Isomers Recap

• Isomers are compounds with the same molecular formula but a different arrangement of atoms in space.
• Structural isomers are defined as having the same molecular formulae but different structural formulae.
• What is an optical isomer?

  When there are 4 different substituents attached to 1 carbon atom.
• Isomers

  • These are different arrangement of the same atoms
  • Have the same molecular formula but can have different structures or shapes.
• Three types of isomers

  • Structural isomers - atoms are connected in different ways
  • Stereoisomers - same structural formula but different arrangement in space
  • Optical isomers - compounds that are not superimposable (mirror images)
• Chain isomers

  • The chains have different arrangements
• Positional Isomers

  • Same skeletal formula and attached groups but they are attached at different carbon atoms.
• Functional Group Isomers

  • Same atoms, but arranged into different functional groups
  • e.g. alkenes and cyclic alkanes
• Stereoisomers

  • Unlike single bonds, double bonds cannot rotate, so the carbons lie in the same plane
  • The restricted rotation means groups are rigid in space
• E/Z isomers

  • Due to the rigid structure of the double bond the stereoisomers produced have different names
  • One is the E isomer and the other is the Z isomer
  • The Z isomer has the groups both above or both below the double bond
  • The E isomer has the groups across the double bond
• E/Z isomers

  • The groups around the double bond do not need to be the same to be E/Z isomers. There can be 4 different groups.
  • The Cahn-Ingold-Prelog priority rules can be used to identify the type of stereoisomer.
• Cahn-Ingold-Prelog system of priorities

  • In this system each bonded atom is prioritised according to its atomic number. If this does not differentiate them, then the next atom in the chain is used etc.
  • If the two groups with the highest priority lie on the same side the molecule is designated the letter Z. If the highest priority groups are on opposite sides of the double bond the isomer is labelled E.
• Cahn-Ingold-Prelog Priority

  1. Assign a priority to two atoms attached to each carbon atom
  2. The atoms bonded to the carbon atoms with the highest atomic number has the highest priority
  3. Look at the groups with the highest priority on each carbon to work out isomers