$2^{o}$ 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.