Azo coupling: practice and mechanism
The diazonium ion is a weak electrophile and only attacks very electron-rich rings (phenols, dimethylanilines), preferably at para. The N=N group stays: an azo dye forms.
Example
The diazonium ion is a weak electrophile and only attacks very electron-rich rings: phenols (as phenoxide) and dialkylanilines. It couples at the para position, giving an azo compound (Ar–N=N–Ar), usually a dye.
Mechanism step by step
- 1
The aryl diazonium ion (Ar–N≡N⁺) is made beforehand in the cold from aniline, NaNO₂ and HCl.
- 2
The very electron-rich ring (phenoxide or dimethylaniline) attacks the terminal N atom, preferably at para (SEAr).
- 3
Loss of H⁺ restores aromaticity: Ar–N=N–Ar forms, a coloured azo dye.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
Often confused with: Sandmeyer reaction (chloride) · Nitration (SEAr)
Frequently asked questions
Why are azo compounds coloured?
The N=N group links two aromatic rings into a long conjugated π system that absorbs visible light (yellow to red).
What pH is needed?
Weakly basic for phenols (the phenoxide is richer), weakly acidic for amines. Too basic and the diazonium ion decomposes.