Rules of Caen - Ingold - Prelog

In the modern stereochemical nomenclature of IUPAC, the configurations of double bonds, stereocenters, and also other chiral elements are assigned based on the relative positions of the substituents (ligands) for these elements. The rules of Kahn - Ingold - Prelog establish the seniority of deputies, according to the following mutually subordinate provisions ^{[2] [3]} .

1. An atom with a large atomic number is older than an atom with a lower atomic number . Comparison of substituents is carried out on an atom that is directly connected to a stereo center or double bond. The higher the atomic number of this atom, the deputy is older. If the first atom of the substituents is the same, the comparison is carried out for atoms remote from the stereo center (double bond) by the distance of two bonds (the so-called atoms of the second layer). For this, these atoms for each substituent are written out in the form of a list in the 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, etc., until the first difference.
2. An atom with a larger atomic mass is older than an atom with a lower atomic mass . This rule is usually applied to isotopes, since they cannot be distinguished by atomic number.
3. Sectionis substituents are older than sectrans substituents . This rule applies to substituents containing double bonds or planar four-coordinated fragments.
4. Diastereomeric substituents with similar ( English like ) designations are older than diastereomeric substituents with unlike ( English unlike ) designations . The former include substituents with the designations RR , SS , MM , PP , secississis , sectransectrans , Rsecis , Sectrans , Msecis and RM , SP . The second group includes substituents with the designations RS , MP , RP , SM , secissectrix , Rectrans , Ssecis , Psecis and Mectrans .
5. The substituent with the designation R or M is older than the substituent with the designation S or P.

The rules are applied sequentially one after another, if using the previous it is impossible to determine the seniority of deputies. The exact wording of rules 4 and 5 is currently under discussion ^{[4] [5]} .

In the R / S nomenclature

The assignment of the configuration R or S to the stereo center is based on the relative position of the substituents ( ligands ) around the stereo center. At the same time, their precedence is determined in the beginning according to the Kahn – 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 seniority of the remaining substituents is determined. If seniority decreases clockwise, then the stereo center configuration is denoted by R ( lat. Rectus - right). In the opposite case, the configuration is denoted by S ( lat. Sinister - left) ^{[6] [7]}

In the E / Z nomenclature

π-Diastereomers ( geometric isomers ) can have a different configuration depending on the location of the substituents at double bond . In this case, the double bond receives the designation E ( German: entgegen - on the contrary) if the two highest substituents at each carbon atom of the double bond are on opposite sides of this double bond. If the senior substituents are located on one side of the double bond plane, then such a double bond has the Z configuration ( German zusammen - together). Moreover, the seniority of deputies is also determined, according to the rules of Kahn - Ingold - Prelog ^[8] .

In the list of top parties

The Kahn – Ingold – Prelog rules are also used to indicate the sides of planar trigonal molecules, for example, ketones . For example, the sides of acetone are identical, since the attack of a nucleophile on both sides of a planar molecule leads to a single product. If the nucleophile attacks butanone-2 , then the sides of butanone-2 are not identical (enantiotopic), because when attacking on different sides, enantiomeric products are formed. If the ketone is chiral, then joining to the opposite sides will lead to the formation of diastereomeric products, therefore, such parties are called diastereotopic.

To indicate the top sides, the notation re and si are used , which respectively reflect the direction of decreasing seniority of substituents at the trigonal carbon atom of the carbonyl group . For example, in the illustration, the acetophenone molecule is visible from the re- side.

• Potapov V.M. Stereochemistry. - M: Chemistry, 1988. - ISBN 5-7245-0376-X .
• Kahn R., Dermer O. Introduction to Chemical Nomenclature = Introduction to Chemical Nomenclature / Per. from English N. N. Shcherbinovskaya, under the editorship of V. M. Potapova, R. A. Lidina. - M: Chemistry, 1983.- 224 p.