Alkenes · Grade 11
Alkenes are hydrocarbons with a carbon–carbon double bond, C=C, so they are unsaturated. Their general formula is CnH2n. The double bond makes them much more reactive than alkanes: other atoms can add across it. This gives a simple test, bromine water, and lets alkenes be made into alcohols and polymers. Alkenes are made by cracking.
Key facts
- Alkenes have a C=C double bond and the general formula CnH2n: ethene C2H4, propene C3H6, butene C4H8.
- The C=C double bond is the functional group of the alkenes.
- Test: alkenes turn orange bromine water colourless; alkanes leave it orange.
- Addition reactions: the atoms add across the double bond. With hydrogen (nickel catalyst) an alkane forms; with bromine, a dibromoalkane; with steam (phosphoric acid catalyst, about 300 °C, 60–70 atm), an alcohol.
- Alkenes burn with smoky flames, because there is often not enough oxygen for complete combustion.
- Alkenes can join together into long chains: addition polymers such as poly(ethene).