Question

Sodium fusion extract, obtained from aniline, on treatment with iron (II) sulphate and ${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$ in presence of air gives a Prussian blue precipitate. The blue colour is due to the formation of:
A. ${\text{F}}{{\text{e}}_{\text{4}}}{\left[ {{\text{Fe}}{{\left( {{\text{CN}}} \right)}_{\text{6}}}} \right]_{\text{3}}}$
B. ${\text{F}}{{\text{e}}_{\text{3}}}{\left[ {{\text{Fe}}{{\left( {{\text{CN}}} \right)}_{\text{6}}}} \right]_{\text{2}}}$
C. ${\text{F}}{{\text{e}}_{\text{4}}}{\left[ {{\text{Fe}}{{\left( {{\text{CN}}} \right)}_{\text{6}}}} \right]_{\text{2}}}$
D. ${\text{F}}{{\text{e}}_{\text{3}}}{\left[ {{\text{Fe}}{{\left( {{\text{CN}}} \right)}_{\text{6}}}} \right]_{\text{3}}}$

Answer 1

We know that the sodium fusion extract is used in Lassaigne’s test. Lassaigne’s test is used for the detection of nitrogen, sulphur, phosphorus and halogen in an organic compound.

Complete answer:
Lassaigne’s test is a qualitative test and it is used for the detection of nitrogen, sulphur, phosphorus and halogen in an organic compound.
In Lassaigne’s test, the organic compound is fused with sodium metal. The carbon and nitrogen in the organic compound react with sodium and produce sodium cyanide $\left( {{\text{NaCN}}} \right)$. The reaction is as follows:
${\text{Na + C + N }} \to {\text{ NaCN}}$
The sodium cyanide is boiled in distilled water. This extract after boiling is known as sodium fusion extract. The sodium fusion extract is then boiled with ferrous sulphate $\left( {{\text{FeS}}{{\text{O}}_{\text{4}}}} \right)$ solution. The reaction is as follows:
${\text{FeS}}{{\text{O}}_{\text{4}}} + {\text{2NaOH}} \to {\text{Fe}}{\left( {{\text{OH}}} \right)_{\text{2}}} + {\text{N}}{{\text{a}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$
${\text{Fe}}{\left( {{\text{OH}}} \right)_{\text{2}}} + 6{\text{NaCN}} \to {\text{N}}{{\text{a}}_{\text{4}}}\left[ {{\text{Fe}}{{\left( {{\text{CN}}} \right)}_{\text{6}}}} \right] + {\text{2NaOH}}$
To this solution ferric chloride solution is added along with excess of concentrated hydrochloric acid.
${\text{N}}{{\text{a}}_{\text{4}}}\left[ {{\text{Fe}}{{\left( {{\text{CN}}} \right)}_{\text{6}}}} \right] + 4{\text{FeC}}{{\text{l}}_3} \to {\text{F}}{{\text{e}}_{\text{4}}}{\left[ {{\text{Fe}}{{\left( {{\text{CN}}} \right)}_{\text{6}}}} \right]_{\text{3}}} + 12{\text{NaCl}}$
The complex formed ${\text{F}}{{\text{e}}_{\text{4}}}{\left[ {{\text{Fe}}{{\left( {{\text{CN}}} \right)}_{\text{6}}}} \right]_{\text{3}}}$ develops a Prussian blue colour. The Prussian blue colour confirms the presence of nitrogen in the organic compound.
Thus, sodium fusion extract, obtained from aniline, on treatment with iron (II) sulphate and ${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$ in presence of air gives a Prussian blue precipitate. The blue colour is due to the formation of ${\text{F}}{{\text{e}}_{\text{4}}}{\left[ {{\text{Fe}}{{\left( {{\text{CN}}} \right)}_{\text{6}}}} \right]_{\text{3}}}$.

**Thus, the correct option is (A) ${\text{F}}{{\text{e}}_{\text{4}}}{\left[ {{\text{Fe}}{{\left( {{\text{CN}}} \right)}_{\text{6}}}} \right]_{\text{3}}}$.

Note:**
Lassaigne’s test is used to detect the presence of nitrogen, sulphur and halogens in an organic compound. The carbon and nitrogen present in the compound fuses with sodium and produces sodium cyanide. The test for nitrogen is then done using the formed sodium cyanide.