GCSE Chemistry revision

Structure and formulae of alkenes

Reactions of alkenes and alcohols (chemistry only)

AQA 4.7.2.1 Foundation & Higher

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What Are Alkenes?

Alkenes are unsaturated hydrocarbons, meaning they contain at least one carbon-carbon double bond (C=C), which acts as their functional group.
Because alkenes only contain hydrogen and carbon atoms, they are classified as hydrocarbons.
The presence of the C=C double bond makes alkenes more reactive than alkanes, enabling them to undergo addition reactions.

1. What Are Alkenes? AQA

Notes

Alkenes are unsaturated hydrocarbons, meaning they contain at least one carbon-carbon double bond (C=C), which acts as their functional group.
Because alkenes only contain hydrogen and carbon atoms, they are classified as hydrocarbons.
The presence of the C=C double bond makes alkenes more reactive than alkanes, enabling them to undergo addition reactions.

Alkenes as a Homologous Series

Alkenes form a homologous series — a group of compounds with similar structures and properties — just like alkanes do.
The key structural difference is that alkenes contain a carbon–carbon double bond (C=C), making them unsaturated, whereas alkanes only have single bonds, making them saturated.
Because alkenes are unsaturated, they are more reactive than alkanes.

2. Alkenes as a Homologous Series AQA

Notes

Alkenes form a homologous series — a group of compounds with similar structures and properties — just like alkanes do.
The key structural difference is that alkenes contain a carbon–carbon double bond (C=C), making them unsaturated, whereas alkanes only have single bonds, making them saturated.
Because alkenes are unsaturated, they are more reactive than alkanes.

The Bromine Water Test for Alkenes

Alkenes decolourise bromine water, turning it from orange to colourless, which is the standard test used to identify the presence of an alkene.
Alkanes do not react with bromine water, so the solution remains orange, distinguishing them from alkenes.

Unsaturated Nature

Alkenes are described as unsaturated because the carbon–carbon double bond means they contain fewer hydrogen atoms than the maximum possible capacity for that number of carbon atoms.
Example: Ethene (C₂H₄) has two fewer hydrogen atoms than the saturated alkane ethane (C₂H₆), enabling alkenes to open their double bond and undergo addition reactions.

4. Unsaturated Nature AQA

Notes

Alkenes are described as unsaturated because the carbon–carbon double bond means they contain fewer hydrogen atoms than the maximum possible capacity for that number of carbon atoms.
Example: Ethene (C₂H₄) has two fewer hydrogen atoms than the saturated alkane ethane (C₂H₆), enabling alkenes to open their double bond and undergo addition reactions.

Uses of Alkenes

Alkenes are not waste products; they are valuable as starting materials for producing a wide range of other chemicals.
Because the double bond in an alkene can break to form two single bonds, alkene molecules can join together to form polymers in a process called addition polymerisation.