E- and Z-alkenes

Introductory 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, but the E/Z is the more useful method.


cis-2-butene or Z-2-butene	cis-2-chloro-2-butene or E-2-chloro-2-butene

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 group 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: 2-butene

Isomer 1 :


Step 1 : split the alkene	Step 2 : assign the relative priorities	Step 3 : look at the relative positions of the higher priority groups : same side = Z, hence (Z)-2-butene.

Isomer 2 :


The two substituents are -CH₃and 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)-2-butene.

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.