Alkenes are hydrocarbons with at least one carbon-carbon double bond. Naming them involves identifying the longest carbon chain with the double bond, numbering it from the end closest to the bond, and using the -ene suffix.

Substituents are named and numbered based on their position. The double bond's location is specified before the -ene suffix. Cis-trans isomerism and E-Z nomenclature are used to describe the arrangement of groups around the double bond.

Alkene Nomenclature

Naming alkenes with -ene suffix

• Alkenes are unsaturated hydrocarbons contain at least one carbon-carbon double bond ($\ce{C=C}$)
• Determine the parent chain by identifying the longest continuous chain of carbon atoms that includes the double bond
  • Establish the root name based on the number of carbons in the longest chain
    • 2 carbons (ethene), 3 carbons (propene), 4 carbons (butene), 5 carbons (pentene), 6 carbons (hexene)
  • Replace the -ane suffix of the corresponding alkane with -ene to denote the presence of a double bond
• Follow IUPAC rules for systematic naming of alkenes

Numbering alkene carbon chains

• Assign numbers to the carbon atoms in the longest chain
  • Begin numbering from the end of the chain that gives the lowest number for the double bond position
• Specify the position of the double bond by its lower-numbered carbon
  • Include the double bond position in the name before the -ene suffix
    • 1-butene $\ce{CH2=CHCH2CH3}$ has the double bond between C1 and C2
    • 2-pentene $\ce{CH3CH=CHCH2CH3}$ has the double bond between C2 and C3
• Use structural formula to clearly show the arrangement of atoms and bonds in alkenes

Alkene nomenclature conventions

• Name any substituents (branches) attached to the parent chain
  • Use prefixes like methyl-, ethyl-, propyl- for alkyl groups
  • Indicate the number of the carbon to which each substituent is attached
    • Number the carbons based on the previously determined double bond position
• Construct the complete alkene name
  • Arrange the substituents in alphabetical order before the parent chain name
  • Specify the double bond position before the -ene suffix
    • 4-methyl-2-pentene $\ce{CH3CH=CHCH(CH3)CH3}$ has a methyl group on C4 and double bond at C2-C3
    • 3-ethyl-1-hexene $\ce{CH2=CHCH(CH2CH3)CH2CH2CH3}$ has an ethyl group on C3 and double bond at C1-C2

Alkene Isomerism and Advanced Nomenclature

• Cis-trans isomerism occurs in alkenes due to restricted rotation around the double bond
• E-Z nomenclature is used to unambiguously describe the configuration of substituents around the double bond
• Molecular formula represents the types and numbers of atoms in an alkene molecule