IUPAC nomenclature for alkenes extends alkane rules by prioritizing the C=C double bond in chain numbering, using the -ene suffix, and adding E/Z designators for substituent geometry. The longest chain must contain the double bond, numbered to give it the lowest position; E/Z stereoisomerism is determined by Cahn-Ingold-Prelog priority rules applied to the sp² carbons.
Practice naming unsymmetrical alkenes with varying substituent patterns, then assign E/Z configuration based on atomic number priorities. Compare E/Z to older cis/trans terminology.
You already know how to name alkanes using IUPAC rules: find the longest continuous carbon chain, number it to give substituents the lowest locants, and list substituents alphabetically with their position numbers. Naming alkenes follows the same framework with one critical modification — the carbon-carbon double bond takes priority in determining both the parent chain and the numbering direction.
The first rule change is parent chain selection. In alkanes, you simply find the longest chain. In alkenes, the parent chain must contain the double bond, even if a longer all-single-bond chain exists elsewhere in the molecule. For example, if a molecule has a seven-carbon chain with no double bond and a six-carbon chain that includes the C=C, the parent chain is the six-carbon one — hex-ene, not heptane. The suffix changes from -ane to -ene, and a locant indicates the lower-numbered carbon of the double bond: but-1-ene, pent-2-ene, hex-3-ene. Number the chain so that the double bond gets the lowest possible locant; if there is a tie, use the same rules as for alkanes (lowest set of substituent locants).
The second major addition is E/Z stereochemistry. Because rotation around a double bond is restricted, substituents can be locked on the same side or opposite sides of the double bond — creating stereoisomers. To assign configuration, apply the Cahn-Ingold-Prelog (CIP) priority rules to the two substituents on each sp² carbon: the atom with the higher atomic number gets higher priority. If the two higher-priority groups are on the same side of the double bond, the configuration is Z (from German *zusammen*, together). If they are on opposite sides, it is E (*entgegen*, opposite). The E/Z designation goes in parentheses before the name: (E)-pent-2-ene.
A common source of confusion is the relationship between E/Z and the older cis/trans labels. Cis means "same side" and trans means "opposite side," but these terms only work unambiguously for disubstituted alkenes (one substituent on each carbon of the double bond). Once you have three or four different groups, "same side as what?" becomes ambiguous, and E/Z — which uses formal priority rules — is the only reliable system. Practice by drawing several tri- and tetrasubstituted alkenes, assigning CIP priorities, and determining E or Z. The skill becomes automatic quickly, and it is essential for every subsequent topic involving alkene reactivity and stereochemistry.