Question

Which of the following compounds does not show Lassaigne’s test for nitrogen? (A) Urea
(B) Hydrazine
(C) Phenylhydrazine
(D) Azobenzene

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

Hint:

Lassaigne’s test detects nitrogen only if it can be converted to cyanide ions (\(\text{CN}^-\)) during the fusion process. Compounds without carbon (such as hydrazine) do not produce cyanide ions and thus do not show a positive test.

Answer 1

The Correct Option is B

Step 1: Comprehending Lassaigne’s Test for Nitrogen
Lassaigne’s test identifies nitrogen in organic compounds.
The process involves fusing the compound with sodium metal, followed by testing the resultant mixture for cyanide ions (\(\text{CN}^-\)).
Step 2: Evaluating the Provided Compounds
(A) Urea: Possesses nitrogen, thus yielding a positive Lassaigne’s test.
(B) Hydrazine: Lacks carbon; consequently, it cannot form sodium cyanide (\(\text{NaCN}\)) and will not produce a positive Lassaigne’s test.
(C) Phenylhydrazine: Contains nitrogen, hence exhibiting a positive Lassaigne’s test.
(D) Azobenzene: Features nitrogen in an azo group (\(-N=N-\)). While this configuration typically does not yield cyanide ions in Lassaigne’s test, it may produce a faint or false-positive outcome.
Step 3: Ascertaining the Correct Conclusion
Hydrazine (\(\text{NH}_2\text{NH}_2\)), lacking carbon, does not form sodium cyanide upon reaction with sodium metal and therefore does not yield a positive Lassaigne’s test.Final Answer: Hydrazine does not yield a positive result in Lassaigne’s test for nitrogen.