4.4 : エナンチオマーの命名

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.