Stereoisomers: Compounds with same structural formula but different arrangement of atoms in
space. So chemically different.
• If asks what type of stereoisomerism- E/Z.
• Why E/Z Isomerism: Stereoisomerism arises because…
– Must have double bond as rotation around double bond restricted.
– And each carbon atom of the double bond is bonded to two different groups.
• Cis- Trans Isomerism: Type of E/Z isomerism so same conditions needed. However two of the
substituent groups (one on each carbon atom of the C=C group) are the same.
• Molecules described as both E/Z isomers and cis- trans. But cis isomer can both be either E or Z
isomer depending on priority.
• Cis Isomer: Cis isomer has same group on each carbon in the double bond on the same side
(below/ above) of the molecule. It makes a C shape.
• Trans Isomer: Trans isomer has same group diagonally opposite.
• Cahn- Ingold- Prelog Nomenclature: (Back to E/Z isomerism). The immediate atom attached to
each carbon in double bond, given higher priority if higher atomic number.
– Z isomer is if groups of higher priority are on the same side (below/ above) of the double bond.
– E isomer is if groups of higher priority are diagonally placed across the double bond.
Point of Difference: If two individual atoms immediately attached to a double atom are the same,
need to continue down to the first point of difference in atom, where can give a priority. For
example if one side it is CH2OH, write out as CHHOH.
• On left hand side, start from atom closest to double bond.
• Always draw out structurally in correct double bond form.
• In questions, mainly they will want E/ Z isomerism.
• How does the structure differ? One is E isomer and one is Z isomer