.2.2. Six-membered ring conformations

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

Different conformations of cyclohexane arise as a result of partial rotation around σ-bonds between carbon atoms. Of several nonplanar conformations, the most energetically favorable conformation is armchairs(Fig. 7.7), since in it all bond angles between the C-C bonds are ~ 110 °, and the hydrogen atoms at the neighboring carbon atoms do not overshadow each other.

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

In the presence of a substituent in the ring, the conformation with the equatorial position of the substituent is more favorable, such as conformation (I) of methylcyclohexane (Fig. 7.8).

The reason for the lower stability of conformation (II) with an axial arrangement of the methyl group is 1,3-diaxial repulsioncH groups3 and H atoms in positions 3 and 5. In this

Figure: 7.7.Cyclohexane in chair conformation:

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

Figure: 7.8.Cycle inversion of the methylcyclohexane molecule (not all hydrogen atoms are shown)

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

In nature, there are many derivatives of the cyclohexane series, among which hexatomic 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 myoinositis.The myo-inositol molecule has a stable chair conformation, in which five of the six OH groups are in equatorial positions.

Concept chirality- one of the most important in modern stereochemistry. A model is chiral if it does not have any elements of symmetry (plane, center, mirror-rotary axes), except for simple rotation axes. We call the molecule, which is described by such a model, chiral (which means "like a hand", from the Greek ... hiro - hand) for the reason that, like hands, molecules are not compatible with their mirror images. 1 shows a number of simple chiral molecules. Two facts are quite obvious: first, the pairs of the given molecules represent mirror reflections of each other, and secondly, these mirror reflections cannot be combined with each other. You will notice that in each case the molecule contains a carbon atom with four different substituents. Such atoms are called asymmetric. An asymmetric carbon atom is a chiral or stereogenic center. This is the most common type of chirality. If a molecule is chiral, then it can exist in two isomeric forms, related as an object and its mirror image, and incompatible in space. Such isomers (pair) are called enantiomers.

The term "chiral" is not open to interpretation. When a molecule is chiral, then, by analogy with a hand, it must be either left or right. When we call a substance or a certain sample of it chiral, this simply means that it (he) consists of chiral molecules; in this case, it is not at all necessary that all molecules are the same in terms of chirality (left-handed or right-handed, R or S, see section 1.3). Two limiting cases can be distinguished. In the first, the sample consists of molecules identical in terms of chirality (homochiral, only R or only S); such a sample is called enantiomerically pure... In the second (opposite) case, the sample consists of the same number of molecules different from the point of view of chirality (heterochiral, molar ratio R: S\u003d 1: 1); such a sample is also chiral, but racemic... There is also an intermediate case - a nonequimolar mixture of enantiomers. This mixture is called skelemic or non-racemic. Thus, the statement that a macroscopic sample (as opposed to an individual molecule) is chiral should be considered not entirely clear and therefore insufficient in some cases. Additional indication may be required as to whether a sample is racemic or non-racemic. The lack of accuracy in understanding this leads to a certain kind of misconception, for example, in the headings of articles when the synthesis of a certain chiral compound is declared, but it remains unclear whether the author simply wants to draw attention to the very fact of the chirality of the structure discussed in the article, or the product was actually obtained in the form a single enantiomer (i.e., an ensemble of homochiral molecules; this ensemble, however, should not be called a homochiral sample). Thus, in the case of a chiral nonracemic sample, it is more correct to say "Enantiomerically enriched" or " enantiomerically pure ".

Methods for imaging optical isomers

The image method is chosen by the author solely for reasons of convenience in conveying information. In Figure 1, images of enantiomers are given using perspective images. In this case, it is customary to draw connections lying in the image plane with a solid line; connections going beyond the plane - dotted line; and the connections directed towards the observer are shown in bold. This image method is quite informative for structures with one chiral center. The same molecules can be depicted in the form of the Fisher projection. This method was proposed by E. Fisher for more complex structures (in particular, carbohydrates) with two or more chiral centers.

Mirror plane

Figure: 1

To construct Fisher's projection formulas, the tetrahedron is rotated so that two bonds lying in the horizontal plane are directed towards the observer, and two bonds lying in the vertical plane are directed away from the observer. Only an asymmetric atom falls on the image plane. In this case, the asymmetric atom itself, as a rule, is omitted, keeping only the crossing lines and symbols of the substituents. To remember the spatial arrangement of the substituents, a dashed vertical line is often kept in projection formulas (the upper and lower substituents are removed beyond the plane of the drawing), but this is often not done. Below are examples of different ways of depicting the same structure with a certain configuration (Fig. 2)

Fisher projection

Figure: 2

Here are some examples of Fisher's projection formulas (Fig. 3)

(+)-(L) -alanine (-) - 2-butanol (+) - ( D) -glyceric aldehyde

Figure: 3

Since the tetrahedron can be viewed from different sides, each stereoisomer can be represented by twelve (!) Different projection formulas. To standardize projection formulas, certain rules for their writing have been introduced. So, the main (nomenclature) function, if it is at the end of the chain, is usually placed at the top, the main chain is depicted vertically.

