Chemistry Grade 12 – Chapter 2: Carbohydrates

Chemistry Grade 12 – Chapter 2: Carbohydrates

# 1. Concept of Carbohydrates

Carbohydrates are complex organic compounds, usually with the general formula C_n(H₂O)_m.

# 2. Classification of Carbohydrates

Carbohydrates are divided into three main groups:

• Monosaccharides: The simplest group of carbohydrates, which cannot be hydrolyzed. Examples: glucose, fructose.

• Disaccharides: A group of carbohydrates that, when hydrolyzed, each molecule produces two monosaccharide molecules. Example: sucrose.

• Polysaccharides: A group of carbohydrates that, when hydrolyzed, each molecule produces many monosaccharide molecules. Examples: starch, cellulose.

# 3. Glucose (C₆H₁₂O₆)

• Physical properties: Glucose is a solid, colorless, easily soluble in water, and has a sweet taste but not as sweet as cane sugar.

• Source: Glucose is found in most parts of the plant such as leaves, flowers, roots, especially in ripe fruits. Glucose is also found in the human and animal body.

• Chemical properties:

• Silver mirror reaction: Glucose has a silver mirror reaction and is oxidized by bromine water to form gluconic acid, indicating that glucose has an aldehyde group.

• Reaction with Cu(OH)₂: Glucose reacts with Cu(OH)₂ to form a blue solution, indicating that glucose has adjacent OH groups.

• Ester formation: Glucose forms esters containing 5 CH₃COO- groups, indicating that glucose has 5 adjacent OH groups.

• Complete reduction: Complete reduction of glucose yields hexane, indicating that the 6 carbon atoms in glucose form an unbranched chain.

• Structure: Glucose is a complex compound, in the open-chain form, the molecule has the structure of a monofunctional aldehyde and a 5-functional alcohol. In practice, the structure of glucose is mainly in the ring form, including ?-glucose and ?-glucose.

• Characteristic chemical properties:

• Oxidation: Oxidation of glucose with Ag₂O solution (silver mirror reaction) or Cu(OH)₂ (forming blue solution, sodium gluconate).

• Reduction: Reduction of glucose with H₂ to form a polyalcohol called sorbitol.

• Fermentation: In the presence of enzyme catalysts, glucose in solution ferments to form ethyl alcohol and carbon dioxide.

• Preparation: Hydrolysis of starch or cellulose.

• Applications: Glucose is used as a tonic for the elderly, children, sick people, mirror plating, and thermos flask plating.

# 4. Fructose (C₆H₁₂O₆)

• Physical properties: Fructose is one of the isomers of glucose, with an open-chain structural formula. Fructose is a crystalline solid, colorless, easily soluble in water, and sweeter than cane sugar.

• Source: Fructose is found in sweet fruits, especially honey (40%).

• Chemical properties: Fructose has similar chemical properties to glucose.

• Reaction with Cu(OH)₂: Fructose reacts with Cu(OH)₂ to form a blue complex.

• Reduction: Addition of H₂ to form a polyalcohol.

• Oxidation: Oxidized by AgNO₃/NH₃ solution and by Cu(OH)₂ in alkaline medium. In alkaline medium, fructose is converted to glucose.

# 5. Sucrose (C₁₂H₂₂O₁₁)

• Physical properties: Sucrose is the most common sugar, found in many plants such as sugarcane, sugar beet, and palmyra palm flowers. Sucrose is a crystalline solid, colorless, sweet, and dissolves well in water.

• Molecular structure:

• Sucrose does not have a silver mirror reaction and does not decolorize bromine solution, indicating that sucrose does not have an aldehyde group.

• Hydrolyzing sucrose produces fructose and glucose (which have silver mirror reactions).

• Structure: Sucrose is a disaccharide composed of one glucose unit and one fructose unit linked together by an oxygen atom. Sucrose does not have an aldehyde group, only an alcohol group.

• Chemical properties:

• No reducing properties: Sucrose does not have an aldehyde group, so it does not have the reducing properties of glucose.

• Properties of polyhydric alcohols: Sucrose has the properties of polyhydric alcohols.

• Hydrolysis reaction: Sucrose is hydrolyzed in acidic medium (dilute H₂SO₄).

• Applications: Sucrose is used in industry: making cakes, candies, soft drinks, canned food; in the pharmaceutical industry: preparing medicine. Sucrose is a raw material for hydrolyzing into glucose and fructose, which are then used for mirror plating and thermos flask plating.

# 6. Starch (C₆H₁₀O₅)_n

• Physical properties: Starch is an amorphous white powder, insoluble in cold water. In hot water, starch granules absorb water and swell to form a colloidal solution called starch paste.

• Molecular structure: Starch is a polysaccharide, the molecule consists of many ?-glucose units linked together to form two forms: amylose and amylopectin.

• Amylose: Has a straight chain structure, ?-1,4-glycosidic bond.

• Amylopectin: Has a branched chain structure, ?-1,4-glycosidic bond and ?-1,6-glucosidic bond.

• Source: Starch is formed in green plants through photosynthesis.

• Chemical properties: Starch reacts with iodine to form a blue-violet complex.

• Applications: Starch is used as food, raw material for producing alcohol, paste, and glucose sugar.

# 7. Cellulose (C₆H₁₀O₅)_n

• Physical properties: Cellulose is a white fibrous solid, insoluble in water and solvents, but soluble in Schweizer’s reagent (a solution of Cu(OH)₂ dissolved in ammonia). Cotton contains nearly 98% cellulose.

• Molecular structure: Cellulose has an unbranched chain structure, a polysaccharide, the molecule consists of many ?-glucose units linked together.

• Chemical properties:

• Hydrolysis reaction: Cellulose is hydrolyzed to form glucose.

• Reaction with nitric acid: Cellulose reacts with nitric acid to form cellulose trinitrate.

• Soluble in Schweizer’s reagent: Cellulose does not react with Cu(OH)₂ but dissolves in Schweizer’s reagent.

• Applications: Cellulose is used in paper processing, production of artificial fibers such as viscose, acetate, manufacture of smokeless powder, and film.