Diazotization is the reaction of a primary aromatic amine with nitrous acid ($HN{O}_2$).

• Reagents: $NaN{O}_2$ and $HCl$ at $0-5^{\circ }C$.
• Reaction: $C_6{H}_{5}N{H}_2+NaN{O}_2+2HCl\to C_6{H}_5{N}_2^{+}C{l}^{-}+NaCl+2H_{2}O$
• Product: Benzenediazonium chloride.
• The low temperature is critical because diazonium salts are thermally unstable and decompose above $5^{\circ }C$ to release $N_2$ gas. These salts are important intermediates for Sandmeyer, Gatterman, and Coupling reactions.