Statement I: Primary aliphatic amines react with $\mathrm{HNO}{}_2$ to give unstable diazonium salts. Statement II: Primary aromatic amines react with $\mathrm{HNO}{}_2$ to form diazonium salts which are stable even above 300 K. Choose the most appropriate answer from the options given below:

Solution & Explanation

\textbf{Answer:} (C) Aliphatic diazonium salts are indeed unstable, but aromatic diazonium salts are stable only at low temperature (273$-$278 K), not above 300 K. \textbf{Solution:} Statement I (correct): Primary aliphatic amines react with nitrous acid to form highly unstable aliphatic diazonium salts that decompose immediately, releasing $\mathrm{N}{}_2$ quantitatively: $\mathrm{R}-\mathrm{NH}{}_2+\mathrm{HNO}{}_2\to [\mathrm{R}-\mathrm{N}{}_2{}^{+}]\to \mathrm{R}{}^{+}+\mathrm{N}{}_2\uparrow +\mathrm{H}{}_2\mathrm{O}$ Statement II (incorrect): Primary aromatic amines give arenediazonium salts that are stabilised by resonance with the ring, but they are stable \textbf{only at low temperature} (typically 273$-$278 K). Above this they decompose: $\mathrm{C}{}_6\mathrm{H}{}_5\mathrm{NH}{}_2\underset{273-278\mathrm{K}}{\overset{\mathrm{NaNO}{}_2/\mathrm{HCl}}{\to }}\mathrm{C}{}_6\mathrm{H}{}_5\mathrm{N}{}_2{}^{+}\mathrm{Cl}{}^{-}$ $\mathrm{C}{}_6\mathrm{H}{}_5\mathrm{N}{}_2{}^{+}\mathrm{Cl}{}^{-}+\mathrm{H}{}_2\mathrm{O}\stackrel{>278\mathrm{K}}{\to }\mathrm{C}{}_6\mathrm{H}{}_5\mathrm{OH}+\mathrm{N}{}_2\uparrow +\mathrm{HCl}$ Thus they are NOT stable above 300 K; Statement II is false. Since Statement I is correct and Statement II is incorrect, option (C) is the answer.