Which type of amine produces $N_2$ when treated with HONO?

Primary amine

The correct answer is Option (1) → Primary amine

Primary amines react with nitrous acid (HONO) to form unstable diazonium salts which decompose to release nitrogen gas ($N_2$). This is a characteristic reaction used to distinguish primary amines from secondary and tertiary amines.

Explanation

Option 1: Primary amine

Primary aliphatic amines react with nitrous acid to form diazonium salts. These salts are unstable and immediately decompose to produce alcohol along with evolution of nitrogen gas. The liberation of $N_2$ is a key identifying feature of primary amines.

$CH_3NH_2 + 2HONO \longrightarrow \underset{methylnitrite} {CH_3ON=O}+ N_2 + 2H_2O$

$2CH_3NH_2 + 2HONO \longrightarrow CH_3OCH_3 + 2N_2 + 2H_2O$

Option 2: Secondary amine

Secondary amines react with nitrous acid to form N-nitrosamines. These compounds are stable oily substances and do not release nitrogen gas. Hence, no $N_2$ evolution occurs in this reaction.

Secondary amines will form nitroso alkyl amine.

Option 3: Tertiary amine

Tertiary amines do not form diazonium salts because they lack a hydrogen attached to nitrogen. Instead, they form nitrite salts or undergo mild reactions without gas evolution. Therefore, nitrogen gas is not produced.

Tertiary amines react with nitrous acid and form nitrites.

Option 4: Quaternary ammonium salt

Quaternary ammonium salts do not react with nitrous acid in a way that produces nitrogen gas. Their nitrogen is fully substituted and cannot undergo diazotization, so no $N_2$ is evolved.