Chemical Reactions of Carbohydrates PDF

Oxidation reactions of cellulose and starch Oxidation reactions involve the introduction of oxygen-containing functional groups such as aldehyde, ketone, and carboxyl groups into the structure of cellulose or starch. This process alters the molecular structure and physicochemical properties, leadin...

Oxidation reactions of cellulose and starch Oxidation reactions involve the introduction of oxygen-containing functional groups such as aldehyde, ketone, and carboxyl groups into the structure of cellulose or starch. This process alters the molecular structure and physicochemical properties, leading to significant changes in solubility, reactivity, and mechanical properties. 1. Oxidation of Cellulose a. Cellulose Structure Cellulose is a polysaccharide composed of β-D-glucopyranose units linked by β-1,4 glycosidic bonds. Each glucose unit has three hydroxyl groups at the C2, C3, and C6 positions, which can be targeted by oxidizing agents. b. Common Oxidizing Agents for Cellulose 1. Sodium Periodate (NaIO₄): Specificity: Sodium periodate selectively cleaves the C2-C3 bond in an anhydroglucose unit (AGU) of cellulose. Reaction Mechanism: Periodate oxidation involves the formation of dialdehyde cellulose (DAC) by breaking the C2- C3 bond of the glucopyranose ring, leading to the introduction of two aldehyde groups. Applications: Dialdehyde cellulose is more reactive than native cellulose and is used in producing antimicrobial materials, hydrogels, and as a precursor for other derivatives. Reaction: Cellulose + NaIO4 → Dialdehyde Cellulose This modification disrupts the crystalline structure, making the material more soluble and easier to process. 2. TEMPO-Mediated Oxidation: TEMPO (2,2,6,6-Tetramethylpiperidine-1-oxyl) is a nitroxyl radical used in the presence of a primary oxidant (such as sodium hypochlorite, NaClO) and a co-oxidant (sodium bromide, NaBr). Selectivity: TEMPO is highly selective for oxidizing primary alcohol groups at the C6 position of glucose units, converting them into carboxyl groups (-COOH), forming 6-carboxycellulose. Reaction Mechanism: TEMPO forms a nitroxyl cation, which abstracts an electron from the C6 primary alcohol, leading to the formation of a carboxyl group. Reaction: Cellulose + TEMPO + NaClO → 6-Carboxycellulose + NaCl Properties and Applications: 6-Carboxycellulose has a higher water solubility and better dispersibility in water, making it useful for hydrogels, superabsorbents, and in drug delivery systems. 3. Potassium Permanganate (KMnO₄): Potassium permanganate is a strong oxidizer and can oxidize both primary and secondary hydroxyl groups in cellulose. Reaction Mechanism: KMnO₄ oxidizes the C6 primary hydroxyl groups, forming carboxyl groups (-COOH) and, under harsh conditions, breaks down the cellulose backbone. Applications: Oxidation with KMnO₄ leads to significant degradation of cellulose, making it less common for controlled oxidation applications. c. Properties of Oxidized Cellulose Increased Reactivity: The introduction of carbonyl and carboxyl groups increases the reactivity, enabling further chemical modifications. Altered Solubility: Oxidation disrupts the crystalline structure, improving solubility in water and organic solvents. Applications: Oxidized cellulose is used in wound dressings, drug carriers, paper coatings, and food packaging. 2. Oxidation of Starch a. Starch Structure Starch is a polysaccharide consisting of amylose (linear chains of α-D-glucopyranose units linked by α-1,4 glycosidic bonds) and amylopectin (branched chains with α-1,4 and α-1,6 glycosidic bonds). The primary hydroxyl group at C6 and secondary hydroxyl groups at C2 and C3 positions are susceptible to oxidation. b. Common Oxidizing Agents for Starch Sodium Hypochlorite (NaClO): Reaction Mechanism: NaClO is the most commonly used oxidizing agent for starch. It preferentially oxidizes the primary hydroxyl group at the C6 position to form carboxyl groups (-COOH) or the C2 and C3 positions to form carbonyl groups (-CHO). Reaction Conditions: The reaction is typically carried out under mild alkaline conditions (pH 9-11) to control the extent of oxidation. Reaction: Starch + NaClO → Oxidized Starch + NaCl Degree of Oxidation: The extent of oxidation can be controlled by adjusting the NaClO concentration, pH, and reaction time. Properties: Oxidized starch has lower viscosity, reduced molecular weight, and increased solubility. Applications: Oxidized starch is used as a thickener, emulsifier, and stabilizer in food products and adhesives in paper and textile industries.  Ozone (O₃): Ozone is used for eco-friendly oxidation of starch, targeting both primary and secondary hydroxyl groups. Reaction Mechanism: Ozone reacts with the hydroxyl groups, converting them to carbonyl and carboxyl groups. Reaction: Starch + O3 → Ozonated Starch Properties and Applications: Ozonated starch has improved water-binding capacity and increased biodegradability, making it suitable for biodegradable plastics and packaging. Periodate Oxidation (NaIO₄): Periodate selectively oxidizes the C2-C3 bond in starch, similar to its action on cellulose, resulting in the formation of dialdehyde starch (DAS). Reaction Mechanism: Periodate oxidation breaks the C2-C3 bond, forming aldehyde groups at both C2 and C3 positions. Reaction: Starch + NaIO4 → Dialdehyde Starch Properties: Dialdehyde starch has increased reactivity, and cross-linking potential, and can form biodegradable hydrogels. Applications: DAS is used in adhesives, biodegradable films, and as a matrix for immobilization of biomolecules.  