Problem 71 - Chemistry: Structure and Properties

Question

One kind of polyester is a condensation copolymer formed from terephthalic acid and ethylene glycol. Draw the structure of the dimer. [Hint: Water (circled) is eliminated when the bond between the monomers forms.]

Step-by-Step Solution

Answer

The dimer of polyester formed from terephthalic acid and ethylene glycol consists of two terephthalic acid units and two ethylene glycol units connected by ester linkages, with water being eliminated during the reaction.

Step 1: Identify the Monomers

Find the chemical structure of terephthalic acid (benzene-1,4-dicarboxylic acid) and ethylene glycol (ethane-1,2-diol). Terephthalic acid has the formula \(C_8H_6O_4\) and ethylene glycol has the formula \(C_2H_6O_2\).

Step 2: Drawing the Monomers

Draw the two monomers placing -COOH groups of terephthalic acid and -OH groups of ethylene glycol at the ends so that they can react with each other.

Step 3: Show the Condensation

Indicate the condensation reaction between the -COOH group of the terephthalic acid and the -OH group of ethylene glycol. During the reaction, a molecule of water \(H_2O\) is eliminated.

Step 4: Formation of Ester Linkage

Draw the formation of an ester linkage (\( -CO-O- \) bond) between terephthalic acid and ethylene glycol. Repeat this action to show the dimer, which involves two of these reactions, as it is the simplest polyester segment that contains the repeating unit.

Step 5: Draw the Dimer Structure

Complete the structure by including the ester linkages between two terephthalic acid and two ethylene glycol molecules, making sure to exclude the two molecules of water that are eliminated during the condensation polymerization process.

Key Concepts

Polyester Synthesis

Polyester synthesis is a chemical process that creates long-chain polymers through a reaction known as condensation polymerization. The process typically involves combining organic acids and alcohols—monomers that can form a repetitive network of bonds.

In the synthesis of polyester, the monomers are usually di-carboxylic acids and di-alcohols, which means each monomer has two functional groups capable of bonding. When these monomers react, they release a small molecular byproduct, such as water, hence the term 'condensation'. The polyester chains grow longer as more monomers join in, forming a high molecular weight material commonly used in fabrics, plastics, and resins. Easy to understand, picture each monomer as a link in a chain that's joined together in a sequence, removing a tiny piece from each link to lock them firmly in place.

Terephthalic Acid

Terephthalic acid, a white crystalline compound, is one of the primary raw materials in the manufacture of polyesters. Chemically, it is known as benzene-1,4-dicarboxylic acid, with a formula of \(C_8H_6O_4\).

It features a benzene ring—a hexagonal structure of carbon atoms with alternating single and double bonds—attached to two carboxylic acid groups (\( -COOH \)) at opposite ends. These carboxylic acids are reactive sites where terephthalic acid links up with another molecule, like ethylene glycol, during polyester synthesis. Because the bonding happens at two points, terephthalic acid is especially good at forming long, stable polymer chains.

Ethylene Glycol

Ethylene glycol is a colorless, sweet-tasting organic compound used extensively as a monomer in the production of polyesters. It's represented by the simple formula \(C_2H_6O_2\).

Commonly known by its more everyday use in antifreeze solutions, ethylene glycol has two hydroxyl groups (\( -OH \)) capable of reacting with the acid groups of terephthalic acid. These reactions form the ester linkages that are the backbone of polyester chains. Just imagine ethylene glycol as a flexible connection that holds two stiff pieces of a puzzle together—those stiff pieces being units of terephthalic acid.

Ester Linkage

An ester linkage is a chemical bond specifically formed between an acid and an alcohol when they undergo a condensation reaction. The characteristic bond sequence in an ester linkage is \( -CO-O- \).

In the context of polyester synthesis, these linkages result from the reaction between terephthalic acid's carboxyl groups and ethylene glycol's hydroxyl groups, with the elimination of water molecules. The ester linkage is a pivotal point of the polymer structure, providing flexibility and resistance to the material. It's these strong yet versatile bridges that give polyester its valuable properties like durability and resistance to stretching and shrinking.