In nylon-66 production (a polyamide used for fibres and engineering plastics), which organic diacid monomer combines with hexamethylene diamine during polycondensation?

Introduction / Context:
Nylons are polyamides formed by condensation of diamines with diacids (or their derivatives). Nylon-66 is one of the most important, prized for strength, abrasion resistance, and thermal stability. Distinguishing among possible acid monomers is essential in polymer chemistry.

Given Data / Assumptions:

• Target polymer: nylon-66 (the “66” denotes 6 carbons in diamine + 6 carbons in diacid).
• Common diamine: hexamethylene diamine (6 carbons).
• Need to identify the matching diacid with 6 carbons.

Concept / Approach:

Nylon naming convention aids recall: nylon-66 comes from a 6-carbon diamine (hexamethylene diamine) and a 6-carbon diacid, adipic acid (HOOC-(CH2)4-COOH). Condensation yields repeating amide linkages with water elimination and produces a semi-crystalline engineering thermoplastic.

Step-by-Step Solution:

Verification / Alternative check:

Standard references list adipic acid as the diacid monomer for nylon-66; sebacic acid (C10) is used in nylon-610; terephthalic acid is used in polyesters (PET) or in aramids with diamines; benzoic acid is monofunctional and cannot form long linear polyamides with diamines.

Why Other Options Are Wrong:

Sebacic acid: Yields different nylon types (e.g., 610). Terephthalic acid: Aromatic diacid used mainly in PET. Benzoic acid: Monocarboxylic, not suitable for linear step-growth polymerization with diamines.

Common Pitfalls:

Confusing nylon numbering; overlooking the requirement for bifunctional monomers for step-growth polymerization.

Final Answer:

Adipic acid