Problem 10 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Which one of the following gives positive Fehling's solution test? |CBSE 2001] (a) Sucrose (b) Glucose (c) Fats (d) Protein

Step-by-Step Solution

Answer

Glucose gives a positive Fehling's solution test.

Step 1: Understanding Fehling's Solution Test

Fehling's solution test is used to detect the presence of reducing sugars. A reducing sugar is one that can donate electrons to another chemical, reducing it. This is often because they have a free aldehyde group or a free ketone group.

Step 2: Analyzing Each Option

- Sucrose is a disaccharide compound that is not a reducing sugar because it lacks a free aldehyde or ketone group due to its glycosidic linkage. - Glucose is a simple sugar (monosaccharide) with a free aldehyde group, making it a reducing sugar. - Fats usually consist of fatty acids and glycerol and do not have a reducing property. - Proteins are composed of amino acids and do not have reducing properties similar to carbohydrates.

Step 3: Identifying the Reducing Sugar

Among the given options, the compound with a free aldehyde group capable of reducing Fehling's solution to produce a positive result is glucose. The other options do not have the necessary structure to reduce Fehling's solution.

Key Concepts

Reducing Sugars

Reducing sugars are carbohydrates that can act as reducing agents because they possess free aldehyde or ketone groups. These groups allow the sugar to donate electrons to other molecules, undergoing oxidation in the process. Reducing sugars play an important role in many biochemical reactions and are commonly tested for using Fehling’s solution. This test helps to identify the presence of these sugars in substances.

• Common reducing sugars include glucose, fructose, and maltose.
• These sugars often taste sweet and can undergo Maillard reactions, which are important in cooking and baking.

When a sugar is reducing, it means it has the ability to reduce another compound while being oxidized itself. For example, glucose, a reducing sugar, can donate electrons and reduce the copper ions in Fehling's solution, resulting in a color change from blue to red.

Aldehyde Group

The aldehyde group is a functional group present in organic chemistry that has significant importance. It is characterized by the presence of a carbon atom double-bonded to an oxygen atom and single-bonded to a hydrogen atom ( \[\text{-CHO}\]). Aldehyde groups are highly reactive and are often found in several important biochemical molecules, particularly carbohydrates.

• They play a key role in determining whether a sugar is reducing or non-reducing.
• An aldehyde group that is free (not bound in a glycosidic bond) can participate in redox reactions, typical for reducing sugars.

In the Fehling's test, the presence of a free aldehyde group in glucose allows it to reduce the copper ions in the solution, resulting in the characteristic red precipitate indicative of a positive reaction.

Carbohydrates

Carbohydrates are essential biomolecules made up of carbon, hydrogen, and oxygen. They are one of the primary sources of energy for living organisms and play numerous roles in biological functions. Carbohydrates vary greatly in complexity, ranging from simple sugars to very large polysaccharides.

• Simpler forms like monosaccharides and disaccharides can be sweet-tasting and are critical in energy metabolism.
• They can be classified into monosaccharides, disaccharides, and polysaccharides based on the number of sugar units.

In the context of reducing sugars, only some carbohydrates have the structural characteristics needed to act as reducing agents. Sucrose, despite being a carbohydrate, is not a reducing sugar because its glycosidic linkage involves both its reducing ends, preventing it from donating electrons in reactions.

Monosaccharides

Monosaccharides are the simplest form of carbohydrates and consist of single sugar units. Due to their basic structure, they are fundamental in the makeup of more complex carbohydrates like oligosaccharides and polysaccharides.

• They typically contain three to seven carbon atoms and are categorized based on the number of carbon atoms they possess, such as triose, tetrose, pentose, hexose, etc.
• Monosaccharides such as glucose, fructose, and galactose are notable examples.

Monosaccharides like glucose are particularly important in chemistry due to their free aldehyde or ketone groups, which enable them to behave as reducing sugars. This ability makes them key molecules in energy production and storage within biological systems. In terms of chemical reactions, their simplicity allows them to readily participate in oxidation-reduction reactions, making them an ideal candidate for Fehling's solution test.