Ch. 23 - Carbohydrates and Nucleic Acids

Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook

Wade 9th Edition

Ch. 23 - Carbohydrates and Nucleic Acids

Problem 42

Chapter 23, Problem 42

Cellulose is converted to cellulose acetate by treatment with acetic anhydride and pyridine. Cellulose acetate is soluble in common organic solvents, and it is easily dissolved and spun into fibers. Show the structure of cellulose acetate.

Verified step by step guidance

Understand the structure of cellulose: Cellulose is a polysaccharide composed of β-D-glucose units linked by β-1,4-glycosidic bonds. Each glucose unit contains three hydroxyl (-OH) groups at positions C2, C3, and C6.

Recognize the role of acetic anhydride: Acetic anhydride reacts with hydroxyl groups in the presence of a base (pyridine) to form acetate esters. This reaction replaces the hydroxyl groups (-OH) with acetyl groups (-OCOCH₃).

Identify the reaction mechanism: The reaction proceeds via nucleophilic attack of the hydroxyl oxygen on the carbonyl carbon of acetic anhydride, forming an ester bond and releasing acetic acid as a byproduct.

Determine the structure of cellulose acetate: Replace the hydroxyl groups on the glucose units of cellulose with acetyl groups (-OCOCH₃). The degree of substitution (DS) can vary, meaning not all hydroxyl groups may be acetylated. For full acetylation, all three hydroxyl groups on each glucose unit are replaced with acetyl groups.

Draw the structure: Represent the repeating unit of cellulose acetate, showing the β-1,4-glycosidic linkage between glucose units and the acetyl groups attached to the C2, C3, and C6 positions of each glucose unit. Ensure the stereochemistry of the glucose units is preserved.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Cellulose Structure

Cellulose is a polysaccharide composed of linear chains of β-D-glucose units linked by β(1→4) glycosidic bonds. This structure provides cellulose with high tensile strength and makes it insoluble in water. Understanding the basic structure of cellulose is crucial for comprehending how it can be chemically modified to form cellulose acetate.

Acetylation Reaction

Acetylation is a chemical reaction that introduces an acetyl group (–COCH3) into a compound. In the case of cellulose acetate, acetic anhydride reacts with the hydroxyl groups of cellulose, replacing some of them with acetyl groups. This modification alters the solubility and thermal properties of cellulose, making it suitable for applications like fiber production.

Solubility of Polymers

The solubility of polymers like cellulose acetate in organic solvents is influenced by their chemical structure and the presence of functional groups. The introduction of acetyl groups in cellulose acetate reduces hydrogen bonding between polymer chains, enhancing its solubility in solvents such as acetone and chloroform. This property is essential for processing cellulose acetate into fibers and films.

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Trehalose is a nonreducing disaccharide (C₁₂H₂₂O₁₁) isolated from the poisonous mushroom Amanita muscaria. Treatment with an α-glucosidase converts trehalose to two molecules of glucose, but no reaction occurs when trehalose is treated with a β-glucosidase. When trehalose is methylated by dimethyl sulfate in mild base and then hydrolyzed, the only product is 2,3,4,6-tetra-O-methylglucose. Propose a complete structure and systematic name for trehalose.

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Raffinose is a trisaccharide (C₁₈H₃₂O₁₆) isolated from cottonseed meal. Raffinose does not reduce Tollens reagent, and it does not mutarotate. Complete hydrolysis of raffinose gives D-glucose, D-fructose, and D-galactose. When raffinose is treated with invertase, the products are D-fructose and a reducing disaccharide called melibiose. Raffinose is unaffected by treatment with a β-galactosidase, but an α-galactosidase hydrolyzes it to D-galactose and sucrose. When raffinose is treated with dimethyl sulfate and base followed by hydrolysis, the products are 2,3,4-tri-O-methylglucose, 1,3,4,6-tetra-O-methylfructose, and 2,3,4,6-tetra-O-methylgalactose. Determine the complete structures of raffinose and melibiose, and give a systematic name for melibiose.

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Is gentiobiose a reducing sugar? Does it mutarotate? Explain your reasoning

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