Reactions of alkenes
The C=C functional group
A is the part of a molecule that decides how it reacts. For alkenes, it is the .
Alkenes burn in air, but they often burn incompletely, with a smoky yellow flame.
In an , atoms add across the double bond: one bond of the C=C breaks, and the two carbon atoms each join to a new atom. The product has only single bonds, and it is the only product.
Key terms
Three addition reactions
| Reacts with | Conditions | Product | Example |
|---|---|---|---|
| Hydrogen | Catalyst, e.g. nickel | Alkane | C₂H₄ + H₂ → C₂H₆ |
| Water (steam) | High temperature, catalyst | Alcohol | C₂H₄ + H₂O → C₂H₅OH |
| (Cl₂, Br₂, I₂) | Room temperature | Two halogen atoms added | C₂H₄ + Br₂ → C₂H₄Br₂ |
- Adding hydrogen is called . Ethene becomes ethane.
- Adding water is called . Ethene becomes ethanol.
- Adding bromine is the reaction behind the .
Key terms
Saturated and unsaturated
Alkenes are : their C=C double bond means they have fewer hydrogen atoms than the alkane with the same number of carbon atoms. Their general formula is CₙH₂ₙ.
Addition reactions make them , with only single bonds. The same reactions work for every alkene, not just ethene:
- propene + hydrogen → propane: C₃H₆ + H₂ → C₃H₈
- propene + steam → propanol: C₃H₆ + H₂O → C₃H₇OH
- propene + bromine → dibromopropane: C₃H₆ + Br₂ → C₃H₆Br₂
In each product, the new atoms are joined to the two carbons that had the double bond.
Key terms
Testing for alkenes
is orange. Shake it with a hydrocarbon:
- An turns it from orange to colourless, because the bromine adds across the C=C double bond and is used up.
- An has no double bond, so the bromine water stays orange.
This lets you tell an alkene from an alkane, for example to show that cracking an alkane has made alkenes.
Exam tip
Say the bromine water is decolourised or turns colourless. 'Clear' is wrong: orange bromine water is already clear, because you can see through it.