Properties of Oxidized Starch Lowered Viscosity: Oxidation breaks down the starch molecules, resulting in reduced viscosity. Increased Reactivity: Introduction of aldehyde and carboxyl groups enhances chemical reactivity for further modifications. Improved Film-Forming Ability: Oxidized starches have better film-forming properties due to increased cross- linking potential. Applications: Used in the food industry (as thickeners), paper and textile industry (as adhesives), and in biodegradable plastic formulations. Oxidizing agent Cellulose Starch Sodium Periodate Dialdehyde cellulose Dialdehyde Starch (selective C2-C3 cleavage) TEMPO 6-Carboxycellulose Minimal effect (not widely used) Sodium hypochlorite Non-selective oxidation C6 oxidation to carboxyl gp Potassium Permanganate C6 carboxylation Limited use due to harsh reaction Ozone Minimal use High selectivity, eco-friendly oxidation  Reduction reactions of cellulose and starch Reduction reactions in cellulose and starch typically involve the transformation of carbonyl (C=O) groups into hydroxyl (-OH) groups. This can occur in both naturally oxidized or chemically modified carbohydrates and is used to reverse or further modify their chemical properties. Below is a detailed explanation of the reduction reactions for both cellulose and starch, focusing on the mechanisms, reducing agents, and the impact on material properties. 1. Reduction Reactions of Cellulose a. Cellulose Structure Cellulose is a polysaccharide composed of β-D-glucopyranose units. The primary reactive sites for reduction are carbonyl groups, which can form due to previous oxidation processes, such as the introduction of aldehyde or carboxyl groups. b. Common Reducing Agents for Cellulose Reduction reactions typically convert carbonyl groups (aldehyde or ketone) into hydroxyl groups, restoring the more native, alcohol-like structure. 1. Sodium Borohydride (NaBH₄): Reaction Mechanism: Sodium borohydride is a mild reducing agent that selectively reduces aldehyde groups (-CHO) formed during the oxidation of cellulose (e.g., in dialdehyde cellulose) back to primary alcohols (-CH₂OH). Selectivity: NaBH₄ is selective for aldehydes and does not reduce carboxylic acids (-COOH) or esters. Reaction: Dialdehyde Cellulose+NaBH4→Reduced Cellulose (Alcohol Groups)+NaBO2 Properties: This process restores the flexibility and mechanical strength of the cellulose by reducing its oxidation state. It also lowers its reactivity, making the material more chemically stable. Applications: Reduced cellulose is used in bio-based composites, textiles, and packaging where enhanced mechanical properties and stability are desired. 2. Lithium Aluminum Hydride (LiAlH₄): Reaction Mechanism: LiAlH₄ is a stronger reducing agent than NaBH₄ and can reduce a broader range of functional groups, including aldehydes, ketones, and even esters and carboxyl groups. Reaction Conditions: This reagent must be handled under anhydrous conditions (no water) as it reacts violently with water, producing hydrogen gas. Reaction: Oxidized Cellulose + LiAlH4 →Fully Reduced Cellulose (Alcohol Groups) Properties: This reduction yields a cellulose structure with restored hydroxyl groups at the C2, C3, and C6 positions, reversing previous oxidation processes. Applications: LiAlH₄-reduced cellulose is more chemically stable and can be used in high-performance applications requiring greater resilience, such as advanced composites. 3. Catalytic Hydrogenation: Reaction Mechanism: Catalytic hydrogenation involves using a metal catalyst (such as palladium on carbon, Pd/C) and hydrogen gas (H₂) to reduce carbonyl groups in oxidized cellulose back to hydroxyl groups. Reaction: Oxidized Cellulose + 𝐻2 → Reduced Cellulose (Alcohol Groups) Applications: Catalytically reduced cellulose is used in the production of cellulose-based films and fibers with enhanced physical and chemical properties. c. Properties of Reduced Cellulose Improved Mechanical Strength: Reducing oxidized cellulose restores its native structure, improving flexibility and strength. Enhanced Stability: Reduction lowers the reactivity of cellulose, making it more resistant to further chemical changes. Applications: Reduced cellulose is used in textiles, and packaging, and as a reinforcing agent in biodegradable composites. 2. Reduction Reactions of Starch a. Starch Structure Starch consists of two main components, amylose (linear) and amylopectin (branched). The reduction of starch typically targets carbonyl groups formed by oxidation processes, converting them back to hydroxyl groups. b. Common Reducing Agents for Starch Similar to cellulose, starch reduction reactions are primarily carried out to convert carbonyl groups (generated from oxidation processes) back into hydroxyl groups. 