Configuration

🔍 How to Identify Cis or Trans Isomers (Geometric Isomers)

Cis-trans isomerism (also called geometric isomerism) occurs in compounds with restricted rotation, typically:

• Alkenes (C=C double bonds)

• Cyclic compounds

✅ Requirements for Cis-Trans Isomerism:

1. A double bond between two carbon atoms (C=C)

2. Each carbon in the double bond must have two different groups attached

🔹 Cis vs. Trans (for Alkenes):

Example: But-2-ene

• Cis-but-2-ene: both CH₃ groups on the same side

• Trans-but-2-ene: CH₃ groups on opposite sides

     H       CH3             CH3      H
      \     /                 \      /
       C = C       vs.         C = C
      /     \                 /      \
   CH3      H               H       CH3
     Cis                       Trans

For cyclic compounds

Rotation is restricted due to the ring, so if two substituents are:

• On the same face of the ring → Cis

• On opposite faces of the ring → Trans

🔹 E/Z System (for complex molecules):

When four different groups are attached to the C=C, use the Cahn-Ingold-Prelog priority rules:

• Z (zusammen) = higher priority groups on same side

• E (entgegen) = higher priority groups on opposite side