19.23 : 1° アミンからジアゾニウム塩またはアリールジアゾニウム塩へ: NaNO_2 によるジアゾ化の概要

HNO₂ is a weaker acid compared to HNO₃. It ionizes to give hydronium and nitrite ions and has a pK_a value of 3.37.

As HNO₂ is unstable, it is prepared in situ by reacting NaNO₂ with a strong mineral acid, like hydrochloric or sulfuric acid.

Under the same acidic condition, the –OH group of nitrous acid undergoes protonation to give an oxonium ion, which loses a water molecule to generate the nitrosyl cation or the nitrosonium ion.

The electrophilic nitrosonium ion can then react with the nucleophilic amine nitrogen.

The reaction of HNO₂ with primary, secondary, and tertiary aliphatic and aromatic amines results in different products.

For instance, primary amines, upon treatment with HNO₂, yield diazonium salts, while secondary amines yield N-nitrosamines.

When reacted with HNO₂, tertiary aliphatic amines undergo protonation to yield water-soluble salts and N-nitrosoammonium compounds. In contrast, tertiary arylamines undergo electrophilic aromatic substitution to form C-nitroso aromatic compounds.