Geometric Isomerism in Alkenes

Why does geometric isomerism exist in Alkenes?

• The carbons in the double bond are sp2 hybridized, so they form one sigma bond and one pi bond.
• Sigma bonds have freedom of rotation, but pi bonds do not.
• Therefore, the groups attached to the double bond are locked in place.
• If there are different groups attached to the double bond, this makes for 2 possible arrangements.

Cis- and Trans- naming

• If the molecule has an identical pair of atoms on the same side of the double bond, it is a **cis-alkene.

• If the molecule has an identical pair of atoms on opposite sides of the double bond, it is a **trans-alkene.

  

• The problem with using the cis-trans system is that you need at least 1 identical pair of atoms.

• So we sometimes use another system of naming

E-Z Naming

• First you have to rank the substituents based on their atomic number. Elements with the higher atomic number get higher priority.
  • E.g. a chlorine atom is ranked higher than an ethyl group which is ranked higher than a hydrogen
• If the higher ranked atoms/groups are on the same side, it is a (Z)-alkene
• If the higher ranked atoms/groups are on opposite sides, it is a (E)-alkene