Question

One kind of polyester is a condensation copolymer formed from tere- phthalic acid and ethylene glycol. Draw the structure of the dimer and circle the ester functional group. [Hint: Water (circled) is eliminated when the bond between the monomers forms.

Short answer

Draw the chemical structure of the dimer formed by connecting two monomer units of terephthalic acid and ethylene glycol via ester linkages created from the condensation reaction (where water is eliminated), and circle the two ester functional groups in the dimer.

Step-by-step solution

Identify the Monomers

Determine the chemical structure of the two monomers involved in the reaction: terephthalic acid (C8H6O4) and ethylene glycol (C2H6O2). Terephthalic acid has two carboxyl groups (-COOH) and ethylene glycol has two hydroxyl groups (-OH).

Formation of the Ester Bond

Visualize the condensation reaction where one hydroxyl group from ethylene glycol reacts with one carboxyl group from terephthalic acid to form an ester linkage. In this process, a water molecule is eliminated. Show this ester bond formation between two monomer units to create a dimer.

Draw the Dimer Structure

Draw the resulting dimer which includes two repeat units of the monomers connected through ester linkages. Make sure to draw two ester functional groups within the dimer structure.

Circle the Ester Functional Group

Identify and circle the ester functional group in the dimer structure. An ester functional group can be identified by its molecular formula -COO-. Make sure to circle both instances of the ester group in the dimer.

Key concepts

Polyester Condensation Copolymer

Polyester copolymers are a crucial type of plastic that can be found in a broad range of products, from fabrics to plastic bottles. The process of forming a polyester involves a reaction known as condensation polymerization, which typically occurs between a dicarboxylic acid and a diol.

In the case of polyesters, the reaction involves the removal of water molecules as the molecules link together, hence 'condensation'. A copolymer is a polymer derived from more than one species of monomer, distinguishing it from a homopolymer where only one monomer is used. Polyester condensation copolymers, such as those formed from terephthalic acid and ethylene glycol, are bespoke materials designed to exhibit specific properties suited for various applications, such as flexibility, durability, and resistance to environmental factors.

The unique properties of these copolymers can be tweaked by altering the types and ratios of the monomers used, making them incredibly versatile materials in the field of plastics engineering.

Terephthalic Acid

Terephthalic acid is a white, crystalline substance primarily used as a precursor to the polyester PET (polyethylene terephthalate), which is widely used in textile fibers and beverage bottles. It contains two carboxyl groups (denoted as -COOH), which are highly reactive and essential for the polyester-making process.

Each carboxyl group can react with hydroxyl groups (-OH) to form ester linkages through condensation reactions. This bi-functional structure allows terephthalic acid to serve as a 'building block' in the creation of polyester chains. The acid's structure contributes to the rigidity and strength of the resulting polyester, making it suitable for materials that require a high degree of durability.

Understanding the role of terephthalic acid in polyester production is key to grasping how polymer properties can be engineered by choosing specific monomers.

Ethylene Glycol

Ethylene glycol is a colorless, sweet-tasting chemical that is both toxic and inedible. This compound plays a central role in the production of polyesters, where it serves as the diol component. Ethylene glycol features two hydroxyl groups, which under the right conditions, react with carboxyl groups of acids like terephthalic acid to form ester bonds.

The properties of ethylene glycol, such as its melting and boiling points, solubility, and reactivity, play a crucial role in its capability to create sturdy and flexible plastic materials. Moreover, due to ethylene glycol's reactivity and the nature of the condensation reaction, it helps in building the polymer chain, contributing significantly to the characteristics of the resulting copolymer.

Ethylene glycol's contribution goes beyond industrial applications; it is also the primary ingredient in automotive antifreeze and coolant solutions due to its ability to lower the freezing point and raise the boiling point of aqueous solutions.