Problem 3 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following is a non-reducing sugar? (a) glucose (b) sucrose (c) fructose (d) arabinose

Step-by-Step Solution

Answer

The non-reducing sugar is (b) sucrose.

Step 1: Understanding Reducing Sugars

Reducing sugars are those that have a free hemiacetal or hemiketal group, which can act as a reducing agent due to the presence of an aldehyde group or an alpha-hydroxy ketone group.

Step 2: Identifying the Hemiacetal Group

Glucose, fructose, and arabinose all contain free hemiacetal groups in their structures. This makes them capable of acting as reducing agents and reacting with Tollen's or Benedict's reagents.

Step 3: Analyzing Sucrose's Structure

Sucrose is a disaccharide composed of glucose and fructose units linked by a glycosidic bond. This specific bond involves the anomeric carbon of glucose and fructose, making both their hemiacetal groups unavailable for reaction.

Step 4: Conclusion: Non-Reducing Sugar Identification

Since sucrose's glycosidic bond prevents the free hemiacetal groups from reacting, it is not a reducing sugar, distinguishing it from glucose, fructose, and arabinose, which are reducing sugars.

Key Concepts

Reducing Sugars

Reducing sugars are carbohydrates that possess the ability to act as reducing agents. This means that they can donate electrons to other molecules during a chemical reaction. They have a specific structure characterized by a free hemiacetal or hemiketal group. These groups have an exposed carbonyl group, either an aldehyde or a ketone, which can react with compounds like Tollen's or Benedict's reagents, often used in laboratory settings to test for these sugars.

• Examples of reducing sugars include glucose, fructose, and galactose.
• They play vital roles in energy metabolism and can be found in various forms in nature and food.

Understanding the properties of reducing sugars helps in identifying non-reducing sugars, which lack this capability.

Hemiacetal Group

The hemiacetal group is a functional group in carbohydrates, defined by the presence of a carbon atom bound to two oxygens: one in the form of an -OH (hydroxyl group) and the other in the form of -OR (alkoxy group). This structure is critical in organic chemistry as it allows sugars to form cyclic structures.
In simple terms, when a sugar molecule cyclizes, it forms a ring where one of its carbons becomes a hemiacetal or hemiketal center. This center remains crucial for determining a sugar as reducing or non-reducing:

• If the hemiacetal group is free, as in glucose, the sugar can act as a reducing agent.
• If it's involved in a bond, as in sucrose, the sugar is non-reducing.

Learning to identify these groups is key to understanding sugar chemistry and reactions.

Glycosidic Bond

A glycosidic bond is a covalent bond that connects two sugar molecules in carbohydrates. It forms when the anomeric carbon of one sugar reacts with the hydroxyl group of another, creating an acetal linkage.
This bond is significant for understanding the behavior of disaccharides like sucrose:

• In sucrose, the glycosidic bond involves the anomeric carbons of both glucose and fructose, preventing them from existing in their reducing form.
• Glycosidic bonds largely determine the properties of carbohydrates like solubility, sweetness, and digestibility.

Recognizing how glycosidic bonds affect sugar properties can explain why sucrose is non-reducing.

Disaccharides

Disaccharides are carbohydrates composed of two monosaccharide units linked together by a glycosidic bond. Famous examples include sucrose, lactose, and maltose.

• Disaccharides are important in nutrition as they are found in everyday food items like sugar (sucrose) and dairy (lactose).
• The properties of disaccharides depend on the types of monosaccharides involved and the nature of the glycosidic bond.

In sucrose, for example, glucose and fructose units are linked by a specific glycosidic bond that prevents the sugar from being reducing. This highlight underscores the importance of understanding the structure for determining their chemical behavior.