This reaction is called diazotization and begins with nucleophilic attack of the amine on the nitrosonium ion. After a proton transfer, an N-nitrosamine is generated which is eventually converted into a diazonium salt by loss of H2O:
Arene diazonium salts represent an important tool in organic chemistry since they’re stable enough to be used as intermediates for preparing substituted benzene rings.
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Below is the summary of arene diazonium salt reactions and more details can be found in this post:
Alkyl diazonium salts, on the other hand, are not of great interest since they very unstable below room temperature forming carbocations with loss of N2.
The reason for the higher stability of arene diazonium salts is the nature or aryl cations which have to put the positive charge in an sp2 orbital perpendicular to the conjugated system of p orbitals:
This makes aryl and vinyl carbocations very unfavorable to form and we have seen this, for example, in the Friedel-Crafts reactions which do not work for aryl and vinyl halides for this exact reason.
Reaction of Secondary Amines with Nitrous Acid
The reaction of secondary amines with nitrous acid can be summarized in the first part of the arene diazonium, salt formation. The reaction stops at this stage because there is no more proton to be removed and facilitate the loss of water:
This reaction is not nearly as applicable in organic synthesis since the product here is a N-Nitrosamines which, despite being carcinogens are still used in some foods such as cured meats, bacon, beer, some cheeses, and some more.