Carbohydrates: Complete Class 12 NCERT Chemistry Guide

What Are Carbohydrates? An Overview

Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen, generally with the formula $C_m(H_2O)_n$. They are essential for energy storage and structural functions in plants and animals. In Class 12 NCERT Chemistry, carbohydrates are studied as biomolecules classified into three main types:

• Monosaccharides: Simple sugars like glucose and fructose.
• Disaccharides: Two monosaccharides linked together, e.g., sucrose and lactose.
• Polysaccharides: Long chains of monosaccharides, such as starch and cellulose.

Carbohydrates are water-soluble due to multiple hydroxyl (-OH) groups that form hydrogen bonds with water molecules. This solubility is crucial for their biological roles.

Classification of Monosaccharides: Carbon Number and Functional Groups

Monosaccharides are the simplest carbohydrates and are classified based on two criteria:

1. Number of Carbon Atoms: Trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C), and heptoses (7C). 2. Functional Group Present: Aldehyde group (aldoses) or keto group (ketoses).

For example:

Glucose is an aldohexose with six carbons and an aldehyde group, making it a primary energy source in cells. Fructose is a ketohexose with a ketone group and six carbons, commonly found in fruits.

Structure and Properties of Glucose

Glucose, a key monosaccharide, exists mainly in two forms:

• Open-chain form: Contains an aldehyde group at carbon 1.
• Cyclic form: Forms a six-membered ring called a pyranose through intramolecular hemiacetal formation.

The cyclic form is more stable and predominant in aqueous solutions. Glucose is highly soluble in water due to its five hydroxyl groups, which form hydrogen bonds.

Key properties:

• Reducing sugar: Glucose can reduce mild oxidizing agents due to its free aldehyde group.
• Forms derivatives like pentaacetate where the aldehyde group is masked by acetal formation.

Worked example:

Calculate the molecular formula of glucose if it has 6 carbon atoms and follows the general carbohydrate formula.

$$ C_6(H_2O)_6 = C_6H_{12}O_6 $$

This confirms glucose's molecular formula as $C_6H_{12}O_6$.

Disaccharides: Formation and Hydrolysis

Disaccharides are carbohydrates formed by the condensation of two monosaccharide units via a glycosidic bond. Common examples include:

• Sucrose: Glucose + Fructose (α-1,2 glycosidic bond)
• Lactose: Glucose + Galactose (β-1,4 glycosidic bond)
• Maltose: Two glucose units (α-1,4 glycosidic bond)

Hydrolysis of disaccharides:

When disaccharides undergo hydrolysis (acidic or enzymatic), they break down into their constituent monosaccharides. For example, lactose hydrolyzes into glucose and galactose.

Understanding glycosidic bonds is crucial for recognizing reducing and non-reducing sugars.

Polysaccharides: Structure, Types, and Functions

Polysaccharides are long chains of monosaccharide units linked by glycosidic bonds. They serve as energy storage or structural components.

Major polysaccharides:

• Starch: A storage polysaccharide in plants, composed of amylose (linear α-1,4 linkages) and amylopectin (branched α-1,6 linkages).
• Glycogen: Animal storage polysaccharide, highly branched (α-1,4 and α-1,6 linkages).
• Cellulose: Structural polysaccharide in plant cell walls, made of β-1,4 linked glucose units.

Humans cannot digest cellulose due to the β-1,4 linkages, which is why it acts as dietary fiber.

Solubility and Chemical Properties of Carbohydrates

Carbohydrates are generally soluble in water but insoluble in non-polar solvents like benzene or cyclohexane. This is because:

• Carbohydrates have multiple polar hydroxyl (-OH) groups.
• These groups form hydrogen bonds with water, enhancing solubility.
• Non-polar solvents cannot interact with these polar groups.

Chemical properties include:

• Reducing nature: Monosaccharides and some disaccharides can reduce mild oxidizing agents.
• Formation of derivatives: Such as pentaacetates where hydroxyl groups are acetylated.
• Hydrolysis: Breaking down complex carbohydrates into simpler sugars.

Worked example:

Explain why glucose is soluble in water but cyclohexane is not.

Answer: Glucose has several -OH groups that form hydrogen bonds with water molecules, making it soluble. Cyclohexane is non-polar and cannot form such bonds, so it is insoluble in water.