What is phenylamine and how is it synthesized in A-Level Chemistry?

Phenylamine, also known as aniline, is an aromatic amine with the formula C6H5NH2. In Cambridge International A Levels, it is typically synthesized through the reduction of nitrobenzene using tin and hydrochloric acid, followed by neutralization with sodium hydroxide. This process is a key example of converting a nitro compound to an amine in A-Level Organic Chemistry.

What are azo compounds and how are they formed?

Azo compounds are characterized by the presence of the azo group (-N=N-) linking two aromatic rings. They are commonly formed through a reaction called azo coupling, which involves the reaction of a diazonium salt with a phenol or an aromatic amine. This process is significant in the synthesis of dyes and is an important topic in the study of nitrogen compounds in A-Level Chemistry.

Why are azo compounds important in the dye industry?

Azo compounds are crucial in the dye industry due to their vivid colors and ability to bind strongly to fabrics. The azo group (-N=N-) allows for a wide range of colors by altering the aromatic rings attached to it. This versatility makes azo dyes one of the most important classes of synthetic dyes, a topic often explored in Cambridge International A Levels Chemistry.

How do the properties of phenylamine differ from aliphatic amines?

Phenylamine differs from aliphatic amines in several ways. It has a lower basicity due to the delocalization of the lone pair of electrons on the nitrogen into the aromatic ring, making it less available for protonation. Additionally, phenylamine is less soluble in water compared to aliphatic amines due to its aromatic nature. Understanding these differences is essential for A-Level students studying nitrogen compounds.

What safety precautions should be taken when handling phenylamine in the laboratory?

When handling phenylamine in the laboratory, it is important to wear appropriate personal protective equipment, such as gloves and goggles, due to its toxic and potentially carcinogenic nature. Work in a well-ventilated area or fume hood to avoid inhalation of vapors. Proper storage and disposal according to safety guidelines are also crucial, topics that are emphasized in the practical aspects of Cambridge International A Levels Chemistry.

Why is "Not keeping diazotisation below 10 °C." a common mistake in Phenylamine and azo compounds?

Why it happens: Forgetting the diazonium salt's instability. How to avoid it: Keep below 10 °C — warmer, the diazonium salt decomposes. Source: 9701 Examiner Reports 2022-2024

Why is "Saying phenylamine is a stronger base than ammonia." a common mistake in Phenylamine and azo compounds?

Why it happens: Ignoring ring delocalisation. How to avoid it: The N lone pair is delocalised into the ring → weaker base than NH₃.

Why is "Not linking azo colour to the extended delocalised system." a common mistake in Phenylamine and azo compounds?

Why it happens: Stating 'coloured' with no reason. How to avoid it: Colour arises from the extended conjugation absorbing visible light.

Nitrogen compoundsCambridge International A Level Chemistry 9701

Phenylamine and azo compounds

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Phenylamine and Azo Compounds — Cambridge International AS & A Level Chemistry 9701 (2025-2027 syllabus)

Making phenylamine from nitrobenzene, its weak basicity, diazotisation below 10 °C, coupling to form azo dyes, and why azo compounds are coloured.

What you’ll learn

Mapped to the Cambridge International A Level 9701 syllabus (2025-2027).

34.2 — Describe the formation of phenylamine by the reduction of nitrobenzene.
34.2 — Describe the diazotisation of phenylamine and the coupling reaction to form azo dyes.
34.2 — Explain the colour of azo compounds.

Making phenylamine and its basicity

Reduce nitrobenzene with Sn/HCl then NaOH; the ring delocalisation makes it a weak base.

Phenylamine is made by reducing nitrobenzene, using tin and concentrated hydrochloric acid (with heat), then adding NaOH to release the free amine from its salt: $\text{C}_6\text{H}_5\text{NO}_2 \xrightarrow{\text{Sn / conc HCl, then NaOH}} \text{C}_6\text{H}_5\text{NH}_2$

Phenylamine is a weaker base than ammonia. Its nitrogen lone pair overlaps with (is delocalised into) the benzene ring's π system, so it is less available to accept a proton. (Recall the basicity order: ethylamine > ammonia > phenylamine.)

Nitrobenzene + Sn/conc HCl, then NaOH → phenylamine.
Weaker base than ammonia.
N lone pair delocalised into the ring → less available.

See the full worked example for phenylamine and azo compounds →

Diazotisation

Phenylamine + HNO₂ below 10 °C gives the (unstable) benzenediazonium salt.

