WSA Nomenclature Practice Test

Naming alkenes with IUPAC requires (1) choosing the longest carbon chain that includes the double bond, (2) numbering it so the double bond has the lowest possible locant, and (3) using the -ene ending. In CH3-CH=CH-CH3, the carbon skeleton is four carbons long and the double bond lies between the second and third carbons, giving a locant of 2. Therefore the proper name is but-2-ene (also 2-butene). If stereochemistry were specified, you could have cis-2-butene or trans-2-butene, but the base name remains but-2-ene. The other options imply a longer chain or a double bond at the first carbon, which doesn’t match this structure.