Physical Properties of Diazonium Salts
NCERT Reference: Chapter 9 – Amines – Page 251
Quick Notes
- Benzenediazonium chloride is a colourless crystalline solid.
- Soluble in water and stable in cold aqueous solution.
- Decomposes readily when warmed or in the dry state.
- Benzenediazonium fluoroborate is insoluble in water and more stable, can be stored.
Full Notes
Diazonium salts, though widely used in organic synthesis, are thermally sensitive and must be handled with care. Their physical properties influence how they are used and stored in laboratory conditions.
Physical State and Appearance
- Benzenediazonium chloride, a typical diazonium salt, appears as a colourless crystalline solid
- It is normally used immediately after preparation due to its limited stability
Solubility
- Soluble in cold water: This allows diazonium salts to participate in aqueous reactions.
- However, solubility and reactivity also make them unstable in warm or neutral water, where they decompose rapidly, liberating nitrogen gas and yielding phenols or other derivatives depending on the conditions.
Thermal Stability
- Diazonium salts are unstable in the dry state and can explode or decompose violently if heated
- Hence, they are generally not stored and are prepared fresh for immediate use
Stability of Fluoroborate Salts
- Benzenediazonium fluoroborate (C6H5N2+BF4−) is insoluble in water and more thermally stable
- It can be isolated as a solid and stored for a limited period
- This salt provides a safer way to use diazonium chemistry in practical synthesis
Key Point
The physical instability of most diazonium salts underlines their role as intermediates, not shelf-stable reagents. Their solubility and sensitivity require controlled conditions for use, particularly cold temperatures and aqueous environments.
Summary
- Benzenediazonium chloride is a colourless crystalline solid that is soluble in cold water.
- Diazonium salts decompose readily when warm or dry so are used immediately after preparation.
- Benzenediazonium fluoroborate is water-insoluble and more stable which makes it suitable for limited storage.