Plan a route from phenylamine to an azo compound
Use cold diazotisation followed by alkaline coupling to form the azo link in the correct sequence.
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
What linkage identifies the azo product?
What you will learn
- Form an aromatic diazonium salt
- Choose a coupling partner and condition
- Recognise the azo linkage and route product
Make the diazonium coupling reagent
Phenylamine reacts with nitrous acid, generated from sodium nitrite and dilute acid, below 10°C to form benzenediazonium salt. Keep the mixture cold for this diazotisation stage.
The diazonium ion is an electrophilic coupling partner. In the syllabus example, it reacts with phenol in aqueous sodium hydroxide to form an azo compound.
Recognise the product link
The defining azo group is –N=N– linking two carbon-containing groups. Azo compounds are often coloured and used as dyes.
Do not use the warm-water hydrolysis step if the target is an azo compound; that route produces phenol instead. After the diazonium salt forms, use the coupling partner and alkaline condition specified for azo formation.
Pause and explain
Which coupling partner and condition are used in the syllabus example?
Worked example
Plan an azo-compound route from phenylamine using phenol as the coupling partner.
Show the worked solution
- Diazotise phenylamine with NaNO₂/dilute acid below 10°C.
- React the benzenediazonium salt with phenol in NaOH(aq).
- Identify –N=N– in the azo product.
Answer Phenylamine → benzenediazonium salt below 10°C; couple with phenol in aqueous NaOH to form an azo compound containing –N=N–.
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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.
Give the ordered reagents and conditions to make an azo compound from phenylamine and phenol.
Give me a hint
Separate diazonium formation from coupling.
Compare my reasoning
- Generate nitrous acid with NaNO₂/dilute acid.
- Keep diazotisation below 10°C.
- Couple the diazonium salt with phenol in NaOH(aq).
First form the diazonium salt from phenylamine below 10°C using NaNO₂/dilute acid; then couple with phenol in aqueous NaOH.
Look for these in your work
- Cold formation stated
- Intermediate named
- Coupling condition correct
A product drawing joins two aromatic residues through N=N. Name the group and one common use of the product class.
Give me a hint
Look for the double bond between nitrogen atoms.
Compare my reasoning
- The linkage is –N=N–.
- This is an azo compound.
- Azo compounds are commonly used as dyes.
The –N=N– group is an azo linkage; the product is an azo compound, often used as a dye.
Look for these in your work
- Link named
- Class identified
- Use stated
A synthesis diagram warms the diazonium salt with water, but the target is an azo dye. What route decision should change?
Give me a hint
Warm water consumes the diazonium salt by hydrolysis to phenol.
Compare my reasoning
- Keep diazotisation below 10°C to make the salt.
- Use phenol in aqueous NaOH as the coupling partner.
- Avoid the warm-water conversion that would form phenol instead.
After forming the diazonium salt cold, couple it with phenol in aqueous NaOH; warming with water would direct the route to phenol, not the azo dye.
Look for these in your work
- Intermediate preserved
- Correct partner chosen
- Competing route distinguished
Common mistakes
- Warming the diazonium salt with water instead of coupling
- Using acidic rather than alkaline coupling conditions
- Drawing –N–N– instead of –N=N–
- Forgetting the cold diazotisation condition
What can you explain now?
What linkage identifies the azo product?
Compare with the explanation
–N=N–
The azo group is a nitrogen-nitrogen double bond.
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Make it stick
How do cold diazotisation and alkaline coupling lead to an azo compound rather than phenol?
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.