Question

Lassaigne’s test for the detection of nitrogen will fail in the case of:A. $\text{N}{{\text{H}}_{2}}\text{CON}{{\text{H}}_{2}}$ B. $\text{N}{{\text{H}}_{2}}\text{CON}{{\text{H}}_{2}}.\text{HCl}$C. $\text{N}{{\text{H}}_{2}}\text{N}{{\text{H}}_{2}}$D. ${{\text{C}}_{6}}{{\text{H}}_{5}}\text{NHN}{{\text{H}}_{2}}.2\text{HCl}$

$\text{Na+C+N}\to \text{NaCN}$
$\text{6NaCN+ FeS}{{\text{O}}_{4}}\to \text{N}{{\text{a}}_{4}}[\text{Fe(CN}{{\text{)}}_{6}}]+\text{N}{{\text{a}}_{2}}\text{S}{{\text{O}}_{4}}$
$\text{N}{{\text{a}}_{4}}[\text{Fe(CN}{{\text{)}}_{6}}]+\text{F}{{\text{e}}^{3+}}\to \text{F}{{\text{e}}_{4}}{{[\text{Fe(CN}{{\text{)}}_{6}}]}_{3}}$
Therefore, the correct option is (c) $\text{N}{{\text{H}}_{2}}\text{N}{{\text{H}}_{2}}$.
Note: We should also know that when nitrogen and sulphur both are present in an organic compound, sodium fusion will convert it into sodium thiocyanate which then reacts with $\text{F}{{\text{e}}^{3+}}$ to form blood colour complex${{[\text{Fe(SCN) }\!\!]\!\!\text{ }}^{2+}}$.