In order to compare "non-standard" written projection formulas, you need to know the following rules for transforming projection formulas.

1. The formula cannot be taken out of the plane of the drawing and cannot be rotated by 90 o, although it can be rotated in the plane of the drawing by 180 o without changing their stereochemical meaning (Fig. 4)

Figure: 4

2. Two (or any even number) permutations of substituents at one asymmetric atom do not change the stereochemical meaning of the formula (Fig. 5)

Figure: five

3. One (or any odd number) permutation of substituents at the asymmetric center leads to the formula of the optical antipode (Fig. 6)

Figure: 6

4. Rotation in the plane of the drawing by 90 0 turns the formula into antipodal, unless at the same time the condition for the location of the substituents relative to the plane of the drawing is not changed, i.e. consider that now the side substituents are behind the plane of the drawing, and the upper and lower ones are in front of it. If you use a formula with a dotted line, then the changed orientation of the dotted line will directly remind you of this (Fig. 7)

Figure: 7

5. Instead of permutations, projection formulas can be transformed by rotating any three substituents clockwise or counterclockwise (Fig. 8); the fourth substituent does not change the position (such an operation is equivalent to two permutations):

Figure: 8

Fischer's projections cannot be applied to molecules whose chirality is associated not with the chiral center, but with other elements (axis, plane). In these cases, 3D images are needed.

D , L - Fisher's nomenclature

We discussed one problem - how to depict a three-dimensional structure on a plane. The choice of the method is dictated exclusively by the convenience of presentation and perception of stereo information. The next problem is related to the compilation of the name for each individual stereoisomer. The name should reflect information about the configuration of the stereogenic center. Historically, the first nomenclature for optical isomers was D, L- the nomenclature proposed by Fisher. Until the 1960s, it was more customary to designate the configuration of chiral centers on the basis of plane projections (Fisher), and not on the basis of three-dimensional 3D - formulas, while descriptors were used D andL. Currently D, L - the system is used to a limited extent - mainly for such natural compounds as amino acids, hydroxy acids and carbohydrates. Examples illustrating its application are shown in Fig. 10.

Figure: ten

For α - amino acids, the configuration is indicated by the symbol Lif in Fischer's projection formula the amino (or ammonium) group is located on the left; symbol D used for the opposite enantiomer. For sugars, configuration designation is based on the orientation of the highest-numbered OH group (furthest from the carbonyl end). If OH is a group directed to the right, then this is a configuration D; if HE is left - configuration L.

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. However, the limitations of the Fischer system, as well as the fact that in 1951 an X-ray diffraction method appeared for determining the true arrangement of groups around a chiral center, led to the creation in 1966 of a new, more rigorous and consistent system for describing stereoisomers, known as R, S - the Cahn-Ingold-Prelog (KIP) nomenclature. In the instrumentation system, special descriptors are added to the usual chemical name R or S (in italics in the text), strictly and unambiguously defining the absolute configuration.

NomenclatureKahn-Ingold-Prelog

To define a descriptor R or S for a given chiral center, the so-called chirality rule.Consider four substituents associated with the chiral center. They should be arranged in a uniform sequence of stereochemical precedence; for convenience, let's designate these substituents by the symbols A, B, D and E and agree to consider that in the general sequence of precedence (in other words, by priority) A is older than B, B is older than D, D is older than E (A\u003e B\u003e D\u003e E) ... The chirality rule of the EIA requires that the model be considered from the side opposite to that occupied by substituent E with the lowest priority or stereochemically lower substituent (Fig. 11). Then the other three substitutes form something like a tripod, the legs of which are directed at the viewer.

Figure: eleven

If the drop in the seniority of substituents in the row A\u003e B\u003e D is carried out clockwise (as in Fig. 11), then the center is assigned a configuration descriptor R ( from latin word rectus - right). In another arrangement, when the stereochemical precedence of the substituents falls counterclockwise, the center is assigned a configuration descriptor S (from Latin sinister - left).

By depicting connections using Fisher projections, you can easily define the configuration without building spatial models. The formula must be written so that the junior deputy is at the bottom or at the top, since according to the rules for representing Fisher's projections, vertical links are directed from the observer (Fig. 12). If, in this case, the remaining substituents in decreasing order of precedence are arranged clockwise, the compound is referred to ( R) -series, and if counterclockwise, then k ( S) -a series, for example:

Figure: 12

If the junior group is not on vertical links, then it should be swapped with the lower group, but it should be remembered that the configuration is reversed. You can make any two permutations without changing the configuration.

Thus, the determining factor is stereochemical precedence . Let's discuss now precedence sequence rules, i.e. the rules by which groups A, B, D and E are ranked in order of priority.

