Question

Which of the following compounds cannot give Lassaigne's test of nitrogen?
A.Pyridine
B.Benzene diazonium chloride
C.Hydrazine
D.Hydrazoic acid

Answer 1

Hint: We were asked to find the compound that cannot give Lassaigne’s test of nitrogen. We first discuss Lassaigne's test briefly. We write how the test is done and then we mention some examples of Lassaigne’s test.

Complete step by step answer:
The sodium fusion test, or Lassaigne's test, is used in elemental analysis for the qualitative determination of the presence of foreign elements, namely halogens, nitrogen, and sulphur, in an organic compound. It was developed by J. L. Lassaigne.
The test involves heating the sample strongly with clean sodium metal, "fusing" it with the sample. A variety of techniques has been described. The "fused" sample is plunged into water, and the usual qualitative tests are performed on the resultant solution for the respective possible constituents.
The organic compound is fused with sodium. Carbon and nitrogen present in the organic compound gives sodium cyanide (NaCN). This is further used in the test for nitrogen.
Benzene diazonium chloride on fusion loses nitrogen. hydrazine and hydrazoic acid do not contain carbon. Thus these compounds cannot form NaCN on fusion with sodium.
Hence, benzene diazonium chloride, hydrazine and hydrazoic acid cannot give Lassaigne's test of nitrogen.

Hence, the answers are options B,C and D .

Additional Information:
The halogens, nitrogen and sulphur are covalently bonded to the organic compounds. In order to detect them, the elements need to be converted into their ionic forms. This is done by fusing the organic compound with sodium metal. The ionic compounds formed during the fusion are extracted in aqueous solution and can be detected by simple chemical tests. The extract is called sodium fusion extract or Lassaigne's extract.
When an organic compound is heated strongly with sodium, any halogens, nitrogen, and sulfur will be converted into inorganic sodium salts such as sodium halide (for halides), sodium cyanide (for nitrogen), sodium sulfide (for sulfur), and sodium thiocyanate (for sulfur and nitrogen).The nitrogen is confirmed with ferrous sulfate.



Note:
For Nitrogen element,
$$\begin{array}{l}Na+(C+N)------->NaCN\\ 2NaCN+FeSO4-------->Na2SO4+Fe\left(CN\right)2\\ Fe\left(CN\right)2+4NaCN------->Na4\left[Fe\left(CN\right)6\right]\end{array}$$