Question

The covalent compound which does not give a positive test in Lassaigne’s test is:
A.1,3dinitrobenzene
B.Glycine
C.Urea
D.Hydrazine

Answer 1

Hint: Lassaigne’s test is used to detect nitrogen, Sulphur and halogens present in the organic compounds. Basically, this test 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.

Complete step by step answer:
In Lassaigne’s test we detect nitrogen, Sulphur and various halogens present in organic compounds. In this test a small piece of Na metal is heated in a fusion tube with the organic compound. The principle of this test is that the Na converts all the elements present into ionic form. The equations are as shown:
$\Rightarrow$ $Na + C + N \to NaCN$
$\Rightarrow$ $2Na + S \to N{a_2}S$
$\Rightarrow$ $Na + X \to NaX$ Where X can be Cl, Br or I.
Now, the ionic salts that are formed are extracted from the fused mass by boiling it with distilled water. This process is further known as sodium fusion extract.
Further the compounds that do not contain C atoms, do not give Lassaigne’s test. So, form the above given options hydrazine i.e. $N{H_2}N{H_2}$ does not contain C atoms and hence it does not give Lassaigne’s test. Further, both C and N atoms are required to form CN ions.

Hence, option D is correct.

Note:
This test is not given by diazonium salts as they decompose to give nitrogen gas on heating. Moreover, Prussian blue color confirms the presence of nitrogen, violet color indicates the presence of Sulphur and for halogens we have different color ppt soluble in water.