\(2^{o}\) nitro compound react with \(\mathrm{HNO}_{2}\) acid

but \(3^{o}\) nitro compound does not react with nitrous acid.

\((i)\)  \({\mkern 1mu} \mathop {\begin{array}{*{20}{c}}
  {\,\,\,\,\,\,\begin{array}{*{20}{c}}
  {\,\,\,\,\,C{H_2}} \\ 
  {|\,\,} 
\end{array}} \\ 
  {C{H_3} - C - CH - N{O_2}} \\ 
  {||\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \\ 
  {O\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} 
\end{array}}\limits_{{1^o}\,\,nitro} \) \(\xrightarrow{{HN{O_2}}}\) \(\begin{matrix}
   \,\,\,\,\,\,\,\,\,\begin{matrix}
   \,\,\,\,\,\,C{{H}_{3}}  \\
   |  \\
\end{matrix}  \\
   C{{H}_{3}}-C-C-N{{O}_{2}}  \\
   \,||\,\,\,\,\,\,\,\,\,|\,\,  \\
   \,\,\,\,\,\,\,\,\,\,\,\,O\,\,\,\,\,\,\,N=O  \\
\end{matrix}\)

\((ii)\)  \(\mathop {C{H_3} - C{H_2} - C{H_2} - N{O_2}}\limits_{{1^o}\,nitro} \) \(\xrightarrow{{HO - N = O}}\) \(\begin{array}{*{20}{c}}
  {C{H_3} - C{H_2} - C - N{O_2}} \\ 
  {\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,||} \\ 
  {\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,N - OH} 
\end{array}\)

\((iii)\)  \(\mathop {\begin{array}{*{20}{c}}
  {C{H_3} - CH - C{H_2} - N{O_2}} \\ 
  {|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \\ 
  {C{H_3}\,\,\,\,\,\,\,\,\,\,\,\,} 
\end{array}\,}\limits_{{1^o}\,\,nitro} \)  \(\xrightarrow{{HO - N = O}}\)  \(\begin{matrix}
   C{{H}_{3}}-CH-C-N{{O}_{2}}  \\
   |\,\,\,\,\,\,\,\,\,\,\,\,\,||  \\
   \,\,\,\,\,\,\,\,\,\,\,\,\,\,C{{H}_{3}}\,\,\,\,\,\,N-OH  \\
\end{matrix}\)

\(3^o\) nitro \(\xrightarrow{HN{{O}_{2}}}\) No reaction.