Question

We have studied Fischer esterification, in which a carboxylic acid is reacted with an alcohol in the presence of an acid catalyst to form an ester. Suppose that you start instead with a dicarboxylic acid such as terephthalic acid and a diol such as ethylene glycol. Show how Fischer esterification in this case can lead to a macromolecule with a molecular weight several thousand times that of the starting materials. O=C(O)c1ccc(C(=O)O)cc1 (PET) 1,4-Benzenedicarboxylic acid \(\quad 1,2\)-Ethanediol (Terephthalic acid) (Ethylene glycol) As we shall see in Section 29.5B, the material produced in this reaction is a highmolecular-weight polymer, which can be fabricated into Mylar films and into the textile fiber known as Dacron polyester.

Short answer

Question: Describe the process of Fischer esterification when a dicarboxylic acid (terephthalic acid) reacts with a diol (ethylene glycol) and how it results in a macromolecule with a high molecular weight. Answer: In Fischer esterification, a carboxylic acid reacts with an alcohol in the presence of an acid catalyst to form an ester. In this case, terephthalic acid (a dicarboxylic acid) reacts with ethylene glycol (a diol) and forms ester linkages in both carboxylic acid groups. The reaction continues, with another terephthalic acid molecule reacting with the remaining alcohol group of ethylene glycol, linking molecules to form a macromolecule with a significantly higher molecular weight. This process creates a polymer chain known as polyethylene terephthalate (PET).

Step-by-step solution

Understand Fischer esterification

Fischer esterification is a reaction between a carboxylic acid and an alcohol in the presence of an acid catalyst to form an ester. In this exercise, instead of having a simple carboxylic acid, we have a dicarboxylic acid (terephthalic acid) and a diol (ethylene glycol), which contains two alcohol groups.

Write down the reactants

We have terephthalic acid (1,4-Benzenedicarboxylic acid) with the structure O=C(O)c1ccc(C(=O)O)cc1 and ethylene glycol (1,2-Ethanediol) with the structure HO-CH2-CH2-OH.

Fischer esterification of dicarboxylic acid with a diol

During the Fischer esterification, the acid and the alcohol groups interact with each other to form an ester linkage. Since there are two carboxylic acid groups in terephthalic acid and two alcohol groups in ethylene glycol, the formation process will result in the ester linkages at both carboxylic acid groups. Reaction of the first carboxylic acid group of terephthalic acid with one alcohol group of ethylene glycol: O=C(O)c1ccc(C(=O)O)cc1 \(\ + \) HO-CH2-CH2-OH \(\rightarrow\) O=C(O)C1=CC=C(C(=O)OC-CH2-CH2-OH)C=C1

Formation of macromolecules

The reaction can continue, with another terephthalic acid molecule reacting with the alcohol group left on ethylene glycol, linking the two molecules to each other. This process can be repeated, leading to the formation of a macromolecule with a molecular weight that is several thousand times that of the starting materials. Polymer formation: O=C(O)C1=CC=C(C(=O)OC-CH2-CH2-OH)C=C1 + O=C(O)c1ccc(C(=O)O)cc1 \(\rightarrow\) O=C(O)C1=CC=C(C(=O)OC-CH2-CH2-O-C(=O)c2ccc(C(=O)O)cc2)C=C1 Here, the ester linkages formed between terephthalic acid and ethylene glycol create a polymer chain known as polyethylene terephthalate (PET), which is used in the production of Mylar films and Dacron polyester textile fibers. In summary, the Fischer esterification of terephthalic acid (a dicarboxylic acid) with ethylene glycol (a diol) leads to the formation of a macromolecule with a significantly higher molecular weight than the starting materials, due to the repeatable formation of ester linkages that create a polymer chain.