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: Understanding Lassaigne’s Test for Nitrogen
Lassaigne’s test is used to detect the presence of nitrogen in organic compounds.
The compound is fused with sodium metal, and the resulting mixture is tested for the presence of cyanide ions (\(\text{CN}^-\)).
Step 2: Analyzing the Given Compounds
(A) Urea: Contains nitrogen and will show a positive Lassaigne’s test.
(B) Hydrazine: Does not contain carbon, so it cannot form sodium cyanide (\(\text{NaCN}\)) and does not show a positive Lassaigne’s test.
(C) Phenylhydrazine: Contains nitrogen and will show a positive Lassaigne’s test.
(D) Azobenzene: Contains nitrogen in the form of an azo group (\(-N=N-\)), which does not produce cyanide ions during Lassaigne’s test but may give a weak or false-positive result.
Step 3: Determining the Correct Answer
Since hydrazine (\(\text{NH}_2\text{NH}_2\)) does not contain carbon, it does not form sodium cyanide upon reaction with sodium metal and therefore does not give a positive Lassaigne’s test. Final Answer: Hydrazine does not show Lassaigne’s test for nitrogen.