Lassaigne’s test for the detection of nitrogen will fail in the case of:
A. NH2CONH2
B. NH2CONH2.HCl
C. NH2NH2
D. C6H5NHNH2.2HCl

Lassaigne's test is used to detect the presence of nitrogen in an organic compound by converting it into sodium cyanide, which then forms Prussian blue upon treatment with ferric chloride. However, certain nitrogen-containing compounds may not give a positive test due to the nature of the functional groups present.

Let's analyze the options:

A. NH2CONH2 - This is urea, which contains a carbonyl group and an amine group. It will give a positive Lassaigne's test.

B. NH2CONH2.HCl - This is also urea hydrochloride, which behaves similarly to urea. It will give a positive test.

C. NH2NH2 - This is hydrazine, which is a strong reducing agent. It will decompose sodium cyanide formed during the Lassaigne's test, preventing the formation of Prussian blue. So, it will fail the test.

D. C6H5NHNH2.2HCl - This is phenylhydrazine hydrochloride, which contains a hydrazine group. Like hydrazine, it will fail the Lassaigne's test due to its reducing properties.

Therefore, the correct answer is option C. NH2NH2.