Treating phenylamine with nitrous acid (HNO₂) — made in situ from NaNO₂ + HCl — at a temperature below 10 °C forms a diazonium salt: $\text{C}_6\text{H}_5\text{NH}_2 + \text{HNO}_2 + \text{HCl} \rightarrow \text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + 2\text{H}_2\text{O}$

The reaction must be kept below 10 °C because the benzenediazonium ion decomposes (to phenol and N₂) if it warms up. Note that only aromatic amines form a stable diazonium salt this way — an aliphatic amine with HNO₂ does not.

HNO₂ from NaNO₂ + HCl; keep below 10 °C.
Forms benzenediazonium chloride (C₆H₅N₂⁺Cl⁻).
Decomposes if warmed; only aromatic amines work.

See the full worked example for phenylamine and azo compounds →

Coupling and the colour of azo dyes

The diazonium salt couples with a phenol to give a coloured azo dye; colour comes from extended delocalisation.

Adding the benzenediazonium salt to an alkaline solution of a phenol gives a coupling reaction: the diazonium ion substitutes onto the activated ring, forming an azo compound in which two aromatic rings are joined by an –N=N– group. The products are brightly coloured azo dyes.

Why are they coloured? The –N=N– link joins the two aromatic rings into an extended delocalised (conjugated) system. The energy gap between levels in this system is small enough that it absorbs part of visible light; the colour we see is the complement of the absorbed light. Azo dyes make excellent dyes because they are intensely coloured and chemically stable, and the colour can be tuned by changing the groups on the rings.

Diazonium salt + phenol (alkali) → coloured azo compound (–N=N–).
Colour from the extended delocalised system absorbing visible light.
Stable, intense, tunable → good dyes.

See the full worked example for phenylamine and azo compounds →

How it’s examined

Paper 4 asks for the preparation of phenylamine, the diazotisation conditions (and why < 10 °C), the coupling reaction, and the origin of azo colour. Examiner reports flag missing the temperature condition and saying phenylamine is a strong base.

Sources: Cambridge International AS & A Level Chemistry 9701 syllabus (2025-2027), section 34.2; 9701 Examiner Reports 2022-2024; 9701/41 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-20.

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Step-by-step worked examples — Phenylamine and azo compounds

Step-by-step solutions to past-paper-style questions on phenylamine and azo compounds, written exactly the way a tutor would explain them at the board.

1Preparing phenylamine (3 marks)
Getting started• preparation
Question
State the reagents and conditions to make phenylamine from nitrobenzene.
Step-by-step solution
1. Step 1
  Reduce nitrobenzene with tin (Sn) and concentrated HCl, heated.
2. Step 2
  Then add NaOH to liberate the free amine from its salt.
  $\text{C}_6\text{H}_5\text{NO}_2 \xrightarrow{\text{Sn / conc HCl, then NaOH}} \text{C}_6\text{H}_5\text{NH}_2$
Answer
Reduce with Sn + conc HCl (heat), then NaOH → phenylamine.
Examiner tip
Sn/conc HCl reduces –NO₂ to –NH₃⁺; NaOH then frees the amine (–NH₂).
2Why phenylamine is a weak base (3 marks)
Getting started• basicity
Question
Explain why phenylamine is a weaker base than ammonia.
Step-by-step solution
1. Step 1
  The nitrogen lone pair overlaps with the benzene ring's delocalised π system.
2. Step 2
  The lone pair is delocalised into the ring, so it is less available to accept a proton.
3. Step 3
  A less available lone pair → weaker base than ammonia.
Answer
The N lone pair is delocalised into the ring, making it less available to accept H⁺ → weaker base than NH₃.
3Diazotisation (4 marks)
Building confidence• diazotisation
Question
Give the reagents, conditions and equation for converting phenylamine into a diazonium salt.
Step-by-step solution
1. Step 1
  Reagents: nitrous acid (HNO₂) made in situ from NaNO₂ + HCl, kept below 10 °C.
2. Step 2
  Equation:
  $\text{C}_6\text{H}_5\text{NH}_2 + \text{HNO}_2 + \text{HCl} \rightarrow \text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + 2\text{H}_2\text{O}$
3. Step 3
  Below 10 °C because the benzenediazonium salt decomposes if warmed.
Answer
NaNO₂ + HCl (makes HNO₂), < 10 °C; C₆H₅NH₂ → C₆H₅N₂⁺Cl⁻ (benzenediazonium chloride).
Examiner tip
Temperature control (< 10 °C) is essential — warmer, the diazonium salt decomposes.
4Azo coupling (3 marks)
Building confidence• azo coupling
Question
Describe what happens when a benzenediazonium salt is added to an alkaline solution of phenol.
Step-by-step solution
1. Step 1
  The diazonium ion couples with the phenol (an electrophilic substitution on the activated ring).
2. Step 2
  An azo compound forms, with two aromatic rings joined by an –N=N– group.
3. Step 3
  The product is a brightly coloured azo dye (e.g. yellow/orange).
Answer
Coupling gives a coloured azo dye — two rings joined by –N=N– (e.g. (4-hydroxyphenyl)azobenzene).
Examiner tip
The reaction is done in alkali so the phenol is present as the more reactive phenoxide.
5Why azo compounds are coloured (4 marks)
Stretch• colour, dyes
Question
Explain why azo compounds are coloured and why they make good dyes. (4)
Step-by-step solution
1. Step 1
  The –N=N– group links two aromatic rings into an extended delocalised (conjugated) system.
2. Step 2
  This delocalised system absorbs part of visible light (a small energy gap between levels).
3. Step 3
  The colour seen is the complement of the light absorbed.
4. Step 4
  They make good dyes because they are strongly coloured and chemically stable; varying the rings/groups tunes the colour.
Answer
The extended delocalised system (via –N=N–) absorbs visible light; stable, intense colours make good, tunable dyes.
Examiner tip
Colour comes from the extended conjugation; substituents on the rings shift the absorbed wavelength (and so the colour).
6Distinguishing phenylamine reactions (3 marks)
Stretch• reactions
Question
State what phenylamine forms with (a) HCl and (b) HNO₂ below 10 °C.
Step-by-step solution
1. Step 1
  (a) With HCl → a salt (phenylammonium chloride), as it is a base.
2. Step 2
  (b) With HNO₂ (< 10 °C) → a diazonium salt (C₆H₅N₂⁺Cl⁻).
Answer
(a) phenylammonium chloride (salt); (b) benzenediazonium chloride (diazonium salt).
Examiner tip
An aliphatic amine with HNO₂ does NOT give a stable diazonium salt — only aromatic amines do (below 10 °C).

