4. Alkenes
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What is the general formula for alkenes?
CnH2n
What is the functional group for alkenes?
All alkenes contain a double carbon bond, which is shown as two lines between two of the carbon atoms i.e. C=C Alkenes are unsaturated hydrocarbons
What is the maximum number of bonds a carbon atom can make?
4 Bonds
Why are alkenes (C=C double bond) more reactive than alkanes?
• The C=C double bond can open up, letting new atoms form single bonds with each carbon. • Each carbon then forms 4 single bonds (instead of 1 double + 2 single).
What happens when alkenes react with bromine, and why is this an example of an addition reaction?
• Alkenes undergo addition reactions, where atoms add across the C=C double bond. • When bromine reacts with an alkene, a dibromoalkane is formed. • Example: ethene + bromine → dibromoethane. • The same process works with any halogen and any alkene, with halogen atoms always adding across the C=C bond. Dihaloalkane
How can you distinguish alkanes from alkenes using bromine water?
• Alkanes = saturated (no C=C) → bromine water stays orange. • Alkenes = unsaturated (C=C present) → bromine atoms add across double bond → solution decolourises (loses orange colour). *Bromine water is used because it is safer and easier to handle than bromine.
