Question

The reactive dyes are applied to Cellulosic fiber in an alkaline dyeing bath, by chemically reacting to form \(\underline{\hspace{2cm}}\) with the hydroxyl group of the fiber.

• A. Salt linkage
• B. Hydrogen bond
• C. Ionic bond
• D. Covalent bond

Hint:

Associate dye classes with their primary bond type: - Reactive Dyes on Cotton \(\rightarrow\) Covalent - Direct Dyes on Cotton \(\rightarrow\) Hydrogen bonds, Van der Waals forces - Vat/Sulphur Dyes on Cotton \(\rightarrow\) Mechanical entrapment - Acid Dyes on Wool/Nylon \(\rightarrow\) Ionic (salt linkage) - Disperse Dyes on Polyester \(\rightarrow\) Solid solution (Van der Waals forces)

Answer 1

The Correct Option is D

Step 1: Understand the name "reactive dye." The name itself implies that the dye molecule chemically reacts with the fiber molecule. 
 

Step 2: Analyze the bonding mechanism. Cellulosic fibers (like cotton) are polymers of glucose and are rich in hydroxyl (-OH) groups. In an alkaline medium (which activates the hydroxyl groups), the reactive group on the dye molecule forms a direct, strong, and permanent chemical bond with the fiber. 
 

Step 3: Define the bond types. 
- Salt linkage / Ionic bond: Involves attraction between opposite charges. This is typical for acid dyes on wool. 
- Hydrogen bond: A weaker attraction between a hydrogen atom and an electronegative atom like oxygen or nitrogen. This contributes to dye affinity but is not the primary bond for reactive dyes. 
- Covalent bond: Involves the sharing of electron pairs between atoms. This is a very strong and stable chemical bond. 

Conclusion: The reaction between a reactive dye and the hydroxyl group of a cellulosic fiber forms a strong covalent bond, which accounts for the excellent wash fastness of these dyes.