Nomenclature of aldehydes with double bonds and stereoisomerism
IUPAC has changed some of the rules of nomenclature as of 2013. In aldehyde 1 both the aldehyde group and double bond are included in the base name (noun) and the methyl group is the substituent (adjective) because both chains are $\ce{C4}$. In structure 2 the methyl group of 1 has been changed to ethyl. Now the longest chain is $\ce{C5}$ containing the aldehyde group and the double bond, as a substituent is the modifier methylene. In structure 3 the branch at $\ce{C3}$ has two $\ce{C3}$ units. The one containing the double bond takes priority as the &quot;noun&quot;. In structure 4 the chain containing the double bond is shorter, which becomes the &quot;adjective&quot;. In your compound 5, the noun is heptanal (in black) and the adjective, which contains the double bond, is in red. The two entities are numbered independently.