The Cahn-Ingold-Prelog R-S Notational System

Just as it makes sense to have a nomenclature system by which we can specify the constitution of a molecule in words rather than pictures, so too is it helpful to have one that lets us describe stereochemistry. We have already had some experience with this idea when we distinguished between E and Z stereoisomers of alkenes. 

As outlined in Table 7.1, (+)-2-butanol has the S configuration. Its mirror image is (—)-2-butanol, which has the R configuration. 

the R or S configuration and the sign of rotation are incorporated into the name of the compound, as in (R)-(—)-2-butanol and (S)-(+)-2-butanol. 

TABLE 7.1 Absolute Configuration According to the Cahn-Ingold-Prelog Notational System 

Identify the substituents at the chirality center, and rank them in order of decreasing precedence according to the system described in Section 5.4. Precedence is determined by atomic number, working outward from the point of attachment at the chirality center. 

Orient the molecule so that the lowest ranked substituent points away from you. 

The rules for establishing precedence were given in Table 5.1. 

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Biological reduction of pyruvic acid, catalyzed by the enzyme lactate dehydrogenase, gives (+)-lactic acid, represented by the Fischer projection shown. What is the configuration of (+)-lactic acid according to the Cahn-Ingold-Prelog R-S notational system ... 

Fischer projections and d-l notation have proved to be so helpful m representing carbohydrate stereochemistry that the chemical and biochemical literature is replete with their use To read that literature you need to be acquainted with these devices as well as the more modern Cahn-Ingold-Prelog R S system... 

The role of nitronium ion in the nitration of benzene was demonstrated by Sir Christo pher Ingold—the same per son who suggested the SnI and Sn2 mechanisms of nu cleophilic substitution and who collaborated with Cahn and Prelog on the R and S notational system... 

Cahn-Ingold-Prelog notation (Section 7.6) System for specifying absolute configuration as R or S on the basis of the order in which atoms or groups are attached to a chirality center. Groups are ranked in order of precedence according to rules based on atomic number. 

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