Problem 23 - A Complete Resource Book in Chemistry for JEE Main

Question

The number of chiral carbons in \(\beta-\mathrm{D}-(+)\)-glucose is (a) 3 (b) 4 (c) 5 (d) 6

Step-by-Step Solution

Answer

4 chiral carbons.

Step 1: Understanding Chiral Carbons

A chiral carbon atom is a carbon atom that is attached to four different groups. For a molecule to be chiral, it should have at least one chiral carbon, making it asymmetric.

Step 2: Analyzing the Structure of Glucose

Glucose is an aldohexose, meaning it has six carbon atoms, with an aldehyde group on one end. In its 3D-f-D form, it forms a six-membered pyran ring, specifically in the 3D-glucose model.

Step 3: Identifying Chiral Centers

In 3D-3D-glucose, the chiral centers are at the C-2, C-3, C-4, and C-5 positions. These carbons are bonded to four different groups, making them asymmetric, hence chiral.

Step 4: Counting the Chiral Carbons

By identifying the chiral centers, we count that there are 4 chiral carbons in 3D-3D-glucose. No other carbons in the ring meet the criteria for chirality as they do not connect to four different groups.

Key Concepts

Chirality in Organic Molecules

Chirality in organic molecules is a fascinating topic that involves the study of carbon atoms which are connected to four different substituents, leading to an asymmetrical structure. This asymmetry creates a condition where the molecule cannot be superimposed onto its mirror image, similar to how our left and right hands are not identical.

To identify chirality, one must look for a chiral carbon, which is defined as a carbon atom bonded to four distinct groups. The presence of such atoms in a molecule indicates that the molecule displays chirality. Molecular chirality plays a vital role in biochemistry and pharmaceuticals because the mirror images, or enantiomers, could have vastly different biological effects.

• Chiral Carbon: Attached to four different groups.
• Non-Superimposable: Like left and right hands.
• Importance: Affects molecular interaction and reactions.

Aldohexose Structure

An aldohexose is a type of monosaccharide, or simple sugar, which contains six carbon atoms including an aldehyde functional group at one end. Such sugars play a crucial role in biology, serving as energy sources and structural components. Glucose is a well-known example of an aldohexose.

The backbone of an aldohexose consists of a straight chain of six carbon atoms, numbered from the aldehyde group. This structure is significant because it determines the sugar's properties and reactivity.

• Six Carbon Atoms: Fundamental element of structure.
• Aldehyde Group: Characteristic functional group of aldohexoses.
• Biological Role: Integral in energy metabolism.

Pyran Ring Formation

Pyran ring formation is an intriguing process in carbohydrate chemistry, where the straight-chain form of sugars such as glucose cyclizes to form a ring structure. In the case of D-glucose, this typically results in a six-membered ring known as a pyranose.

The formation of the pyran ring involves the attack of the hydroxyl group on the aldehyde group, leading to a hemiacetal linkage. Once cycled, the glucose molecule can exist in different configurations, known as anomers, designated as α or β, depending on the orientation of the hydroxyl group on the anomeric carbon.

• Six-Membered Ring: Common in glucose and fructose.
• Hemiacetal Linkage: Bonds forming between hydroxyl and aldehyde groups.
• Anomers: Variants differing by orientation at the anomeric carbon.