Naming Alkenes and Alkynes
Identifying and Numbering the Longest Chain
These 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.
Substituents are added to the name as prefixes to the longest chain. Because rotation is restricted around the double bond, 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 also added as a 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.
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, for example, 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.
Some examples of the names and structures of hex-1-enes are shown in Figure 1.