IUPAC Nomenclature of Alkynes
Triple Bonds & Enynes | Organic Chemistry
1. Basics of Alkynes
Alkynes are unsaturated hydrocarbons containing at least one carbon-carbon triple bond ($C \equiv C$).
- General Formula: $C_nH_{2n-2}$ (for acyclic mono-alkynes).
- Suffix: -yne (e.g., Ethyne, Propyne).
2. Rule 1: Longest Chain Rule (Selection)
Critical Rule: Select the longest continuous carbon chain that contains the triple bond.
The parent chain must include the triple bond, even if a longer carbon chain is available without it.
3. Rule 2: Lowest Locant for Triple Bond
Number the parent chain from the end that gives the triple bond the lowest possible number.
- Priority: Triple Bond > Substituents (Alkyl groups, Halogens).
- If the triple bond gets the same number from both ends, 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 triple bond is indicated before the suffix 'yne'.
$\text{Prefix} + \text{Word Root} + \text{Locant} + \text{yne}$
Example: $\text{CH}_3-\text{C} \equiv \text{C}-\text{CH}_3$ is But-2-yne.
5. Poly-ynes (Multiple Triple Bonds)
If a molecule has more than one triple bond:
- Use suffixes -diyne, -triyne, etc.
- Important: An 'a' is added to the word root (e.g., Buta-1,3-diyne).
6. Hydrocarbons with both Double & Triple Bonds (Enynes)
When both double ($=$) and triple ($\equiv$) bonds are present:
Priority Rule for Numbering:
- Lowest Locant Set: Number the chain from the end that gives the lowest number to the first multiple bond (whether it is double or triple).
- Tie-Breaker: If the positions are identical from both ends, the Double Bond ($=$) gets priority (lower number) over the Triple Bond ($\equiv$).
Naming Format:
$\text{Root} + \text{en} + \text{yne}$
(The 'e' of 'ene' is dropped because it is followed by 'y')
$\text{Root} + \text{en} + \text{yne}$
(The 'e' of 'ene' is dropped because it is followed by 'y')
Examples:
- Case 1 (No Tie): $HC \equiv C - CH_2 - CH = CH_2$.
Numbering from Left: 1, 4. From Right: 1, 4. Tie!
Priority Rule: Double bond gets 1.
Name: Pent-1-en-4-yne. - Case 2 (Lowest Locant): $HC \equiv C - CH = CH - CH_3$.
Numbering from Left: 1, 3. From Right: 2, 4.
Set (1,3) is lower than (2,4). Triple bond gets 1.
Name: Pent-3-en-1-yne.
Practice Quiz
Test your naming skills for Alkynes & Enynes.
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