Stereoisomerism

• Stereoisomers have the same structural formula but a different arrangement of the atoms in space
• There are two types of stereoisomerism, E/Z isomerism and optical isomerism
• E/Z isomerism only occurs in compounds with a C=C double bond, whereas optical isomerism can occur in a much wider range of compounds, including alkanes with no functional groups
• Stereoisomerism around double bonds 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 a special case of E/Z isomerism. Molecules must have a C=C double bond and each carbon atom must be attached to two different groups, as for all E/Z isomers.

  HOWEVER in cis-trans isomers one of the attached groups on each carbon must be the same!
• The cis isomer has the same groups on each carbon on the same side of the molecule
• In the trans isomer the same groups attached to each carbon atom are on opposite sides (diagonally) of the molecule
• Where there is a hydrogen on each of the double-bonded carbon atoms in cis-trans isomerism:

  • the cis isomer is the Z isomer
  • the trans isomer is the E isomer
• The cis-trans system of naming can only be used if each carbon atom in the C=C has a hydrogen atom attached to it
• Cahn-Ingold-Prelog rules:

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

  • If the groups of higher priority are on the same side of the double bond, the compound is the Z isomer
  • If the groups of higher priority are diagonally placed across the double bond, the compound is the E isomer
• Assigning priority - Step 1

  Examine the atoms attached directly to the carbon atoms of the double bond and decide which of the two atoms has the highest priority. The higher the atomic number, the higher the priority.
• Assigning priority - Step 2

  If the two atoms attached to a carbon in the double bond are the same, the you will need to find the first point of difference. The group which has the higher atomic number at the first point of difference is given the highest priority