Question

25.41 Why is cellulose insoluble in water?

Short answer

Answer: Cellulose is insoluble in water due to its structure, which consists of long, straight chains of glucose molecules linked by beta-glycosidic bonds and held closely together by strong hydrogen bonds between the hydroxyl (-OH) groups on adjacent glucose molecules. This tight network of hydrogen bonds and the ribbon-like structure formed by the beta-glycosidic bonds make it difficult for water molecules to penetrate or dissolve the cellulose structure. As a result, cellulose remains insoluble in water, as the forces between its chains are too strong for water molecules to disrupt.

Step-by-step solution

Understand the structure of cellulose

Cellulose is a complex carbohydrate (polysaccharide) composed of thousands of glucose molecules linked together. These glucose units are connected to each other by beta-glycosidic bonds between carbon atoms 1 and 4 (C1 and C4) on adjacent glucose molecules. The resulting structures form long, straight chains.

Examine the beta-glycosidic bonds

In cellulose, glucose units are bonded by beta-glycosidic bonds. These bonds cause each glucose molecule to be flipped 180 degrees compared to the next one in the chain. This arrangement of the glucose molecules creates a flat ribbon-like structure, allowing the individual cellulose chains to line up closely with each other, enhancing the stability and strength of the overall structure.

Investigate hydrogen bonding in cellulose

Cellulose chains are held together by hydrogen bonds between the hydroxyl (-OH) groups on adjacent glucose molecules. The presence of multiple hydrogen bonds between cellulose chains forms a tight network that contributes to the strength of the cellulose fibers. Due to these strong hydrogen bonds, the fibers are tightly packed and water molecules cannot easily penetrate or dissolve the cellulose structure.

Understand the role of water in solubility

To dissolve a substance in water, water molecules must be able to surround and interact with the substance, eventually breaking intermolecular forces such as hydrogen bonds, ionic bonds, or van der Waals forces in the substance. These interactions allow water molecules to disperse the substance and create a homogeneous solution.

Explain why cellulose is insoluble in water

Given the strong hydrogen bonding between cellulose chains and the tight, ribbon-like structure formed by the beta-glycosidic bonds, water molecules cannot easily interact with cellulose to break these hydrogen bonds and dissolve the chains. Consequently, cellulose remains insoluble in water as the forces between its chains are too strong for water molecules to disrupt.