Balancing A Redox Reaction

Step 1: Understanding the Concept:
Lassaigne's test is a qualitative chemical test used to detect the presence of foreign elements like nitrogen, sulfur, and halogens in organic compounds. The color observed at the end of the specific chemical sequence indicates which elements are present.
Step 2: Key Formula or Approach:
The theoretical approach involves knowing the specific indicator colors for Lassaigne's extract:
- Nitrogen alone yields a Prussian blue complex.
- Sulfur alone yields a purple color with nitroprusside or a black precipitate with lead acetate.
- The presence of both Nitrogen and Sulfur together yields a distinct blood-red color.
Step 3: Detailed Explanation:
When an organic compound contains both nitrogen and sulfur, fusion with sodium metal produces sodium thiocyanate ( ):

When this extract is boiled with Iron(II) sulfate ( ) and then acidified, some oxidizes to . The ferric ions react with the thiocyanate ions to form the blood-red iron(III) thiocyanate complex:

To get this positive result, the unknown compound must contain both N and S atoms. Let's analyze the options:
- A) Urea ( ): Contains nitrogen, but no sulfur.
- B) Aniline ( ): Contains nitrogen, but no sulfur.
- C) Benzenesulfonic acid ( ): Contains sulfur, but no nitrogen.
- D) Sulfanilic acid ( ): Contains an amino group (nitrogen) and a sulfonic acid group (sulfur). Therefore, it contains both N and S.
Step 4: Final Answer:
Because it contains both Nitrogen and Sulfur, sulfanilic acid (Option D) will produce the blood-red color.