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Alkenes: Introduction

Alkenes: Nomenclature

According to IUPAC conventions, the names of alkenes are derived from those of the corresponding alkanes by changing the infix -an to -en. Frequently, the older names with an ending of -ylene are being used. The basic rules for naming molecules are summarized on the page describing IUPAC nomenclature. In order to name alkenes correctly, the following rules in addition to the basic rules of nomenclature have to be followed:

  • The double bond must be part of the carbon chain which forms the parent system of the alkene.

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  • The position of the double bond in the chain is indicated by a number in the name similar to the way substituents are handled.
  • Double bonds which are part of a substituent are named alkenyls, e.g. ethenyl (or vinyl) and 2-propenyl (or allyl).
Ethenyl (vinyl)
2-Propenyl (allyl)
Vinyl chloride
Allyl alcohol
  • In the case of a non-terminal double bond, the chain can extend on either the same or the opposite side of the double bond. In the first case, the stereochemical assignment is called cis or (Z) while the latter is designated trans or (E).

The E/Z-system follows the CIP (Cahn, Ingold, Prelog)-system which is based on the priorities of the individual substituents. Highest priority is assigned to the atom with the highest atomic number which is attached to one of the olefinic C atoms. If an unanimous decision is not possible based on this rule, atoms in the second sphere, i.e. two bonds removed from the double bond, are taken into account. Again, if no decision can be made based on the atomic number, then the bond orders of the involved atoms have to be considered.


In the (Z)-isomer (from the German zusammen) the two substituents with priorities 1 and 2 are located on the same side of the double bond. If, however, these substituents are on opposite sides then the molecule possesses the (E)- (from the German entgegengesetzt) configuration.

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