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.

Solution

Chemistry ma’am · 3 steps

Test (ii) asks for N and S together, and Fehling's asks for an aliphatic CHO. Only (a) has both.

1. Read test (ii). During sodium fusion, a compound with both N and S can give NaSCN instead of NaCN. Thiocyanate gives the blood red colour. Prussian blue needs free CN⁻, so its absence means N never appears alone as cyanide.
2. Read test (i). Fehling's solution is reduced by aliphatic aldehydes. An aromatic aldehyde, where -CHO sits directly on an aromatic ring, gives a negative Fehling's test.

   Option	N and S both?	Where the CHO sits	Fehling's
   (a) thiazole with -CH₂CH₂CHO	yes, blood red	on a CH₂ chain, aliphatic	positive
   (b) oxazole-2-carbaldehyde	no, N and O only	on the aromatic ring	negative
   (c) imidazole-2-carbaldehyde	no, N only	on the aromatic ring	negative
   (d) thiazole-2-carbaldehyde	yes, blood red	on the aromatic ring	negative

3. So. Options (a) and (d) both pass test (ii). Only (a) has its -CHO separated from the ring by two CH₂ groups, so only (a) reduces Fehling's solution. The answer is (a).

The idea to keep. Blood red from the fusion extract means N and S are present together as thiocyanate. Fehling's works only for aliphatic aldehydes, so check whether the CHO touches the aromatic ring.

Answer from the NTA final answer key. Solution by PrepWiser.