Stereoisomers II - Geometric or Cis/Trans Isomers

Geometric or Cis/Trans Isomers

Geometric (or cis/trans) isomers arise when there there a rigid bond (usually a mutiple bond between two atoms as these contain a minimum of one pi bond). For 1,2-disubstituted alkenes, those with substituents on the same side are called cis and those with substituents on opposite sides are called trans.

The trans isomer is considered more stable due to less steric repulsion.

With tri- or tetrasubstituted alkenes, we use a different nomenclature called E and Z.

In order to assign E/Z:

1. cut the alkene in half vertically.
2. assign high/low priorities on the two substituents using the Cahn-Ingold-Prelog rules (see R/S nomenclature).
3. if the high priority groups are on the same side, the alkene is (Z).
4. if the high priority groups are on opposite sides, the alkene is (E).