Problem 130 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

(+)-Sucrose is made up of (a) \(\mathrm{L}\) -fructose and \(\mathrm{L}\) -glucose (b) D-glucose and \(\mathrm{L}\) -fructose (c) D-fructose and \(\mathrm{L}\) -glucose (d) D-glucose and \(\mathrm{L}\) -fructose

Step-by-Step Solution

Answer

None of the options are correct; sucrose is made of D-glucose and D-fructose.

Step 1: Understand the Components of Sucrose

Sucrose is a disaccharide, which means it consists of two monosaccharides joined together. It is commonly found naturally in plants.

Step 2: Identify the Monosaccharides in Sucrose

Sucrose is composed of two specific monosaccharides: glucose and fructose. In sucrose, these monosaccharides are linked by a glycosidic bond.

Step 3: Determine the Isomeric Forms Present

In nature, sucrose is made up of the 0D1-isomers of both glucose and fructose. This means that the glucose and fructose in sucrose are both in the D-configuration, which refers to the orientation of the hydroxyl groups around the chiral carbon atoms.

Step 4: Select the Correct Option

The options provided are various combinations of D- and L- forms of glucose and fructose. As established, sucrose consists of D-glucose and D-fructose, which means that none of the options provided are correct, as all options incorrectly list the configurations.

Key Concepts

Disaccharide Structure

A disaccharide is a type of carbohydrate that forms when two monosaccharide molecules join. This is a kind of sugar. Sucrose, for example, is a disaccharide. It combines two simpler sugars, linking them together. Typically, this structure involves a specific type of bond called a glycosidic bond, which serves as the connecting bridge.

The formation of a disaccharide often involves a dehydration reaction—a water molecule is removed during the bonding process. In nature, disaccharides like sucrose play a fundamental role in transporting and storing energy across plants. This makes them essential for various biological functions.

Monosaccharides in Sucrose

Sucrose is specifically composed of two monosaccharides: glucose and fructose. These are the basic building blocks of carbohydrates, commonly referred to as simple sugars. In sucrose, both glucose and fructose unite to form the more complex sugar.

Each monosaccharide contributes its unique characteristics:

• Glucose is a primary energy source for cells.
• Fructose, known for its sweetness, is commonly found in fruits.

The combination of these two sugars in sucrose creates a common dietary sugar found naturally in many plants.

Glycosidic Bond

In sucrose, glucose and fructose are linked by a glycosidic bond. This is a covalent bond formed via a dehydration reaction, where a water molecule is removed. This reaction typically occurs between two hydroxyl groups, one from each monosaccharide.

In terms of structure, the linkage in sucrose is between the anomeric carbon of glucose and the anomeric carbon of fructose. This creates a specific bond often termed an "α,β-1,2-glycosidic bond." Glycosidic bonds are crucial in stabilizing the sugar molecules, allowing them to function within cells and energy usage.

D-isomers

D-isomers refer to a specific orientation of atoms around the chiral carbon atoms in sugars. For sucrose, the arrangement known as the "D" form means that the hydroxyl group (-OH) on the second last carbon of the sugar is on the right in a standard Fischer projection.

In nature, sucrose is composed of D-glucose and D-fructose, meaning the D-isomer configuration dictates the structural conformation. This orientation is biologically active and hence, critical for the function of sucrose as a natural sugar.

Chiral Carbon

Chiral carbon atoms are carbon atoms attached to four different groups. This unique structure allows them to exist in multiple forms, or isomers, which are mirror images of each other.

In sugars, each chiral carbon plays a significant role, as they determine the optical activity of the sugar molecules. This is essential for distinguishing between different isomers, such as D- and L- forms. In the case of sucrose, the presence of chiral carbons in both glucose and fructose is what causes the sugar to have the specific properties and functionalities it does in nature.