Priority preference is given to atoms with a large atomic number. If the numbers are the same (in the case of isotopes), then the atom with the highest atomic mass becomes the oldest (for example, D\u003e H). The youngest "substitute" is a lone electron pair (for example, in nitrogen). Thus, the seniority increases in the row: lone pair

Consider a simple example: in bromochlorofluoromethane CHBrCIF (Fig. 13) there is one stereogenic center, and two enantiomers can be distinguished as follows. First, the substituents are ranked according to their stereochemical seniority: the higher the atomic number, the older the substituent. Therefore, in this example, Br\u003e C1\u003e F\u003e H, where “\u003e” means “more preferred” (or “older”). The next step is to look at the molecule from the side opposite to the lowest substituent, in this case hydrogen. It can be seen that the other three substituents are located at the corners of the triangle and are directed towards the observer. If the seniority in this triplet of substituents decreases clockwise, then this enantiomer is designated as R... In another arrangement, when the seniority of the substituents falls counterclockwise, the enantiomer is designated as S. Designations R and S write in italics and placed in parentheses before the name of the structure. Thus, the two considered enantiomers are named ( S) -bromochlorofluoromethane and ( R) -bromochlorofluoromethane.

Figure: thirteen

2. If two, three or all four identical atoms are directly connected to an asymmetric atom, the precedence is established by the atoms of the second belt, which are no longer connected with the chiral center, but with those atoms that had the same precedence.

Figure: fourteen

For example, in the molecule of 2-bromo-3-methyl-1-butanol (Fig. 14), according to the first belt, the oldest and the lowest substituents are easily determined - these are bromine and hydrogen, respectively. But according to the first atom of the CH 2 OH and CH (CH 3) 2 groups, it is not possible to establish the seniority, since in both cases it is a carbon atom. In order to determine which of the groups is older, the sequence rule is applied again, but now the atoms of the next belt are considered. Compare two sets of atoms (two triplets), written in order of decreasing seniority. Seniority is now determined by the first point where a difference is found. Group FROMH 2 OH - oxygen, hydrogen, hydrogen FROM(ABOUTНН) or in numbers 6 ( 8 eleven). Group FROMH (CH 3) 2 - carbon, carbon, hydrogen FROM(FROMCH) or 6 ( 6 61). The first point of difference is underlined: oxygen is older than carbon (by atomic number), therefore the CH 2 OH group is older than CH (CH 3) 2. Now we can designate the configuration of the enantiomer depicted in Figure 14 as ( R).

If such a procedure did not lead to the construction of an unambiguous hierarchy, it is continued at increasing distances from the central atom, until, finally, differences are encountered, and all four substituents receive their seniority. Moreover, any preference acquired by one or another deputy at one of the stages of seniority approval is considered final and cannot be reassessed at subsequent stages.

3. If there are branch points in the molecule, the procedure for establishing the seniority of atoms should be continued along the molecular chain of the highest seniority. Suppose you want to determine the order of precedence of the two substituents shown in Figure 15. Obviously, the solution will not be achieved either in the first (C) or in the second (C, C, H) or in the third (C, H, F, C, H, Br) layers. In this case, you will have to go to the fourth layer, but this should be done along the path, the advantage of which is set in the third layer (Br\u003e F). Therefore, the decision on the priority of the deputy AT over the deputy AND is done on the basis that in the fourth layer Br\u003e CI for the branch to which the transition to which is dictated by the seniority in the third layer, and not on the basis that the highest atomic number in the fourth layer is possessed by atom I (which is on the less preferred and therefore not branch under study).

Figure: fifteen

4. Multiple links are represented as the sum of the corresponding simple links. In accordance with this rule, each atom linked by a multiple bond is associated with an additional “phantom” atom (or atoms) of the same kind located at the other end of the multiple bond. Complementary (additional or phantom) atoms are enclosed in parentheses, and they are considered to carry no substituents in the next layer. As an example, consider the representations of the following groups (Fig. 16).

Group Representation

Figure: sixteen

5. An artificial increase in the number of substituents is also required when the substituent (ligand) is bidentate (or tri- or tetradentate), as well as when the substituent contains a cyclic or bicyclic fragment. In such cases, each branch of the cyclic structure is dissected after the branch point [where it bifurcates on its own], and the atom that is the branch point is placed (in parentheses) at the end of the chain resulting from the dissection. In Fig. 17, using the example of a tetrahydrofuran derivative (THF), the case of a bidentate (cyclic) substituent is considered. The two branches of the five-membered ring (separately) are cleaved at bonds with a chiral atom, which is then added to the end of each of the two newly formed chains. It is seen that as a result of the dissection AND a hypothetical substituent —CH 2 OCH 2 CH 2 - (C) is obtained, which turns out to be older than the real acyclic substituent —CH 2 OCH 2 CH 3 due to the advantage of the phantom (C) at the end of the first substituent. On the contrary, formed as a result of dissection AT the hypothetical ligand –CH 2 CH 2 OCH 2 - (C) is lower in precedence than the real substituent –CH 2 CH 2 OCH 2 CH 3, since the latter has three hydrogen atoms attached to the terminal carbon, while the former has none in this layer. Therefore, taking into account the established order of precedence of substituents, the configuration symbol for a given enantiomer turns out to be S.

Determine seniority

AT\u003e A