Question

An ester is a compound formed by a condensation reaction between a carboxylic acid and an alcohol. Use the index to find the discussion of esters in Chapter 25, and give an example of a reaction forming an ester. How might this kind of reaction be extended to form a polymer (a polyester)?

Short answer

Esters are formed through condensation reactions between carboxylic acids and alcohols, in which a water molecule is released as the hydroxyl group of the acid and a hydrogen atom from the alcohol's hydroxyl group are removed. An example of this reaction is the formation of ethyl ethanoate from ethanoic acid and ethanol: \(CH_3COOH + CH_3CH_2OH \rightarrow CH_3COOCH_2CH_3 + H_2O\). Polyesters are formed by extending the ester-forming process using a dicarboxylic acid and a diol. For instance, the condensation reaction between ethylene glycol and terephthalic acid creates a polyester called polyethylene terephthalate (PET), with the repeating unit -\(\!-\!OC_6H_4COOCH_2CH_2O-\!\!-\).

Step-by-step solution

Understand Esters and Condensation Reaction

Esters are formed through a condensation reaction between a carboxylic acid and an alcohol. In a condensation reaction, two molecules combine and release a smaller molecule as a by-product, usually water. The ester linkage occurs when the hydroxyl (OH) group of the carboxylic acid and a hydrogen (H) atom from the alcohol's hydroxyl group are removed, forming a water molecule. The remaining oxygen from the alcohol then forms a bond with the carbonyl carbon of the carboxylic acid, resulting in an ester.

Provide an Example of a Reaction Forming an Ester

An example of a reaction forming an ester can be shown with the reaction between ethanoic acid (acetic acid) and ethanol: Ethanoic acid: \(CH_3COOH\) Ethanol: \(CH_3CH_2OH\) In this reaction, the hydroxyl group of ethanoic acid and the hydrogen atom from ethanol's hydroxyl group combine to form a water molecule. The remaining components then bond to form the ester: Ethyl ethanoate (ester): \(CH_3COOCH_2CH_3\) This can be represented by the following equation: \(CH_3COOH + CH_3CH_2OH \rightarrow CH_3COOCH_2CH_3 + H_2O\)

Explain the Formation of Polyesters

To form a polyester, the process of forming esters is extended by using a dicarboxylic acid and a diol (an alcohol with two hydroxyl groups). The condensation reaction between these two compounds introduces ester linkages into a long, repeating chain. For example, the reaction between ethylene glycol (a diol) and terephthalic acid (a dicarboxylic acid) forms a polyester known as polyethylene terephthalate (PET): Ethylene glycol: \(HOCH_2CH_2OH\) Terephthalic acid: \(HOOC-C_6H_4-COOH\) The condensation reaction between ethylene glycol and terephthalic acid forms ester linkages, while water is continuously eliminated. This process leads to the creation of a long, linear polyester chain (polyethylene terephthalate) with repeating ester units: Polyethylene terephthalate: -\(\!-\!OC_6H_4COOCH_2CH_2O-\!\!-\) The formation of a polyester demonstrates how the process of forming esters can be extended to create polymers with various properties and applications, such as fabrics, bottles, and other materials.