Functional Groups
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Two molecules can share the same carbon chain and still behave differently. How one small group of atoms gives a compound its chemical identity: –OH, –CHO, C=O, –COOH and –NH₂.
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Homologous SeriesMethane, ethane, propane, butane: four different compounds, one family. How each member differs from the next by one CH₂ unit, and what that means for their properties.Chemistry · simulation
Key points
- A functional group is an atom or group of atoms that largely decides a compound's characteristic chemical properties and reactions.
- Think of it this way: the carbon chain is the body, the functional group is the identity.
- –OH alcohol, –CHO aldehyde, C=O in the middle of a chain ketone, –COOH carboxylic acid, –NH₂ amine.
- An aldehyde and a ketone both have C=O. In an aldehyde it is at the end of the chain, with an H on it.
- Compounds with the same group react alike, so learn groups, not single molecules.
One chain, five identities
Keep the chain CH₃CH₂– and change only the group. The carbon in –CHO and –COOH belongs to the group, so the chain stays the same.
| Group | Family | Example | Name |
|---|---|---|---|
| –OH | Alcohol | CH₃CH₂–OH | Ethanol |
| –CHO | Aldehyde | CH₃CH₂–CHO | Propanal |
| C=O (between two carbons) | Ketone | CH₃CH₂–CO–CH₃ | Butanone |
| –COOH | Carboxylic acid | CH₃CH₂–COOH | Propanoic acid |
| –NH₂ | Amine | CH₃CH₂–NH₂ | Ethanamine |
Why "largely" decides
Propanoic acid fizzes with sodium hydrogencarbonate and gives CO₂. Ethanol does not, even though both carry the same chain. That difference comes from the group.
The group does not decide everything. Longer chains still change physical properties such as boiling point: ethanol boils at about 78 °C and propan-1-ol at about 97 °C.
Try it yourself
Goal: Change the chain and the group separately and find which one decides each test result.
- Build a molecule from a chain and a group.
- Run a test and read the result.
- Keep the group, change the chain, and run the same test.
- Keep the chain, change the group, and run it again.
- Compare the boiling points as the chain grows.
Quick quiz
5 questions. Choose an answer for each, then submit to see your score and the explanations.
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