Problem 54 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Glucose comes under the class: (a) ketohexose (b) aldohexose (c) disaccharide (d) polysaccharide

Step-by-Step Solution

Answer

Glucose is an aldohexose (option b).

Step 1: Understanding Classes of Sugars

To determine what class glucose belongs to, we first need to understand the different categories involved: a ketohexose is a sugar with a ketone group and six carbon atoms, aldohexose is a sugar with an aldehyde group and six carbon atoms, a disaccharide is made up of two monosaccharides, and a polysaccharide consists of multiple sugar units.

Step 2: Structure Analysis of Glucose

Glucose is a monosaccharide with the chemical formula \(C_6H_{12}O_6\). It has an aldehyde group, which is a characteristic of an aldose sugar, and it contains six carbon atoms, classifying it specifically as a hexose.

Step 3: Determine the Correct Class

Based on the structure, glucose is not a disaccharide or polysaccharide because it contains only one sugar unit. Since glucose has an aldehyde group and six carbon atoms, it is classified as an aldohexose.

Key Concepts

Structure of Glucose

Glucose is one of the most important and well-known sugars, particularly because of its role in biology as a primary energy source. It is a simple sugar, also known as a monosaccharide, which means it is a single sugar unit. The chemical formula for glucose is \(C_6H_{12}O_6\), indicating it contains six carbon atoms, twelve hydrogen atoms, and six oxygen atoms. These atoms are arranged to form a hexagonal ring structure, which is typical for many sugars.
The structure of glucose is characterized by the presence of an aldehyde group \(( -CHO )\), which is located at one end of the molecule. This aldehyde group is a defining feature, differentiating glucose as an aldose. Having six carbon atoms classifies glucose as a hexose, hence the term "aldohexose." Additionally, the arrangement of hydroxyl \((-OH)\) groups around the carbon ring determines whether glucose is in the alpha form or beta form, which are important in distinguishing how glucose behaves in forming larger carbohydrate structures.

Classification of Sugars

Sugars can be classified in several ways, primarily by their functional groups and the number of carbon atoms. The classification helps us to understand the chemical properties and biological roles of different sugars.
A fundamental classification is based on the functional group present in the sugar molecule:

• An **aldose** contains an aldehyde group \((-CHO)\).
• A **ketose** features a ketone group \((-C=O-)\). For example, fructose is a well-known ketose.

Another way to classify sugars is by the number of carbon atoms they contain:

• **Triose:** 3 carbon atoms
• **Tetrose:** 4 carbon atoms
• **Pentose:** 5 carbon atoms
• **Hexose:** 6 carbon atoms, like glucose

By combining these, sugars can be more specifically classified. For glucose, which has both an aldehyde group and six carbons, the correct classification is aldohexose. This classification assists in predicting glucose's reactivity and its role in metabolism.

Monosaccharides

Monosaccharides are the simplest form of carbohydrates and are fundamental units that cannot be broken down into simpler sugars through hydrolysis. They serve as the building blocks for more complex carbohydrates, such as disaccharides and polysaccharides.
These simple sugars are crucial because:

• They are the initial source of energy for cells, being easily absorbed by the body.
• They play a role in cellular communication and recognition processes.
• They can exist in open-chain or ring forms, with the ring form being more prevalent in aqueous solutions.

Common monosaccharides include glucose, galactose, and fructose. Each has unique properties and roles within biological systems, but they all share the quality of being a single sugar unit. Monosaccharides vary in their chemical structure, contributing to their different functions in metabolism and energy production. Understanding the characteristics of monosaccharides aids in comprehending their significance in larger biological molecules and pathways.