4.D, L - Stereoisomer designation system.
In a number of cases, it is preferable to use not the R,S -system for designating the absolute configuration, but another, the D,L -system. The choice of D or L isomer expression is based on the specific location. referee group in the Fisher projection . D , L - Nomenclature is widely used in the names of -amino, -hydroxy acids and carbohydrates.
According to this system, the L-configuration is assigned to a stereosomer, in which, in the Fisher projections, the reference group is located to the left of the vertical line (from Latin "laevus" - left). Accordingly, if the reference group located in the Fisher projection on the right, the stereoisomer has a D-configuration (from Latin "dexter" - right):
Of course, we must remember that in the Fischer projection, the most oxidized carbon atom is at the top (that is, the COOH group in amino- and hydroxyl acids and the CH=O group in carbohydrates).
Amino and hydroxy acids
In -amino- and -hydroxy acids, the reference groups are, respectively, the NH 2 and OH groups:
If there are several amino or hydroxy groups in an amino or hydroxy acid, then their mutual arrangement is indicated using the prefixes "erythro", "threo" sp. The assignment of the acid to the D- or L-series in this case determines the NH 2 or OH group, which is in the - position to the COOH group located at the top in the Fischer projection:
In this case, the letters D and L, indicating the position of the reference group, are provided with the index "S". This is done to avoid confusion. The index "S" emphasizes that the configuration of the upper chiral center is indicated, which is located in the - position relative to the carboxyl group, as in the amino acid serine ("S" - from the word "serine").
For hydroxy acids with several OH groups, as well as amino hydroxy acids, an alternative designation of the configuration is also used, in which the reference group is the lowest HO group in the Fischer projection. In this case, the configuration descriptors D and L are subscripted with "g" (from "glyceric aldehyde"). In this case, the amino acids shown in Fig. 123 and 124 are named: D g -threonine (L s - threonine) and L g -threonine (D s -threonine).
Carbohydrates.
In carbohydrates, the reference group is lowest in the Fischer projection, the hydroxyl group associated with an asymmetric carbon atom
Obviously, in the case of molecules with one asymmetric atom, the D,L nomenclature, like the R,S nomenclature, unambiguously indicates the absolute configuration of the center of chirality. The same applies to the use of D,L - the names of stereoisomers with several asymmetric atoms, since in this case the configuration of the remaining centers of chirality is given by the prefixes erythro-, threo-, ribo-, lyxo-, etc. So, if we say "threose", we will only set relative configuration of asymmetric atoms in a molecule. Then it will not be clear which enantiomer we are talking about: (26) or (27), If we say "D-threose", then we will unequivocally indicate that the isomer (26) is meant, since in it the OH reference group is located on the right in the Fisher projection:
Thus, the name "D-threose" (as well as "L-threose") refers to the absolute configuration of both asymmetric atoms in the molecule.
Like the R,S nomenclature, the D,L stereoisomer notation is not related to the optical rotation sign.
It should be noted that lowercase letters d
(right) and l
(left). Do not confuse the use of these letters with the use of capital letters D
and L to indicate the configuration of the molecules. At present, the direction of rotation of the plane of polarization of light is usually denoted by the symbols (+) and (-).
5. Chiral molecules without asymmetric atoms
In the previous sections, molecules were considered whose chirality is due to a certain spatial arrangement of four different atoms or groups of atoms relative to a certain center, called the center of chirality.
Cases are possible when there are no such centers in the molecule, but nevertheless the molecule is chiral, since it lacks symmetry elements of the S n group. In such cases, enantiomers differ in the arrangement of atoms
about some axis or plane, which is called the axis of chirality or the plane of chirality. The chirality axis occurs, for example, in cumulene molecules.
The structure of the molecule of the simplest cumulene - allene - such that its two fragments CH 2 are in two mutually perpendicular planes:
The allene molecule is achiral: it has two planes of symmetry (shown in the figure). Molecules of butadiene-1,2 and 3-methyl-butadiene-1,2 are also achiral
If we consider the pentadiene-2,3 molecule, we will see that it has no planes of symmetry (just as there are no other symmetry elements of the Sn group). This diene exists as a pair of enantiomers:
The chirality of molecules (28) and (29) is due to a certain spatial arrangement of substituents relative to the axis (shown in the figure) passing through the carbon atoms linked by double bonds. This axis is called chirality axis. Molecules like (28) and (29) are said to have axial chirality.
