Write the reactions of (i) aromatic and (ii) aliphatic primary amines with nitrous acid.
aromatic primary amines with nitrous acid
Aromatic primary amines react with nitrous acid at low temperatures (273-278 K) to form diazonium salts. For example, aniline reacts with nitrous acid (prepared in situ from NaNO₂ and HCl) to form benzenediazonium chloride:
C₆H₅-NH₂ + NaNO₂ + 2HCl → C₆H₅-N₂Cl + NaCl + 2H₂O
These diazonium salts are stable at low temperatures and are used for synthesis of various aromatic compounds.
Aliphatic primary amines react with nitrous acid to form aliphatic diazonium salts which are unstable. These unstable diazonium salts decompose to liberate nitrogen gas quantitatively and form alcohols:
R-NH₂ + HNO₂ → [R-N₂Cl] → ROH + N₂ + HCl
The quantitative evolution of nitrogen is used in estimation of amino acids and proteins.
Explanation
The textbook clearly distinguishes between the two reactions. Aromatic primary amines form stable diazonium salts at low temperatures (273-278 K), which are important synthetic intermediates. Aliphatic primary amines, however, form unstable diazonium salts that rapidly decompose to release nitrogen gas and form alcohols. This difference is due to the resonance stabilization of aromatic diazonium ions, which is absent in aliphatic diazonium ions.