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Basic IUPAC Organic Nomenclature
Stereoisomers of Alkenes

As we have already described, alkenes with two different substituents at each end of the C=C can exist as a pair of stereoisomers.

The alkene can only exist as stereoisomers if R1 is not equal to R2 AND R3 is not equal to R4.

subs alkene

As we have already seen, there are two ways to name these types of isomers: (for a review details follow the links)

The E- and Z- style is more reliable (i.e. potentially less ambiguous) and particularly suited to highly substituted alkenes, especially when the substituents are not alkyl groups.

The E- and Z-alkene nomenclature system is based on the Cahn-Ingold-Prelog priority rules. They can also be used for naming chirality center 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