Problem 28 - A Complete Resource Book in Chemistry for JEE Main

Question

Terylene is a condensation polymer of ethylene glycol and (a) benzoic acid (b) phthalic acid (c) salicylic acid (d) terephthalic acid

Step-by-Step Solution

Answer

Terylene is a condensation polymer of ethylene glycol and terephthalic acid (d).

Step 1: Understanding Terylene

Terylene is a type of polyester that is synthesized through a condensation reaction. The process involves combining a diol (such as ethylene glycol) with a dicarboxylic acid to form a polymer.

Step 2: Identifying Diol Component

In the question, ethylene glycol is given as one of the components. Ethylene glycol is a common diol used in the production of polyesters.

Step 3: Analyzing Acid Options

Let's review the acids given in the options: 1. Benzoic acid is a mono-carboxylic acid, which usually does not form polyesters by itself. 2. Phthalic acid can form polyesters but typically makes different kinds of resins. 3. Salicylic acid has phenol and carboxylic groups which are not typically used for creating Terylene. 4. Terephthalic acid is a dicarboxylic acid suitable for forming polyesters, specifically Terylene, when coupled with ethylene glycol.

Step 4: Identifying Correct Condensation Partner

The correct dicarboxylic acid needed to form Terylene with ethylene glycol is terephthalic acid. This combination results in the production of a polyester with the desired properties characteristic of Terylene.

Key Concepts

Condensation Polymerization

Condensation polymerization is a process where small molecules join together to form a large polymer. These small molecules typically have two functional groups that react with each other, releasing a small byproduct like water or methanol.

During the formation of polyesters like Terylene, a diol and a dicarboxylic acid react. The diol provides hydroxyl groups (-OH), and the dicarboxylic acid provides carboxyl groups (-COOH).

When these groups react, they form an ester linkage, releasing a molecule of water ( H_2O ) for each ester bond formed. This process repeats over and over, building a long chain of repeating units that make up the polyester. The repeated removal of small molecules, like water, distinguishes condensation polymerization from addition polymerization, where no molecules are lost.

Terephthalic Acid

Terephthalic acid is a crucial component in the creation of many polyesters, including Terylene. It is a dicarboxylic acid, meaning it has two carboxylic acid groups (-COOH) that allow it to react with two molecules of a diol, such as ethylene glycol.

This structure is ideal for making linear polymers because each of the two carboxylic groups can react in a condensation polymerization reaction, linking each molecule of terephthalic acid to two diol molecules in a chain.

Some important points about terephthalic acid include:

• It is a white crystalline solid.
• It is insoluble in water, but soluble in alkaline solutions, like those containing sodium hydroxide.
• It forms the backbone of many key polyester materials used widely in textiles and packaging.

Its role in polyester chemistry is instrumental in creating fibers that are strong, durable, and have excellent resistance to various environmental factors.

Ethylene Glycol

Ethylene glycol is what is called a diol, meaning it has two hydroxyl groups (-OH). It plays a critical role in the creation of polyesters as a reactant in condensation polymerization.

Unlike other diols, ethylene glycol is particularly valued for its:

• High boiling point, which helps in forming stable polyesters.
• Low volatility, which means it doesn’t easily evaporate during the polymerization process.
• Compatibility with a variety of acids, including terephthalic acid, making it versatile.

The presence of two hydroxyl groups means it can react effectively with dicarboxylic acids to form ester linkages, thus extending the polymer chain.

This ability makes it a staple ingredient in manufacturing not only fibers like Terylene, but also other polyesters used in plastics and resins, which find application in industries like automotive and electronics.