Terylene, also known as Dacron, is a polyester obtained by condensation polymerisation of ethylene glycol with which dicarboxylic acid?

Introduction / Context:
Synthetic fibres and polymers are important topics in general and organic chemistry. Terylene, also marketed as Dacron, is a widely used polyester fibre found in fabrics, ropes and packaging materials. It is formed by a condensation polymerisation reaction between a diol and a dicarboxylic acid. This question asks which specific dicarboxylic acid reacts with ethylene glycol to form terylene, reinforcing knowledge of monomers used in common polyesters.

Given Data / Assumptions:
- The polymer is terylene, a polyester of ethylene glycol and an aromatic dicarboxylic acid.
- Ethylene glycol is a diol with two hydroxyl groups, HO-CH2-CH2-OH.
- Options include benzoic, salicylic, phthalic, terephthalic and oxalic acids.
- We assume standard textbook descriptions of polyester formation.

Concept / Approach:
Terylene is formed by condensation polymerisation between ethylene glycol and terephthalic acid. Terephthalic acid is a benzene dicarboxylic acid with carboxyl groups in the para (1,4) positions, giving linear, symmetrical repeat units that can pack well in fibres. In the polymerisation reaction, the hydroxyl groups of ethylene glycol react with the carboxyl groups of terephthalic acid, releasing water and forming ester linkages. Benzoic acid and salicylic acid are monocarboxylic or have different functional groups, while phthalic acid has carboxyl groups in the ortho position, leading to different polymers. Oxalic acid is a very small dicarboxylic acid and does not give terylene.

Step-by-Step Solution:
Step 1: Recall that terylene is a polyester and therefore consists of repeating ester units formed by condensation of a diol and a dicarboxylic acid. Step 2: Identify ethylene glycol as the diol component, with two hydroxyl groups that can form ester bonds. Step 3: Recognise that the aromatic dicarboxylic acid used in terylene has its carboxyl groups opposite each other on the benzene ring to produce a linear polymer. Step 4: Recall that terephthalic acid has carboxyl groups at the para positions on benzene and is therefore the correct partner for ethylene glycol in forming terylene. Step 5: Conclude that among the options, terephthalic acid is the correct monomer used with ethylene glycol in the formation of terylene.

Verification / Alternative check:
Standard polymer chemistry summaries and school textbooks describe terylene or Dacron as polyethylene terephthalate. The name itself contains terephthalate, indicating that the acid component is terephthalic acid. Structural diagrams show repeating units derived from ethylene glycol and terephthalic acid residues linked by ester bonds. Meanwhile, phthalic acid with ortho positioned groups is used to make other polyesters and plasticisers but not terylene. This confirms that terephthalic acid is the specific dicarboxylic acid involved.

Why Other Options Are Wrong:
Benzoic acid has only one carboxyl group and cannot form a long chain polyester with a diol in the same way as a dicarboxylic acid. Salicylic acid contains both carboxyl and hydroxyl groups on a benzene ring but does not match the known terylene synthesis. Phthalic acid is an ortho dicarboxylic acid and forms different types of polyesters, not the linear fibre forming polymer terylene. Oxalic acid is a very small dicarboxylic acid and does not produce the familiar fibre polymer when condensed with ethylene glycol. Therefore, none of these alternatives are correct for terylene.

Common Pitfalls:
Students may confuse phthalic and terephthalic acids because their names sound similar and both are benzene dicarboxylic acids. Another pitfall is not remembering the structural significance of para substitution, which allows linear polymer chains to form easily. Learners might also generalise and assume that any dicarboxylic acid could be used to make any polyester, without considering the specific polymers that result. Associating the name polyethylene terephthalate with terephthalic acid helps to avoid these confusions.

Final Answer:
Terylene is a condensation polymer of ethylene glycol and Terephthalic acid.