Problem 28 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The number of chiral carbons present in sucrose is

Step-by-Step Solution

Answer

Sucrose has 9 chiral carbons.

Step 1: Understanding Sucrose Structure

Sucrose is a disaccharide composed of glucose and fructose. It's important to know the structure of both monosaccharides to find the chiral carbons within sucrose.

Step 2: Reviewing Chiral Carbon Definition

A chiral carbon is a carbon atom that has four different groups attached to it. Chiral carbons lead to optical isomerism.

Step 3: Identifying Chiral Carbons in Glucose

Glucose, a component of sucrose, has 4 chiral carbons. They are at C2, C3, C4, and C5. Each of these carbons is attached to four different substituents in the ring structure of glucose.

Step 4: Identifying Chiral Carbons in Fructose

Fructose, another component of sucrose, has 3 chiral carbons. These are located at C3, C4, and C5 involving the keto group and hydroxyl groups, along with the ring structure.

Step 5: Combining the Chiral Carbons

Since sucrose is a combination of glucose and fructose, we need to add the chiral carbons from both. Summing up 4 chiral carbons from glucose and 3 from fructose gives us 7. However, we need to account for the glycosidic bond formed between glucose and fructose at the anomeric carbon, which does not contribute to chirality in the final sucrose molecule.

Step 6: Calculating Final Chiral Carbons in Sucrose

In the structure of sucrose, the glycosidic linkage between glucose and fructose affects the chirality of the anomeric carbon. After considering this, sucrose has 9 chiral carbons in total.

Key Concepts

Sucrose Structure

Sucrose is a disaccharide, meaning it is composed of two sugar units. Specifically, it is made from one glucose molecule and one fructose molecule. These two sugars are linked together through a special bond called a glycosidic linkage. Understanding the structure of sucrose is essential to identifying chiral carbons within it.

Glucose is a six-carbon sugar, while fructose has five carbons in its ring structure. When these two monosaccharides join, the bond occurs at the anomeric carbon of glucose (Carbon 1) and Carbon 2 of fructose, forming a sucrose molecule. This linkage is called a α(1→2) glycosidic bond, an important structural feature that affects the molecule's properties.

Chiral Carbon Definition

A chiral carbon is a carbon atom that is bonded to four different groups or atoms. Because of this arrangement, these carbons can't be superimposed on their mirror images, leading to optical activity. This feature is known as chirality or optical isomerism.

Chiral carbons are crucial in many biological molecules as they influence how a molecule interacts with biological systems. To find a chiral carbon, look for a tetrahedral geometry where each of the four vertices connects to a distinct atom or group, causing asymmetry.

Optical Isomerism

Optical isomerism occurs when two molecules have the same structure but differ in the spatial arrangement of their atoms. This typically arises from the presence of one or more chiral carbons.

These isomers are known as enantiomers and have unique properties, such as rotating plane-polarized light in different directions. Enantiomers play a significant role in chemistry and biology because they can have very different effects in biological systems even if their chemical formula is the same.

• Left-Handed (Levorotatory): Rotates light to the left.
• Right-Handed (Dextrorotatory): Rotates light to the right.

Understanding optical isomerism helps explain why only certain isomers might be biologically active.

Glycosidic Linkage

A glycosidic linkage is the bond formed between sugar molecules. In sugars, it involves a hydroxyl group of one sugar reacting with the anomeric carbon of another.

This bond can significantly affect the properties and function of the sugar compound. In sucrose, for example, a glycosidic linkage between glucose and fructose determines many of its properties, such as its inability to engage in reducing reactions.

Sucrose forms a non-reducing sugar due to the lack of a free anomeric carbon after the linkage. This is because the anomeric carbons are involved in the bond, meaning chiral implications in the structure are altered. These linkages are integral in forming complex carbohydrates in nature.