Identify functional groups in a molecule with several groups
Scan a structure systematically and identify every functional group before predicting its reactions.
Section 36 · Organic synthesis
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 36. Routes use reactions listed in the syllabus and state the relevant reagents and conditions. Check every intermediate and possible by-product; no route implies a practical procedure. Written practice is self-checked
Before you begin
- Use the functional-group reactions from Topics 13–19 and 29–35, including their reagents, conditions, products and limitations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which structural feature distinguishes an ester from a carboxylic acid?
What you will learn
- Identify multiple functional groups in one structure
- Use connectivity to distinguish similar groups
- Connect each group to a reaction in the syllabus
Read the complete structure
Check each part of a molecule rather than stopping at the first recognisable group. The syllabus includes alkenes, halogenoalkanes, alcohols, phenols, aldehydes, ketones, carboxylic acids, esters, amines, nitriles, amides and aromatic rings.
Use the atoms around a carbonyl carbon to distinguish groups: –CHO is an aldehyde, >C=O within a carbon chain is a ketone, –COOH is a carboxylic acid, –COO– is an ester and –CONH– is an amide. An aromatic ring with –OH directly attached is a phenol, not an alcohol.
Name evidence for each group
A functional group can be recognised structurally and supported by a characteristic syllabus reaction. For example, 2,4-DNPH detects a carbonyl but does not alone distinguish an aldehyde from a ketone; Tollens’ or Fehling’s reagent can distinguish an aldehyde in the stated test context.
When a structure contains several groups, report them separately. A positive test for one group does not erase or identify the others.
Pause and explain
What does a positive 2,4-DNPH test establish by itself?
Worked example
Identify the functional groups in H₂NCH₂CH₂COOCH₃.
Show the worked solution
- Locate the nitrogen group at the left end.
- Inspect the carbonyl and the two oxygen connections on the right.
- Name each group by its bonding pattern.
Answer The molecule contains a primary amine (–NH₂) and an ester (–COO–).
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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.
Identify the functional groups in CH₃COCH₂CH₂OH.
Give me a hint
Read both ends of the structure; do not treat it as one group.
Compare my reasoning
- The internal C=O is bonded to carbon groups on both sides.
- The terminal –OH is on an aliphatic carbon.
- Name both functional groups.
It contains a ketone and a primary alcohol group.
Look for these in your work
- Carbonyl class correct
- Alcohol distinguished from phenol
- Both groups listed
A structure contains –COOH, C=C and a halogen on an alkyl carbon. List its functional groups.
Give me a hint
Check the carboxyl, alkene and carbon–halogen parts independently.
Compare my reasoning
- –COOH is a carboxylic acid.
- C=C is an alkene group.
- A halogen bonded to an alkyl carbon is a halogenoalkane group.
The molecule contains a carboxylic acid, an alkene and a halogenoalkane group.
Look for these in your work
- Acid identified
- Alkene identified
- Halogenoalkane identified
A student labels C₆H₅OH an alcohol. Correct the classification by describing the bond to –OH.
Give me a hint
Check whether oxygen is bonded directly to an aromatic carbon.
Compare my reasoning
- The –OH is attached directly to the aromatic ring.
- That makes the group a phenol.
- An alcohol has –OH on an aliphatic carbon.
C₆H₅OH is phenol because –OH is directly attached to the aromatic ring, not an alcohol group on an aliphatic carbon.
Look for these in your work
- Attachment site identified
- Phenol named
- Contrast explained
Common mistakes
- Calling every carbonyl a ketone
- Confusing an ester with a carboxylic acid
- Stopping after identifying one group
- Calling a phenol an alcohol
What can you explain now?
Which structural feature distinguishes an ester from a carboxylic acid?
Compare with the explanation
An ester has –COO– between carbon groups
A carboxylic acid has –COOH; an ester has a carbon group attached through the single-bond oxygen.
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Make it stick
How do you distinguish common carbonyl groups and identify every group in a multi-functional structure?
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.