Problem 94 - A Complete Resource Book in Chemistry for JEE Main

Question

Glucose is different from fructose as (a) does not undergo hydrolysis (b) gives silver mirror with Tollen's reagent (c) is a monosaccharide (d) none of these

Step-by-Step Solution

Answer

(a) does not undergo hydrolysis

Step 1: Understand Hydrolysis

Hydrolysis is a reaction where a compound reacts with water, leading to the breakdown of that compound. Monosaccharides, like glucose, do not undergo hydrolysis because they are single sugar units and there are no bonds to break using water.

Step 2: Reaction with Tollen's Reagent

Tollen's reagent is used to identify reducing sugars. Glucose, being an aldose, is a reducing sugar that can react with Tollen's reagent, resulting in a silver mirror. Fructose can also isomerize to glucose in the presence of a base, so it may give a silver mirror as well, but indirectly.

Step 3: Identify Monosaccharide Nature

Glucose is a monosaccharide, a simple sugar consisting of one sugar unit, which differentiates it from other carbohydrate types such as disaccharides and polysaccharides.

Step 4: Determine the Correct Answer

Evaluate which of the provided options distinguishes glucose from fructose. Since both are monosaccharides (hence option c doesn't work), and both give a silver mirror test (option b doesn't work), the correct distinguishing feature given in the options is (a), as both do not undergo hydrolysis because they are already monosaccharides.

Key Concepts

Understanding Hydrolysis

Hydrolysis is a fundamental chemical reaction where a compound reacts with water, leading to its breakdown. Let's break it down with a simple analogy. Imagine trying to dismantle a Lego model piece by piece. This is similar to what hydrolysis does – it separates larger molecules into their basic building blocks by adding water. However, monosaccharides like glucose do not undergo hydrolysis.
They are already the simplest form of sugar, consisting of a single unit. There are no additional bonds to break with water.

In the realm of carbohydrates, hydrolysis is essential for breaking down larger sugar chains like disaccharides and polysaccharides into monosaccharides for absorption. But, since glucose and fructose are individual sugar units, hydrolysis does not apply.

Exploring Tollen's Reagent

Tollen's reagent is a chemical solution used for detecting reducing sugars. It's named after a German chemist, Bernhard Tollens. The basis of this test is the ability of some sugars to reduce metal ions, leading to a visible change.
In particular, Tollen's reagent contains ammoniacal silver nitrate, and when a reducing sugar like glucose is present, it gets reduced by the sugar.

This results in the formation of a beautiful silver mirror on the inner surface of the test tube. Among sugars, glucose is a classic example of a reducing sugar due to its free aldehyde group. However, fructose, although a ketose, can undergo enediol rearrangement in a base to form glucose, allowing it to also give a silver mirror indirectly. This principle has made Tollen's reagent an invaluable tool in distinguishing reducing sugars from non-reducing ones.

Characteristics of Reducing Sugars

Reducing sugars are sugars that can donate electrons to another chemical species, essentially getting oxidized themselves. This characteristic makes them essential in various chemical tests and processes, including food chemistry.
Reductions involve the presence of free aldehyde or ketone groups, and in the case of monosaccharides like glucose, the aldehyde group is freely available to engage in redox reactions.

The importance of reducing sugars extends beyond labs. In the culinary world, reducing sugars like glucose participate in the Maillard reaction, a crucial process responsible for the browning and flavor development in cooked foods. However, not all sugars are reducing sugars. For example, sucrose, a disaccharide, lacks a free aldehyde group and doesn't exhibit reducing properties under normal conditions. Recognizing which sugars are reducing is fundamental for understanding their behavior in both biological and chemical contexts.