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Amines with Nitrous Acid

Amines of different classes react with nitrous acid to yield different products. Nitrous acid being an unstable acid is prepared in situ by the reaction of sodium nitrite and dilute hydrochloric acid.

Primary aromatic amines react with nitrous acid at low temperature (273 – 278 K) to give aromatic diazonium salts. This reaction is known as diazotization.

Primary aliphatic amines also react with nitrous acid to form diazonium salt, however, the aliphatic diazonium salts being unstable, decompose to yield mixture of alcohols, alkenes along with nitrogen gas.

Secondary aliphatic and aromatic amines react with nitrous acid to produce nitroso-amines that are insoluble in the aqueous solution and separate out as a yellow oil layer.

The nitrosoamines on warming with a little phenol and concentrated sulphuric acid produce red solution which turns blue on treatment with sodium hydroxide. This colour change provides an excellent test for secondary amines and is known as Liebermann’s nitroso reaction.

Tertiary aliphatic amines on reaction with nitrous acid form nitrites while tertiary aromatic amines undergo electrophilic substitution at the ring.