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Basic IUPAC Organic Nomenclature

E- and Z-nomenclature of alkenes

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

Misconception Alert!

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).

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

Example: but-2-ene

Example: 3-methylpent-2-ene

organic chemistry ©Dr. Ian Hunt, Department of Chemistry University of Calgary