Naming Enantiomers The R,S-System

If we name these two enantiomers nsing only the lUPAC system of nomenclature that we have learned so far, both enantiomers will have the same name 2-butanol (or sec-butyl alcohol) (Section 4.3F). This is undesirable because each compound must have its own distinct name. Moreover, the name that is given a compound should allow a chemist 

Three chemists, R. S. Cahn (England), C. K. Ingold (England), and V. Prelog (Switzerland), devised a system of nomenclature that, when added to the lUPAC system, solves both of these problems. This system, called the R, S -system or the Cahn-Ingold-Prelog system, is part of the lUPAC rules. 

According to this system, one enantiomer of 2-butanol should be designated (R)-2-butanol and the other enantiomer should be designated (S)-2-butanol. [(R) and (S) are from the Latin words rectus and sinister, meaning right and left, respectively.] These molecules are said to have opposite configurations at C2. 

We can illustrate the application of the rule with the 2-butanol enantiomer, II  

When a priority cannot be assigned on the basis of the atomic number of the atoms that are directly attached to the chirality center, then the next set of atoms in the unassigned groups is examined. This process is continued until a decision can be made. We assign a priority at the first point of difference.  

---

One enantiomer is distinguished from the other enantiomer in a pair of enantiomers by placing a prefix before the name of the compound. There are two nomenclature systems for this purpose. The most general system is the lUPAC R-S system where the prefixes R- and S- are used to distinguish between the enantiomers. We will not consider the R-S system since it is not universally used by biological scientists. The latter more often use an older nomenclature system which uses the prefixes d- and l-. 

What we need is a way to name each enantiomer of ibuprofen (or any other pair of enantiomers for that matter) so that we can refer to them in conversation or in writing. To do so, chemists have developed the R,S system. The first step in assigning an R or S... 

Amino acids exist in nature as only one of these enantiomers. Except when the R group is CH2SH, the stereogenic center on the a carbon has the S configuration. An older system of nomenclature names the naturally occurring enantiomer of an amino acid as the L isomer, and its unnatural enantiomer the D isomer. 

Oxynitrilase as catalyst. It is possible to treat a racemic cyanohydrin with a (R)- or (S)-HNL to decompose selectively one enantiomer of this mixture (exemplified in Fig. 14.7-3.). The (R)-HNL from Prunus amygdalus was used for the resolution of racemic cyanohydrins. Employing a biphasic system, namely citrate buffer/diisopropyl ether (40 1) at 39 °C, catalytic amounts of PI1NH2 and semicarbazide were added for... 

Naming enantiomers with the prefixes R or S is called the Cahn-Ingold-Prelog system after the three chemists who devised it. 

To name enantiomers unambiguously, we need a system that allows us to indicate the handedness in the molecule, a sort of left-hand versus right-hand nomenclature. Such a system was developed by three chemists, R. S. Cahn, C. Ingold, and V. Prelog. ... 

How do we name the different stereoisomers of a compound like 2-bromobutane so that we know which one we are talking about We need a system of nomenclature that indicates the arrangement of the atoms or groups around the asymmetric center. Chemists use the letters R and S for this purpose. For any pair of enantiomers with one asymmetric center, one member... 

---