Alkenes
Alkenes (or olefins) are hydrocarbons with at least one double bond between carbons. The double bond makes the molecule more reactive than an alkane — it's the starting point for addition reactions (halogenation, hydration, polymerization).
General formula
Naming rule
Name = numerical prefix + infix -en- + suffix -e, with the double bond's locant before the name (e.g., but-1-ene). Chain numbering starts from the end that gives the lowest locant to the double bond.
How to name it with IUPAC
Use this sequence to move from the structure to the systematic name:
- Choose the longest chain containing the double bond.
- Number from the end nearest the double bond and use -ene with its locant.
- CH₃CH=CHCH₃ = but-2-ene: four carbons and a double bond starting at C2.
Examples
| Name | Formula |
|---|---|
| Ethene (ethylene) | C₂H₄ |
| Propene | C₃H₆ |
| But-1-ene | C₄H₈ |
| But-2-ene | C₄H₈ |
| Buta-1,3-diene | C₄H₆, 2 double 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 pi bond is electron-rich, so alkenes undergo addition and cannot freely rotate around C=C.
Properties and applications
Ethene produces polyethylene and propene produces polypropylene; hydrogenation converts C=C into C–C.
- C₄H₈ may be an alkene or a ring, and but-2-ene can have cis/trans or E/Z geometry.
- Compare polarity, chain size, and hydrogen bonding before predicting physical properties.