In polymer chemistry, the synthetic fibre known as Nylon 6 is prepared from which monomer or monomers?

Introduction / Context:
This question falls under the topic of synthetic polymers and man made fibres. It asks specifically about Nylon 6, which is a widely used polyamide fibre. Knowing which monomer or monomers are used to manufacture different nylons is a standard part of polymer chemistry in school and entrance examinations. Nylon 6 is structurally different from Nylon 6,6, and this difference arises from the nature of the monomer units used in their polymerisation, so identifying the correct monomer is essential to answer the question.

Given Data / Assumptions:
Nylon 6,6 is a polyamide formed from a diamine and a dicarboxylic acid, typically hexamethylenediamine and adipic acid.
Nylon 6 is formed from a single monomer containing six carbon atoms, which undergoes ring opening polymerisation.
Urea and formaldehyde are used to produce a different type of polymer, urea formaldehyde resin, which is a thermosetting plastic.
Phenol and formaldehyde are used to make phenolic resins such as Bakelite, also a thermosetting polymer.
We assume standard industrial processes for the production of these polymers.

Concept / Approach:
To approach this question, you need to recall the specific monomer structures. Nylon 6,6 is made by condensation polymerisation between hexamethylenediamine and adipic acid, leading to a repeating unit with six carbon atoms on each side of the amide linkage. Nylon 6, however, is made from caprolactam, a cyclic amide with six carbon atoms in the ring. Caprolactam undergoes ring opening polymerisation to form a linear polyamide chain with repeating units derived from aminohexanoic acid. By contrasting these known facts with the options given, we can conclude which monomer corresponds specifically to Nylon 6.

Step-by-Step Solution:
Step 1: Recall that Nylon 6,6 is produced by condensation of hexamethylenediamine with adipic acid. Therefore, the pair hexamethylenediamine and adipic acid is associated with Nylon 6,6, not Nylon 6. Step 2: Consider caprolactam. Caprolactam is a six membered cyclic lactam that can open up under polymerisation conditions to give a linear chain with amide linkages, forming Nylon 6. Step 3: Urea and formaldehyde react to form urea formaldehyde resin, which is a thermosetting polymer used in adhesives and moulded objects, not a nylon. Step 4: Phenol and formaldehyde react to form phenolic resins such as Bakelite, again a thermosetting polymer and not a polyamide like Nylon 6. Step 5: From these considerations, Nylon 6 is clearly associated with caprolactam as its monomer, so caprolactam is the correct answer.

Verification / Alternative check:
Industrial chemistry references describe Nylon 6 as polycaprolactam, emphasising that it is obtained by polymerising caprolactam. They also show reaction schemes where caprolactam undergoes ring opening to yield the amide linked polymer chain. Nylon 6,6 is uniquely noted as being derived from hexamethylenediamine and adipic acid. In contrast, urea formaldehyde and phenol formaldehyde resins are listed as separate categories of thermosetting plastics. These independent points confirm that caprolactam is the monomer used to synthesise Nylon 6, validating our choice.

Why Other Options Are Wrong:
Hexamethylenediamine and adipic acid are incorrectly chosen if linked to Nylon 6, because they are used to manufacture Nylon 6,6, not Nylon 6, so option A is wrong in this context.
Urea and formaldehyde form urea formaldehyde resin, which is a thermosetting plastic, not a linear polyamide fibre; therefore option C does not correspond to Nylon 6.
Phenol and formaldehyde form phenolic resins like Bakelite and are not used for making nylon type polyamides, so option D is also incorrect for this question.

Common Pitfalls:
A common confusion arises between Nylon 6 and Nylon 6,6 because both names contain the number six, and students may wrongly assume they come from the same pair of six carbon units. Another pitfall is to assume that any mention of formaldehyde indicates a type of plastic, which is true, but not necessarily a nylon. Remembering that Nylon 6 comes from a single monomer caprolactam, while Nylon 6,6 comes from two monomers, a diamine and a diacid, helps to keep these fibres clearly distinguished in memory.

Final Answer:
Nylon 6 is manufactured by ring opening polymerisation of the monomer Caprolactam.