Question

Which of the following will not give Lassaigne's test?

• A. Urea
• B. Azobenzene
• C. Hydrazine
• D. Phenylhydrazine

Answer 1

The Correct Option is C

To determine which compound does not give a positive Lassaigne's test, let's first understand the test and the nature of the compounds involved.

Understanding Lassaigne's Test:

Lassaigne's test is used to detect the presence of nitrogen, sulfur, and halogens in an organic compound. During this test, the organic compound is fused with sodium, converting these elements into sodium cyanide (NaCN), sodium sulfide (Na₂S), and sodium halides (e.g., NaCl), respectively. These are then detected through various reactions.

Analysis of Options:

1. Urea (NH₂CONH₂): Contains nitrogen and when fused with sodium, gives NaCN, which can be detected, thus gives Lassaigne's test.
2. Azobenzene (C₆H₅-N=N-C₆H₅): Contains nitrogen in the azo group. On fusion with sodium, nitrogen would be converted to a detectable form, giving a positive result.
3. Hydrazine (N₂H₄): Though it contains nitrogen, the nature of hydrazine prevents it from producing sodium cyanide efficiently upon fusion, leading to a negative result.
4. Phenylhydrazine (C₆H₅NHNH₂): Contains nitrogen, and on fusion with sodium, it can be converted to a detectable form, thus giving a positive test result.

The only compound in the list that does not consistently give a positive Lassaigne's test is Hydrazine.

Conclusion:

Hence, among the given options, Hydrazine does not give Lassaigne's test due to its inability to form a detectable amount of sodium cyanide on fusion.