Problem 83 - A Complete Resource Book in Chemistry for JEE Main

Question

In most of the cases glycosidic linkage is formed between (a) \(\left(1 \longrightarrow 4^{\prime}\right)\) (b) \(\left(4 \longrightarrow 4^{\prime}\right)\) (c) \(\left(2 \longrightarrow 2^{\prime}\right)\) (d) \(\left(1 \longrightarrow 3^{\prime}\right)\)

Step-by-Step Solution

Answer

(a) (1 → 4')

Step 1: Understand Glycosidic Linkages

A glycosidic linkage refers to a bond formed between the hydroxyl group of a sugar molecule and another molecule, which may be another sugar or a different compound. Most commonly, glycosidic bonds form between the first carbon (anomeric carbon) of one sugar and a specific carbon of another sugar.

Step 2: Recognize Common Linkages

In disaccharides and polysaccharides, the most common glycosidic linkage is the (1 ightarrow 4) linkage. This involves the first carbon ( C_1 ) of the first monosaccharide bonding with the fourth carbon ( C_4 ) of the second monosaccharide.

Step 3: Compare the Options with Common Practice

From the options given, (1 ightarrow 4') ( C_1 of one sugar to C_4 of another) is the most common glycosidic bond, typically seen in disaccharides like maltose.

Key Concepts

Disaccharides

Disaccharides are a type of carbohydrate made up of two monosaccharide units. These simple sugars form a glycosidic bond through a dehydration reaction, where a molecule of water is removed. In this process, a common connection is made between the anomeric carbon of one sugar and the fourth carbon of another. This is referred to as a \((1 \rightarrow 4)\) linkage.
This linkage is predominant in sugars like maltose and lactose. Maltose is formed from two glucose units, whereas lactose combines glucose with galactose. Disaccharides serve various roles in both human diet and biological systems, such as energy sources and sweetness providers.

Polysaccharides

Polysaccharides are larger carbohydrate molecules composed of long chains of monosaccharide units linked by glycosidic bonds. These complex sugars fulfill important biological roles, such as energy storage and structural support.

• Energy storage polysaccharides, like starch and glycogen, usually have \((1 \rightarrow 4)\) and \((1 \rightarrow 6)\) linkages. These allow for compact storage and rapid glucose release.
• Structural polysaccharides like cellulose feature strong \((1 \rightarrow 4)\) linkages between glucose units, providing rigidity to plant cell walls.

Understanding how different linkages affect the properties and functions of polysaccharides can help in fields from nutrition to material science.

Anomeric Carbon

The anomeric carbon is a special carbon found in carbohydrate molecules. It plays a crucial role in forming glycosidic linkages, which are pivotal connections in carbohydrate chemistry. Located at the first carbon atom of a sugar, the anomeric carbon can form a bond with another sugar or molecule.
In glucose, when it cyclizes to form a ring, the anomeric carbon becomes the new chiral center. This allows for the formation of different isomers, called anomers. Alpha and beta configurations result depending on the orientation of the hydroxyl group at the anomeric carbon.
This flexibility is significant in the diversity of glycosidic linkages, influencing the structure and function of disaccharides and polysaccharides.

Monosaccharides

Monosaccharides are the simplest form of carbohydrates, consisting of a single sugar unit. They are the building blocks for more complex carbohydrates like disaccharides and polysaccharides. Common examples include glucose, fructose, and galactose.
Monosaccharides possess multiple hydroxyl groups and a carbonyl group, allowing them to form cyclic structures. This cyclic form includes the anomeric carbon, a key player in glycosidic linkage formation.

• Glucose is a primary energy source for many organisms.
• Fructose, found in fruits, is sweeter and often used in sweeteners.
• Galactose forms part of lactose when linked with glucose.

A good grasp of monosaccharides is foundational for understanding larger carbohydrate structures and their biochemical roles.