Overall reaction
C₆H₅N₂⁺ + C₆H₅O⁻ (pH 9–10, 0–5 °C) → p-HO–C₆H₄–N=N–C₆H₅ (orange)
A diazonium ion attacks an activated ring: azo dyes.
Name reaction · mechanism
Azo Coupling Reaction
In azo coupling an arenediazonium ion acts as a weak electrophile towards strongly activated rings — phenoxides and anilines. Attack at the para carbon (ortho if para is blocked) followed by loss of the ring proton gives an azo compound Ar–N=N–Ar′, whose extended conjugation makes it coloured.
Mechanism step by step
- Electrophilic attack. Phenoxide attacks through its para carbon
- Rearomatisation. Hydroxide removes the para C–H
Each step is checked electron by electron: the curly arrows give exactly the structures drawn.
Key points
- The diazonium salt is made from aniline with NaNO₂/HCl at 0–5 °C (diazotisation).
- A weak electrophile: only phenoxides and anilines react, mainly para.
- Phenols couple at pH 9–10; anilines at pH 4–5.
- The extended conjugation (Ar–N=N–Ar) absorbs visible light: methyl orange, Sudan dyes.
For JEE and NEET
- Phenol gives p-hydroxyazobenzene (orange); aniline gives p-aminoazobenzene (aniline yellow); β-naphthol gives an orange-red dye.
- Electrophilic aromatic substitution; the electrophile is the terminal N of Ar–N₂⁺.
- Phenols couple best as phenoxides (pH 9–10); too basic turns the diazonium salt into diazotate.
- Used for dyes and indicators such as methyl orange.
FAQ
Why does coupling happen at the para position?
The para carbon is activated by O⁻ (or NH₂) and is less hindered than ortho; ortho attack occurs only if para is blocked.
Why is the pH important?
Phenol must be ionised to phenoxide to be reactive enough, but at very high pH the diazonium ion is converted to an unreactive diazotate.
Why are azo compounds coloured?
The –N=N– group links two aromatic rings into one long conjugated system that absorbs visible light.
Draw it in MolDraw
The button above opens the complete figure — overall reaction, numbered mechanism and key points — as ordinary editable structures. Change substituents, recolour atoms, or export a PNG, SVG or chemfig code for notes and slides. The same figure is in the Templates panel under Name reactions.