Basic IUPAC Organic Nomenclature

E- and Z-nomenclature of alkenes

On the previous pages, we looked at naming alkenes as cis- and trans-.

It is important to note that the two methods are different (i.e. they are based on different rules) and they are NOT interchangeable, see below for an example.
The cis- / trans- style is based on the longest chain whereas the E/Z style is based on a set of priority rules. 
You need to know both styles.

cis-but-2-ene or (Z)-but-2-ene	cis-2-chlorobut-2-ene or (E)-2-chlorobut-2-ene

The E- and Z- style is more reliable and particularly suited to tri- or tetra-substituted alkenes, and especially when the substituents are not alkyl groups.

The Cahn-Ingold-Prelog priority rules are used for naming geometric isomers (e.g. E- or Z-alkenes) and other stereoisomers (see later).

In order to apply the Cahn-Ingold-Prelog priority rules to alkenes:

• Imagine splitting each alkene into two pieces, each piece containing one of the sp² C atoms from the C=C (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.	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