Reactions of Alkenes (Addition Reactions) — AQA GCSE Chemistry (Separate)
Alkenes are reactive because of their C=C double bond, which can take part in addition reactions.
The functional group
Alkenes have a carbon–carbon double bond (C=C) — this is their functional group, the part of the molecule responsible for their reactions. In an addition reaction, the double bond opens up and atoms add across it.
Key addition reactions
Combustion — alkenes burn in air. With limited oxygen they tend to produce more soot (smoky flame) than alkanes because of their higher proportion of carbon.
Addition of hydrogen (hydrogenation) — alkene + hydrogen → alkane, using a nickel catalyst. For example, ethene + hydrogen → ethane.
Addition of water (steam) — alkene + steam → alcohol, using a catalyst. For example, ethene + steam → ethanol. This is an industrial way of making ethanol.
Addition of halogens — e.g. ethene + bromine → dibromoethane. This is why alkenes decolourise bromine water (orange → colourless), the test for a double bond.
Exam tips
- The reactive part is the C=C double bond (functional group).
- In addition reactions, the double bond opens and atoms add across it.
- Learn the four reactions: combustion, + hydrogen (→ alkane), + steam (→ alcohol), + halogen.
- Bromine water test: orange → colourless confirms the C=C bond.