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Diazotization

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Diazotization is the reaction of a primary aromatic amine with a nitrosating agent, such as sodium nitrite or to a lesser extent with nitrosylsulfuric acid NOSO4H, nitrous gases or organic nitrites in an aqueous acidic solution at a temperature between 0 and 5oC, converting the amine to its diazonium salt. 

Diazotization
The diazotization reaction was discovered in 1858 by Peter Griess. Following a suggestion by Kolbe, he treated picramic acid with nitrous gases in an ethanol solution in an attempt to replace the amino function by an OH group. Kolbe who had been the first to carry out this substitution on p-aminobenzoic acid, worked at elevated temperature, so that the formation of the intermediate diazonium compound escaped. 

On the industrial scale, diazotization reactions are carried out by dissolving the aromatic amine in hydrochloric or sulfuric acid. Despite the fact that 2 equivalents of acid er equivalent of amino group should theoretically suffice as much as 2.5 to 3 equivalents per amino function are actually required to ensure complete diazonium salt formation.

Mechanism

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The aryl ammonium ion is not able to undergo diazotization since the crucial step in the diazotization mechanism is the electrophilic nitrosation of the free amino group in the free base primary aromatic amine. 

Diazotization Mechanism

Formation of the diazonium ion proceeds via the diazohydroxide. 

Formation of The Diazonium Ion

The diazotization reaction requires an excess of acid, formation of the active nitrosating agent XNO proceeds via the underlying equilibrium. 

Formation of The Active Nitrosating Agentn

Diazo Coupling

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Being positively charged, aryldiazonium ions are electrophiles. They are weak electrophiles, however because the positive charge can be delocalised through resonance. Aryldiazonium ions react with strongly activated aromatic rings to give azo compounds. For example

Diazo Coupling

The nitrogen atoms are retained in the product. This electrophilic aromatic substitution reaction is called diazo coupling, because in the product, two aromatic rings are coupled by the azo or -N=N- group. In the above equation para coupling is prefered. But if the para position is blocked by another substituent ortho coupling can occur. All azo compounds are colored and many are used commercially as dyes for cloth and in color photography.