Phenylamine and azo compounds (diazotisation, coupling)
Chemistry· 12 min read
1. Structure and Physical Properties of Phenylamine★★☆☆☆⏱ 3 min
Phenylamine (C₆H₅NH₂) is the simplest primary aromatic amine, with an amine group directly bonded to a benzene ring. The lone pair on the nitrogen atom delocalises partially into the benzene π system, making phenylamine a weaker base than aliphatic amines like ethylamine.
Phenylamine
A colourless oily liquid at room temperature that darkens on exposure to air due to oxidation, and is sparingly soluble in water.
Explain why phenylamine is less basic than ethylamine in aqueous solution.
- 1
First, compare the availability of the nitrogen lone pair for protonation.
- 2
In phenylamine, the nitrogen lone pair delocalises into the benzene ring, reducing its electron density.
- 3
In ethylamine, the electron-donating ethyl group increases nitrogen lone pair density, making it more available to accept H⁺.
- 4
Which of the following is a physical property of pure phenylamine?
Strongly alkaline pH in water
Colourless oily liquid
Fully miscible with water
Solid at 25°C
Reveal answer
Colourless oily liquid —Pure unoxidised phenylamine is a colourless oily liquid that darkens only on contact with air.
2. Diazotisation Reaction★★★☆☆⏱ 4 min
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Diazotisation is the reaction of phenylamine with nitrous acid (HNO₂) to form a benzenediazonium salt. Nitrous acid is unstable, so it is generated in situ by reacting sodium nitrite (NaNO₂) with a strong dilute acid like HCl.
Outline the laboratory procedure to prepare a stable sample of benzenediazonium chloride from phenylamine.
- 1
Add phenylamine to dilute hydrochloric acid in a flask, and cool the mixture to 0-5°C using an ice bath.
- 2
Add a stoichiometric amount of sodium nitrite solution slowly, with constant stirring, maintaining temperature below 5°C at all times.
- 3
Continue stirring for 10 minutes after full addition to ensure complete reaction, and use the product immediately for coupling.
3. Azo Coupling Reactions★★★★☆⏱ 3.5 min
Azo coupling is an electrophilic aromatic substitution reaction, where the positively charged diazonium ion acts as a weak electrophile, attacking highly activated aromatic rings like phenol or phenylamine. The reaction almost exclusively substitutes at the 4 (para) position relative to the activating group.
Predict the product formed when benzenediazonium chloride reacts with phenol in alkaline conditions.
- 1
First, identify the activated site on phenol: the para (4) position relative to the -OH group is the most electron rich.
- 2
The electrophilic diazonium ion attacks the para position, losing a proton to restore aromaticity.
- 3
The final product is p-hydroxyazobenzene, a bright orange crystalline azo dye with the trans -N=N- group linking the two benzene rings.
4. Uses of Azo Compounds★★☆☆☆⏱ 1.5 min
Azo compounds are the largest and most widely used class of synthetic dyes, accounting for over 60% of all commercial textile dyes. Their intense colour comes from the extended delocalised π system across the two aromatic rings and the azo group, which absorbs visible light.
Azo Compound | Common Use | Characteristic Colour |
|---|---|---|
Methyl Orange | pH indicator for acid-base titrations | Red in acid, yellow in alkali |
p-hydroxyazobenzene | Orange textile dye | Bright orange |
p-aminoazobenzene | Yellow food colourant | Pale yellow |
5. Common Pitfalls
Wrong move:
Performing diazotisation at room temperature
Why:
Benzenediazonium chloride decomposes rapidly above 5°C to form phenol and release toxic nitrogen gas, destroying the reactive intermediate
Correct move:
Strictly maintain a 0-5°C reaction temperature using an ice-water bath at all stages
Wrong move:
Using concentrated HCl for diazotisation
Why:
Excess high concentration HCl fully protonates phenylamine, reducing its nucleophilicity and slowing the reaction
Correct move:
Use dilute HCl with a 1:1 mole ratio relative to sodium nitrite
Wrong move:
Drawing the azo -N=N- group in cis configuration
Why:
CIE mark schemes exclusively accept the more stable trans configuration for full marks
Correct move:
Always draw the two aromatic groups on opposite sides of the N=N double bond
Wrong move:
Coupling benzenediazonium chloride with phenol in acidic conditions
Why:
Neutral phenol is not a sufficiently activated nucleophile to undergo electrophilic substitution
Correct move:
Use mild alkaline NaOH conditions to deprotonate phenol to the more reactive phenoxide ion
Wrong move:
Storing benzenediazonium salt for later use
Why:
It is highly unstable and will decompose spontaneously even at cool temperatures
Correct move:
Prepare the benzenediazonium salt in situ immediately before the coupling reaction
6. Quick Reference Cheatsheet
Reaction | Required Conditions | Key Product | Observation |
|---|---|---|---|
Diazotisation of phenylamine | 0-5°C, dilute HCl, NaNO₂ | Benzenediazonium chloride | Pale yellow solution |
Coupling with phenol | Alkaline NaOH, <10°C | p-hydroxyazobenzene | Bright orange precipitate |
Coupling with phenylamine | Weakly acidic, <10°C | p-aminoazobenzene | Yellow crystalline solid |
When this came up on past exams
AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2025 · 42
Diazotisation reaction conditions
- 2024 · 41
Azo coupling product prediction
- 2023 · 43
Azo dye structure drawing
What's Next
Mastering diazotisation and azo coupling completes your study of aromatic amine reactivity, and is a high-weight topic for A2 Paper 4 organic synthesis and structural elucidation questions. You will frequently see azo compounds appear as intermediate products in multi-step synthesis pathways linking benzene, amines, and phenol derivatives. This content also builds directly to your upcoming study of organic dye chemistry and applied industrial organic processes, which are regularly tested in extended response questions. Ensure you can draw all reaction mechanisms and recall exact temperature conditions to avoid losing easy marks in exams.