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and V. Prelog in 1966.

The Kahn-Ingold-Prelog rules differ from other chemical nomenclatures, because 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

Subtitles

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 Cahn-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, that of fluorine is 9, that of carbon is 6, and, finally, that of 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 arrange 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 Cahn-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 is 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 stand for, and if we're going to make that out of it, we're going to have to literally disconnect and reconnect 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 the modern IUPAC stereochemical nomenclature, configurations of double bonds, stereocenters, and other chirality elements are assigned based on the mutual arrangement of 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 the formation of diastereomeric products, so such 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. four . - P. 385–415. - DOI:10.1002/anie.196603851 .
3. Preferred IUPAC Names. Chapter 9 . Retrieved February 5, 2013.

2. Kahn-Ingold-Prelog notation (R-S-nomenclature)

Since it is impossible to use the D-L-nomenclature without setting the direction of orientation of the projection formula, and since many compounds contain more than one asymmetric carbon, in 1956 R. S. Kahn, D. K. Ingold and V. Prelog developed the R-S notation system for the spatial connection configuration, in which R stands for right (rectus) and S for left (sinister). (Note that R and S are also Kahn's initials.)

The spatial configuration of substituents near each asymmetric carbon atom is designated according to the following rules:

1. Note the atomic number of each of the atoms directly attached to the asymmetric carbon atom in question.

2. Arrange these atoms in descending order of atomic number.

3. If the substituents on the asymmetric carbon atom are two atoms with the same atomic number (for example, two other carbon atoms), the atomic number of the substituents on these attached atoms is taken into account. An atom with a substituent having a higher atomic number is placed in front of an atom whose substituent has a lower atomic number. The order of precedence of frequently occurring substituents on asymmetric carbon is as follows: I, Br, CI, SH, OH, NO 2, NH 2, COOR, COOH, CHO, CR 2 OH, CHOHR, CH 2 OH, C 6 H 5 , CH 2 R , CH 3 , H. Atoms connected by double and triple bonds are counted twice or thrice, respectively. For example:

4. Position the asymmetric carbon atom so that the atom with the lowest atomic number (most often H) is facing away from the observer's eye.

Note that any pair of Fisher's two-dimensional projections can be swapped or the positions of three substituents can be changed without changing the true spatial structure. For example, the position of H, OH and CH 2 OH in the Fischer projection for D (+) -glyceraldehyde can be depicted in different ways:

If you rotate this model 120° to the right, it will match model (1).

5. Consider three substituents located in front of the asymmetric carbon atom. (Recall that the atom with the lowest atomic number is behind the asymmetric carbon atom.) Determine how the atoms are arranged in descending order of atomic number - clockwise (right R configuration) or counterclockwise (left S configuration).

For example, in glyceraldehyde, the order of the substituents attached to the asymmetric carbon atom, according to the above rules, will be OH, CHO, CH 2 OH and H. In order to determine whether the asymmetric carbon will be R or S, we will arrange the molecule so: so that the H atom is at the bottom in a two-dimensional formula or behind an asymmetric carbon atom in a three-dimensional formula (see rule 4).

STEREOCHEMICAL NOMENCLATURE

(from Latin in menclatura - list, list), is intended to designate spaces. chemical structures. connections. The general principle of N. with. (rules , section E) is that spaces. the structure of the connection denoted by prefixes added to the names without changing these names. and numbering in them (although sometimes stereochemical features can determine the choice between possible alternative numbering methods and the choice of the main chain).

At the heart of most stereochem. notation lies the sequence rule, which unambiguously establishes the precedence of substituents. Those of them are considered senior, for which with the considered chiral (see. Chirality) element (eg, asymmetric atom, double bond, cycle) is directly related to a large atomic number (see table). If these atoms are identical in seniority, then consider the "second layer", which includes atoms associated with the atoms of the "first layer", etc., until the first difference appears; the numbers of atoms linked by a double bond are doubled when determining seniority. Naib. a general approach to designating the configuration of enanthiomers is to use R,S-systems. The designation R (from lat. Rectus-right) gets one of the enantiomers, in which, when considering the model from the side opposite to the junior substituent, the seniority of the remaining substituents falls clockwise. Falling seniority counterclockwise corresponds to the S-designation (from Latin sinister-left) (Fig. 1).

