Which of the following nitrogen containing compound does not give Lassaigne’s test ?
- Phenyl hydrazine
- Glycene
- Urea
- Hydrazine
The Correct Option is D
Approach Solution - 1
Understanding Lassaigne’s Test:
Lassaigne’s test is used to detect the presence of nitrogen, sulfur, and halogens in organic compounds. In this test, the compound is fused with sodium to convert these elements into ionic forms (e.g., NaCN for nitrogen).
Requirement for Lassaigne’s Test:
To give a positive result for nitrogen in Lassaigne’s test, the compound must contain a carbon-nitrogen bond. During fusion with sodium, nitrogen forms cyanide ions (\( \text{CN}^- \)), which can be detected in subsequent steps.
Analyzing Each Compound:
Phenyl hydrazine (\( \text{C}_6\text{H}_5\text{NHNH}_2 \)): Contains a carbon-nitrogen bond and will give a positive result for Lassaigne’s test.
Glycine (\( \text{NH}_2\text{CH}_2\text{COOH} \)): Contains a carbon-nitrogen bond in the amino acid structure and will give a positive result for Lassaigne’s test.
Urea (\( \text{NH}_2\text{CONH}_2 \)): Contains a carbon-nitrogen bond in the amide linkage and will give a positive result for Lassaigne’s test.
Hydrazine (\( \text{NH}_2\text{NH}_2 \)): Does not contain any carbon atoms; therefore, it cannot form \( \text{NaCN} \) and does not give a positive Lassaigne’s test for nitrogen.
Conclusion:
Hydrazine does not contain a carbon-nitrogen bond and hence does not give Lassaigne’s test.
The correct answer is Option (4).
Approach Solution -2
We are asked which nitrogen-containing compound does not give Lassaigne’s test for nitrogen detection.
Step 2: Concept of Lassaigne’s test.
In Lassaigne’s test, the compound is fused with sodium metal to convert elements like nitrogen, sulfur, or halogens into ionic forms that can be tested in solution.
For nitrogen, the reaction produces sodium cyanide (NaCN):
\[ \text{Na} + \text{C} + \text{N} \rightarrow \text{NaCN} \] The NaCN formed then reacts with FeSO₄ and ferric ions to form Prussian blue (confirming the presence of nitrogen).
Step 3: Why Hydrazine fails the test.
Hydrazine (\( \text{N}_2\text{H}_4 \)) does not contain carbon. Therefore, during fusion with sodium, no NaCN can be formed because the cyanide ion requires both carbon and nitrogen.
Hence, hydrazine does not give the Lassaigne’s test for nitrogen.
Step 4: Final Answer.
\[ \boxed{\text{Hydrazine}} \]
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