Question

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)?

Short answer

An ester is formed by a condensation reaction between a carboxylic acid and an alcohol, where the hydroxyl group from the carboxylic acid and a hydrogen atom from the alcohol's hydroxyl group are removed, forming water as a side product. For example, the reaction between acetic acid and ethanol forms ethyl acetate: \(CH_3COOH + CH_3CH_2OH \longrightarrow CH_3COOCH_2CH_3 + H_2O\) To form a polyester, a di-carboxylic acid and a di-alcohol are used. They react through a condensation reaction, forming long chains with ester linkages. For example, adipic acid reacts with ethylene glycol to form polyethylene terephthalate (PET): \(n \cdot HOOC-(CH_2)_4-COOH + n \cdot HO-CH_2-CH_2-OH \longrightarrow \\ \\\ [-OOC-(CH_2)_4-COO-CH_2-CH_2-O-]_n + (2n-1)\cdot H_2O\)

Step-by-step solution

Understanding ester formation

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.

Example of an ester formation reaction

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.

Extending the reaction to form a polyester

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.

Key concepts

Condensation Reaction

A condensation reaction is a transformative chemical process where two or more molecules combine to form a larger, more complex molecule. During this reaction, a smaller molecule is expelled, usually water, which is why it's sometimes simply referred to as a dehydration reaction. Let's consider ester formation where a carboxylic acid and an alcohol come into play. They react together by losing a water molecule, which results in the creation of an ester.
This elimination of water occurs as the hydrogen atom from the alcohol and the hydroxyl group from the carboxylic acid unite to form water. The remaining components of the molecules bond and create the ester. This bond is the ester linkage, characterized by the (-COO-) group.
Here's an overview:

• Joining of two different molecules (a carboxylic acid and an alcohol).
• Elimination of a small molecule (often water).
• Creation of a new molecule (an ester in this case) with a new chemical structure.

By understanding the basic mechanics of condensation reactions, we can explore how similar reactions can create various compounds, like esters, and extend to synthesize larger, more complex materials.

Polyester Synthesis

Polyester synthesis is an extension of the concept of ester formation. Instead of forming a single ester molecule, the goal is to create long chains of repeated units, known as polymers. This is where di-carboxylic acids and di-alcohols come into play—they each have two reactive groups, allowing the formation of continuous, repeating chains through condensation reactions. This results in the polymer known as polyester.
In a typical polyester formation, a di-carboxylic acid reacts with a di-alcohol. Each carboxyl group reacts with a hydroxyl group, eliminating water and forming an ester linkage each time. This linking repeats many times, forming a long chain. The length of this chain, often represented as 'n', affects the properties of the polyester, like flexibility and strength.

• Repetition of ester formation between di-carboxylic acids and di-alcohols.
• Each reaction forms an ester linkage (COO) and eliminates a water molecule.
• Growth of long repeating chains leads to polyester formation.

The synthesis of common polyester, such as polyethylene terephthalate (PET), involves these precise reactions, offering a glimpse into how fundamental chemical processes create versatile synthetic materials widely used in textiles and packaging.

Carboxylic Acids and Alcohols

Carboxylic acids and alcohols are the fundamental building blocks in the formation of esters and polyesters. Carboxylic acids are organic compounds characterized by a carboxyl group, (-COOH), which is responsible for their acidic nature. Alcohols, on the other hand, contain a hydroxyl group, (-OH), which makes them reactive and crucial for these types of reactions.
In ester formation, an alcohol and a carboxylic acid undergo a condensation reaction, where the OH from the acid and the H from the alcohol's OH group unite to form water. This leaves behind components that bond to form an ester.
Di-carboxylic acids contain two carboxyl groups, while di-alcohols have two hydroxyl groups. These compounds are key in polyester synthesis, enabling the continuous linking required to form long chains.

• Carboxylic acids contain a (-COOH) group, imparting acidic properties.
• Alcohols feature a (-OH) group, reactive in forming esters.
• The reactivity and structure of these compounds enable the formation of complex molecules like esters and polyesters.

Understanding their roles and reactions helps one appreciate the chemical foundations of everyday materials and the types of substances these reactions can yield.