Increasing seniority of substituents at the chiral center:

Rice. 1. Scheme for determining the seniority of substituents in organic compounds.

For carbohydrates, a-hydroxy acids, a-amino acids, the D, L-system is also widely used, based on a comparison of the configuration of the considered asymmetric. center with the configuration of the corresponding enantiomer of glyceraldehyde. When considering projection Fisher foremule the location of the OH or NH 2 groups on the left is indicated by the symbol L (from lat. laevus - left), on the right - by the symbol D (from lat. dexter - right):

Fig.2. dihedral angle.

To designate the conformations of the molecule, indicate the value of the dihedral (dihedral) angle j between the two senior substituents in the SChS bond (Fig. 2), which is counted clockwise and expressed in arbitrary units (one unit is equal to 60 °), or use verbal designations of the location senior deputies in Newman f-lakhs (Fig. 3).

Rice. 3. Designations of butane conformers (asterisk marked recommended by IUPAC rules).

Lit.: IUPAC nomenclature rules for chemistry, v.3, semi-volume 2, M., 1983, p. 5-118; Nogradi M., Stereochemistry. Basic concepts and application, trans. from English, M., 1984. V. M. Potapov, M. A. Fedorovskaya.

Chemical encyclopedia. - M.: Soviet Encyclopedia. Ed. I. L. Knunyants. 1988 .

See what "STEREOCHEMICAL NOMENCLATURE" is in other dictionaries:

Section of stereochemistry that studies the conformations of molecules, their interconversions and the dependence of physical. and chem. sv from conformation. characteristics. Molecule conformations decomp. spaces. the shape of the molecule that occurs when the ratio changes. orientation of its individual ... Chemical Encyclopedia

Not to be confused with the term "Isomerism of atomic nuclei". Isomerism (from izos equal and meros share, part of Greek, cf. iso), the existence of compounds (mainly organic), identical in elemental composition and molecular weight, but different in ... ... Wikipedia

Not to be confused with the term "Isomerism of atomic nuclei". Isomerism (from izos equal and meros share, part of Greek, cf. iso), the existence of compounds (mainly organic), identical in elemental composition and molecular weight, but different in ... ... Wikipedia

Not to be confused with the term "Isomerism of atomic nuclei". Isomerism (from izos equal and meros share, part of Greek, cf. iso), the existence of compounds (mainly organic), identical in elemental composition and molecular weight, but different in ... ... Wikipedia

Not to be confused with the term "Isomerism of atomic nuclei". Isomerism (from izos equal and meros share, part of Greek, cf. iso), the existence of compounds (mainly organic), identical in elemental composition and molecular weight, but different in ... ... Wikipedia

Not to be confused with the term "Isomerism of atomic nuclei". Isomerism (from izos equal and meros share, part of Greek, cf. iso), the existence of compounds (mainly organic), identical in elemental composition and molecular weight, but different in ... ... Wikipedia

Not to be confused with the term "Isomerism of atomic nuclei". Isomerism (from izos equal and meros share, part of Greek, cf. iso), the existence of compounds (mainly organic), identical in elemental composition and molecular weight, but different in ... ... Wikipedia

- (Greek anti prefix meaning opposite; Greek syn prefix meaning compatibility), prefixes denoting: 1) geometric. double bond isomers =NCh and ChN=NCh. For example, in the isomers of benzaldoxime, syn indicates the proximity ... ... Chemical Encyclopedia

- (from iso ... and Greek meros share, part), the existence of compounds (ch. arr. organic), identical in composition and mol. mass, but different in physical. and chem. St. you. Such Comm. called isomers. As a result of the controversy between J. Liebig and F. Wöhler, it was established ... ... Chemical Encyclopedia

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 does 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 valency 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

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 had been arbitrarily assigned to it at first. As a result, most of the D-centers and, very importantly, glyceraldehyde itself, have the (R)-configuration, while 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 on 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.