7.2 : Nomenclature of Alkenes

The IUPAC names of alkenes are derived by replacing the suffix -ane of the parent alkane with -ene. The chain is numbered from the end nearest the double bond while assigning the lowest possible consecutive numbers to the double-bond carbon atoms.

In branched alkenes, the longest carbon chain containing the double bond is identified as the principal or parent chain. For instance, the parent alkene is pentene and not butene. The lowest number is allocated to the double-bond carbon atoms over the substituents. However, the name and position of the substituent group are cited before the position of the double bond, and the name is 2-ethyl-4-methyl-1-pentene.

Non-identical substituents are listed alphabetically. Thus, an ethyl substituent is cited before methyl, as in 4-ethyl-2-methyl-1-octene.

If more than one double bond is present in the parent chain, the position of each one is specified while the infix -en- is replaced by -⁠adien- for two, -atrien- for three double bonds, and so on.

The principal chain is the one with the maximum number of double bonds, even if it is not the longest chain. Hence, the correct name of this molecule is 4-butyl-5-methyl-1,4-hexadiene.

However, if more than one chain has an equal number of double bonds, the longest chain is the parent, as illustrated by 5-methylene-1,9-decadiene.

In cycloalkenes, the lowest numbers are assigned to the double-bond carbon atoms, while the direction of numbering is such that the lowest possible number is allocated to the first point of difference.

When substituents containing double bonds, like vinyl, allyl, and isopropenyl groups, are present, the ring is a principal chain.

IUPAC has recognized that smaller alkenes are usually referred to by their common names. For example, ethene is called ethylene, propene as propylene, and 2-methylpropene as isobutylene.