Recognise functional groups in organic compounds
Identify the functional groups listed for AS organic chemistry and relate them to compound classes.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the published functional-group scope and nomenclature limits; draw formula and mechanism evidence with correct connectivity, electron movement and stereochemistry. Written practice is self-checked
Before you begin
- Use covalent structures, valency, molecular formulas, lone pairs, and single, double and triple bonds.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which group identifies a carboxylic acid?
What you will learn
- Recognise the syllabus functional groups
- Match a group to its compound class
- Explain how a functional group influences characteristic properties
The functional group identifies a reaction family
A functional group is the atom or group of atoms that gives a class of organic compounds characteristic chemical properties. The carbon skeleton may change while the functional group remains the same.
The AS table includes C=C in alkenes; X in halogenoalkanes; –OH in alcohols; –CHO in aldehydes; >C=O in ketones; –COOH in carboxylic acids; –COO– in esters; –NH₂ in primary amines; and –C≡N in nitriles. Here X represents a halogen.
Recognise groups in context
Alkanes have no functional group in this classification. The carbonyl group C=O occurs in both aldehydes and ketones, but its position and bonding context distinguish the two classes.
The group influences chemical behaviour and can also affect physical properties such as polarity and intermolecular attraction. The same group on different carbon skeletons usually retains a recognizable pattern of reactivity.
Pause and explain
What is the functional-group difference between an aldehyde and a ketone?
Worked example
Identify the functional group and class in CH₃CH₂CHO and CH₃COCH₃.
Show the worked solution
- CH₃CH₂CHO has a terminal –CHO group.
- CH₃COCH₃ has a C=O group between carbon atoms.
- They are an aldehyde and a ketone, respectively.
Answer CH₃CH₂CHO is an aldehyde; CH₃COCH₃ is a ketone. Both contain C=O, but the aldehyde has a terminal –CHO group.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Match –NH₂, –C≡N and –COO– with their classes.
Give me a hint
Look at the characteristic atoms and bonds.
Compare my reasoning
- –NH₂ is an amine group.
- –C≡N is a nitrile group.
- –COO– is an ester link.
–NH₂: primary amine; –C≡N: nitrile; –COO–: ester.
Look for these in your work
- All groups matched
- Nitrile distinguished
- Ester link identified
Identify the functional group in CH₃CH₂OH and CH₃COOH.
Give me a hint
Compare –OH alone with –COOH.
Compare my reasoning
- CH₃CH₂OH contains an alcohol –OH group.
- CH₃COOH contains the complete –COOH carboxyl group.
- The acid is not classified as an alcohol just because –OH appears within –COOH.
CH₃CH₂OH is an alcohol; CH₃COOH is a carboxylic acid.
Look for these in your work
- Alcohol identified
- Carboxyl identified
- Overlap clarified
A molecule has CH₃–CO–CH₃. Explain why it is a ketone rather than an aldehyde.
Give me a hint
Locate the carbonyl carbon relative to the carbon chain.
Compare my reasoning
- The C=O carbon is bonded to carbon groups on both sides.
- It is not a terminal –CHO group.
- Therefore the molecule is a ketone.
The carbonyl is internal, between carbon groups, so CH₃COCH₃ is a ketone.
Look for these in your work
- Carbonyl position stated
- Aldehyde pattern compared
- Class concluded
Common mistakes
- Calling every C=O compound a ketone
- Treating the whole carbon chain as a functional group
- Forgetting that X stands for a halogen
What can you explain now?
Which group identifies a carboxylic acid?
Compare with the explanation
–COOH
The carboxyl group is –COOH.
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Make it stick
Which functional group distinguishes a nitrile from a primary amine?
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