1. Sodium Borohydride (NaBH₄): Reaction Mechanism: Sodium borohydride is commonly used to reduce oxidized starch (which contains carbonyl groups such as aldehydes and ketones) back to its original form. NaBH₄ selectively reduces aldehydes and ketones without affecting carboxyl groups. Reaction: Oxidized Starch + NaBH4 → Reduced Starch (Alcohol Groups) + NaBO2 Effect on Properties: Reducing the carbonyl groups in starch restores its molecular integrity and increases the viscosity and stability of starch solutions. Applications: Reduced starch is used in the food industry as a thickening agent, stabilizer, and in biodegradable packaging materials. 2. Catalytic Hydrogenation: Reaction Mechanism: Catalytic hydrogenation uses hydrogen gas (H₂) and a metal catalyst (such as palladium on carbon, Pd/C) to reduce carbonyl groups in oxidized starch back to hydroxyl groups. Reaction: Oxidized Starch+H2 Reduced Starch (Alcohol Groups) Properties: Catalytic hydrogenation yields starch with restored hydroxyl groups, making it more stable and less prone to degradation. Applications: Reduced starch is used in the production of starch-based films, biodegradable packaging, and in food applications where a more stable, thickening agent is required. 3. Hydrogenation of Dialdehyde Starch (DAS): Dialdehyde starch (DAS), produced through periodate oxidation, can be reduced to regenerate hydroxyl groups. Reaction Mechanism: DAS contains aldehyde groups at the C2 and C3 positions. Reducing these aldehyde groups restores hydroxyl groups and modifies the mechanical properties of the starch. Reaction: Dialdehyde Starch (DAS)+NaBH4 → Reduced Starch (Alcohol Groups)+NaBO2​ Properties: Reduced DAS has improved mechanical and barrier properties, which is particularly useful in packaging applications. c. Properties of Reduced Starch Restored Functionality: The reduction of oxidized starch restores its original functionality, improving its viscosity, water-binding capacity, and solubility. Increased Stability: Reduction lowers the reactivity of starch, preventing further degradation and extending its shelf life. Applications: Reduced starch is used in food products as a thickening agent, in adhesives, and as a biodegradable material in packaging applications. Reducing agent Cellulose Starch Sodium borohydride Reduces aldehyde groups to Reduces oxidized starch to hydroxyl groups, restoring restore hydroxyl groups and flexibility viscosity Lithium aluminum hydride Reduces a wide range of carbonyl Rarely used due to harsh compounds (aldehydes, ketones, conditions but can reduce acids) carbonyl groups Catalytic hydrogenation Uses hydrogen gas and a metal Reduces oxidized starch carbonyl catalyst to reduce carbonyl groups back to hydroxyl groups groups Hydrogenation of DAS Not typically used Reduces dialdehyde starch (DAS) to restore hydroxyl groups  Acid-Modified Celluloses and Starches Acid hydrolysis plays a key role in breaking down carbohydrate polymers like cellulose and starch. a. Cellulose Hydrolysis with Acids: Acids Used: Sulfuric acid (H₂SO₄) and hydrochloric acid (HCl) are typically used for cellulose hydrolysis. Mechanism: Acid treatment breaks down cellulose into smaller fragments, known as cellulose nanocrystals (CNCs). This occurs through the cleavage of glycosidic bonds, particularly in the amorphous regions, leaving behind crystalline cellulose. Applications: CNCs are used in reinforcing composites and films due to their high mechanical strength and biodegradability. b. Starch Hydrolysis with Acids: Acids Used: Starch is commonly treated with hydrochloric or sulfuric acid for partial hydrolysis. Mechanism: Acid hydrolysis breaks down starch into dextrins (smaller carbohydrate units) or glucose. The reaction selectively cleaves α-1,4 glycosidic bonds while leaving the α-1,6 bonds relatively unaffected. Applications: Acid-hydrolyzed starches are used as food thickeners, adhesives, and in the textile industry.  Alkali-Modified Celluloses and Starches Treatment with alkali leads to the modification of the structure and properties of carbohydrates. a. Cellulose Modification with Alkali: Alkaline Agents: Sodium hydroxide (NaOH) is the most commonly used alkali for cellulose modification. Mechanism: When cellulose is treated with NaOH, it undergoes swelling and dissolution, leading to the formation of alkali cellulose. This is the first step in the production of cellulose derivatives like carboxymethyl cellulose (CMC). Carboxymethyl Cellulose Formation: In this process, the hydroxyl groups of cellulose react with chloroacetic acid in the presence of NaOH to introduce carboxymethyl groups (-CH₂COOH), forming CMC. Properties: CMC has improved solubility in water and is used as a stabilizer and thickener in food, pharmaceuticals, and cosmetics. b. Starch Modification with Alkali: Alkaline Agents: NaOH is also used to modify starch. Mechanism: Alkali treatment partially breaks down the starch molecules, leading to alkali starch. It disrupts the granule structure and promotes solubility. Cross-linking with Alkali: Starch can be further modified by cross-linking its chains using agents like epichlorohydrin in an alkaline medium. This produces cross-linked starch, which has enhanced stability and viscosity. Applications: Alkali-modified starches are used in applications where higher stability under heat and shear is required, such as in canned and frozen foods.