Animated Solution for Chemistry - Organic Chemistry: Seliwanoff test and xanthoproteic test are used for the identification of ......... and ......... respectively.

Welcome, future scientists! Today, we are diving into the fascinating world of biochemical qualitative analysis. When you are working in a chemistry lab, you often encounter unknown samples. How do you figure out what is inside them? You use specific chemical tests that act like molecular detectives, revealing the identity of the compounds through beautiful and distinct colour changes.

In this problem, we are asked to identify the specific targets of two very famous tests: the Seliwanoff's test and the Xanthoproteic test. Let's break down the chemistry behind each of them so you never have to just memorize the answer again.

Hunting for Ketoses

Imagine you have a solution containing a sugar, but you don't know if it's an aldose (like glucose) or a ketose (like fructose). This is where Seliwanoff's test comes to the rescue.

The reagent used in this test is a mixture of concentrated hydrochloric acid ($\text{HCl}$) and resorcinol. The principle behind this test relies on the fact that when heated with an acid, ketoses undergo dehydration much more rapidly than aldoses.

When a ketose is heated with concentrated $\text{HCl}$, it quickly loses water molecules to form a compound called 5-hydroxymethylfurfural. This newly formed furfural derivative then reacts (condenses) with two molecules of resorcinol. The result of this condensation is a highly conjugated, large molecular complex that absorbs specific wavelengths of light, appearing to our eyes as a brilliant, deep cherry red colour.

While aldoses can also undergo this dehydration, the process is incredibly slow. Therefore, if you see that rapid formation of a cherry red colour, you can confidently declare: "We have a ketose!"

Unmasking Proteins

Now, let's shift our focus to the Xanthoproteic test. The prefix "xantho-" comes from the Greek word for yellow, which is a massive hint about what we are going to see. This test is a classic qualitative method used to detect the presence of proteins in a solution.

But it doesn't just react with any part of the protein; it specifically targets amino acids that contain activated aromatic rings, such as tyrosine and tryptophan.

Here is how the magic happens. First, you add concentrated nitric acid (${\text{HNO}}_3$) to your protein sample and heat it. The nitric acid acts as an electrophile and attacks the electron-rich aromatic rings of tyrosine and tryptophan in a classic electrophilic aromatic substitution reaction known as nitration. The nitrated aromatic compounds that are formed have a distinct yellow colour.

But the test isn't over yet! To confirm the result, we cool the test tube and carefully add a strong base, typically a $40\%$ solution of sodium hydroxide ($\text{NaOH}$). The base neutralizes the acid and deprotonates the phenolic hydroxyl group of the nitrated tyrosine. This deprotonation increases the electron delocalization (conjugation) across the molecule. In the world of photochemistry, increased conjugation leads to a bathochromic shift—meaning the absorbed light shifts to a longer wavelength. Visually, this causes the solution's colour to dramatically shift from yellow to a vibrant orange.

By understanding the underlying chemical mechanisms, the answer to our question becomes crystal clear.

1. The Seliwanoff's test relies on rapid acid-catalyzed dehydration to identify ketoses. 2. The Xanthoproteic test relies on the nitration of aromatic amino acids to identify proteins.

Matching this with our given choices, we see that option (c) perfectly aligns with our chemical deductions. Chemistry isn't just about memorizing facts; it's about understanding the beautiful, logical dance of molecules!