Key Definitions and Keywords — Phenylamine and azo compounds

Definitions to memorise and the exact keywords mark schemes credit for phenylamine and azo compounds answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9701 sitting.

Phenylamine
Examiner keyword
An aromatic amine (C₆H₅NH₂), made by reducing nitrobenzene; a weaker base than ammonia (lone pair delocalised into the ring).
Diazotisation
Examiner keyword
Reaction of an aromatic amine with HNO₂ (NaNO₂ + HCl) below 10 °C to form a diazonium salt (C₆H₅N₂⁺).
Azo coupling
Examiner keyword
Reaction of a diazonium salt with an activated arene (e.g. phenol in alkali) to form a coloured azo compound (–N=N–).
Azo dye
Examiner keyword
A coloured compound with an –N=N– group linking aromatic rings; the extended delocalised system absorbs visible light.

Common Mistakes and Misconceptions — Phenylamine and azo compounds

The traps other students keep falling into on phenylamine and azo compounds questions — taken from recent Cambridge International A Level 9701 examiner reports and mark schemes — and how to avoid them.

✕Not keeping diazotisation below 10 °C.
9701 Examiner Reports 2022-2024
Why it happens
Forgetting the diazonium salt's instability.
How to avoid it
Keep below 10 °C — warmer, the diazonium salt decomposes.
✕Saying phenylamine is a stronger base than ammonia.
Why it happens
Ignoring ring delocalisation.
How to avoid it
The N lone pair is delocalised into the ring → weaker base than NH₃.
✕Not linking azo colour to the extended delocalised system.
Why it happens
Stating 'coloured' with no reason.
How to avoid it
Colour arises from the extended conjugation absorbing visible light.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Phenylamine and azo compounds — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Nitrogen compoundsCambridge International A Level Chemistry 9701

Phenylamine and azo compounds

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next.

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Short Study Notes — Phenylamine and azo compounds

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Phenylamine and Azo Compounds — Cambridge International AS & A Level Chemistry 9701 (2025-2027 syllabus)

Making phenylamine from nitrobenzene, its weak basicity, diazotisation below 10 °C, coupling to form azo dyes, and why azo compounds are coloured.

What you’ll learn

Mapped to the Cambridge International A Level 9701 syllabus (2025-2027).

34.2 — Describe the formation of phenylamine by the reduction of nitrobenzene.
34.2 — Describe the diazotisation of phenylamine and the coupling reaction to form azo dyes.
34.2 — Explain the colour of azo compounds.

