Question

Which compound does not give a positive test in Lassaigne’s test for nitrogen?
A.Urea
B.Hydrazine
C.Azobenzene
D.Phenyl hydrazine

Answer 1

Hint: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.

Complete step-by-step answer:In Lassaigne’s test, the given compound is fused with sodium metal. The carbon and nitrogen in the compound react with sodium and produce sodium cyanide $\left(\text{NaCN}\right)$.
The sodium cyanide $\left(\text{NaCN}\right)$ is then further used to test the presence of nitrogen.
Thus, the compound must contain carbon and nitrogen to give a positive Lassaigne’s test for nitrogen.
The molecular formula for urea is $\text{C}{\text{H}}_{\text{4}}{\text{N}}_{\text{2}}\text{O}$. Thus, urea contains carbon and nitrogen. On reaction with sodium it can form sodium cyanide and thus gives a positive Lassaigne’s test for nitrogen.
The reaction of urea with sodium metal is as follows:
${\left(\text{N}{\text{H}}_2\right)}_2-\text{C}=\text{O}+\text{Na}\to \text{NaCN}$
Thus, option (A) is not correct.
The molecular formula for hydrazine is ${\text{N}}_{\text{2}}{\text{H}}_{\text{4}}$. Thus, hydrazine contains nitrogen but it does not contain carbon. On reaction with sodium it cannot form sodium cyanide and thus does not give a positive Lassaigne’s test for nitrogen.
The reaction of hydrazine with sodium metal is as follows:
${\text{N}}_{\text{2}}{\text{H}}_{\text{4}}+\text{Na}\to \text{NaCN cannot be formed}$
Thus, option (B) is correct.
The molecular formula for azobenzene is ${\text{C}}_{\text{12}}{\text{H}}_{\text{10}}{\text{N}}_{\text{2}}$. Thus, azobenzene contains carbon and nitrogen. On reaction with sodium it can form sodium cyanide and thus gives a positive Lassaigne’s test for nitrogen.
The reaction of azobenzene with sodium metal is as follows:
${\text{C}}_{\text{12}}{\text{H}}_{\text{10}}{\text{N}}_{\text{2}}+\text{Na}\to \text{NaCN}$
Thus, option (C) is not correct.
The molecular formula for phenyl hydrazine is ${\text{C}}_6{\text{H}}_8{\text{N}}_{\text{2}}$. Thus, phenyl hydrazine contains carbon and nitrogen. On reaction with sodium it can form sodium cyanide and thus gives a positive Lassaigne’s test for nitrogen.
The reaction of phenyl hydrazine with sodium metal is as follows:
${\text{C}}_6{\text{H}}_8{\text{N}}_{\text{2}}+\text{Na}\to \text{NaCN}$
Thus, option (D) is not correct.
Thus, hydrazine does not give a positive test in Lassaigne’s test for nitrogen.

Thus, the correct option is (B) hydrazine.

Note:The carbon and nitrogen in the compound fuse with sodium metal to produce sodium cyanide. 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 $\text{FeS}{\text{O}}_{\text{4}}$ and then acidified using concentrated sulphuric acid. If Prussian blue colour is observed then the presence of nitrogen is confirmed.