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.
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.
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.