Why does amine give an electrophilic substitution

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Similar to primary aliphatic amines, benzene amine (aniline) reacts with nitrous acid to form a diazonium ion. A nitrosyl cation is formally transferred to the amine.

Unlike aliphatic diazonium ions, which decompose spontaneously with elimination of nitrogen, aromatic diazonium salts are generally stable below 5 ° C. The reason for this lies in the delocalization of the positive charge by the aromatic.

The nitrous acid that one commonly uses in situ from and a mineral acid is first protonated and forms the actual reagent, which differs depending on the mineral acid used.

Accordingly, dinitrogen trioxide (X =), nitrosyl halide (X = Cl, Br) or (X =) are conceivable as reactive agents, and in strongly acidic solution also the nitrosyl cation itself. This species then attacks the aromatic amine and acts as nitrosyl -Transfer. The attack also takes place in a strongly acidic solution on the unprotonated amine, since aromatic amines have only a low basicity.

By deprotonation, a nitrosamine is formed from the cation formed primarily, which tautomerizes to form the diazo hydroxide. Protonation of the hydroxyl group and elimination of water lead to the diazonium ion.

Diazonium ions are only present in solution under acidic or weakly alkaline conditions; in more alkaline media they react to form diazohydroxides and finally to diazotate anions.

Aromatic diazonium salts can also be isolated, but they are explosive when dried. They are starting compounds for the Sandmeyer reaction, which allows the introduction of substituents such as or in aromatics, and for the azo coupling.