Chirality axes are also present in the molecules of some other compounds, for example, spiro compounds (spirans):
The anthropoisomers of ortho-disubstituted biphenyls mentioned are also molecules with axial chirality. Examples of molecules with chirality plane molecules of para-cyclophanes can serve:
The enantiomers depicted here cannot be converted into each other by rotation around α-bonds due to the spatial requirements of the fragments that make up these molecules.
The R,S nomenclature can be used to designate the configuration of molecules with axial and planar chirality. Those interested can find a description of the principles for assigning a configuration to R or S for such molecules in the VINITI publication: IUPAC Nomenclature Rules for Chemistry, vol. 3, semi-volume 2, M., 1983.
6. To the sequence rule in R,S - nomenclature.
In a number of cases, there are complications in determining the order of precedence of substituents. Let us consider some of them.
Example 1
Obviously, in this case, the junior substituents at the asymmetric carbon atom marked with an asterisk are H (d) and CH 3 (c). Let us consider the two remaining complex substituents, arranging the atoms in them in layers.
In the first layer of both substituents, the atoms are the same. In the second layer, the set of atoms is also the same. (H, C, O). Therefore, we need to turn to the third layer of atoms. In this case, in the left and right substituents, one should first of all compare layer III atoms bonded to senior atoms of layer II(that is, consider the "higher branches" of both deputies). In this case, we are talking about the atom associated with the oxygen atom of the P layer. Since the C atom is bonded to the oxygen atom in the right substituent, and the H atom is bonded to the left substituent, the right substituent gains an advantage in the majority:
The compound should be assigned the R-configuration:
If the atoms of the "senior branch" in the third layer turned out to be the same, for example, both C, then it would be necessary to compare the atoms of the same third layer, but already in the younger branch. Then the left deputy would have learned the advantage. However, we do not reach this point in our comparisons, since we can make a choice already on the basis of the difference of the III atoms layer of the older branch.
Quite similarly, the choice of order of precedence is carried out, for example, between the following deputies:
There may be a situation when, in order to select a senior deputy, it is necessary to "pass" through a multiple bond. In this case, resort to the help of the so-called phantom atoms, having a zero atomic number (that is, a priori the youngest) and a valence equal to 1.
In this example, the Nadr example is to make a choice between left and right carbon-containing substituents. Let us consider them, having previously "opened" the double C=C bond of the first substituent. In this case, duplicated atoms will appear (highlighted by circles). We add phantom atoms to duplicates of atoms (we denote them by the letter f) so as to bring the valence of each to 4:
Now we can compare the left and right substituents:
The difference in the third layer of atoms makes it possible to give preference in seniority to the right substituent:
Therefore, the connection has an R-configuration.
Example 3 In a number of cases, two substituents at an asymmetric atom are structurally identical, but differ only in the absolute configuration of the chiral centers. Then accept that- R-configuration older than S-configuration. In accordance with this, the central carbon atom in the example below should be assigned the S-configuration:
Example 4. The above principles are also applicable to the description of the absolute configuration of asymmetric atoms with three substituents (nitrogen, phosphorus, sulfur atoms). In this case, a phantom atom is used as the fourth substituent, which is always the youngest (a lone pair of electrons can be considered as a phantom atom):
Example 5 Sometimes, in order to choose the seniority of substituents, it is necessary to "open" the cycle, just as the "opening" of a multiple bond is performed.
In this case, it is easy to determine the oldest (O) and the youngest (H) substituents at the carbon atom marked with an asterisk. In order to make a choice between carbon atoms 1 Cu 2 C, one should "open" the cycle on the 2 C-O bond according to the following scheme (doubles of atoms are highlighted in circles):
In this case, in contrast to the "opening" of multiple bonds, the duplicated atoms no longer represent "dead-end" branches, but continue in the repetition of the atom marked with an asterisk. That is, the cycle "opening" procedure ends when the same atom (or rather, its duplicate) appears at the ends of both branches. Now we can compare the atoms 1 C 2 C by considering the corresponding layers of atoms:
The difference in the third layer allows you to give priority in seniority - carbon atom 2 C. Therefore, the chiral center under consideration has an S-configuration:
1.E.Iliel, Fundamentals of stereochemistry. M.: Mir, 1971, 107 s,
2.V.M.Potapov, Stereochemistry. Moscow: Chemistry, 1988, 463 p.
