Halogens & Alkenes (Hydration + Hydrogenation + Combustion + Reactions + Bromine Water Test)

What is an alkene?

An unsaturated hydrocarbon containing a carbon-carbon double bond, C

What does unsaturated mean?

The molecule contains at least one carbon-carbon double bond.

Why do alkenes contain fewer hydrogen atoms than equivalent alkanes?

Because they contain a C

What are the first four alkenes?

Ethene — C₂H₄ -- Propene — C₃H₆ -- Butene — C₄H₈ -- Pentene — C₅H₁₀

How do you test for an alkene?

Add bromine water.

What is the positive result for an alkene?

Bromine water changes from: orange → colourless

Why does bromine water decolourise?

Bromine adds across the carbon-carbon double bond, forming a saturated product.

Would an alkane decolourise bromine water under normal test conditions?

No.

What is an addition reaction?

A reaction in which atoms are added across the C

Why are alkenes more reactive than alkanes?

Because alkenes contain a reactive carbon-carbon double bond.

Which substances does AQA require you to know can add to an alkene?

Hydrogen -- Water/steam -- Halogens — chlorine, bromine and iodine

What happens when an alkene reacts with hydrogen?

Hydrogen adds across the C

What happens to the C

C bond during hydrogenation?

What happens when an alkene reacts with steam/water?

Water adds across the C

What type of reaction is alkene + steam?

An addition reaction.

What happens when an alkene reacts with a halogen?

The halogen atoms add across the carbon-carbon double bond, forming a saturated product.

Why is the bromine-water test evidence for a C

C bond?

Can alkenes burn?

Yes. Like other hydrocarbons, they react with oxygen in combustion reactions.

How do alkenes tend to burn in air?

With smoky flames because of incomplete combustion.