GCSE Chemistry revision

Reactions of alkenes

Reactions of alkenes and alcohols (chemistry only)

AQA 4.7.2.2 Foundation & Higher

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In an addition reaction, the carbon-carbon double bond opens up, allowing the two carbon atoms to bond to atoms from another molecule.
The result is a single product with no double bond remaining, meaning the product is saturated.

When an alkene reacts with hydrogen gas (H₂) in the presence of a catalyst, the double bond breaks and hydrogen atoms bond to each carbon.
This type of addition reaction is called hydrogenation and converts an alkene into an alkane.
For example, propene reacts with hydrogen to form propane, which is now saturated and contains no double bond.

2. Reaction of Alkenes with Hydrogen AQA

When an alkene reacts with hydrogen gas (H₂) in the presence of a catalyst, the double bond breaks and hydrogen atoms bond to each carbon.
This type of addition reaction is called hydrogenation and converts an alkene into an alkane.
For example, propene reacts with hydrogen to form propane, which is now saturated and contains no double bond.

Alkenes react with steam (water vapour) in the presence of a catalyst at high temperatures to produce alcohols.
The water molecule splits into a hydrogen atom (H) and a hydroxyl group (OH), which each bond to the carbon atoms at the double bond.
For example, ethene reacts with steam to produce ethanol, the same alcohol found in alcoholic drinks and used in industrial processes.
To separate the ethanol product from unreacted ethene, the mixture is cooled so the ethene remains as a gas while ethanol and water condense into liquid.
Ethanol and water are then separated using fractional distillation, as ethanol has a lower boiling point and evaporates first, condensing into a separate container.

3. Reaction of Alkenes with Water (Steam) AQA

Alkenes react with steam (water vapour) in the presence of a catalyst at high temperatures to produce alcohols.
The water molecule splits into a hydrogen atom (H) and a hydroxyl group (OH), which each bond to the carbon atoms at the double bond.
For example, ethene reacts with steam to produce ethanol, the same alcohol found in alcoholic drinks and used in industrial processes.
To separate the ethanol product from unreacted ethene, the mixture is cooled so the ethene remains as a gas while ethanol and water condense into liquid.
Ethanol and water are then separated using fractional distillation, as ethanol has a lower boiling point and evaporates first, condensing into a separate container.

Alkenes react with halogens in an addition reaction that requires no catalyst, with the double bond opening to allow halogen atoms to bond to the carbon atoms.
For example, ethene reacts with bromine (Br₂) to form dibromoethane, using up all the bromine in the process.
Bromine solution is orange in colour, and when it reacts with an alkene, the orange colour disappears (decolourises), indicating a reaction has taken place.

4. Reaction of Alkenes with Halogens AQA

Alkenes react with halogens in an addition reaction that requires no catalyst, with the double bond opening to allow halogen atoms to bond to the carbon atoms.
For example, ethene reacts with bromine (Br₂) to form dibromoethane, using up all the bromine in the process.
Bromine solution is orange in colour, and when it reacts with an alkene, the orange colour disappears (decolourises), indicating a reaction has taken place.

The bromine water test is used to distinguish between alkenes and alkanes: alkenes decolourise bromine water from orange to colourless.
Alkanes are saturated and have no double bonds, so bromine water cannot react with them and the solution remains orange.
This difference in reactivity is due to alkenes having a C=C double bond available for addition reactions, while alkanes do not.

5. Testing for Alkenes Using Bromine Water AQA

The bromine water test is used to distinguish between alkenes and alkanes: alkenes decolourise bromine water from orange to colourless.
Alkanes are saturated and have no double bonds, so bromine water cannot react with them and the solution remains orange.
This difference in reactivity is due to alkenes having a C=C double bond available for addition reactions, while alkanes do not.

Alkenes undergo addition reactions with hydrogen, water (steam), and halogens due to the presence of the carbon-carbon double bond.
Each addition reaction produces a single saturated product: hydrogenation gives an alkane, reaction with steam gives an alcohol, and reaction with a halogen gives a dihalogenoalkane.
The conditions vary: hydrogenation requires a catalyst, reaction with steam requires a catalyst and high temperature, and halogenation requires no catalyst.

6. Summary / Key Terms AQA

Alkenes undergo addition reactions with hydrogen, water (steam), and halogens due to the presence of the carbon-carbon double bond.
Each addition reaction produces a single saturated product: hydrogenation gives an alkane, reaction with steam gives an alcohol, and reaction with a halogen gives a dihalogenoalkane.
The conditions vary: hydrogenation requires a catalyst, reaction with steam requires a catalyst and high temperature, and halogenation requires no catalyst.