3. V. I. Sokolov, Introduction to theoretical stereochemistry, M., Nauka, 1979, 243 p.
To determine the absolute configuration of the chiral center, you must perform the following operations:
1. Position the chiral center so that the line of sight is directed from the chiral carbon to the junior substituent.
2. In the resulting projection, the three remaining substituents will be located at an angle of 120 o. If the decrease in the seniority of substituents occurs clockwise- This R-configuration (the following change of precedence is assumed: A > D > B):
If counterclock-wise - S-configuration:
The absolute configuration can be determined using the Fisher formula. To do this, by actions that do not change the Fisher formula, the junior deputy is placed down. Thereafter, a change in the seniority of the three remaining deputies is considered. If the decreasing order of precedence of the substituents occurs clockwise, this is the R-configuration, if against, the S-configuration. The junior deputy is not taken into account.
Example
Consider the definition of the configuration of chiral centers using the example of 3-bromo-2-methyl-2-chlorobutanol-1, which has the following structure:
Let us define the absolute configuration C 2 . To do this, we represent C 3 and C 4, as well as everything connected with them in the form of a radical A:
Now the original formula will look like this:
We determine the seniority of the substituents (from the oldest to the youngest): Cl> A> CH 2 OH> CH 3. We make an even number of permutations (this does not change the stereochemical meaning of the formula!) so that the junior substituent is at the bottom:
Now consider the top three substituents in the Fisher formula at the chiral center C 2:
It can be seen that the bypass of these substituents in descending order of precedence occurs counterclockwise, hence the configuration of this chiral center is S.
We will perform similar actions for another chiral center associated with C 3 . Imagine again, this time C 2 and everything connected with it, as a radical IN:
Now the original formula will look like this:
Again, we determine the seniority of the deputies (from the oldest to the youngest): Br\u003e B\u003e CH 3\u003e H. We make an even number of permutations so that the junior deputy is again at the bottom:
Let's determine in which direction the seniority decreases (we don't take into account the lowest, the youngest deputy!):
The decrease in the seniority of the substituents occurs counterclockwise, therefore the configuration of this chiral center is S.
The name of the starting substance, taking into account the absolute configuration of chiral centers - 3-/S/-bromo-2-/S/-methyl-2-chlorobutanol-1
How to designate the configuration of the compound so that the name can depict the spatial arrangement of groups at the chiral carbon atom? For this use R,S-system proposed by K. Ingold, R. Kahn, Z. Prelog. R,S-system is based on determining the seniority of substituents around the chiral center. Group precedence is determined as follows:
1). An atom with a higher atomic number is superior to an atom with a lower atomic number.
2). If the C* atoms directly connected to carbon are the same, then it is necessary to consider the seniority of subsequent atoms.
For example, how to determine the eldest of the groups: -C 2 H 5 and CH (CH 3) 2 in the compound
In the ethyl group, the atom connected to the chiral center is followed by H, H and C, and in the isopropyl group - by H, C and C. Comparing these groups with each other, we establish that the isopropyl group is older than the ethyl one.
3). If the chiral carbon C* is connected to an atom that has a multiple bond, then the bonds of this atom should be represented as simple bonds.
4). In order to establish the configuration of the molecule, it is positioned so that the bond of the chiral center with the junior group at number 4 is directed away from the observer, and the location of the remaining groups is determined (Fig. 2.6).
Rice. 2.6. Definition R,S-configurations
If the seniority of the groups decreases (1®2®3) clockwise, then the configuration of the chiral center is defined as R(from the Latin word "rectus" - right). If the seniority of the substituents decreases counterclockwise, then the configuration of the chiral center is S(from the Latin "sinister" - left).
The optical rotation sign (+) or (-) is determined experimentally and is not related to the designation of the configuration ( R) or ( S). For example, dextrorotatory 2-butanol has ( S)-configuration.
In order to determine the configuration of the compound depicted by the Fisher projection formula, proceed as follows.
1). Perform an even number of permutations of the substituents at the chiral center (an odd number of permutations will result in an enantiomer) so that the junior substituent number 4 is at the top or bottom.
2). Determine the location of the remaining groups, bypassing them in descending order of precedence. If the seniority of substituents decreases clockwise, then the initial configuration is defined as R-configuration, if counterclockwise, then the configuration is defined as S-configuration.
