The Colorful World of Qualitative Organic Analysis
Welcome to the fascinating realm of qualitative organic chemistry! When you walk into a chemistry laboratory, you aren't just looking at clear liquids and white powders; you are looking at a world of hidden identities waiting to be revealed through the magic of chemical tests. This problem is a beautiful exercise in matching organic compounds with their signature chemical reagents. Let's break down the chemistry behind each of these colorful reactions.
The Ninhydrin Test
Unmasking Amino Acids
Our first compound is Lysine, a fundamental building block of proteins. If you look closely at its structure, you will notice it is an α-amino acid, possessing both an amine group (−NH2) and a carboxylic acid group (−COOH).
How do we detect such a molecule? Enter the Ninhydrin test. Ninhydrin is a powerful oxidizing agent. When it reacts with an α-amino acid, it causes oxidative deamination and decarboxylation. The amino acid is broken down into an aldehyde, ammonia, and carbon dioxide. The liberated ammonia then reacts with another molecule of ninhydrin and its reduced form (hydrindantin) to form a deeply colored blue-purple complex known as Ruhemann's purple.
Because Lysine produces this striking violet colouration, we confidently match (A) Lysine with (Q) Ninhydrin.
The Molisch Test
The Signature of Carbohydrates
Next, we encounter Furfural. Furfural is an organic compound derived from the dehydration of sugars, specifically pentoses. It is the central player in the Molisch test, which is the universal test for carbohydrates.
In the Molisch test, a carbohydrate is treated with the Molisch reagent, which is simply a solution of 1-naphthol in ethanol. When concentrated sulfuric acid is carefully added down the side of the test tube, it dehydrates the carbohydrate to form furfural (or hydroxymethylfurfural for hexoses). This newly formed furfural immediately condenses with two molecules of 1-naphthol to form a stunning violet or purple ring at the junction of the two liquids.
Thus, the presence of furfural directly links to the reagent 1-naphthol, giving us the match (B) Furfural with (P) 1-naphthol.
The CAN Test
Identifying Alcohols
Our third molecule is Benzyl alcohol, a classic primary alcohol. To identify the presence of an alcoholic hydroxyl group (−OH), chemists frequently rely on the Ceric Ammonium Nitrate (CAN) test.
Ceric Ammonium Nitrate, (NH4)2Ce(NO3)6, is a yellow-orange coordination complex. When an alcohol is added to a solution of CAN, the alcohol molecules displace the nitrate ligands around the central Cerium(IV) ion. This ligand exchange alters the electronic transitions within the complex, resulting in a dramatic color shift from yellow to a deep red or pink complex.
This color change is a definitive positive test for alcohols. Therefore, we match (C) Benzyl alcohol with (S) Ceric ammonium nitrate.
Baeyer's Test
Exposing Unsaturation
Finally, we look at Styrene. Styrene features a benzene ring attached to a vinyl group (−CH=CH2). The presence of this carbon-carbon double bond means the molecule is unsaturated.
To test for unsaturation, we use Baeyer's reagent, which is a cold, dilute, alkaline solution of potassium permanganate (KMnO4). Potassium permanganate is a brilliant purple-pink color. When it encounters a double bond, it undergoes a syn-dihydroxylation reaction, adding two hydroxyl groups across the double bond to form a vicinal diol. In the process, the Mn(VII) in permanganate is reduced to Mn(IV), precipitating as brown manganese dioxide (MnO2).
The visual result is the rapid discharge of the pink color, confirming the presence of the double bond. Hence, we match (D) Styrene with (R) KMnO4.
Bringing It All Together
By understanding the specific functional groups and their corresponding chemical reactions, we have successfully decoded the matrix:
A → Q
B → P
C → S
D → R
This perfectly aligns with option (c). Mastering these qualitative tests not only helps in solving complex matrix matches but also builds a strong intuitive foundation for practical organic chemistry.