Naming Alkenes and Alkynes



 

Learning Objective

  • Translate between the structure and the name of an alkene or alkyne compound

Key Points

    • Alkenes and alkynes are named by identifying the longest chain that contains the double or triple bond.
    • The chain is numbered to minimize the numbers assigned to the double or triple bond.
    • The suffix of the compound is “-ene” for an alkene or “-yne” for an alkyne.

Terms

  • AlkenesAn unsaturated hydrocarbon containing at least one carbon–carbon double bond.
  • alkyneAn unsaturated hydrocarbon containing at least one carbon—carbon triple bond between two carbon atoms.
  • stereoisomerOne of a set of the isomers of a compound that exhibits stereoisomerism.

Alkenes are hydrocarbons that contain one or more double bonds, while alkynes contain one or more triple bonds. The naming conventions for these compounds are similar to those for alkanes.

Identifying and Numbering the Longest Chain

Alkene and alkyne compounds are named by identifying the longest carbon chain that contains both carbons of the double or triple bond. This longest chain is named by the alkane series convention: “eth-” for two carbons; “prop-” for three carbons; “but-” for four carbons; etc. The carbon backbone is numbered from the end that yields the lowest positioning for the double or triple bond.

Adding Substituents

Substituents are added to the name as prefixes to the longest chain. Rotation is restricted around the double bond, so prefixes can be added to differentiate stereoisomers. Cis or trans is used to indicate whether higher priority substituents are located on the same or opposite sides of the bond. If the compound is cyclic, this information is noted by adding the “cyclo-” prefix.

Changing the Suffix

Next, the position of the double or triple bond is indicated using the position of the carbon in the bond with the lower backbone number, and the suffix for the compound is changed to “-ene” for an alkene and “-yne” for an alkyne. For cycloalkenes, the carbons in the double bond are numbered as positions 1 and 2.

Naming hexene with different substituentsExamples of substituted hex-1-enes. Remember that the 1 could go in front of the word hexene as well. For example, the middle molecule could be named as 4-methyl-1-hexene.

For multiple double or triple bonds, “di-,” “tri-,” or “tetra-” can be added prior to the “-ene” or “-yne.” In these cases, an extra “a” is appended to the end of the name of the alkyl chain, like in the case of butadiene. For compounds containing both double and triple bonds, the “-ene” suffix precedes the “-yne,” and the compound is numbered to minimize the bond positions.

Naming alkenesThis video shows you how to name alkene molecules using IUPAC conventions.