Question

Carbohydrates are classified as monosaccharides, oligosaccharides and polysaccharides depending on their behaviour towards hydrolysis. All monosaccharides containing five or six carbon atoms can have two cyclic structures namely furanose and pyranose form. Oligosaccharides are further classified as di, tri, tetra etc saccharides depeding upon the number of monosaccharide units actually obntained on hyrolysis. In oligosaccharides and polysaccharides, the different monomer units are joined together by glycosidic linkages. The incorrect statement about sucrose is (a) It does not reduce Tollen's reagent for Fehling's solution. (b) It is not optically active. (c) It is an a-D-glucoside. (d) It does not form osazone with phenyl hydrazine.

Short answer

a) It does not reduce Tollen's reagent or Fehling's solution. b) It is not optically active. c) It is an α-D-glucoside. d) It does not form osazone with phenyl hydrazine. Answer: b) It is not optically active.

Step-by-step solution

Understanding Sucrose Structure

Sucrose is a disaccharide composed of one glucose molecule and one fructose molecule joined together via the glycosidic linkage. The glycosidic bond is formed between the C1 (anomeric carbon) of the α-D-glucose and the C2 (also an anomeric carbon) of the β-D-fructose. As a result, it has neither a free aldehyde nor a free ketone group.

Analysis of Option A

Option A states that sucrose does not reduce Tollen's reagent or Fehling's solution. Tollen's reagent and Fehling's solution are used to detect the presence of free aldehyde or ketone groups in a carbohydrate. Since sucrose has neither free aldehyde nor ketone groups, this statement is correct.

Analysis of Option B

Option B states that sucrose is not optically active. Optical activity is the ability of a molecule to rotate plane-polarized light. Both glucose and fructose are optically active. When they form a sucrose molecule, the odd-order glycosidic bond between them results in an optically active sucrose molecule. Therefore, this statement is incorrect.

Analysis of Option C

Option C states that sucrose is an α-D-glucoside. As mentioned in Step 1, sucrose is a disaccharide composed of one α-D-glucose and one β-D-fructose molecule. Hence, this statement is correct.

Analysis of Option D

Option D states that sucrose does not form osazone with phenyl hydrazine. Osazone formation is a chemical reaction that occurs when a carbohydrate containing a free aldehyde or ketone group reacts with phenyl hydrazine. As sucrose has neither a free aldehyde nor a ketone group, it will not form osazone with phenyl hydrazine. Thus, this statement is correct. Based on the step-by-step analysis, we can conclude that the incorrect statement about sucrose is: (b) It is not optically active.

Key concepts

Monosaccharides and Polysaccharides

Carbohydrates are organic compounds classified into three primary categories based on their behavior during hydrolysis. These are monosaccharides, oligosaccharides, and polysaccharides. Understanding these classifications is crucial in grasping carbohydrate chemistry.

Monosaccharides are the simplest form of carbohydrates, consisting of a single sugar unit. Common examples include glucose and fructose. These molecules can form cyclic structures such as furanose and pyranose rings depending on the number of carbon atoms. Such structural versatility is key to their biological functions.

Oligosaccharides, like sucrose, consist of a few linked monosaccharides. A disaccharide like sucrose is composed of two monosaccharide units - specifically glucose and fructose. Polysaccharides are long chains formed by repeating monosaccharide units. They play vital roles in storing energy and providing structural support in living organisms.

This classification provides a framework for understanding how carbohydrates function in biology and chemistry, depending on their complexity and the number of sugar units they possess.

Glycosidic Linkage

Glycosidic linkage is a type of covalent bond that connects carbohydrate (sugar) molecules together. It plays a fundamental role in the formation of complex carbohydrates.

When two monosaccharides bond to form a disaccharide, such as when glucose and fructose form sucrose, a glycosidic bond is created. This bond typically connects the anomeric carbon (tail end of the sugar molecule) of one carbohydrate to the hydroxyl group of another.

Specifically, in the case of sucrose, a glycosidic linkage is formed between the C1 of α-D-glucose and the C2 of β-D-fructose. This combination results in the disaccharide known as sucrose. Because of this linkage, sucrose does not possess a free aldehyde or ketone group, which would normally be free if the glycosidic bond wasn’t formed.

Understanding glycosidic linkages is essential in biochemistry as they determine the structure, properties, and digestibility of carbohydrates in various organisms.

Optical Activity

Optical activity is a property of chiral molecules, allowing them to rotate the plane of polarized light. This phenomenon is vital in distinguishing different forms of sugars, proteins, and other biomolecules.

In the context of carbohydrates like sucrose, optical activity results from the arrangement of atoms in space, particularly involving carbon-center asymmetry. Both glucose and fructose, the constituent monosaccharides of sucrose, are optically active due to their chiral nature.

When these monosaccharides form a glycosidic bond, the resulting disaccharide (sucrose) retains this property of optical activity. Each chiral center contributes to the rotation of polarized light when it passes through a solution of the sugar.

Recognizing optical activity in carbohydrates is crucial for understanding their reactions and interactions with other molecules, which can have significant implications in food science, pharmacology, and other fields.