Which of the following compounds would give the following set of qualitative analysis?
(i) Fehling's Test: Positive
(ii) Na fusion extract upon treatment with sodium nitroprusside gives a blood red colour but not prussian blue.

1. Fehling's Test (Positive):

   A positive Fehling's test indicates the presence of an aliphatic aldehyde group ($$\text{--CHO}$$). This eliminates any option that contains a ketone, ester, or other non-reducing functional groups, focusing our attention on compounds with an active formyl functionality.




2. Sodium Fusion Extract (Lassaigne's Test) with Sodium Nitroprusside:

   When an organic compound containing both Nitrogen ($$\text{N}$$) and Sulfur ($$\text{S}$$) is fused with sodium metal, they react together with carbon to form sodium thiocyanate ($$\text{NaSCN}$$):

   $$\text{Na} + \text{C} + \text{N} + \text{S} \rightarrow \text{NaSCN}$$
   • Blood-Red Coloration: When ferric ions ($$\text{Fe}^{3+}$$) or specific complexing reagents like sodium nitroprusside are introduced into an acidified thiocyanate extract solution, it yields a characteristic blood-red color due to the formation of ferric thiocyanate complexes:
   $$\text{Fe}^{3+} + \text{SCN}^\ominus \rightarrow [\text{Fe}(\text{SCN})]^{2+} \quad \text{(Blood-Red Solution)}$$
   • Absence of Prussian Blue: Because both nitrogen and sulfur are fixed together as thiocyanate ($$\text{SCN}^\ominus$$) rather than isolated cyanide ($$\text{CN}^\ominus$$) ions, there are no free cyanide ions available to form the classic Prussian blue complex ($$\text{Fe}_4[\text{Fe}(\text{CN})_6]_3$$).

   Therefore, the compound must contain both nitrogen and sulfur atoms alongside its aldehyde group.

Evaluating the Structural Candidates:

• Oxazole derivatives: Contain Nitrogen ($$\text{N}$$) and Oxygen ($$\text{O}$$), but lack Sulfur ($$\text{S}$$). They do not give a blood-red color during Lassaigne's element detection.
• Pyrrole derivatives: Contain Nitrogen ($$\text{N}$$), but lack Sulfur ($$\text{S}$$), giving a Prussian blue test rather than blood-red thiocyanate formations.
• 2-Thiazolecarboxaldehyde ($$\text{C}_4\text{H}_3\text{NOS}$$): The thiazole ring features both an aromatic nitrogen atom and a sulfur heteroatom in its cyclic core, along with a directly attached formyl group ($$\text{--CHO}$$). This perfectly satisfies both the structural and chemical criteria.

Conclusion:

The simultaneous presence of the aldehyde moiety and both nitrogen and sulfur within the thiazole ring framework satisfies all requirements for a positive Fehling's reaction and a blood-red Lassaigne's outcome.

Answer: Option C (2-Thiazolecarboxaldehyde)