E- and Z-alkenes

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 highly substituted alkenes, 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).

These rules are based on atomic number, and the first point of difference.

Imagine each alkene as two pieces, each piece containing one of the sp² C
Assign the priority (high = 1, low = 2) to each atom on each sp² C based on atomic number
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 location needs to be included with the locant, e.g. (2E,4Z)-

Example: but-2-ene

Isomer 1 :


Step 1 : split the alkene	Step 2 : assign the relative priorities. The two attached atoms are C and H, so since the atomic numbers C > H then the -CH₃ group is higher priority.	Step 3 : look at the relative positions of the higher priority groups : same side = Z, hence (Z)-but-2-ene.

Isomer 2 :


The two attached atoms are C and H, so since the atomic numbers C > H then the -CH₃ group is higher priority. Therefore the two high priority groups are on the opposite side, then this is (E)-but-2-ene.

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 one unit at a time, locate the first point of difference and apply rules there.