Carbohydrates: Complete Guide for Class 12 NCERT Chemistry

What Are Carbohydrates? An Overview for Class 12 Students

Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen, usually in the ratio $C:H:O = 1:2:1$. They serve as a primary energy source in living organisms and are essential in various biological functions.

In Class 12 NCERT Chemistry, carbohydrates are introduced as polyhydroxy aldehydes or ketones. Their general formula is $C_n(H_2O)_n$. Examples include glucose, fructose, and sucrose.

Key features:

• Contain multiple hydroxyl (-OH) groups
• Can exist in linear or cyclic forms
• Soluble in water due to hydrogen bonding

Understanding carbohydrates is crucial for grasping biochemical processes and their industrial applications.

Classification of Carbohydrates Based on Hydrolysis

Carbohydrates are classified into three main groups depending on their hydrolysis products:

Monosaccharides are reducing sugars, while polysaccharides are generally non-reducing and non-sweet. Oligosaccharides include disaccharides like sucrose, which hydrolyzes into glucose and fructose.

Monosaccharides: The Building Blocks

Monosaccharides are the simplest carbohydrates and cannot be hydrolyzed into smaller carbohydrates. They have the general formula $C_nH_{2n}O_n$.

Common monosaccharides:

• Glucose: An aldohexose with six carbon atoms and an aldehyde group.
• Fructose: A ketohexose with six carbon atoms and a ketone group.
• Ribose: A pentose sugar important in nucleic acids.

Properties:

• Sweet taste
• Soluble in water due to multiple -OH groups
• All are reducing sugars because they can reduce Fehling's solution or Tollens' reagent

Example: Glucose exists mainly in cyclic form (pyranose ring) in solution, which is important for its biological function.

Oligosaccharides and Disaccharides Explained

Oligosaccharides consist of 2 to 10 monosaccharide units linked by glycosidic bonds. The most common oligosaccharides are disaccharides, which have two monosaccharide units.

Examples of disaccharides:

• Sucrose: Glucose + Fructose; non-reducing sugar
• Lactose: Glucose + Galactose; reducing sugar
• Maltose: Two glucose units; reducing sugar

Hydrolysis of disaccharides breaks the glycosidic bond, yielding monosaccharides. For example, lactose hydrolyzes to glucose and galactose.

Polysaccharides: Complex Carbohydrates

Polysaccharides are large molecules made of many monosaccharide units linked together. They are generally non-sweet and insoluble or partially soluble in water.

Important polysaccharides:

• Starch: Energy storage in plants; composed of amylose and amylopectin
• Cellulose: Structural component in plant cell walls; linear chains of glucose
• Glycogen: Energy storage in animals; highly branched glucose polymer

Properties:

• Non-reducing sugars
• Not sweet
• Insoluble or sparingly soluble in water

Polysaccharides are hydrolyzed into many monosaccharide units under acidic or enzymatic conditions, providing energy or structural support.

Reducing vs Non-Reducing Sugars: Key Differences

Carbohydrates can be classified based on their ability to reduce certain reagents:

• Reducing sugars: Can reduce Fehling's solution or Tollens' reagent due to free aldehyde or ketone groups.
• Non-reducing sugars: Do not reduce these reagents because their aldehyde or ketone groups are involved in glycosidic bonds.

All monosaccharides are reducing sugars. Disaccharides like lactose and maltose are reducing, while sucrose is non-reducing.

Solubility and Physical Properties of Carbohydrates

Carbohydrates are generally soluble in water because of their multiple hydroxyl (-OH) groups, which form hydrogen bonds with water molecules.

For example:

• Glucose and sucrose dissolve easily in water.
• Non-polar compounds like cyclohexane or benzene do not dissolve in water due to lack of polarity.

Physical properties:

• Sweet taste (mostly monosaccharides and disaccharides)
• Crystalline solids
• High melting points due to hydrogen bonding

Understanding solubility helps explain biological transport and industrial applications of carbohydrates.

Worked Example: Hydrolysis of Lactose

Problem: What are the products of lactose hydrolysis?

Solution:

• Lactose is a disaccharide composed of glucose and galactose linked by a β-1,4-glycosidic bond.
• Hydrolysis (acidic or enzymatic) breaks this bond.
• Products formed are one molecule of glucose and one molecule of galactose.

Equation:

$$ \text{Lactose} + H_2O \xrightarrow{\text{enzyme/acid}} \text{Glucose} + \text{Galactose} $$

This reaction is important in digestion and food chemistry.