Which of the following amine forms a stable diazonium salt?

Primary aromatic amines

The correct answer is Option (3) → Primary aromatic amines

• Primary aromatic amines (e.g., aniline, C₆H₅NH₂) react with NaNO₂/HCl at 0–5 °C to form stable diazonium salts (Ar–N₂⁺ Cl⁻) at low temperature. These are useful in synthesis (Sandmeyer, azo dyes, etc.).

Why others do NOT form stable diazonium salts:

• Primary aliphatic amines (R–NH₂): Form diazonium salts (R–N₂⁺), but they are highly unstable and immediately decompose to N₂ gas + carbocation.
• Secondary aliphatic amines (R₂NH): Form nitrosamines (R₂N–NO), not diazonium salts.
• Secondary aromatic amines (Ar–NHR): Also form nitrosamines, not diazonium salts.

Only primary aromatic amines (like C₆H₅NH₂) give stable (at low temp) diazonium salts.