Basic IUPAC Organic Nomenclature
E- and Z-nomenclature of alkenes
On the previous pages, we looked at naming alkenes as cis- and trans-.
cis ¹ Z and trans ¹ E
In general terms there is NO specific relationship between cis and trans / E and Z as they are based on fundamentally different naming rules.
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.
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).
These rules are based on atomic number, and the first point of difference.
|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.
|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.|
|©Dr. Ian Hunt, Department of Chemistry|