Rule 1. Alkenes are hydrocarbons that conform to the formula CnH2n. They are named using the same prefix as alkanes (meth-, eth-, prop-, but-...) but changing the suffix -ane to -ene.
[1] Ethene
[2] Propene
[3] 2-Butene
[4] Cyclopentene
Rule 2. The principal chain selected is the longest one containing the double bond. If there are multiple double bonds, the principal chain is the one with the most double bonds (even if it is not the longest).
[5] 5-Methylhept-2-ene
[6] 2-Ethylhexa-1,4-diene
[7] 3-Propylhepta-1,4-diene
Rule 3. Numbering starts from the end of the chain that gives the double bond the lowest possible locator. Double bonds take precedence over substituents.
[8] 4-Bromo-3-methylpent-1-ene
[9] 5-Bromo-4-chloropent-2-ene
[10] 5,5-Dimethylhepta-1,3,5-triene
Rule 4. Alkenes can exist in the form of spatial isomers, which are distinguished by cis/trans notation.
[11] cis-2-butene
[12] trans-2-butene
[13] trans-2-pentene
[14] cis-2-pentene
The notation cis- indicates identical groups on the same side of the double bond. The notation trans- is used when identical groups are on opposite sides of the alkene. It is written in italic, always in lowercase, and separated from the name by a hyphen.