An ester is a compound formed by a condensation reaction between a carboxylic acid and an alcohol that eliminates a water molecule. Read the discussion of esters in Section 24.4 and then give. an example of a reaction forming an ester. How might this kind of reaction be extended to form a polymer (a polyester)?

An ester is formed by a condensation reaction between a carboxylic acid and an alcohol. A condensation reaction is a type of chemical reaction where two molecules combine to form a single, more complex molecule with the elimination of a smaller one, typically water. In the case of ester formation, the hydroxyl (OH) group from the carboxylic acid and a hydrogen atom from the alcohol's hydroxyl group are removed, forming water as a side product.

One example of a reaction forming an ester is the reaction between acetic acid (ethanoic acid) and ethanol: \(CH_3COOH + CH_3CH_2OH \longrightarrow CH_3COOCH_2CH_3 + H_2O\) In this reaction, acetic acid and ethanol combine to form ethyl acetate (an ester) and water.

To extend this kind of reaction to form a polymer (a polyester), we need to use a di-carboxylic acid and a di-alcohol. A di-carboxylic acid is a molecule with two carboxylic acid groups, and a di-alcohol is a molecule with two alcohol groups. When a di-carboxylic acid reacts with a di-alcohol, they can form long chains by repeatedly reacting with each other through a condensation reaction. These chains contain ester linkages and are called polyesters. For example, if we react adipic acid (a di-carboxylic acid) with ethylene glycol (a di-alcohol), we can form a polyester called polyethylene terephthalate (PET). The reaction can be represented as follows: \(n \cdot HOOC-(CH_2)_4-COOH + n \cdot HO-CH_2-CH_2-OH \longrightarrow \\ \\\ [-OOC-(CH_2)_4-COO-CH_2-CH_2-O-]_n + (2n-1)\cdot H_2O\) In this reaction, adipic acid and ethylene glycol combine by eliminating water molecules, and the number of repeating units (n) in the polymer chain determines the size and properties of the polyester.