Recognise the A Level organic functional groups
Identify the functional groups and homologous-series families added in A Level organic chemistry, including aromatic groups and amides.
Section 29 · An introduction to A Level organic chemistry
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 29. Follow the formula, naming and optical-isomerism limits stated in the syllabus. Benzene-ring displayed formulas are not expected; use the supplied convention for aromatic skeletal formulas. Do not extend the optical-isomerism scope to naming diastereoisomers or meso compounds. Written practice is self-checked
Before you begin
- Use covalent formula conventions, functional groups, reaction terminology, and the AS organic chemistry foundations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What feature identifies an amide?
What you will learn
- Identify an organic functional group from its bonding pattern
- Classify arenes, halogenoarenes, phenols, acyl chlorides, amines, amides and amino acids
- Explain that a functional group influences characteristic physical and chemical behaviour
Find the group in the structure
A functional group is the part of an organic molecule responsible for a characteristic pattern of reactions. Locate the group by its connectivity, not just by counting atoms. For example, a carbonyl C=O bonded to chlorine is an acyl chloride, while a carbonyl bonded to nitrogen is an amide.
The A Level extension includes arenes (an aromatic benzene ring), halogenoarenes (a halogen attached to that ring), phenols (an –OH directly attached to the aromatic ring), acyl chlorides (–COCl), secondary and tertiary amines, primary/secondary/tertiary amides, and amino acids with both amine and carboxyl groups.
Distinguish groups that look similar
A phenol has –OH attached directly to an aromatic carbon; an alcohol has –OH attached to an aliphatic carbon. An amine has nitrogen bonded to carbon groups and/or hydrogen; an amide has nitrogen bonded directly to a carbonyl carbon.
A halogenoarene has a C–X bond to the benzene ring. A halogenoalkane has C–X on a non-aromatic carbon chain. The location of the bond changes the compound's class and reactivity.
Pause and explain
What makes a molecule a phenol?
Worked example
Classify the key group in CH₃COCl and in C₆H₅OH.
Show the worked solution
- In CH₃COCl, chlorine is bonded to a carbonyl carbon.
- That pattern is an acyl chloride, not a halogenoalkane.
- In C₆H₅OH, the hydroxy group is directly attached to an aromatic ring.
Answer CH₃COCl is an acyl chloride; C₆H₅OH is phenol because –OH is attached directly to the benzene ring.
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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.
Classify the functional group in C₆H₅Cl and distinguish it from chloroethane.
Give me a hint
Check which carbon is bonded to chlorine.
Compare my reasoning
- In C₆H₅Cl, chlorine is attached to an aromatic-ring carbon.
- That makes it a halogenoarene.
- In chloroethane, chlorine is attached to an aliphatic carbon.
C₆H₅Cl is a halogenoarene; chloroethane is a halogenoalkane because its C–Cl bond is on an aliphatic chain.
Look for these in your work
- Bond location identified
- Correct class named
- Comparison made
Explain why CH₃CONH₂ is an amide rather than an amine.
Give me a hint
Locate the bond immediately next to nitrogen.
Compare my reasoning
- The nitrogen is attached to the carbonyl carbon.
- The C(=O)–N linkage is the amide group.
- An amine lacks that direct carbonyl–nitrogen link.
CH₃CONH₂ is an amide because nitrogen is bonded directly to a carbonyl carbon; an amine has no C(=O)–N linkage.
Look for these in your work
- Connectivity inspected
- Amide linkage stated
- Amine distinction explained
A structure contains an aromatic ring, –OH on that ring, and –CH₂NH₂ on a side chain. Which two functional classes are present?
Give me a hint
Classify each group by the atom to which it is attached.
Compare my reasoning
- The ring-bound –OH is phenol.
- The side-chain –NH₂ is a primary amine.
- Do not treat the side-chain amine as part of the aromatic ring.
The molecule contains a phenol group and a primary amine group on its side chain.
Look for these in your work
- Ring-bound hydroxy classified
- Side-chain amine classified
- Distinct locations kept clear
Common mistakes
- Calling every C–X compound a halogenoalkane
- Classifying phenol as an alcohol
- Calling an amide an amine without checking whether N is bonded to C=O
- Naming a functional group by formula alone without inspecting connectivity
What can you explain now?
What feature identifies an amide?
Compare with the explanation
Nitrogen is bonded directly to a carbonyl carbon
The C(=O)–N connection identifies an amide group.
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Make it stick
How do connectivity and the location of a group distinguish phenol, amine, amide and halogenoarene?
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.