Identify functional groups in a synthesis problem
Read a molecule as a set of functional groups, then choose syllabus reactions that distinguish or transform them.
AS Level; Cambridge International Chemistry 9701, syllabus for 2028–2030. Synthetic routes use reactions in this syllabus; reagent hazards are not practical instructions. Written practice is self-checked
Before you begin
- Use the functional-group reactions, reagents and conditions from Topics 13–20. Check connectivity and carbon count at each step.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a positive 2,4-DNPH test establish by itself?
What you will learn
- Identify more than one functional group in a molecule
- Use syllabus reactions as evidence for functional groups
- Separate structural recognition from reaction prediction
Scan the structure systematically
Read from left to right and mark each characteristic group: C=C, C–X, –OH, –CHO, >C=O, –COOH, –COO–, –C≡N and –NH₂ where relevant. A single molecule can contain several groups, so do not stop after finding the first one.
Then link each group to a reaction in the syllabus. For example, a C=C can decolourise aqueous bromine; a carbonyl reacts with 2,4-DNPH; aldehydes can be distinguished from ketones using Fehling’s or Tollens’ reagent; carboxylic acids react with carbonates to release CO₂; a nitrile can be hydrolysed to an acid.
Use evidence to distinguish possibilities
A test result supports a group only when the reagent and observation match. A positive 2,4-DNPH result indicates a carbonyl, but does not by itself distinguish an aldehyde from a ketone. A negative result should be interpreted within the test conditions and the groups being considered.
When a molecule has multiple groups, predict which part reacts under the stated conditions and whether another group remains unchanged. Use the syllabus reaction scope rather than inventing a test or assuming every group reacts at once.
Pause and explain
Which observation is expected when an alkene reacts with aqueous bromine?
Worked example
A structure contains –COOH and C=C. Name one syllabus test for each group and the expected observation.
Show the worked solution
- Use carbonate reaction for the carboxylic acid.
- Use aqueous bromine for the alkene.
- Keep the two observations associated with their own functional groups.
Answer The –COOH group reacts with a carbonate to release CO₂ bubbles. The C=C group decolourises aqueous bromine under the stated test conditions.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
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.
A molecule contains an aldehyde and a nitrile group. Give one syllabus reaction that tests for the aldehyde and one route that changes the nitrile.
Give me a hint
Match each reagent to the group it detects or transforms.
Compare my reasoning
- Use Tollens’ or Fehling’s reagent for an aldehyde.
- Hydrolyse –CN with dilute acid or dilute alkali followed by acidification.
- State that these reactions address different groups.
Tollens’ reagent can give a silver mirror with the aldehyde. The nitrile can be hydrolysed to a carboxylic acid; under alkaline hydrolysis, acidify the carboxylate afterward.
Look for these in your work
- Aldehyde test correctly named
- Nitrile hydrolysis route stated
- Groups distinguished
An unknown gives a positive 2,4-DNPH result but no silver mirror with Tollens’ reagent. What can you conclude, and what can you not conclude?
Give me a hint
Separate evidence for carbonyl from evidence that distinguishes aldehydes.
Compare my reasoning
- 2,4-DNPH supports the presence of a carbonyl.
- Tollens’ result does not support an aldehyde under the test conditions.
- A ketone is consistent, but account for test limitations.
The compound contains a carbonyl group. The result is consistent with a ketone rather than an aldehyde, but conclusions depend on reliable test conditions and controls.
Look for these in your work
- Carbonyl inference correct
- Aldehyde distinction used
- Conclusion appropriately qualified
A student sees –COOH and –COO– in the same formula and calls both ‘carboxylic acid’. Correct the group identification.
Give me a hint
Check whether the carbonyl carbon is bonded to OH or to an oxygen that connects to another carbon group.
Compare my reasoning
- –COOH is a carboxylic acid group.
- –COO– between carbon groups is an ester linkage.
- Give each part its own reaction behaviour.
–COOH is a carboxylic acid; –COO– linking two carbon groups is an ester. The structure contains two different functional groups.
Look for these in your work
- Carboxylic acid identified
- Ester identified
- Groups distinguished by connectivity
Common mistakes
- Treating a positive carbonyl test as proof of an aldehyde
- Ignoring a second functional group
- Assigning an observation to the wrong reagent
- Using a test outside the syllabus without evidence
What can you explain now?
What does a positive 2,4-DNPH test establish by itself?
Compare with the explanation
A carbonyl compound is present
The test detects carbonyl compounds but does not alone distinguish aldehyde from ketone.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do you use functional-group structure and syllabus reactions together without over-interpreting one test?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.