JEE Main 2020
LEVELJEE Advanced
Visualized Solution
The Sigma Insight: Chemicals in Food
This problem is a beautiful intersection of two distinct areas of chemistry: the structural knowledge of everyday chemicals (artificial sweeteners) and the analytical techniques of qualitative organic analysis (Lassaigne's extract and specific functional group tests). Let's break down the detective work step by step.
The Detective's Setup We are given four unknown samples labeled A, B, C, and D
We know that these correspond to four specific artificial sweeteners: Aspartame, Saccharin, Sucralose, and Alitame. To unmask them, we are provided with three chemical clues based on their reactions with specific reagents.
Clue 1
The Ninhydrin Test
The first clue states that samples A and D form a blue-violet color with ninhydrin. The ninhydrin test is a classic biochemical assay used to detect primary amines, specifically amino acids. A positive test indicates the presence of a free amino group ().
When we look at the structures of our candidates, both Aspartame and Alitame are dipeptide derivatives. They are synthesized using aspartic acid, which retains a free primary amino group in the final molecule. Saccharin and Sucralose do not possess this group. Therefore, we can confidently deduce that A and D must be Aspartame and Alitame, in some order.
Clue 2
The Halogen Hunt
The second clue focuses on sample C. Its Lassaigne's extract gives a positive test but a negative Prussian blue test.
A positive silver nitrate () test confirms the presence of a halogen, which in the context of these sweeteners, is chlorine. Conversely, a negative Prussian blue test (which involves the formation of the complex ) means that nitrogen is completely absent from the molecule.
Looking at our list, Sucralose is a trichloro derivative of sucrose. It contains three chlorine atoms and, being a modified carbohydrate, has absolutely no nitrogen. Thus, sample C is definitively identified as Sucralose.
Clue 3
The Sulfur Signature
The final clue reveals that the Lassaigne's extract of samples B and D gives a positive sodium nitroprusside test. This test is highly specific for detecting sulfur. When sulfur is present, it forms sodium sulfide in the extract, which then reacts with sodium nitroprusside to yield a deep purple complex.
Among our candidates, Saccharin (a sulfimide) and Alitame (which contains a thietane ring) both have sulfur atoms. Therefore, B and D must be Saccharin and Alitame.
Putting the Pieces Together Now, let's synthesize our findings: 1
From Clue 1, A and D are {Aspartame, Alitame}.
2. From Clue 3, B and D are {Saccharin, Alitame}.
The common element in both sets is sample D, which must be the sweetener that contains both an amino group and sulfur. That sweetener is Alitame.
If D is Alitame, then A must be Aspartame (to satisfy Clue 1), and B must be Saccharin (to satisfy Clue 3). We already established that C is Sucralose.
Our final mapping is:
- A: Aspartame
- B: Saccharin
- C: Sucralose
- D: Alitame
This perfectly matches option (d). By systematically applying qualitative analysis to molecular structures, we've successfully solved the puzzle!