IUPAC Rules for Alkene and Cycloalkene Nomenclature
The ene suffix (ending) indicates an alkene or cycloalkene.
The longest chain chosen for the root name must include both carbon atoms of the double bond.
The root chain must be numbered from the end nearest a double bond carbon atom. If the double bond is in the center of the chain, the nearest substituent rule is used to determine the end where numbering starts.
The smaller of the two numbers designating the carbon atoms of the double bond is used as the double bond locator. If more than one double bond is present the compound is named as a diene, triene, or equivalent prefix indicating the number of double bonds, and each double bond is assigned a locator number.
In cycloalkenes the double bond carbons are assigned ring locations #1 and #2. Which of the two is #1 may be determined by the nearest substituent rule.
