Answer: Dimethylamine (secondary amine).
- A Dimethylamine (secondary amine)
- B Aniline, which forms benzenediazonium chloride at 0-5°C
- C p-Toluidine, which forms a stable diazonium salt below 5°C
- D Sulfanilic acid, which forms an internal diazonium zwitterion
Correct answer: A. Dimethylamine (secondary amine)
Explanation: Diazotisation requires a primary amine; secondary amines react with HNO<sub>2</sub> to form N-nitroso compounds, not diazonium salts.
In aniline, the nitrogen's lone pair is pulled into the benzene ring through resonance, leaving less electron density available to accept a proton - which is why aniline is a much weaker base than methylamine, where the lone pair is fully available.
Concept context
Nitrogen-containing organic compounds. Covers classification (primary, secondary, tertiary), basicity comparison, preparation methods, and reactions including diazotization and coupling, key for understanding dyes and pharmaceuticals.