Diazonium salts: preparation, properties, reactions and use in synthesis

What diazonium salts are

Diazonium salts have the general formula RN₂⁺X⁻, where X⁻ may be Cl⁻, Br⁻, HSO₄⁻ or BF₄⁻. The -N₂⁺ group is the diazonium group. C₆H₅N₂⁺Cl⁻ is benzenediazonium chloride, and C₆H₅N₂⁺HSO₄⁻ is benzenediazonium hydrogensulphate. Primary aliphatic amines give alkyldiazonium salts that are very unstable. Primary aromatic amines give arenediazonium salts that are stable for a short time in solution at 273 to 278 K. Their extra stability comes from resonance: the positive charge is spread over the benzene ring. Physical properties: benzenediazonium chloride is a colourless crystalline solid. It dissolves readily in water and is stable in the cold, but reacts with water when warmed. It decomposes easily when dry, so it is made in solution and used at once. Benzenediazonium fluoroborate is insoluble in water and stable at room temperature.

Preparation: diazotisation

1. 1
   Dissolve aniline in excess dilute HCl and cool the solution in an ice bath to 273 to 278 K.
2. 2
   NaNO₂ + HCl → HNO₂ + NaCl

   Add cold NaNO₂ solution slowly, with stirring; nitrous acid forms in the mixture. Keep the temperature below 278 K
3. 3★
   C₆H₅NH₂ + NaNO₂ + 2HCl → (273 to 278 K) C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O

   The salt is not stored; it is used at once

★ key step

Reactions in which N₂ is replaced

1. 1★
   Sandmeyer: ArN₂⁺Cl⁻ → (Cu₂Cl₂/HCl) ArCl + N₂; ArN₂⁺Br⁻ → (Cu₂Br₂/HBr) ArBr + N₂; ArN₂⁺Cl⁻ + CuCN → (KCN) ArCN + N₂ + CuCl

   Copper(I) salts bring in Cl, Br or CN; Cu₂Cl₂ is copper(I) chloride, also written CuCl. In the cyanide reaction CuCN is used up, so CuCl appears on the right; your textbook writes this equation without the CuCl
2. 2
   Gattermann: ArN₂⁺X⁻ → (Cu powder, HX) ArX + N₂

   X = Cl or Br. Copper powder acts as a catalyst, so the equation balances without it; your textbook also shows CuX among the products. The yield is lower than in the Sandmeyer reaction
3. 3★
   Iodide: ArN₂⁺Cl⁻ + KI → ArI + KCl + N₂

   Iodine is hard to put on a benzene ring directly, so this is how iodobenzene is made
4. 4
   Fluoride: ArN₂⁺Cl⁻ + HBF₄ → ArN₂⁺BF₄⁻ + HCl; then ArN₂⁺BF₄⁻ → (heat) ArF + BF₃ + N₂

   The fluoroborate precipitates; heating it gives the aryl fluoride
5. 5
   Hydrogen: ArN₂⁺Cl⁻ + H₃PO₂ + H₂O → ArH + N₂ + H₃PO₃ + HCl; ArN₂⁺Cl⁻ + CH₃CH₂OH → ArH + N₂ + CH₃CHO + HCl

   Mild reducing agents, hypophosphorous acid (phosphinic acid) or ethanol, replace N₂ by H and are oxidised to phosphorous acid or ethanal
6. 6
   Hydroxyl: ArN₂⁺Cl⁻ + H₂O → (warm, 283 K) ArOH + N₂ + HCl

   If the solution warms up, phenol forms. That is why it is kept cold
7. 7
   Nitro: C₆H₅N₂⁺BF₄⁻ + NaNO₂ → (Cu, heat) C₆H₅NO₂ + NaBF₄ + N₂

   The fluoroborate heated with aqueous NaNO₂ and copper gives nitrobenzene

★ key step

Coupling reactions: the nitrogen stays

1. 1★
   C₆H₅N₂⁺Cl⁻ + C₆H₅OH + OH⁻ → (mildly alkaline) 4-HOC₆H₄-N=N-C₆H₅ + Cl⁻ + H₂O

   p-Hydroxyazobenzene, an orange dye. Phenol couples at its para position
2. 2★
   C₆H₅N₂⁺Cl⁻ + C₆H₅NH₂ → (mildly acidic) 4-H₂NC₆H₄-N=N-C₆H₅ + HCl

   p-Aminoazobenzene, a yellow dye (aniline yellow)
3. 3
   Ar-N=N-Ar': azo compounds are coloured

   The -N=N- (azo) bond joins two aromatic rings into one long conjugated system that absorbs visible light. Coupling is an electrophilic substitution in which the diazonium ion is the electrophile

★ key step

Importance in the synthesis of aromatic compounds

• Diazonium salts are excellent intermediates for putting -F, -Cl, -Br, -I, -CN, -OH and -NO₂ groups on a benzene ring.
• Aryl fluorides and iodides cannot be made well by direct halogenation, but diazonium salts give them easily.
• -CN cannot be put on a ring by nucleophilic substitution of chlorobenzene, but CuCN/KCN with a diazonium salt does it. The benzonitrile can then be hydrolysed to benzoic acid.
• Replacing N₂ by H removes an -NH₂ group after it has done its job as a director: aniline → (Br₂ water) 2,4,6-tribromoaniline → (NaNO₂ + HCl, 273 to 278 K) diazonium salt → (H₃PO₂) 1,3,5-tribromobenzene.
• Azo dyes for cloth, paper and leather are made by coupling reactions.

Activity: prepare an azo dye (2-naphthol aniline dye or aniline yellow)

1. 1
   Diazotise: dissolve 2 mL of aniline in 6.5 mL of conc. HCl diluted with 6.5 mL of water. Cool to 0 to 5 °C (273 to 278 K) in an ice bath. Slowly add a solution of 1.6 g of NaNO₂ in 8 mL of water, with stirring.
2. 2★
   2-Naphthol aniline dye: dissolve 3.2 g of 2-naphthol (β-naphthol) in 18 mL of 10% NaOH and add about 25 g of crushed ice. Add the cold diazonium solution very slowly, with constant stirring.

   An orange-red dye, phenyl-azo-2-naphthol, forms. Always add the diazonium solution to the alkaline 2-naphthol, never the reverse
3. 3
   Aniline yellow: dissolve 4 mL of aniline in 4 mL of 1.0 M HCl and add this cold solution slowly to the cold diazonium solution.

   Yellow p-aminoazobenzene forms; coupling with an amine needs a weakly acidic medium
4. 4
   Leave the dye in the ice bath for about 30 minutes, then filter and wash it with cold water. Recrystallise a little: the 2-naphthol dye from glacial acetic acid, aniline yellow from carbon tetrachloride. Record the yield and the melting point.
5. 5
   The same coupling is the test for a primary aromatic amine.

   A primary aliphatic amine gives only N₂ gas with nitrous acid, and no dye

★ key step