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In red is the substituent which determines the final priority. In organic chemistry, the Cahn–Ingold–Prelog (CIP) sequence rules (also the CIP priority convention; named after Robert Sidney Cahn, Christopher Kelk Ingold, and Vladimir Prelog) are a standard process to completely and unequivocally name a stereoisomer of a molecule.
Absolute configuration uses a set of rules to describe the relative positions of each bond around the chiral center atom. The most common labeling method uses the descriptors R or S and is based on the Cahn–Ingold–Prelog priority rules. R and S refer to rectus and sinister, Latin for right and left, respectively.
The designations are based on the same Cahn–Ingold–Prelog priority rules used for tetrahedral stereocenters. [3] The chiral axis is viewed end-on and the two "near" and two "far" substituents on the axial unit are ranked, but with the additional rule that the two near substituents have higher priority than the far ones. [4]
The Cahn–Ingold–Prelog priority rules (often abbreviated as the CIP system) were first published in 1966; allowing enantiomers to be more easily and accurately described. [53] [54] The same year saw first successful enantiomeric separation by gas chromatography [55] an important development as the technology was in common use at the time.
Alternately, all four groups can be ranked by Cahn–Ingold–Prelog priority rules, with overall priority given to the two groups on the "front" atom of the Newman projection. The two configurations determined in this way are termed R a and S a, in analogy to the traditional R/S for a traditional tetrahedral stereocenter. [12]
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See: Cahn–Ingold–Prelog priority rules Configuration assignment of the stereo center "X", the substituents are decreasingly prioritized from "A" → "D" according to the CIP rules. The stereochemical descriptors ( R ) (from Latin rectus = right) and ( S ) (from lat. sinister = left) [ 36 ] are used to describe the absolute configuration of ...
Where A has a greater priority than B according to the Cahn–Ingold–Prelog priority rules, the configuration of the axial chirality can be determined by considering the substituents on the front atom followed by the back atom when viewed along the allene axis. For the back atom, only the group of higher priority need be considered.