Making phenylamine and its basicity

Reduce nitrobenzene with Sn/HCl then NaOH; the ring delocalisation makes it a weak base.

Nitrobenzene + Sn/conc HCl, then NaOH → phenylamine.
Weaker base than ammonia.
N lone pair delocalised into the ring → less available.

See the full worked example for phenylamine and azo compounds →

Diazotisation

Phenylamine + HNO₂ below 10 °C gives the (unstable) benzenediazonium salt.

HNO₂ from NaNO₂ + HCl; keep below 10 °C.
Forms benzenediazonium chloride (C₆H₅N₂⁺Cl⁻).
Decomposes if warmed; only aromatic amines work.

See the full worked example for phenylamine and azo compounds →

Coupling and the colour of azo dyes

The diazonium salt couples with a phenol to give a coloured azo dye; colour comes from extended delocalisation.

Diazonium salt + phenol (alkali) → coloured azo compound (–N=N–).
Colour from the extended delocalised system absorbing visible light.
Stable, intense, tunable → good dyes.

See the full worked example for phenylamine and azo compounds →

How it’s examined

Master this Topic

Worked examples, formulae, definitions and the mistakes examiners flag — everything you need to push from a pass to an A*.

Take this whole topic with you

Download a branded revision sheet — worked examples, formulae, definitions and common mistakes for Phenylamine and azo compounds, ready to print or save as PDF.

Step-by-step worked examples — Phenylamine and azo compounds

Step-by-step solutions to past-paper-style questions on phenylamine and azo compounds, written exactly the way a tutor would explain them at the board.

1Preparing phenylamine (3 marks)
Getting started• preparation
Question
State the reagents and conditions to make phenylamine from nitrobenzene.
Step-by-step solution
1. Step 1
  Reduce nitrobenzene with tin (Sn) and concentrated HCl, heated.
2. Step 2
  Then add NaOH to liberate the free amine from its salt.
  $\text{C}_6\text{H}_5\text{NO}_2 \xrightarrow{\text{Sn / conc HCl, then NaOH}} \text{C}_6\text{H}_5\text{NH}_2$
Answer
Reduce with Sn + conc HCl (heat), then NaOH → phenylamine.
Examiner tip
Sn/conc HCl reduces –NO₂ to –NH₃⁺; NaOH then frees the amine (–NH₂).
2Why phenylamine is a weak base (3 marks)
Getting started• basicity
Question
Explain why phenylamine is a weaker base than ammonia.
Step-by-step solution
1. Step 1
  The nitrogen lone pair overlaps with the benzene ring's delocalised π system.
2. Step 2
  The lone pair is delocalised into the ring, so it is less available to accept a proton.
3. Step 3
  A less available lone pair → weaker base than ammonia.
Answer
The N lone pair is delocalised into the ring, making it less available to accept H⁺ → weaker base than NH₃.
3Diazotisation (4 marks)
Building confidence• diazotisation
Question
Give the reagents, conditions and equation for converting phenylamine into a diazonium salt.
Step-by-step solution
1. Step 1
  Reagents: nitrous acid (HNO₂) made in situ from NaNO₂ + HCl, kept below 10 °C.
2. Step 2
  Equation:
  $\text{C}_6\text{H}_5\text{NH}_2 + \text{HNO}_2 + \text{HCl} \rightarrow \text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- + 2\text{H}_2\text{O}$
3. Step 3
  Below 10 °C because the benzenediazonium salt decomposes if warmed.
Answer
NaNO₂ + HCl (makes HNO₂), < 10 °C; C₆H₅NH₂ → C₆H₅N₂⁺Cl⁻ (benzenediazonium chloride).
Examiner tip
Temperature control (< 10 °C) is essential — warmer, the diazonium salt decomposes.
4Azo coupling (3 marks)
Building confidence• azo coupling
Question
Describe what happens when a benzenediazonium salt is added to an alkaline solution of phenol.
Step-by-step solution
1. Step 1
  The diazonium ion couples with the phenol (an electrophilic substitution on the activated ring).
2. Step 2
  An azo compound forms, with two aromatic rings joined by an –N=N– group.
3. Step 3
  The product is a brightly coloured azo dye (e.g. yellow/orange).
Answer
Coupling gives a coloured azo dye — two rings joined by –N=N– (e.g. (4-hydroxyphenyl)azobenzene).
Examiner tip
The reaction is done in alkali so the phenol is present as the more reactive phenoxide.
5Why azo compounds are coloured (4 marks)
Stretch• colour, dyes
Question
Explain why azo compounds are coloured and why they make good dyes. (4)
Step-by-step solution
1. Step 1
  The –N=N– group links two aromatic rings into an extended delocalised (conjugated) system.
2. Step 2
  This delocalised system absorbs part of visible light (a small energy gap between levels).
3. Step 3
  The colour seen is the complement of the light absorbed.
4. Step 4
  They make good dyes because they are strongly coloured and chemically stable; varying the rings/groups tunes the colour.
Answer
The extended delocalised system (via –N=N–) absorbs visible light; stable, intense colours make good, tunable dyes.
Examiner tip
Colour comes from the extended conjugation; substituents on the rings shift the absorbed wavelength (and so the colour).
6Distinguishing phenylamine reactions (3 marks)
Stretch• reactions
Question
State what phenylamine forms with (a) HCl and (b) HNO₂ below 10 °C.
Step-by-step solution
1. Step 1
  (a) With HCl → a salt (phenylammonium chloride), as it is a base.
2. Step 2
  (b) With HNO₂ (< 10 °C) → a diazonium salt (C₆H₅N₂⁺Cl⁻).
Answer
(a) phenylammonium chloride (salt); (b) benzenediazonium chloride (diazonium salt).
Examiner tip
An aliphatic amine with HNO₂ does NOT give a stable diazonium salt — only aromatic amines do (below 10 °C).

