Problem 51 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Which of the following is not a disaccharide? (a) sucrose (b) mannose (c) lactose (d) maltose

Step-by-Step Solution

Answer

Mannose is not a disaccharide.

Step 1: Understand What a Disaccharide Is

Disaccharides are carbohydrates composed of two monosaccharide molecules bonded together. They are a type of sugar formed when two sugars are joined and a molecule of water is removed (a reaction known as a dehydration reaction). Common examples of disaccharides include sucrose, lactose, and maltose.

Step 2: Identify the Options Given

The options provided for identifying which one is not a disaccharide are: (a) sucrose, (b) mannose, (c) lactose, and (d) maltose.

Step 3: Recognize Known Disaccharides

From general chemistry knowledge, we know that sucrose, lactose, and maltose are indeed disaccharides. Sucrose is composed of glucose and fructose, lactose is composed of glucose and galactose, and maltose is composed of two glucose units.

Step 4: Evaluate Mannose

Mannose is a monosaccharide, not a disaccharide. It is a simple sugar and is found in some fruits and vegetables, as well as a component of polysaccharides in some bacteria.

Step 5: Conclusion Based on Analysis

Based on the identification of disaccharides and evaluation of mannose as a monosaccharide, the item "mannose" is not a disaccharide among the given options.

Key Concepts

Monosaccharides

Monosaccharides are the simplest form of carbohydrates. They are the building blocks of more complex sugars and are often referred to as "simple sugars." These molecules cannot be broken down into simpler sugars. Monosaccharides have a general chemical formula of \( C_nH_{2n}O_n \). They typically contain three to seven carbon atoms. Common examples include glucose, fructose, and mannose. Each monosaccharide differs in structure and configuration, making them unique.

• **Glucose** is the most well-known monosaccharide, serving as a primary energy source in many organisms.
• **Fructose**, found naturally in fruits, is another abundant monosaccharide.
• **Mannose** is less common in nature but is crucial in the metabolism of certain compounds.

These sugars can be found in various foods and are vital in energy production and cellular functions.

Carbohydrates

Carbohydrates are organic molecules composed entirely of carbon, hydrogen, and oxygen atoms. They are a significant source of energy for the body and play essential roles in biological processes. Carbohydrates can be classified into three major groups based on their structure:

• **Monosaccharides** – Simple sugars like glucose and fructose.
• **Disaccharides** – Formed by two monosaccharides linked together, such as sucrose, lactose, and maltose.
• **Polysaccharides** – Complex carbohydrates like cellulose, starch, and glycogen.

In addition to providing energy, carbohydrates also contribute to cell structure and signaling processes. Understanding the various forms and functions of carbohydrates is crucial for appreciating their role in nutrition and metabolism.

Dehydration Reaction

A dehydration reaction, also known as a dehydration synthesis, is a chemical reaction that involves the loss of a water molecule from the reacting molecules. This process is vital in the formation of many biological macromolecules, including disaccharides. During this reaction, two monosaccharides join to form a disaccharide. For example, the combination of

• **Glucose and Fructose** forms Sucrose.
• **Glucose and Galactose** results in Lactose.
• **Two Glucose molecules** make Maltose.

Each time a disaccharide forms, a molecule of water is released, which is why it's termed a dehydration reaction. This process is essential in various metabolic pathways and is foundational in the synthesis of complex carbohydrates from simple sugars.