Overview

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-’.

Procedure

Enantiomers, such as 2-butanol and its mirror image, are distinct compounds; accordingly, each enantiomer should have its own distinct name.

The Cahn–Ingold–Prelog system, also called the R–S system, is used to assign unique names to enantiomers. Here, each chiral center in an enantiomer is assigned the prefix R or S based on the spatial arrangement of the substituent groups.

The R–S system involves three steps. In the first step, priorities are assigned to the four substituent groups at the chiral center based on their atomic numbers, with one being the highest priority and four being the lowest priority.

In the second step, the molecule is oriented such that the lowest-priority substituent points away from the observer.

The last step involves determining whether the sequence one-two-three is clockwise or counterclockwise. If the one-two-three sequence is clockwise, the chiral center is labeled as R. Contrarily, if the one-two-three sequence is counterclockwise, the chiral center is labeled as S.

Consider the molecule 2-butanol, which has one hydrogen atom, one oxygen atom and two carbon atoms attached to the chiral center. Of all the substituents, oxygen and hydrogen have the highest and lowest atomic numbers, respectively. Accordingly, the hydroxyl and hydrogen groups are assigned the priority numbers one and four, respectively.

As both the methyl and ethyl groups are attached to the chiral center through carbon atoms, priorities to these groups are assigned at the first point of difference. Here, the ethyl group is assigned the higher priority, as it has a carbon atom at the first point of difference, in comparison to the hydrogen atom in the methyl group.

Next, the molecule is rotated such that hydrogen points away.  Here, the one-two-three sequence is clockwise.

Accordingly, the prefix R is assigned to the chiral center and the enantiomer is labeled as (R)-2-butanol. Similarly, the enantiomer of (R)-2-butanol is labeled (S)-2-butanol.