Prepare phenol through a diazonium salt
Convert phenylamine to a diazonium salt below 10°C, then warm it with water to produce phenol.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 32. Distinguish phenol (–OH directly on an aromatic ring) from an alcohol (–OH on an aliphatic carbon). Use the specified reagents, temperatures and products; phenol reactions are limited to those listed in the syllabus. Written practice is self-checked
Before you begin
- Recognise alcohols and phenols, aromatic substitution, acids and bases, and diazonium ions from the preceding organic topics.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
At what temperature is phenylamine diazotised in the syllabus route?
What you will learn
- Recall diazotisation reagents and low-temperature condition
- Describe the diazonium salt intermediate
- Describe warming the salt with water to form phenol
Make the diazonium salt cold
Treat phenylamine with HNO₂, or generate it in situ from NaNO₂ and dilute acid, below 10°C to form a diazonium salt. A common reagent description is NaNO₂ and dilute HCl at below 10°C.
Keep this low-temperature stage distinct from the next step. The cold condition controls formation of the diazonium intermediate; it is not the condition used to complete the conversion to phenol.
Warm the salt with water
Warm the diazonium salt with water to produce phenol. For benzenediazonium chloride, the organic transformation is C₆H₅N₂⁺Cl⁻ + H₂O → C₆H₅OH + N₂ + HCl.
This is a two-stage route: phenylamine → diazonium salt below 10°C → phenol on warming with water. If a question gives the salt, start from that intermediate rather than repeating the first step.
Pause and explain
What is the next step after forming the diazonium salt?
Worked example
Describe how phenylamine can be converted into phenol.
Show the worked solution
- React phenylamine with HNO₂ or NaNO₂ plus dilute acid below 10°C.
- Form the diazonium salt.
- Warm the salt with water to form phenol.
Answer Phenylamine is diazotised below 10°C using HNO₂ (or NaNO₂/dilute acid); warming the diazonium salt with water produces phenol.
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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 phenol preparation steps in order from phenylamine.
Give me a hint
Separate the cold salt-forming step from the warm hydrolysis step.
Compare my reasoning
- Use HNO₂ or NaNO₂ with dilute acid.
- Keep the diazotisation below 10°C to form the diazonium salt.
- Warm the salt with water to produce phenol.
Phenylamine → diazonium salt using nitrous acid below 10°C → warm with water → phenol.
Look for these in your work
- Reagents identified
- Cold condition applied
- Warm conversion placed second
A student writes ‘warm phenylamine with water’ to make phenol. Correct the missing intermediate and reagents.
Give me a hint
Name the two-stage conversion.
Compare my reasoning
- Phenylamine first reacts with nitrous acid or NaNO₂/dilute acid.
- The diazonium salt forms below 10°C.
- That salt is then warmed with water.
Form the diazonium salt from phenylamine below 10°C using HNO₂ or NaNO₂/dilute acid, then warm the salt with water to obtain phenol.
Look for these in your work
- First-stage reagents added
- Intermediate named
- Second stage corrected
Why should the answer distinguish ‘below 10°C’ from ‘warm with water’?
Give me a hint
Each condition belongs to a different stage.
Compare my reasoning
- Below 10°C is for diazonium salt formation.
- Warming with water is for conversion of that salt to phenol.
- Combining the conditions into one step obscures the sequence.
The salt is formed below 10°C first; the isolated or formed diazonium salt is then warmed with water to produce phenol.
Look for these in your work
- Cold stage assigned
- Warm stage assigned
- Sequence explained
Common mistakes
- Warming the mixture before the diazonium salt forms
- Leaving out the below-10°C condition
- Stopping at the diazonium salt when asked for phenol
- Using NaOH in place of the nitrite/dilute-acid diazotisation reagents
What can you explain now?
At what temperature is phenylamine diazotised in the syllabus route?
Compare with the explanation
Below 10°C
The diazonium salt is formed below 10°C.
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Make it stick
What intermediate connects phenylamine to phenol, and which condition belongs to each step?
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.