Basic IUPAC Organic Nomenclature

As we have already seen, there are two ways to name these types of isomers: (for a review and practice questions follow the links)

• cis / trans method
• E / Z method

The E- and Z- style is more reliable (i.e. potentially less ambiguous and can be used for all scenarios) and particularly suited to highly substituted alkenes, and especially when the substituents are not alkyl groups.

The E- and Z-alkene nomenclature system is based on the Cahn-Ingold-Prelog priority rules. They can also be used for naming chirality center stereoisomers (see later).
In order to apply the Cahn-Ingold-Prelog priority rules to alkenes:

• Imagine splittng each alkene into two pieces, where each piece contains one of the sp² C atoms from the C=C and two substituents (i.e. the ends of the alkene)
• Assign the priority based on the Cahn-Ingold-Prelog priority rules (high = 1, low = 2) to each atom attached to each sp² C based on atomic number 
  

  Subrules:

  • Isotopes: H vs D ? Since isotopes have identical atomic numbers, the mass number is used to discriminate them so D > H
  • Same atom attached ? By moving out away from the C=C one atom at a time, locate the first point of difference and apply the priority rule there.
    
    
• Determine the relative position of the two higher priority groups
  • If they are on the same side then it is a (Z)-alkene (German; zusammen = together) 
  • If they are on opposite sides then it is an (E)-alkene (German; entgegen = opposite)
    
    
• If there is more than one C=C that can be E/Z, then the locant and the stereochemistry of each alkene needs to be included e.g. (2E,4Z)-
  

Example: but-2-ene

Step 1: Split the alkene into two pieces	Step 2: List the attached atoms looking for the first point of difference. Here we have C and H atoms attached.	Step 3: Assign the relative priorities. Since the atomic numbers C > H then the -CH3 group is higher priority than H.	Step 4: Look at the relative positions of the higher priority groups: same side = Z, hence (Z)-but-2-ene
Step 1: Split the alkene into two pieces	Step 2: List the attached atoms looking for the first point of difference. Here we have C and H atoms attached.	Step 3: Assign the relative priorities. Since the atomic numbers C > H then the -CH3 group is higher priority than H.	Step 4: Look at the relative positions of the higher priority groups: opposite side = E, hence (E)-but-2-ene

Example: 3-methylpent-2-ene

Step 1: Split the alkene into two pieces	Step 2: Assign the relative priorities. The two atoms attached to the left end C are C and H, so since the atomic numbers C > H then the -CH3 group is higher priority than H.	Step 3: The two atoms attached to the right end C are both C. So we need to look at the atoms these C are attached to. List them out...we have C (H,H,H) and C (C,H,H) systems. This means that at the first point of difference (see the arrow), a C > H meaning that the ethyl group is higher priority.	Step 4: Look at the relative positions of the higher priority groups : opposite side = E, hence (E)-3-methylpent-2-ene.

©Dr. Ian Hunt, Department of Chemistry