Azo Dye Test: Identification of Aromatic Amines | Chemca

Organic Chemistry

Azo Dye Test: Identification of Aromatic Amines

By Chemca Editorial Team • Last Updated: January 2026 • 9 min read

The Azo Dye Test is a confirmatory test for Primary Aromatic Amines (like Aniline). It involves the conversion of the amine into a diazonium salt, which then undergoes a coupling reaction with a phenol or another amine to form a brightly colored azo compound (dye).

1. General Procedure

The test consists of two distinct steps performed at low temperature ($0-5^\circ C$).

Diazotization: Conversion of amine to diazonium salt.
Coupling: Reaction with a coupling agent (usually $\beta$-Naphthol) to form the dye.

2. Step 1: Diazotization

The primary aromatic amine is dissolved in dilute HCl and cooled to $0-5^\circ C$. An ice-cold solution of Sodium Nitrite ($NaNO_2$) is added.

$$ Ar-NH_2 + NaNO_2 + 2HCl \xrightarrow{0-5^\circ C} \underbrace{Ar-N_2^+ Cl^-}_{\text{Diazonium Salt}} + NaCl + 2H_2O $$

Note: Temperature must be kept low to prevent the decomposition of the unstable diazonium salt into phenol and nitrogen gas.

3. Step 2: Coupling Reaction

The cold diazonium salt solution is added to a cold alkaline solution of $\beta$-Naphthol (2-Naphthol).

Reaction with $\beta$-Naphthol

$$ Ph-N_2^+Cl^- + \beta\text{-Naphthol} \xrightarrow{NaOH, \ pH \ 9-10} \underbrace{\text{Orange-Red Dye}}_{\text{1-Phenylazo-2-naphthol}} + HCl $$

Observation: Formation of a brilliant Orange-Red dye indicates a positive test for primary aromatic amines.

4. Mechanism: Electrophilic Aromatic Substitution

The reaction is an Electrophilic Aromatic Substitution (EAS).

Electrophile: The Diazonium ion ($Ph-N \equiv N^+$) is a weak electrophile.
Nucleophile: The activated aromatic ring of Phenol or Amine (Coupling agent).
Position: Coupling occurs at the Para position (sterically less hindered). If Para is blocked, it occurs at Ortho.

5. pH Dependence (Crucial)

The pH of the medium determines the reactive species.

Coupling with Phenols

Condition: Slightly Alkaline (pH 9-10).
Reason: Alkali converts Phenol ($PhOH$) to Phenoxide ion ($PhO^-$), which is a much stronger activator/nucleophile than phenol itself, necessary for reaction with the weak diazonium electrophile.

Coupling with Amines

Condition: Slightly Acidic (pH 4-5).
Reason: In strong acid, the amine converts to ammonium ion (deactivating). In strong base, the diazonium ion converts to diazotate (non-electrophilic). Weak acid ensures the amine is unprotonated (active) and diazonium is stable.

6. Distinguishing Amine Classes

Amine Type	Reaction with $HNO_2$	Observation
Primary Aromatic	Forms Stable Diazonium Salt	Forms Dye on coupling
Primary Aliphatic	Unstable Diazonium Salt	Rapid $N_2$ gas evolution (Effervescence), forms Alcohol
Secondary	Forms N-Nitrosoamine	Yellow Oily Layer (Liebermann Nitroso Test)
Tertiary Aromatic	C-Nitrosation (EAS)	Green solid (p-Nitroso derivative)

Azo Dye Quiz

Test your concepts on Amine identification. 10 MCQs with explanations.

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