Key Definitions and Keywords — Phenylamine and azo compounds

Phenylamine
Examiner keyword
An aromatic amine (C₆H₅NH₂), made by reducing nitrobenzene; a weaker base than ammonia (lone pair delocalised into the ring).
Diazotisation
Examiner keyword
Reaction of an aromatic amine with HNO₂ (NaNO₂ + HCl) below 10 °C to form a diazonium salt (C₆H₅N₂⁺).
Azo coupling
Examiner keyword
Reaction of a diazonium salt with an activated arene (e.g. phenol in alkali) to form a coloured azo compound (–N=N–).
Azo dye
Examiner keyword
A coloured compound with an –N=N– group linking aromatic rings; the extended delocalised system absorbs visible light.

Common Mistakes and Misconceptions — Phenylamine and azo compounds

✕Not keeping diazotisation below 10 °C.
9701 Examiner Reports 2022-2024
Why it happens
Forgetting the diazonium salt's instability.
How to avoid it
Keep below 10 °C — warmer, the diazonium salt decomposes.
✕Saying phenylamine is a stronger base than ammonia.
Why it happens
Ignoring ring delocalisation.
How to avoid it
The N lone pair is delocalised into the ring → weaker base than NH₃.
✕Not linking azo colour to the extended delocalised system.
Why it happens
Stating 'coloured' with no reason.
How to avoid it
Colour arises from the extended conjugation absorbing visible light.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Phenylamine and azo compounds — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Phenylamine and azo compounds

Short Study Notes in the form of Slides

Short Study Notes — Phenylamine and azo compounds

Detailed Notes

What you’ll learn

How it’s examined

1Preparing phenylamine (3 marks)

2Why phenylamine is a weak base (3 marks)

3Diazotisation (4 marks)

4Azo coupling (3 marks)

5Why azo compounds are coloured (4 marks)

6Distinguishing phenylamine reactions (3 marks)

Phenylamine

Diazotisation

Azo coupling

Azo dye

✕Not keeping diazotisation below 10 °C.

✕Saying phenylamine is a stronger base than ammonia.

✕Not linking azo colour to the extended delocalised system.

Practice questions

Past paper style quiz

Assess your understanding

Phenylamine and azo compounds — frequently asked questions

Phenylamine and azo compounds

Short Study Notes in the form of Slides

Short Study Notes — Phenylamine and azo compounds

Detailed Notes

What you’ll learn

How it’s examined

1Preparing phenylamine (3 marks)

2Why phenylamine is a weak base (3 marks)

3Diazotisation (4 marks)

4Azo coupling (3 marks)

5Why azo compounds are coloured (4 marks)

6Distinguishing phenylamine reactions (3 marks)

Phenylamine

Diazotisation

Azo coupling

Azo dye

✕Not keeping diazotisation below 10 °C.

✕Saying phenylamine is a stronger base than ammonia.

✕Not linking azo colour to the extended delocalised system.

Practice questions

Past paper style quiz

Assess your understanding

Phenylamine and azo compounds — frequently asked questions