AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
It is the generality of reactions of functional groups that determine the…
Full specification wording
Reactions of alkenes: It is the generality of reactions of functional groups that determine the reactions of organic compounds.
Organic compounds are grouped into homologous series, and each series has a characteristic functional group.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 2 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Organic compounds are grouped into homologous series, and each series has a characteristic functional group. The functional group is the reactive part of the molecule, so compounds that share a functional group undergo the same type of reaction, even when their carbon chains differ in length. This generality means a student can predict the chemistry of an unfamiliar member of a series from the behaviour of a known member. For example, because ethene decolourises bromine water by addition across C=C, propene and but-1-ene do the same. The carbon chain affects physical properties such as boiling point, but the functional group determines the reactions.
What earns the marks
- A functional group is the reactive part of an organic molecule and defines its homologous series.
- Compounds with the same functional group react in the same general way, so reactions are characteristic of the group rather than the individual compound.
- The length and branching of the carbon chain mainly affect physical properties, not the general reaction type.
- Predictions about an unfamiliar organic compound can be made by identifying its functional group and applying the known reactions of that group.
- For alkenes, the C=C functional group is responsible for addition reactions, so all alkenes undergo addition with hydrogen, water and halogens.
Common mistakes and how to put them right
Assuming every organic compound reacts differently.
Put it right: Members of a homologous series share the reactions of their functional group.
Attributing the reactions of an alkene to its carbon chain length.
Put it right: The C=C functional group determines the addition reactions.
Treating a functional group as a physical label only.
Put it right: It is the reactive site where bonds are made or broken.
Exam tips
- Name the functional group first, then state the general reaction expected of that group.
- When predicting a reaction, compare the unknown compound with a familiar member of the same homologous series.
- Use phrases such as 'all alkenes' or 'compounds containing C=C' to show that the reaction is general to the functional group.
Try the lesson
The start of the Reactions of alkenes: It is the generality of reactions of functional groups that determine the reactions of organic compounds. lesson, made from checked teaching notes for this specification point.
Have a go first
Why do many different alkenes react with bromine water in a similar way?
Listen
Organic compounds are grouped into homologous series, and each series has a characteristic functional group. The functional group is the reactive part of the molecule, so compounds that share a functional group undergo the same type of reaction, even when their carbon chains differ in length. This generality means a student can predict the chemistry of an unfamiliar member of a series from the behaviour of a known member. For example, because ethene decolourises bromine water by addition across C=C, propene and but-1-ene do the same. The carbon chain affects physical properties such as boiling point, but the functional group determines the reactions. A strong answer earns marks for these points. A functional group is the reactive part of an organic molecule and defines its homologous series. Compounds with the same functional group react in the same general way, so reactions are characteristic of the group rather than the individual compound. The length and branching of the carbon chain mainly affect physical properties, not the general reaction type. Predictions about an unfamiliar organic compound can be made by identifying its functional group and applying the known reactions of that group. A common slip: Assuming every organic compound reacts differently. Instead, members of a homologous series share the reactions of their functional group. Exam tip: Name the functional group first, then state the general reaction expected of that group.
Key words
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Functional groups
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Homologous series
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Patterns in organic chemistry
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Prediction from structure
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See every definition
- Functional groups
- The reactive part of an organic molecule, which defines its homologous series and determines the reactions of the compound.
- Homologous series
- A group of organic compounds that share a characteristic functional group and react in the same general way.
- Patterns in organic chemistry
- The generality that compounds sharing a functional group undergo the same type of reaction, even when their carbon chains differ in length.
- Prediction from structure
- Identifying the functional group of an unfamiliar compound and applying the known reactions of that group to predict its behaviour.
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