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IUPAC Nomenclature

Although the nomenclature is not followed widely, according to IUPAC, an alkene is an acyclic hydrocarbon with just one double bond between carbon atoms. Olefins comprise a larger collection of cyclic and acyclic alkenes as well as dienes and polyenes.

To form the root of the IUPAC names for straight-chain alkenes, change the -an- infix of the parent to -en-. For example, CH -CH is the alkane ethANe. The name of CH =CH is therefore ethENe.

For straight-chain alkenes with 4 or more carbon atoms, that name does not completely identify the compound. For those cases, and for branched acyclic alkenes, the following rules apply:

The position of the double bond is often inserted before the name of the chain (e.g. "2-pentene"), rather than before the suffix ("pent-2-ene").

The positions need not be indicated if they are unique. Note that the double bond may imply a different chain numbering than that used for the corresponding alkane: (H C) C– CH – CH is "2,2-dimethyl pentane", whereas (H C) C– CH = CH is "3,3-dimethyl 1-pentene".

More complex rules apply for polyenes and cycloalkenes.