Plan an aromatic route from nitrobenzene to phenol
Use reduction, diazotisation and hydrolysis in the correct order to convert an aromatic nitro group into phenol.
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 intermediate is required between phenylamine and phenol in this route?
What you will learn
- Order the aromatic conversion steps
- State reagents and temperature limits
- Identify the diazonium intermediate and products
Reduce nitrobenzene to phenylamine
Nitrobenzene can be reduced with tin and concentrated hydrochloric acid under hot conditions to form phenylammonium ions; aqueous sodium hydroxide then liberates phenylamine. Keep the acid reduction and basification stages distinct.
This route changes the –NO₂ substituent to –NH₂ while retaining the benzene ring. Do not attempt diazotisation from nitrobenzene: the amino group must first be formed.
Diazotise cold, then warm with water
React phenylamine with nitrous acid, generated using sodium nitrite and dilute acid, below 10°C to form a diazonium salt. Warming the diazonium salt with water then gives phenol, nitrogen gas and an acid by-product.
The temperature boundary belongs to diazonium-salt formation. Warming applies only after the intermediate has formed; mixing the stages into one condition loses the route logic.
Pause and explain
At what temperature is the diazonium salt formed from phenylamine?
Worked example
Outline a route from nitrobenzene to phenol.
Show the worked solution
- Reduce nitrobenzene using hot Sn/concentrated HCl, then add aqueous NaOH to release phenylamine.
- Use NaNO₂ and dilute acid below 10°C to form the diazonium salt.
- Warm the salt with water to form phenol.
Answer Nitrobenzene → phenylamine → benzenediazonium salt below 10°C → phenol on warming with water.
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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.
Put the nitrobenzene-to-phenol route in order, including the key condition at the diazonium step.
Give me a hint
Reduce first, form the diazonium intermediate cold, then warm with water.
Compare my reasoning
- Reduce nitrobenzene with hot Sn/concentrated HCl and add aqueous NaOH to release phenylamine.
- Diazotise with NaNO₂/dilute acid below 10°C.
- Warm the diazonium salt with water to make phenol.
Nitrobenzene → phenylamine → diazonium salt below 10°C → phenol on warming with water.
Look for these in your work
- Reduction precedes diazotisation
- Cold step specified
- Final hydrolysis step correct
Why does nitrobenzene need to be converted to phenylamine before diazotisation?
Give me a hint
Identify the functional group that forms the diazonium ion.
Compare my reasoning
- Diazotisation uses an aromatic primary amine.
- Nitrobenzene has –NO₂, not –NH₂.
- Reduction provides the amino group needed for the intermediate.
Phenylamine must first be formed because the aromatic –NH₂ group is the substrate for diazotisation; nitrobenzene's –NO₂ group cannot directly supply that route.
Look for these in your work
- Required starting group named
- Nitro group distinguished
- Order justified
A route shows ‘phenylamine, warm water, then NaNO₂/HCl below 10°C’ to produce phenol. Correct the order.
Give me a hint
Formation of the diazonium salt is the cold step before hydrolysis.
Compare my reasoning
- Add NaNO₂/dilute acid to phenylamine below 10°C.
- Form the diazonium salt first.
- Then warm the salt with water to produce phenol.
Phenylamine → NaNO₂/dilute acid below 10°C → diazonium salt → warm with water → phenol.
Look for these in your work
- Reagents placed correctly
- Intermediate explicit
- Warm step last
Common mistakes
- Diazotising nitrobenzene directly
- Warming before the diazonium salt forms
- Leaving out aqueous NaOH after Sn/HCl
- Calling nitrogen gas the organic product
What can you explain now?
Which intermediate is required between phenylamine and phenol in this route?
Compare with the explanation
A diazonium salt
Phenylamine is diazotised below 10°C before warming with water.
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Make it stick
Why must the aromatic route pass through phenylamine and a cold diazonium intermediate before phenol forms?
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.