b
Diethyl oxalate is used for distinguishing \(1^o,\,2^o\) and \(3^o\) amines as three amines react differently as discussed below.

The \(1^0\) amine forms corresponding substituted oxamide which is a crystalline solid; while \(2^o\) amine forms a diethyl oxamic ester which is a liquid and \(3^o\) amine does not react with diethyl oxalate since it does not contain a replacable hydrogen atom.

\(\underset{{{1}^{o}}\,\,amine}{\mathop{RN{{H}_{2}}}}\,\,+\,\begin{matrix}
   COO{{C}_{2}}{{H}_{5}}  \\
   |\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,  \\
   COO{{C}_{2}}{{H}_{5}}  \\
\end{matrix}\) \(\to \) \(\underset{\left( \begin{smallmatrix} 
 oxamide \\ 
 crystalline 
 \\ 
 solid 
\end{smallmatrix} \right)}{\mathop{\begin{matrix}
   CONHR  \\
   |\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,  \\
   CONHR  \\
\end{matrix}}}\,+2{{C}_{2}}{{H}_{5}}OH\)

\(\underset{{{2}^{o}}\,\,amine}{\mathop{R_2N{{H}}}}\,\,+\,\begin{matrix}
   COO{{C}_{2}}{{H}_{5}}  \\
   |\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,  \\
   COO{{C}_{2}}{{H}_{5}}  \\
\end{matrix}\) \(\to \) \(\underset{\begin{smallmatrix} 
 N,N\,-\,dialkyl\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, \\ 
 oxamic\,\,ester 
\end{smallmatrix}}{\mathop{\begin{matrix}
   CO-N{{R}_{2}}  \\
   |\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,  \\
   COO{{C}_{2}}{{H}_{5}}  \\
\end{matrix}+{{C}_{2}}{{H}_{5}}OH}}\,\)

\(\underset{{{3}^{o}}\,\,amine}{\mathop{R_3N}}\,\,+\,\begin{matrix}
   COO{{C}_{2}}{{H}_{5}}  \\
   |\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,  \\
   COO{{C}_{2}}{{H}_{5}}  \\
\end{matrix}\) \(\to \) No reaction