If it is not easy to convert the projection formula, you can set the order of decreasing precedence by discarding the junior substituent standing on the side, but choosing the “reverse” symbol to designate the configuration. For example, in the original connection
discarding the junior deputy (H), we set the order of decreasing precedence: 1→2→3. We get the designation ( S), change it to ( R) and get the correct name: ( R)-2-chloroethanesulfonic acid.
You need to enable JavaScript to run this app.Electronic configuration of an 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 build 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 are like 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 Maybe turn into water, next to sodium into gas, and being next to iron completely turns it into rust. To answer the question why this happens 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 in an atom?
An atom consists of a nucleus and electrons revolving around it, the nucleus consists of protons and neutrons. In the neutral state, each atom has the same number of electrons as the number of protons in its nucleus. The number of protons was indicated by the serial number of the element, for example, sulfur has 16 protons - the 16th element of the periodic system. Gold has 79 protons - the 79th element of the periodic table. Accordingly, there are 16 electrons in sulfur in the neutral state, and 79 electrons in gold.
Where to look for an electron?
Observing the behavior of an electron, certain patterns were derived, they are described by quantum numbers, there are four of them in total:
- 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 - this 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 level of energy, the cloud takes four main forms: a ball, dumbbells and the other two, more complex. In ascending order of energy, these forms are called s-, p-, d- and f-shells. Each of these shells can have 1 (on s), 3 (on p), 5 (on d) and 7 (on f) orbitals. The orbital quantum number is the shell on which the orbitals are located. The orbital quantum number for s, p, d and f orbitals, respectively, takes the values 0,1,2 or 3.
On the s-shell one orbital (L=0) - two electrons
There are three orbitals on the p-shell (L=1) - six electrons
There are five orbitals on the d-shell (L=2) - ten electrons
There are seven orbitals (L=3) on the f-shell - 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=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 its pair is added to each.
Consider a d-shell:
The d-shell corresponds to the value L=2, that is, five orbitals (-2,-1,0,1 and 2), the first five electrons fill the shell, taking the values M l =-2,M l =-1,M l =0 , M l =1,M l =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. Only two electrons with opposite spins can be on the same energy sublevel. The spin quantum number is denoted m s
Principal quantum number n
The main quantum number is the energy level, at the moment seven energy levels are known, each is denoted by an Arabic numeral: 1,2,3,...7. The number of shells at each level is equal to the level number: there is one shell on the first level, two on the second, and so on.
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, let's take the first electron, the lowest energy level is N=1, one shell is located on the first level, the first shell at any level has the shape of a ball (s -shell), i.e. L=0, the magnetic quantum number can take only one value, M l =0 and the spin will be equal to +1/2. If we take the fifth electron (in whatever atom it is), then the main quantum numbers for it will be: N=2, L=1, M=-1, spin 1/2.
The following problem occurs; how to designate a certain configuration in some simpler, more convenient way, so as not to draw its structure every time? For this purpose, the most widely used
symbols This notation was proposed by Kahn (Chemical Society, London), K. Ingold (University College, London) and W. Prelog (Federal Institute of Technology, Zurich).
According to this system, the seniority, or sequence, of the substituents, i.e., the four atoms or groups associated with an asymmetric carbon atom, is first determined based on the rule of precedence (Sec. 3.16).
For example, in the case of an asymmetric carbon atom, four different atoms are linked, and their seniority depends only on the atomic number, and the larger the atomic number, the older the substituent. Thus, in descending order of their precedence, the atoms are arranged in the following order:
Then the molecule is positioned so that the younger group is directed away from the observer, and the location of the remaining groups is considered. If the precedence of these groups decreases clockwise, then the configuration is denoted by the symbol R (from the Latin rectus - right); if the seniority of these groups decreases counterclockwise, then the configuration is denoted by a symbol (from the Latin sinister - left).
So configurations I and II look like this:
and are denoted respectively by the symbols
The full name of the optically active compound reflects both the configuration and the direction of rotation, as for example the racemic modification can be denoted by the symbol eg -sec-butyl chloride.
(The designation of compounds with multiple asymmetric carbon atoms is discussed in Section 3.17.)
Of course, one should not confuse the direction of the optical rotation of a compound (the same physical property of a real substance as the boiling or melting point) with the direction of our gaze when we mentally arrange the molecule in some specific conditional way. Until a connection between the configuration and the sign of rotation has been experimentally established for a particular compound, it is impossible to say whether the sign either corresponds or corresponds to the -configuration.
