Question

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

Answer 1

Hint: the organic sample needs to be heated strongly with the sodium metal. So, the foreign elements in the organic compounds convert into their ionic forms. The organic compounds, which have the presence of carbon and nitrogen, give sodium cyanide \[\left( {NaCN} \right)\] on fusion or Lassaigne’s test for nitrogen.

Complete step by step answer:
First, we are to describe what the Lassigne’s test is. It is also called Sodium fusion test, developed by J. L. Lassaigne. It is used for the qualitative determination of the presence of foreign elements, such as sulphur, halogen, nitrogen, etc. in an organic compound in the elemental analysis.
In the very first step, we have to take the clean sodium metal and any organic compound as a sample. Now, the sample is to be heated strongly with sodium metal. Now, the foreign elements like sulphur, halogen, nitrogen, etc. are covalently bonded with the organic compound. So, our desired goal is to somehow convert the foreign elements into their ionic forms in order to detect them. This conversion is done by fusing the organic compounds with sodium metal, which means heating the organic compounds strongly with the sodium metal. Now, the resultant ionic compounds, which are formed during the fusion reaction, are extracted in aqueous solution and this can be detected by some simple chemical tests. The resultant is thereby called the Lassaigne’s extract or the sodium fusion extract.
So, the organic compounds, which have the presence of carbon and nitrogen, give sodium cyanide \[\left( {NaCN} \right)\] on fusion and can be further used in the test for nitrogen. So, as we can see that hydrazine and hydrazoic acid do not contain carbon and benzene diazonium chloride loses nitrogen on fusion, these compounds cannot form $NaCN$ with fusion. So, these compounds cannot give Lassaigne’s test for nitrogen.

So, the correct answer is Option B,C,D.

Note: We should always remember that, on heating organic compounds with sodium, the halogens, such as sulphur, halogen, nitrogen, etc. are converted into the inorganic sodium salts, such as sodium halide (for halides), sodium sulphide (for sulphur), sodium cyanide (for nitrogen).
For nitrogen element:
$Na + \left( {C + N} \right) \to NaCN$
$2NACN + FeS{O_4} \to N{a_2}S{O_4} + Fe{\left( {CN} \right)_2}$
$Fe{\left( {CN} \right)_2} + 4NaCN \to N{a_4}[Fe{\left( {CN} \right)_6}]$