Question

The compound formed in the positive test for nitrogen with the Lassaigne's solution of an organic compound is:
A) \[F{e_4}{[Fe{(CN)_6}]_3}\]
B) $N{a_3}[Fe{(CN)_6}]$
C) $Fe{(CN)_3}$
D) $N{a_3}[Fe{(CN)_5}NOS]$

Answer 1

Hint: Lassaigen's test is a general method for determining the presence of halogens, phosphorus, and sulphur in an organic compound. The organic compound is fused with sodium metal in this process. The ionic compounds formed during fusion are collected in aqueous solution and identified using basic chemical tests.

Complete answer:
The carbon and nitrogen present in the organic compound on fusion with sodium metal gives sodium cyanide $(NaCN)$ soluble in water.
$Na + C + N \to NaCN$
This is converted into sodium ferrocyanide by the addition of sufficient quantities of ferrous sulphate.
$6NaCN + FeS{O_4} \to N{a_4}[Fe{(CN)_6}] + N{a_2}S{O_4}$
The ferric ions generated during the process react with ferrocyanide to form a prussian blue ferric ferrocyanide precipitate.
$N{a_4}[Fe{(CN)_6}] + F{e^{3 + }} \to F{e_4}{[Fe{(CN)_6}]_3}$
Ferric ferrocyanide turns Prussian blue in this reaction, hence it shows positive test for nitrogen with the Lassaigne's solution.
Therefore, the correct option is A) \[F{e_4}{[Fe{(CN)_6}]_3}\]

Note:
As nitrogen reacts with sodium metal, sodium cyanide $(NaCN)$ is formed, which is water soluble. Because of its electropositive nature, sodium is used in Lassaigne's test to transform covalent to ionic form. Its nature is very reactive.