Which carboxylic acid is used in the manufacturing of nylon-6,6?

Hexanedioic acid

The correct answer is option 4. Hexanedioic acid.

Nylon-6,6 is a type of synthetic polymer known as a polyamide. It is made through a condensation polymerization reaction between two specific monomers: hexamethylenediamine and adipic acid.

Chemical Structure and Role of Hexanedioic Acid

Chemical Formula: \(HO_2C(CH_2)_4CO_2H\)

Adipic acid has a six-carbon chain with carboxylic acid groups \((-COOH)\) at each end. The presence of two carboxyl groups at either end of the molecule allows it to react with amines to form amide bonds.

The polymerization process involves a condensation reaction, where the carboxyl groups of adipic acid react with the amine groups of hexamethylenediamine \((NH_2(CH_2)_6NH_2)\) to form the polyamide nylon-6,6. The reaction releases water as a byproduct.

Reaction:

Adipic acid provides the dicarboxylic acid component needed for the polymerization. Hexamethylene diamine provides the diamine component.

Why Adipic Acid is used:

Chain Length: The six-carbon chain in adipic acid contributes to the properties of nylon-6,6, such as its strength and durability. The specific length and structure are crucial for forming the repeating units in the nylon polymer.

Functional Groups: The carboxyl groups at both ends of the adipic acid molecule are necessary for forming amide linkages with the amine groups in hexamethylenediamine.

Product Properties: The resulting polymer has desirable properties like high melting point, strength, and resistance to wear and chemicals, making it suitable for various applications, including textiles, automotive parts, and industrial components.

Other Options

1. Propanedioic Acid (Malonic Acid):  Has only three carbon atoms and is not suitable for forming the long polymer chains required for nylon-6,6.

2. Butanedioic Acid (Succinic Acid): Has four carbon atoms, also too short to form the desired properties of nylon-6,6.

3. Pentanedioic Acid (Glutaric Acid): Has five carbon atoms, which is still not optimal for the polymer properties needed in nylon-6,6.

Conclusion

Hexanedioic acid (adipic acid) is essential for producing nylon-6,6 due to its chemical structure and ability to form long, stable polymer chains with the desired mechanical properties. This makes it the correct choice for manufacturing nylon-6,6.

Therefore, the correct answer is: Hexanedioic Acid.