Question

Lassaigne's test for the detection of nitrogen fails in

• A. NH$_2$CONHNH$_2$ . HCI
• B. NH$_2NH_2$ . HCI
• C. $NH_2CONH_2$
• D. $C_6H_5NHNH_2 . HCI$

Hint:

Lassaigne's test is given by only those compounds which contain both carbon and nitrogen. 

Answer 1

The Correct Option is B

Lassaigne's test is a qualitative analysis test used to detect the presence of nitrogen, sulfur, and halogens in an organic compound. In this test, the organic compound is fused with sodium, leading to the formation of sodium cyanide if nitrogen is present. This sodium cyanide then reacts with iron sulfate to produce Prussian blue, confirming the presence of nitrogen.

Let's analyze each compound given in the options to understand why Lassaigne's test fails for one:

1. NH$_2$CONHNH$_2$ . HCl: This compound has nitrogen in the amide group as well as in the hydrazine part.
2. NH$_2NH_2$ . HCl: This is hydrazine hydrochloride. The nitrogen here is in the form of a salt (due to the reaction of hydrazine with HCl). In such cases, the nitrogen cannot be converted to sodium cyanide in the Lassaigne's test. Hence, the formation of sodium cyanide does not occur, and the test is unsuccessful.
3. $NH_2CONH_2$: This is urea, which has nitrogen in the form of an amide, and should give a positive result for the Lassaigne's test.
4. $C_6H_5NHNH_2 . HCl$: This is phenylhydrazine hydrochloride. Although it is a salt form, the nitrogen is in the hydrazine group, which can potentially react during fusion with sodium to form cyanide.

The failure of the Lassaigne's test for hydrazine hydrochloride (NH$_2NH_2$ . HCl) is due to the presence of nitrogen in a form that cannot be transformed into sodium cyanide under the test conditions. Therefore, the nitrogen does not get detected.

Conclusion: The correct answer is NH$_2NH_2$ . HCl, as the Lassaigne's test fails to detect nitrogen in this compound.