Cellulose

1.0What Is Cellulose?

Cellulose is a natural carbohydrate polymer composed of β-D-glucose units linked by β(1→4) glycosidic bonds. It is the main structural component of plant cell walls and the most abundant organic compound on Earth.

Chemically, cellulose is a polysaccharide that provides strength, rigidity, and support to plants.

$\text{ General Formula: }{\left({\mathrm{C}}_6{\mathrm{H}}_{10}{\mathrm{O}}_5\right)}_n$

It is insoluble in water and most organic solvents, and serves as an important raw material in industries like textiles, paper, plastics, and pharmaceuticals.

2.0Structure of Cellulose

3.0Occurrence of Cellulose in Nature

Cellulose is widely found in:

• Plant cell walls (cotton fibers, wood, leaves, and stems)
• Algae and certain bacteria (like Acetobacter xylinum)
• Tunicates (marine animals) — contain tunicin, a cellulose-like substance

Percentage of Cellulose in Common Sources:

Thus, plants are the primary natural source of cellulose.

4.0Chemical Composition of Cellulose

Cellulose is made up of β-D-glucose monomers joined by β(1→4) glycosidic linkages.

Each glucose unit is rotated 180° relative to the adjacent one, resulting in a straight, unbranched chain.

5.0Structure of Cellulose

1. Linear Polymer:

• Cellulose is a linear chain polymer of β-D-glucose.
• Each glucose molecule is connected through β(1→4) linkage, forming long, straight chains.

2. Hydrogen Bonding:

• The –OH groups of one chain form hydrogen bonds with neighbouring chains.
• This creates microfibrils, giving cellulose high tensile strength.

3. Crystalline Nature:

• Due to extensive hydrogen bonding, cellulose has crystalline and amorphous regions.
• The crystalline regions are insoluble and rigid, while the amorphous parts are flexible.

Structural Representation:

Repeating β-D-glucose units linked through oxygen bridges at the 1st and 4th carbon atoms.

6.0Physical Properties of Cellulose

Cellulose is hygroscopic (absorbs moisture) and exhibits high tensile strength due to intermolecular hydrogen bonding.

7.0Chemical Properties of Cellulose

1. Reaction with Acids

• Cellulose is hydrolyzed by concentrated acids (like HCl or H₂SO₄) to form glucose molecules.
  ${\left({\mathrm{C}}_6{\mathrm{H}}_{10}{\mathrm{O}}_5\right)}_n+{\mathrm{nH}}_2\mathrm{O}\to {\mathrm{nC}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$
• This reaction demonstrates that cellulose is a polymer of glucose.

2. Reaction with Concentrated Sulfuric Acid

When cellulose is treated with concentrated H₂SO₄, it forms a sticky mass called dextrin and eventually glucose upon dilution.
This process is used in the manufacture of artificial silk (viscose rayon).

3. Reaction with Nitric Acid (Formation of Nitrocellulose)

When cellulose is treated with a mixture of nitric acid (HNO₃) and sulfuric acid (H₂SO₄), it forms cellulose nitrate or nitrocellulose.

${\left({\mathrm{C}}_6{\mathrm{H}}_{10}{\mathrm{O}}_5\right)}_n+{\mathrm{nHNO}}_3\to {\left({\mathrm{C}}_6{\mathrm{H}}_7{\mathrm{O}}_2{\left({\mathrm{ONO}}_2\right)}_3\right)}_n+{\mathrm{nH}}_2\mathrm{O}$

• Trinitrocellulose is used to make gun cotton (explosives).
• Dinitrocellulose is used in lacquers and plastics.

4. Reaction with Acetic Anhydride (Formation of Cellulose Acetate)

Cellulose reacts with acetic anhydride in the presence of sulfuric acid to form cellulose acetate.

${\left({\mathrm{C}}_6{\mathrm{H}}_{10}{\mathrm{O}}_5\right)}_n+\mathrm{n}{\left({\mathrm{CH}}_3\mathrm{CO}\right)}_2\mathrm{O}\to {\left({\mathrm{C}}_6{\mathrm{H}}_7{\mathrm{O}}_2{\left({\mathrm{OCOCH}}_3\right)}_3\right)}_n+{\mathrm{nCH}}_3\mathrm{COOH}$

Cellulose acetate is used in film production, textiles, and plastics.

5. Reaction with Alkalis (Mercerization)

When treated with strong NaOH solution (18%), cellulose fibers swell and become more lustrous and strong — a process known as mercerization.

This process improves dye absorption and texture of cotton fibers.

8.0Hydrolysis of Cellulose

Cellulose can be completely hydrolyzed into D-glucose units using enzymes or acids.

Stepwise Hydrolysis:

1. Partial hydrolysis → Cellobiose (disaccharide)
2. Complete hydrolysis → D-glucose

${\left({\mathrm{C}}_6{\mathrm{H}}_{10}{\mathrm{O}}_5\right)}_n+{\mathrm{nH}}_2\mathrm{O}\to {\mathrm{nC}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$

Biological Hydrolysis:

Certain microorganisms like Trichoderma and Clostridium secrete cellulase enzyme that decomposes cellulose into glucose — important in the carbon cycle.

9.0Preparation of Cellulose Derivatives

Several useful cellulose derivatives are prepared industrially for diverse applications.

1. Cellulose Nitrate (Nitrocellulose)

• Formed by the nitration of cellulose using HNO₃ and H₂SO₄.
• Used in explosives, lacquers, and plastics.

2. Cellulose Acetate

• Produced by treating cellulose with acetic anhydride.
• Used in photographic films, rayon fibers, and eyeglass frames.

3. Methyl Cellulose & Carboxymethyl Cellulose (CMC)

• Formed by reaction with alkyl halides or chloroacetic acid.
• Used as thickening agents in food and pharmaceuticals.

4. Rayon (Artificial Silk)

• Prepared by treating cellulose with NaOH and CS₂ to form viscose, which is extruded into fibers.

$\text{ Cellulose }+\mathrm{NaOH}+{\mathrm{CS}}_2\to \text{ Cellulose xanthate }\text{ → }\text{ Rayon (after regeneration) }$

10.0Distinction Between Starch and Cellulose

11.0Role of Cellulose in Plants

• Provides Mechanical Strength:
  Cellulose microfibrils form a rigid framework in cell walls, maintaining plant shape.
• Controls Growth:
  The arrangement of cellulose fibers affects cell elongation and expansion.
• Acts as a Barrier:
  Prevents entry of pathogens and provides protection from external damage.
• Facilitates Water Transport:
  Its fibrous structure aids in capillary action in plant tissues.

12.0Industrial Uses of Cellulose

• Medical & Antiseptic: Used in mild antiseptic solutions (boric lotion) for eyewash to treat conjunctivitis and to clean wounds. It is also used in antifungal powders (e.g., for athlete's foot).
• Food Preservation: Used as a preservative in food industries (though regulated in many countries).
• Glass Industry: Used in the manufacture of heat-resistant borosilicate glass (e.g., Pyrex) and optical fiberglass.
• Ceramics: used to create glazes for pottery and tiles.
• Nuclear Power: Used as a neutron absorber in the control rods of nuclear reactors to control the rate of fission.
• Insecticide: Used in powders to kill cockroaches, ants, and fleas by damaging their exoskeleton.