Stereoisomerism

• Stereoisomers have the same structural formula but a different arrangement of the atoms in space.
• E/Z isomerism only occurs in compounds with a C=C bond
• Optical isomerism can occur in a much wider range of compounds, including alkanes with no functional groups
• Stereoisomerism around a 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.
• The reason for the rigidity is the position of the Pi bond's electron density above and below the plane of the sigma bond
• If a molecule satisfies both of the following conditions it will have E/Z isomerism:

  1. a C=C bond
  2. different groups attached to each carbon atom of the double bond
• Cis-trans isomerism is the name commonly used to describe a special case of E/Z isomerism. Molecules must have a C=C bond and each carbon in the double bond must be attached to 2 different groups, as far all E/Z isomers.
• For cis-trans, the difference is that one of the attached groups on each carbon atom of the double bond must by hydrogen
• The cis isomer has the hydrogen atoms on each carbon in the double bond on the same side of the molecule
• the trans isomer has the hydrogen atoms diagonally opposite each other
• the cis isomer is the z isomer
• the trans isomer is the e isomer
• Cahn-Ingold-Prelog nomenclature: the cis-trans naming system can only be used when each carbon atom in the double bond is attached to a single hydrogen atom. But, many other isomeric compounds contain C=C bonds and these must be named
• Using 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

  1. if the groups of higher priority are on the same side of the double bond, the compound is the Z isomer
  2. 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 directly attached to the carbon atoms of the double bond and decide which of the 2 has the highest priority - the higher the atomic number, the higher the priority
• step 2 - if the two atoms attached to a carbon atom in the double bond are the same, 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 higher priority