Couple a diazonium salt with phenol to make azo dyes
Couple benzenediazonium chloride with phenol in NaOH(aq) to form an azo compound containing –N=N–; related couplings make other azo dyes.
Section 34 · Nitrogen compounds
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 34. Use the stated reagents, conditions and product scope. Written practice is self-checked This online lesson cannot demonstrate supervised laboratory skills.
Before you begin
- Recall nucleophilic substitution, reduction, acyl-chloride addition–elimination, acid–base equilibria and covalent bonding.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What functional group identifies an azo compound?
What you will learn
- Describe the coupling conditions
- Identify the azo linkage
- State the common dye use and transfer the route to related azo compounds
Couple the activated phenol ring
Benzenediazonium chloride reacts with phenol in aqueous NaOH to form an azo compound. The alkaline solution forms phenoxide, activating the ring for coupling.
The defining azo group is –N=N–, connecting two groups that contain aromatic rings in the named example.
Azo compounds often act as dyes
Many azo compounds are strongly coloured and are often used as dyes. Other azo dyes can be formed by a similar coupling route with a suitable activated aromatic compound.
Use the stated example and identify the N=N bond. Do not mistake the diazonium N₂⁺ group in the reactant for the final azo linkage in the product.
Pause and explain
What condition is specified for coupling benzenediazonium chloride with phenol?
Worked example
State how benzenediazonium chloride and phenol form an azo compound.
Show the worked solution
- Use phenol in NaOH(aq).
- Couple the diazonium species to the activated aromatic ring.
- Identify the N=N linkage in the product.
Answer Benzenediazonium chloride couples with phenol in NaOH(aq) to form an azo compound containing –N=N–; azo compounds are often dyes.
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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.
Describe the reactants and condition for the syllabus azo-coupling example.
Give me a hint
Name both coupling partners and the alkaline medium.
Compare my reasoning
- Use benzenediazonium chloride.
- React it with phenol in NaOH(aq).
- State that an azo compound forms.
Benzenediazonium chloride couples with phenol in NaOH(aq) to form an azo compound.
Look for these in your work
- Diazonium reagent
- Phenol/base
- Product class
A product contains two aromatic fragments joined by –N=N–. Classify it and give a common use.
Give me a hint
Recognise the characteristic nitrogen linkage.
Compare my reasoning
- The N=N group is the azo group.
- A compound with this linkage is an azo compound.
- Azo compounds are often used as dyes.
It is an azo compound; azo compounds are often used as dyes.
Look for these in your work
- Linkage identified
- Class named
- Use stated
A learner draws a single N–N bond and labels the product an azo dye. Correct the structural feature.
Give me a hint
Check the bond order in the defining group.
Compare my reasoning
- The azo group is N=N.
- The two nitrogen atoms are joined by a double bond.
- The product is a coloured azo compound when appropriate.
Show –N=N–, not a single N–N bond; the azo linkage identifies the azo compound.
Look for these in your work
- Bond order
- Group notation
- Class
Common mistakes
- Writing N–N instead of N=N
- Omitting NaOH(aq)
- Calling the product a diazonium salt
- Claiming all azo compounds are a single colour
What can you explain now?
What functional group identifies an azo compound?
Compare with the explanation
–N=N–
The azo linkage is a nitrogen–nitrogen double bond.
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Make it stick
Which partners and condition form the azo linkage, and what is a common use of the products?
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