Figure 2.15. (E) and (Z) Designations of AlkenesMNEMONIC
Z = “z”ame side; E = “e”pposite side(R) AND (S) FORMS
(R) and (S) nomenclature is used for chiral (stereogenic) centers in molecules. We go through a set
sequence to determine this absolute configuration:
Step 1: Assign Priority
Using the Cahn–Ingold–Prelog priority rules described earlier, assign priority to the four
substituents, looking only at the atoms directly attached to the chiral center. Once again, higher
atomic number takes priority over lower atomic number. If the atomic numbers are equal, priority is
determined by the combination of the atoms attached to these atoms; if there is a double bond, it is
counted as two individual bonds to that atom. If a tie is encountered, work outward from the
stereocenter until the tie is broken. An example is shown in Figure 2.16.
Figure 2.16. Applying the Cahn–Ingold–Prelog Priority Rules to Determine Absolute
Configuration
Assign priority by the highest atomic number.