Naming Alkenes

Alkenes are named using IUPAC rules. The name ends in the suffix -ene, and the stem shows the number of carbon atoms in the longest chain that contains the C=C (eth-, prop-, but-, pent-, hex-).From butene onwards the double bond can be in more than one place, so its position is given by a number (a locant). Number the chain from the end nearest the double bond, and quote the lower number of the two C=C carbons, placed just before -ene: but-1-ene is CH₂=CHCH₂CH₃.Worked example: CH₃CH=CHCH₂CH₃ has five carbons. Numbering from the left, the double bond starts at carbon 2; from the right it starts at carbon 3. The lower number wins, so the name is pent-2-ene.Branches are named as prefixes with their own locants, in alphabetical order, using the same numbering. CH₂=C(CH₃)CH₂CH₃ has a four-carbon chain containing the C=C, the double bond at carbon 1 and a methyl group on carbon 2, so it is 2-methylbut-1-ene. Where there is only one possible position, the locant is left out: CH₂=C(CH₃)₂ is methylpropene.A molecule with two C=C bonds is a diene. An extra 'a' is added to the stem and both locants are given, so CH₂=CHCH=CH₂ is buta-1,3-diene.