Alkenes have a carbon–carbon double bond. Hydrogen, water and the halogens add across that double bond, so it becomes a single bond and two molecules join to make one product. Choose an alkene and a reactant, then play the reaction.
Learn the reactant, the conditions and the type of product for each one.
| Add | Conditions | Product type | Example with ethene |
|---|---|---|---|
| Hydrogen H₂ | Nickel catalyst, about 60 °C | Alkane | ethene + hydrogen → ethane |
| Water H₂O (as steam) | About 300 °C, about 70 atmospheres, phosphoric acid catalyst | Alcohol | ethene + steam → ethanol |
| Chlorine Cl₂ | Room temperature | Two Cl atoms on neighbouring carbons | ethene + chlorine → 1,2-dichloroethane |
| Bromine Br₂ | Room temperature (bromine water turns colourless) | Two Br atoms on neighbouring carbons | ethene + bromine → 1,2-dibromoethane |
| Iodine I₂ | Room temperature | Two I atoms on neighbouring carbons | ethene + iodine → 1,2-diiodoethane |
Alkenes are hydrocarbons with a C=C double bond. Their general formula is CnH2n.
They are unsaturated: they have two fewer hydrogen atoms than the alkane with the same number of carbon atoms. The first four are ethene C₂H₄, propene C₃H₆, butene C₄H₈ and pentene C₅H₁₀.
The C=C double bond is the functional group, so all alkenes react in the same way.
Alkenes burn with a smoky flame because of incomplete combustion.
With propene and larger alkenes, water can add in two ways, giving two alcohols. For propene these are propan-1-ol and propan-2-ol.
Shake each hydrocarbon with orange bromine water. Bromine adds across the double bond, so it is used up and the colour disappears.
Ethanol can be made by reacting ethene with water.
Give the conditions used for this reaction.
[3 marks]Describe a test to show the difference between propane and propene. Give the result for each.
[3 marks]“Clear” is not accepted for colourless. Clear means see-through, and orange bromine water is already clear.
Propene reacts with bromine. Name the product and explain why this is called an addition reaction.
[3 marks]Butene has the formula C₄H₈. Give the formula of the product when butene reacts with hydrogen, and name the catalyst.
[2 marks]