19.23:
1° Amines to Diazonium or Aryldiazonium Salts: Diazotization with NaNO2 Overview
HNO2 is a weaker acid compared to HNO3. It ionizes to give hydronium and nitrite ions and has a pKa 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.
19.23:
1° Amines to Diazonium or Aryldiazonium Salts: Diazotization with NaNO2 Overview
Nitrous acid and nitric acids are two types of acids containing nitrogen, among which nitrous acid is weaker than nitric acid. Nitrous acid with a pKa value of 3.37 ionizes in water to give a nitrite ion and the hydronium ion.
The nitrous acid is unstable. Hence, it is formed in situ from a solution of sodium nitrite and cold aqueous acids such as hydrochloric or sulfuric acid. In an acidic solution, the –OH group of nitrous acid undergoes protonation to give oxonium ion, followed by water loss to form nitrosonium ion. Nitrosonium ion is a highly reactive electrophile that undergoes a reaction with the nucleophilic nitrogen atom of an amine.
Nitrous acid can react with different classes of amines, such as primary, secondary, and tertiary aliphatic or aromatic amines, to give different products. For example, nitrous acid reacts with primary amines to form a diazonium salt. The reaction is called the diazotization of amines. Nitrous acid reacts with secondary amines to yield N-nitrosamine. With tertiary aliphatic amines, nitrous acid reacts to generate water-soluble salts and N-nitrosoammonium compounds. In contrast, tertiary arylamines undergo electrophilic aromatic substitution to form aromatic C-nitroso compounds.