4.4: Naming Enantiomers
The naming of enantiomers employs the Cahn–Ingold–Prelog rules that involve assigning priorities to different substituent groups at a chiral center. Each enantiomer, being a distinct molecule, is assigned a unique name by the Cahn–Ingold–Prelog (CIP) rules, also called the R–S system. The prefix R- or S- attached to the chiral centers in an enantiomer is dependent on the spatial arrangement of the four substituents on the chiral center. The R–S system essentially comprises three steps: assignment of priorities to the four substituent groups at the chiral center, reorientation of the molecule such that the lowest-priority substituent points away from the observer, and determining whether the sequence of groups attached is clockwise or counterclockwise. While the former assigns an R- prefix, the latter results in the S- label.
There are some important considerations to be kept in mind while exercising the CIP rules. The priority of functional groups attached to the chiral center is based on their atomic numbers in the periodic table. In the case of two groups attached to the chiral center with the same element, priorities to these groups are assigned based on the first point of difference. For instance, a carboxylic acid group (–COOH) has higher priority than an aldehyde substituent (–CHO), as the former has an oxygen atom at the first point of difference, as compared to the hydrogen atom in the latter.
Figure 1. (1R,2s,3S)-1,2,3-Trichlorocyclopentane
As mentioned earlier, the center with a charter of clockwise rotation is an R- (rectus) center, while the counterclockwise center is an S- (sinister) center. The names ‘rectus’ and ‘sinister’ are derived from Latin for the directions ‘right’ and ‘left’. While naming enantiomers, in the rare but possible cases of two substituents on an atom differing only by their absolute configuration (R or S), R takes priority over S. For example, consider the case of (1R,2s,3S)-1,2,3- trichlorocyclopentane, shown in Figure 1. In such a scenario, the carbon atom with the R configuration is assigned the higher priority, and the prefix of the adjacent carbon atom is given a lowercase ‘s’ configuration instead of the usual uppercase ‘S-’.