Form an azo compound from phenol and a diazonium salt
Use phenol in aqueous sodium hydroxide with a diazonium salt to form an azo compound containing –N=N–.
Section 32 · Hydroxy compounds
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
What linkage identifies an azo compound?
What you will learn
- Recall the coupling conditions for phenol and diazonium salts
- Identify the azo linkage
- Recognise azo compounds as products of aromatic coupling
Phenoxide couples with a diazonium ion
Phenol in aqueous NaOH reacts with a diazonium salt to form an azo compound. Sodium hydroxide converts phenol partly to phenoxide, activating the aromatic ring for coupling.
The defining azo linkage is –N=N– between two aromatic groups. A typical coupling places the azo group on an activated ring position; use the product pattern or position specified in a question.
Recognise the product family
Azo compounds contain the N=N group linking aromatic fragments and many are coloured. The syllabus outcome is to describe formation and identify the azo group; do not assume every possible coupling product has the same shade.
Keep the reagents together: diazonium salt + phenol in NaOH(aq) → azo compound. The alkaline medium is part of the stated conditions.
Pause and explain
Which condition is specified for coupling phenol with a diazonium salt?
Worked example
What product type forms when a diazonium salt reacts with phenol in NaOH(aq)?
Show the worked solution
- NaOH forms some phenoxide and activates the ring.
- The diazonium species couples with the aromatic system.
- The product contains –N=N–.
Answer An azo compound forms, containing an –N=N– linkage between aromatic groups.
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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.
Describe the product feature that confirms phenol has coupled with a diazonium salt.
Give me a hint
Name the group created between the aromatic fragments.
Compare my reasoning
- A diazonium reagent reacts with phenol in NaOH(aq).
- The coupled product contains nitrogen from the diazonium ion.
- Identify the N=N azo linkage.
The product is an azo compound containing an –N=N– link between aromatic groups.
Look for these in your work
- Reagent context stated
- Product family named
- Azo bond identified
Why is NaOH included in the phenol coupling conditions?
Give me a hint
Connect base to phenol's acid-base behavior and ring activation.
Compare my reasoning
- NaOH removes some phenol O–H protons.
- Phenoxide is formed.
- The activated aromatic system couples with the diazonium species.
NaOH forms phenoxide, which activates the phenol ring for coupling with the diazonium salt.
Look for these in your work
- Acid-base step stated
- Phenoxide identified
- Coupling role explained
A student draws an –NH–NH– group in the product and calls it an azo compound. What bond must be corrected?
Give me a hint
Recall the defining functional group.
Compare my reasoning
- Azo compounds contain N=N.
- The nitrogen atoms are linked by a double bond.
- Replace the single N–N representation with –N=N–.
The azo group is –N=N–; the product should show a nitrogen–nitrogen double bond, not –NH–NH– as its defining linkage.
Look for these in your work
- Correct linkage stated
- Bond order corrected
- Product family retained
Common mistakes
- Calling the product an amide because it contains nitrogen
- Omitting NaOH(aq)
- Writing –NH–NH– instead of –N=N–
- Claiming the reaction forms an alkene
What can you explain now?
What linkage identifies an azo compound?
Compare with the explanation
–N=N–
The azo group is a nitrogen–nitrogen double bond.
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Make it stick
Which reagents form an azo compound from phenol, and what linkage defines the product?
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