Question

Write down a test to distinguish between aromatic primary amines and aliphatic primary amines.

Hint:

Only aromatic primary amines form stable diazonium salts at \(0–5^\circ\text{C}\); aliphatic ones do not.

Answer 1

Step 1: Treat the given primary amine with nitrous acid (prepared in situ using NaNO\(_2\) and dilute HCl) at \(0–5^\circ\text{C}\).
Step 2: \begin{itemize} \item Aromatic primary amines form stable diazonium salts at low temperature. \[ \text{ArNH}_2 + \text{HNO}_2 + \text{HCl} \rightarrow \text{ArN}_2^+\text{Cl}^- + 2\text{H}_2\text{O} \] \item Aliphatic primary amines form unstable diazonium salts, which immediately decompose to give alcohols with evolution of nitrogen gas. \[ \text{RNH}_2 + \text{HNO}_2 \rightarrow \text{ROH} + \text{N}_2 \uparrow + \text{H}_2\text{O} \] \end{itemize}
Step 3: Observation: \begin{itemize} \item Formation of stable diazonium salt → Aromatic primary amine \item Brisk effervescence of N\(_2\) gas → Aliphatic primary amine \end{itemize}