2.3 Relative and Absolute Configurations

LEARNING GOALS

After   Chapter 2.3,    you will    be  able    to:

Name    cis,    trans,  E,  and Z   molecules   using   appropriate nomenclature

Apply   Cahn–Ingold–Prelog  priority    rules   to  molecules   with    up  to  four    substituents

The configuration of a stereoisomer refers to the spatial arrangement of the atoms or groups in the
molecule. The relative configuration of a chiral molecule is its configuration in relation to another
chiral molecule (often through chemical interconversion). We can use the relative configuration to
determine whether molecules are enantiomers, diastereomers, or the same molecule. On the other
hand, the absolute conformation of a chiral molecule describes the exact spatial arrangement of
these atoms or groups, independent of other molecules.

(E) AND (Z) FORMS

(E) and (Z) nomenclature is used for compounds with polysubstituted double bonds. Recall that
simpler double-bond-containing compounds can use the cis–trans system. To determine the (E)/(Z)
designation, one starts by identifying the highest-priority substituent attached to each double-
bonded carbon. Using the Cahn–Ingold–Prelog priority rules, priority is assigned based on the
atom bonded to the double-bonded carbons: the higher the atomic number, the higher the priority.
If the atomic numbers are equal, priority is determined by the next atoms outward; again,
whichever group contains the atom with the highest atomic number is given top priority. If a tie
remains, the atoms in this group are compared one-by-one in descending atomic number order
until the tie is broken. The alkene is named (Z) (German: zusammen, “together”) if the two highest-
priority substituents on each carbon are on the same side of the double bond and (E) (entgegen,
“opposite”) if they are on opposite sides, as shown in Figure 2.15.