Question

The compound gives blood red colouration when its Lassaigne's extract is treated with alkali and ferric chloride is

• A. Benzamide
• B. Phenyl hydrazine
• C. Diphenyl sulphide
• D. Thiourea

Hint:

If an organic compound contains both nitrogen and sulfur, the Lassaigne's extract forms sodium thiocyanate (NaCNS), which gives a blood red color with FeCl\(_3\).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Lassaigne's test is used for the detection of nitrogen, sulphur, and halogens in organic compounds. A blood red coloration indicates the presence of both nitrogen and sulphur.

Step 2: Detailed Explanation:
When an organic compound containing both N and S is fused with sodium, NaCNS (sodium thiocyanate) is formed. In Lassaigne's test, the extract is treated with FeCl\(_3\). The Fe\(^{3+}\) ions react with CNS\(^-\) to form Fe(CNS)\(_3\), which is blood red in color. \[ 3\text{NaCNS} + \text{FeCl}_3 \rightarrow \text{Fe(CNS)}_3 + 3\text{NaCl} \] Among the given options, thiourea (H\(_2\)N-CS-NH\(_2\)) contains both N and S, so it gives a positive test.

Step 3: Final Answer:
Thiourea gives a blood red coloration in the Lassaigne test, which corresponds to option (D).