Alkynes
Alkynes are hydrocarbons with at least one triple bond between carbons. The geometry around the triple bond is linear (180°), unlike the double bond (120°) and single bond (109.5°) — this significantly changes the molecule's shape near that point.
General formula
Naming rule
Name = numerical prefix + infix -yn- + suffix -e, with the triple bond's locant (e.g., but-1-yne). Same lowest-locant numbering rule as alkenes.
How to name it with IUPAC
Use this sequence to move from the structure to the systematic name:
- Choose the longest chain containing the triple bond.
- Number from the end nearest the triple bond and use -yne with its locant.
- HC≡CCH₂CH₃ = but-1-yne: four carbons and a triple bond at C1.
Examples
| Name | Formula |
|---|---|
| Ethyne (acetylene) | C₂H₂ |
| Propyne | C₃H₄ |
| But-1-yne | C₄H₆ |
| But-2-yne | C₄H₆ |
| Penta-1,4-diyne | C₅H₄, 2 triple bonds |
Visual examples
See how the functional group appears inside the chain. Compare the drawings with the formulas in the table to practice visual recognition.
Chemical reading
The sp carbons of C≡C are approximately linear at 180°; the triple bond counts as two degrees of unsaturation.
Properties and applications
Ethyne is used in welding and synthesis; complete hydrogenation gives an alkane and partial hydrogenation can stop at an alkene.
- A terminal alkyne has an acidic hydrogen. HC≡CCH₂CH₃ is but-1-yne.
- Compare polarity, chain size, and hydrogen bonding before predicting physical properties.