7.3 Naming Alkenes

Alkene Nomenclature

Naming Alkenes with the -ene Suffix

Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond ($\ce{C=C}$). Because of that double bond, they follow slightly different naming rules than alkanes.

The parent chain is the longest continuous chain of carbon atoms that includes the double bond. This is a key detail: even if there's a longer chain elsewhere in the molecule, the parent chain must pass through the double bond.

Once you've identified the parent chain, replace the -ane ending of the corresponding alkane with -ene:

• 2 carbons → ethene
• 3 carbons → propene
• 4 carbons → butene
• 5 carbons → pentene
• 6 carbons → hexene

Numbering the Carbon Chain

Number the carbons in the parent chain starting from the end that gives the lowest possible number to the first carbon of the double bond. The double bond position is then specified by the number of its lower-numbered carbon, placed before the -ene suffix.

Step-by-step:

1. Identify the longest chain containing the double bond.
2. Number from whichever end places the double bond at the lowest carbon.
3. Name the compound as [double bond position]-[parent name]ene.

A couple of examples to make this concrete:

• 1-butene ($\ce{CH2=CHCH2CH3}$): the double bond sits between C1 and C2. If you numbered from the other end, the double bond would be at C3, which is higher, so C1 is correct.
• 2-pentene ($\ce{CH3CH=CHCH2CH3}$): the double bond sits between C2 and C3. Numbering from either end gives "2-pentene," so both directions work here.

Naming Substituents and Building the Full Name

Any branches (substituents) attached to the parent chain get their own name and position number. Common alkyl substituents include methyl-, ethyl-, and propyl- groups.

The carbon numbers for substituents are based on the numbering you already established for the double bond. To build the complete IUPAC name:

1. Number the parent chain so the double bond gets the lowest locant.
2. Identify and number each substituent based on the carbon it's attached to.
3. List substituents in alphabetical order before the parent chain name.
4. Place the double bond position before the -ene suffix.

Examples:

• 4-methyl-2-pentene ($\ce{CH3CH=CHCH(CH3)CH3}$): the double bond is between C2 and C3, and a methyl group is on C4. "Methyl" comes before the parent name "pentene," and the double bond locant (2) goes right before "-ene."
• 3-ethyl-1-hexene ($\ce{CH2=CHCH(CH2CH3)CH2CH2CH3}$): the double bond is between C1 and C2, with an ethyl group on C3.

Cis-Trans Isomerism and E-Z Nomenclature

Because there's no free rotation around a $\ce{C=C}$ double bond, groups attached to the double-bond carbons can be locked in different spatial arrangements. This creates geometric isomers that are distinct compounds with different physical properties.

Cis-trans nomenclature works when each double-bond carbon has one hydrogen and one other group:

• Cis: the two larger substituents (or identical groups) are on the same side of the double bond.
• Trans: they're on opposite sides.

For more complex cases where cis-trans labels become ambiguous, E-Z nomenclature is used instead. This system assigns priority to each substituent on the double-bond carbons using the Cahn-Ingold-Prelog priority rules (based on atomic number):

• Z (from German zusammen, "together"): the higher-priority groups on each carbon are on the same side.
• E (from German entgegen, "opposite"): the higher-priority groups are on opposite sides.

The E-Z system works for any alkene, regardless of how many different substituents are present, making it the more general and reliable method.