Stereoisomerism

• Stereoisomers have the same structural formula but a different arrangement of the atoms and space.
• Stereoisomerism around the double bond arises because rotation about the double bond is restricted and the groups attached to each carbon atom are therefore fixed relative to each other.
• If a molecule satisfies both of the following conditions it will have E/Z isomerism:

  • a C=C double bond
  • different groups attached to each carbon atom of the double bond
• Cis-trans isomerism is used to describe a special case of E/Z isomerism. The molecules must have the conditions for E/Z isomer isomers however one of the attached groups on each carbon atom of the double bond must be the same.
• Cahn-Ingold-Prelog rules

  In this system, the atoms attached to each carbon atom in a double bond are given a priority based upon their atomic number.

  • If the groups of Higher priority are on the same side of the double bond the compound is a Z isomer .
  • If the groups of higher priority are diagonally placed across the double bond the compound is the E isomer.
• Cis isomers have the same groups on the same side of the double bond.
• Trans isomers have the same groups on the opposite side of the double bond.