In the nomenclature nylon-6, what does the number “6” specifically denote in this polyamide system?

Introduction / Context:
Nylon nomenclature reflects the number of carbon atoms derived from monomers. For homopolyamide nylon-6, the monomer is caprolactam, a cyclic lactam with six carbon atoms. Understanding this convention helps decode structures and relate them to properties such as melting point and moisture uptake.

Given Data / Assumptions:

• Nylon-6 is formed by ring-opening polymerisation of caprolactam.
• Caprolactam contains a six-carbon backbone contributing to the repeating amide unit.
• Numbering conventions: nylon-6,6 uses two numbers for diamine and diacid carbon counts; nylon-6 uses a single number from a single monomer.

Concept / Approach:
For nylons:

• Nylon-6,6: hexamethylene diamine (6 carbons) + adipic acid (6 carbons) → “6,6.”
• Nylon-6: caprolactam (6 carbons) → “6.”

Thus, the “6” represents the number of carbon atoms in the lactam monomer that becomes part of the polyamide backbone after ring opening and polymerisation.

Step-by-Step Solution:
Identify nylon-6 monomer: caprolactam.Count carbons in caprolactam: six.Conclude that “6” denotes the carbon count in the monomer, carried into the repeating unit.

Verification / Alternative check:
Textbook schematics show ring-opening of caprolactam to yield –NH–(CH2)5–CO– repeating units, confirming the six-carbon segment.

Why Other Options Are Wrong:
Total chain carbons: variable with chain length; not fixed by the name.Nitrogen or hydrogen counts: not part of the nylon numbering convention.Repeating unit per nanometer: unrelated to naming.

Common Pitfalls:
Confusing nylon-6 with nylon-6,6; the latter uses two monomer carbon counts (6 and 6).

Final Answer:
Carbon atoms in the monomer ring (caprolactam)