Physical properties of diazonium salts
NCERT Class 12 Chemistry Chapter 9 Section 9.8
Quick answer
Benzenediazonium chloride is a colourless salt, readily water-soluble and comparatively persistent in cold solution; it hydrolyses on warming and is hazardous when dry. The fluoroborate is much less water-soluble and more persistent at room temperature. That relative stability does not make isolated diazonium salts safe to dry or heat.
With your textbook
NCERT Chemistry Part II (Class 12), Chapter 9 “Amines”, Section 9.8 “Physical Properties”, page 275.
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What are diazonium salts like?
Physical properties of diazonium salts
Benzenediazonium chloride is a colourless crystalline solid that dissolves readily in water, is stable in the cold, reacts with water when warmed and decomposes easily when dry; benzenediazonium fluoroborate, by contrast, is insoluble in water and stable at room temperature (NCERT Section 9.8, page 275). NCERT puts it in two sentences:
Benzenediazonium chloride is a colourless crystalline solid. It is readily soluble in water and is stable in cold but reacts with water when warmed.
NCERT Chemistry Part II (Class 12), Section 9.8, page 275
In plain words: the chloride is a salt that behaves like one, dissolving in water, but it is fragile. Keep it cold and in solution, and use it quickly. The fluoroborate is insoluble and stable at room temperature, as NCERT says, so it can be isolated; that compares it with the chloride and is not an instruction to isolate or store it (our safety advice).
Colourless crystalline solids
- Benzenediazonium chloride is a colourless crystalline solid (NCERT page 275).
- It is an ionic salt, C6H5N2+Cl−, made from aniline by diazotisation at 273–278 K (NCERT page 274).
Solubility and stability
- Benzenediazonium chloride is readily soluble in water, is stable in the cold but reacts with water when warmed, and decomposes easily in the dry state (NCERT page 275).
- If the temperature of the solution is allowed to rise to 283 K, the salt is hydrolysed to phenol (NCERT page 276); 283 K is not a sharp threshold (reasoned): C6H5N2+Cl− + H2O → C6H5OH + N2 + HCl.
- Benzenediazonium fluoroborate is insoluble in water and stable at room temperature (NCERT page 275). So when fluoroboric acid is added to a solution of the chloride, the less-soluble fluoroborate precipitates; its thermal decomposition can give an aryl fluoride (NCERT page 275), a route used in preparing haloarenes.
| Salt | In water | Stability |
|---|---|---|
| benzenediazonium chloride | readily soluble | stable in the cold; reacts with water when warmed; decomposes easily when dry |
| benzenediazonium fluoroborate | insoluble | stable at room temperature |
Solved examples
Example 1: Why use the fluoroborate?
Why is benzenediazonium fluoroborate, not the chloride, isolated and heated to make fluorobenzene?
Answer: The fluorine comes from the BF4− ion: heated, the dry fluoroborate decomposes, C6H5N2+BF4− → C6H5F + BF3 + N2, while the chloride has no fluorine to give (reasoned from NCERT page 275). Being insoluble in water (NCERT page 275), it can be isolated, unlike the soluble chloride. Our safety advice: NCERT’s “stable at room temperature” compares it with the chloride, which “decomposes easily in the dry state” (page 275); it is not advice to dry, scrape or heat a diazonium salt outside a supervised procedure.
Example 2: Mass of the fluoroborate
What mass of benzenediazonium fluoroborate, C6H5N2+BF4−, can 0.010 mol of benzenediazonium chloride give at most? (C = 12, H = 1, N = 14, B = 10.8, F = 19.)
Answer: One mole of chloride gives one mole of fluoroborate. M = 72 + 5 + 28 + 10.8 + 76 = 191.8 g mol−1, so 0.010 × 191.8 = 1.92 g (reasoned from NCERT page 275).
Common mistakes to avoid
Common mistake
Storing benzenediazonium chloride as a dry solid
It decomposes easily in the dry state; it is kept cold, in solution, and used at once.
Common mistake
Expecting all diazonium salts to dissolve in water
The chloride does, but the fluoroborate is insoluble, which is why it precipitates.
Quick revision summary
- Benzenediazonium chloride: colourless crystalline solid; readily soluble in water; stable in the cold; reacts with water when warmed (phenol at 283 K); decomposes easily when dry.
- Benzenediazonium fluoroborate: water insoluble and stable at room temperature (NCERT), compared with the chloride; still not safe to dry or heat outside a supervised procedure. Thermal decomposition gives an aryl fluoride.
Formula sheet
- CHNCl: colourless, soluble in water, unstable when warm or dry
- CHNBF: water insoluble, stable at room temperature (NCERT); handle isolated salt with care
- CHNCl + HO (283 K) → CHOH (phenol) + N + HCl
Questions students ask
What is the structure of benzenediazonium chloride?
It consists of CHN and Cl ions. A useful major Lewis representation is CH–N≡N, with formal positive charge on the nitrogen bonded to the ring and an approximately linear C–N–N group. Resonance distributes charge in the arenediazonium ion. Formation of stable N helps explain the usefulness of the group in replacement reactions.
Is dry benzenediazonium chloride explosive?
Yes. Dry diazonium salts can undergo violent decomposition, and greater persistence of a tetrafluoroborate does not remove the hazard. Firth and Fairlamb’s 2020 primary report describes violent decomposition of an aryl diazonium tetrafluoroborate; further thermal studies also show structure-dependent hazards. Keep the school-level chemistry focused on freshly prepared dilute cold solutions under an approved protocol; do not isolate, dry or heat a diazonium salt as an informal experiment.
Why is benzenediazonium fluoroborate more stable than benzenediazonium chloride?
Counter-ion identity affects solubility, crystal packing and decomposition pathways. BF is weakly coordinating and less nucleophilic than chloride, which helps make many arenediazonium tetrafluoroborates more isolable. Stability still depends on the particular cation, solvent, concentration and temperature; weak coordination alone does not prove that an isolated salt is safe.
Practice questions
Try each question first, then open the answer to check your working.
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Q1. Give two properties of benzenediazonium chloride that explain why it is used in cold solution.
Show answerHide answer
It reacts with water when warmed and decomposes easily in the dry state; it is stable in the cold and readily soluble in water (NCERT page 275).
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Q2. How do benzenediazonium chloride and benzenediazonium fluoroborate differ in water?
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The chloride is readily soluble in water; the fluoroborate is insoluble (NCERT page 275).