IUPAC Nomenclature of Alkenes
Unsaturated Hydrocarbons | Organic Chemistry
1. Basics of Alkenes
Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond ($C=C$).
- General Formula: $C_nH_{2n}$ (for acyclic mono-alkenes).
- Suffix: -ene (e.g., Ethene, Propene).
2. Rule 1: Longest Chain Rule (Selection)
Even if a longer chain exists without the double bond, the chain with the $C=C$ is chosen as the parent chain.
Example: In $\text{CH}_3-\text{CH}_2-\text{CH}(\text{CH}_2-\text{CH}_3)-\text{CH}=\text{CH}_2$, the parent chain is 5 carbons long (Pentene) because it must include the double bond, not the 6-carbon chain.
3. Rule 2: Lowest Locant for Double Bond
Number the parent chain from the end that gives the double bond the lowest possible number.
- Priority: Double Bond > Substituents (Alkyl groups, Halogens).
- If the double bond gets the same number from both ends, then follow the Lowest Set of Locants rule for substituents.
- If still tied, use Alphabetical Order for substituents.
4. Rule 3: Writing the Name
The position of the double bond is indicated before the suffix 'ene'.
Example: $\text{CH}_3-\text{CH}(\text{CH}_3)-\text{CH}=\text{CH}_2$ is 3-Methylbut-1-ene.
5. Naming Polyenes (Dienes, Trienes)
If a molecule has more than one double bond:
- Use suffixes -diene, -triene, etc.
- Important: An 'a' is added to the word root. (e.g., Buta-1,3-diene, not But-1,3-diene).
- Numbering should give the lowest set of locants for the double bonds.
6. Cycloalkenes
In cyclic alkenes:
- The double bond carbons are always numbered C1 and C2.
- Numbering proceeds in the direction that gives the substituent the lowest number.
Example: In 3-Methylcyclohexene, the double bond is at 1-2, and the methyl group gets 3. (Not 1-Methylcyclohex-2-ene).
Practice Quiz
Test your naming skills for Alkenes.
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