Diazonium Salts Properties

Physical properties

Benzene diazonium chloride is colourless crystalline solid. It is readily soluble in water and is stable in solution oat low temperature. It reacts with water on warming to form phenol. It decomposes easily in day state also. C6H5N2F is water insoluble and is stable and is stable even at room temperature.

Chemical properties

Arenediazonium salts undergo a large number of reactions. These reactions may be divided into two categories:
    
1. Replacement reactions: in these reactions nitrogen is lost as N₂ and some other atom or group gets attached to the aromatic ring in its place.
    
2. Reduction of benzene diazonium chloride.
    
3. Coupling reactions; In these reactions diazonium salt acts as electrophile and brings about substitution in some other electron rich aromatic ring such as phenols or aryl amines.
    
(i) Replacement reactions
    
(a) Replacement by –Cl, -Br and –CN: Aromatic diazonium salts on reaction with cuprous chloride give any chlorides, on reaction with cuprous bromide give aryl bromides and on reaction with cuprous cyanide give aryl cyanides. These reactions are known as Sandmeyer’s reactions.



The arylchloride or bromides can also be prepared by Gattermann reactions which involves the treatment of diazonium salt with Cu/HCl or Cu/Br respectively


    
(b) Replacement by –I: this is done by warming the diazonium salt with KI or a solution of iodides in KI.


    
(c) Replacement by –F: this involves the treatment of diazonium salt with fluoboric acid results in the formation of arene diazonium fluoborate (a solid which is more stable than benzene diazonium chloride). The fluoborate salt on further decomposes to give fluroarenes.



The reaction is called Balz Schiemann reaction.
    
(d) Replacement by –NO₂ group: this is done by heating diazonium fluoborate with NaNO₂ in the presence of copper powder


    
(e) Replacement by –OH: this involves warming the diazonium salt with water in the presence of mineral acids such as H₂SO₄. The acidic conditions minimize the side reactions.


    
(f) Replacement by H: this is done by treatment of arenediazonium salts with mild reducing agents acid as phosphinic acid (hypophosphoric acid) in the presence of Cu⁺ salts as catalyst or ethanol at room temperature. These reagents get oxidized to phosphorus acid or ethanal and in turn reduce the arenediazonium salts to arenes. These reagents get oxidized to phosphorus acid or ethanal and in turn reduce the arenediazonium salts to arenes.


    
(g) Replacement of Aryl group: This can be done by treatment of arenediazonium chloride with arenes in alkaline medium



This reaction is called Gomberg Bachmann reaction.
    
(ii) Reduction of Benzenediazonium chloride
    
(a) Mild reduction involves the use of reducing agents such as SnCl₂/HCl or Na₂SO₃ or electrolysis and the product is arylhydrazine.


    
(B) Strong vigorous reduction involves the use of Zn/HCl and the product is arylamines.

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