Animated Solution for Chemistry - Organic Chemistry: Match the compounds in LIST-I with the observation in LIST-II, and choose the correct option.

The beauty of qualitative organic analysis lies in its ability to reveal the hidden architecture of molecules through simple, elegant chemical reactions. In this problem, we are presented with four distinct compounds—Aniline, o-Cresol, Cysteine, and Coprolactam—and tasked with matching them to their characteristic chemical tests. Let's embark on this molecular detective journey!

Our first suspect is Aniline. Structurally, it consists of an amino group ($-N{H}_2$) directly attached to a benzene ring.

The presence of the nitrogen atom is a dead giveaway for Lassaigne's test. When Aniline undergoes sodium fusion, the nitrogen and carbon combine to form sodium cyanide ($NaCN$). Boiling this extract with iron(II) sulfate ($FeS{O}_4$) and acidifying it with concentrated sulfuric acid ($H_{2}S{O}_4$) yields the iconic Prussian blue color. Thus, Aniline matches with observation (P).

But Aniline has another trick up its sleeve. The $-N{H}_2$ group is a powerful activating group, pushing electron density into the benzene ring. This makes the ring highly susceptible to electrophilic aromatic substitution. When treated with bromine water, Aniline rapidly undergoes polyhalogenation to form 2,4,6-tribromoaniline, which appears as a white precipitate. This perfectly matches observation (S).

Next, we examine o-Cresol. This molecule features a hydroxyl group ($-OH$) and a methyl group ($-C{H}_3$) on adjacent carbons of a benzene ring.

Because it lacks nitrogen, sulfur, or halogens, Lassaigne's tests will be completely silent here. However, the phenolic $-OH$ group is the star of the show. Phenols are famous for their reaction with neutral iron(III) chloride ($FeC{l}_3$) solution, where they form a complex that produces a striking violet color. Therefore, o-Cresol matches exclusively with observation (T).

Cysteine is a fascinating molecule. It is an amino acid, meaning it packs multiple functional groups into a small framework: a carboxylic acid group ($-COOH$), an amino group ($-N{H}_2$), and a thiol group ($-SH$).

Let's break down its reactivity. The $-COOH$ group is acidic enough to react with a saturated solution of sodium bicarbonate ($NaHC{O}_3$). This acid-base reaction releases carbon dioxide gas, resulting in brisk effervescence. This matches observation (R).

Furthermore, Cysteine contains both nitrogen and sulfur. During the intense heat of sodium fusion, these elements combine with carbon to form sodium thiocyanate ($NaSCN$). When this extract is treated with $F{e}^{3+}$ ions (often tested via sodium nitroprusside or $FeC{l}_3$ depending on the specific lab protocol), it produces a deep blood-red color due to the formation of the $[Fe(SCN){]}^{2+}$ complex. This aligns perfectly with observation (Q).

Coprolactam

Finally, we arrive at Coprolactam (often spelled Caprolactam). This molecule is a 7-membered cyclic amide, widely known as the precursor to the polymer Nylon-6.

The critical feature here is the nitrogen atom embedded within the amide linkage. Just like Aniline, the presence of nitrogen ensures that its sodium fusion extract will form cyanide ions. Subsequent treatment with $FeS{O}_4$ and acid will inevitably produce the classic Prussian blue color. Thus, Coprolactam matches with observation (P).

By carefully analyzing the functional groups and elemental composition of each molecule, we have successfully decoded their chemical behavior: - (I) Aniline $\to$ P, S - (II) o-Cresol $\to$ T - (III) Cysteine $\to$ Q, R - (IV) Coprolactam $\to$ P

This problem is a brilliant reminder that organic chemistry is not just about drawing structures on paper; it is about understanding how those structures interact with the physical world to produce vibrant colors, gases, and precipitates!