Animated Solution for Chemistry - Biomolecules: The correct observation in the following reactions, is $\text{Sucrose}\stackrel{\text{Glycosidic bond Cleavage (Hydrolysis)}}{\to }\text{A}+\text{B}\stackrel{\text{Seliwanoff’s reagent}}{\to }\text{?}$

Hydrolysis of Sucrose

Imagine you have a molecule of sucrose, the common table sugar. Sucrose is a disaccharide, meaning it is composed of two simpler sugar units joined together. These two units are $\alpha$-D-glucose and $\beta$-D-fructose, and they are linked by a specific type of bond called a glycosidic bond.

When sucrose is subjected to hydrolysis—typically by heating it with a dilute acid or using an enzyme like invertase—this glycosidic bond is cleaved. The reaction can be represented as:

This process is also known as the inversion of cane sugar because the optical rotation of the solution changes from dextrorotatory (positive) to levorotatory (negative) due to the high specific rotation of the liberated fructose. So, in our problem, the products A and B are glucose and fructose.

Seliwanoff's Reagent

Now that we have a mixture of glucose and fructose, we introduce our chemical detective: Seliwanoff's reagent. But what exactly is it? Seliwanoff's reagent is a simple mixture of resorcinol (benzene-1,3-diol) dissolved in dilute hydrochloric acid (HCl).

This reagent is specifically designed to distinguish between two types of monosaccharides: aldoses (sugars with an aldehyde group, like glucose) and ketoses (sugars with a ketone group, like fructose).

Why Ketoses Win the Race

When the mixture of glucose and fructose is heated with Seliwanoff's reagent, a fascinating chemical race begins. The acidic environment causes the sugars to undergo dehydration (loss of water molecules).

Here is the catch: Ketoses dehydrate much more rapidly than aldoses. Fructose, being a ketose, quickly loses three water molecules to form a compound called 5-hydroxymethylfurfural.

Once this furfural derivative is formed, it immediately reacts (condenses) with the resorcinol present in the reagent. This condensation reaction produces a deep, cherry-red colored complex.

On the other hand, glucose (an aldose) dehydrates very slowly. By the time the fructose has already turned the solution red, the glucose has barely started reacting. If you were to boil the glucose for a very long time, it might eventually produce a faint pink color, but the rapid formation of the red color is the hallmark of a ketose.

In our reaction sequence, the hydrolysis of sucrose yields fructose. When Seliwanoff's reagent is added, the fructose rapidly reacts to form a red-colored complex. Therefore, the correct observation is the formation of a red colour.

This makes option (c) the correct answer. It is a beautiful example of how specific chemical reagents can exploit the subtle structural differences between molecules to give us a clear, visual signal.