# Phenylamine and azo compounds (diazotisation, coupling)

> Chemistry · CIE A-Level 9701
> Source: https://www.owlsprep.com/study/cie-9701-u23-phenylamine-and-azo-compounds/

This module covers phenylamine properties, diazotisation reaction conditions, azo coupling mechanisms, and exam-focused product prediction for CIE A-Level 9701 Paper 4.

**Prerequisites:** [Aromatic amine basicity and nucleophilicity](https://www.owlsprep.com/study/cie-9701-u23-introduction-to-amines/); [Electrophilic substitution of benzene rings](https://www.owlsprep.com/study/cie-9701-u22-electrophilic-substitution-aromatic/)

## Learning objectives

- Recall the structure and key physical properties of phenylamine
- Describe the full diazotisation reaction conditions to form stable benzenediazonium salts
- Explain azo coupling as an electrophilic substitution reaction and predict products
- Outline common industrial and laboratory uses of azo compounds

## Structure and Physical Properties of Phenylamine

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.

*Notation:* \ce{C6H5NH2}

**Worked example:** 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. $$K_b \text{ of phenylamine} = 4.0 \times 10^{-10} \ll K_b \text{ of ethylamine} = 5.6 \times 10^{-4}$$

**Check your understanding**

1. 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

   *Why:* Pure unoxidised phenylamine is a colourless oily liquid that darkens only on contact with air.

## Diazotisation Reaction

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.

$$\ce{C6H5NH2 + NaNO2 + 2HCl -> C6H5N2+Cl- + NaCl + 2H2O}$$

> **Critical Exam Condition**
>
> Temperature control is non-negotiable: above 5°C, benzenediazonium chloride decomposes immediately to form phenol and release toxic nitrogen gas.

**Worked example:** 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.

*Calculator:* forbidden

## Azo Coupling Reactions

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.

**Exam command terms**

CIE uses specific command terms for this topic with strict marking rules:

- **Draw the azo product** — You must show the trans configuration of the -N=N- group to get full marks

- **State the conditions** — You must specify both temperature range and pH, no partial marks for missing values

**Worked example:** 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.

## Uses of Azo Compounds

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 |

## Common pitfalls

- **Wrong:** Performing diazotisation at room temperature
  - Why it fails: Benzenediazonium chloride decomposes rapidly above 5°C to form phenol and release toxic nitrogen gas, destroying the reactive intermediate
  - Correct: Strictly maintain a 0-5°C reaction temperature using an ice-water bath at all stages
- **Wrong:** Using concentrated HCl for diazotisation
  - Why it fails: Excess high concentration HCl fully protonates phenylamine, reducing its nucleophilicity and slowing the reaction
  - Correct: Use dilute HCl with a 1:1 mole ratio relative to sodium nitrite
- **Wrong:** Drawing the azo -N=N- group in cis configuration
  - Why it fails: CIE mark schemes exclusively accept the more stable trans configuration for full marks
  - Correct: Always draw the two aromatic groups on opposite sides of the N=N double bond
- **Wrong:** Coupling benzenediazonium chloride with phenol in acidic conditions
  - Why it fails: Neutral phenol is not a sufficiently activated nucleophile to undergo electrophilic substitution
  - Correct: Use mild alkaline NaOH conditions to deprotonate phenol to the more reactive phenoxide ion
- **Wrong:** Storing benzenediazonium salt for later use
  - Why it fails: It is highly unstable and will decompose spontaneously even at cool temperatures
  - Correct: Prepare the benzenediazonium salt in situ immediately before the coupling reaction

## 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 |

## 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.

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