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

A condensation reaction between acetic acid (CH3COOH) and ethanol (CH3CH2OH) forms an ester, ethyl acetate (CH3COOCH2CH3), and a water molecule: \[ CH3COOH + CH3CH2OH \rightarrow CH3COOCH2CH3 + H2O \] To form a polymer like polyester, a dicarboxylic acid with two carboxyl groups (-COOH) and a diol with two hydroxyl groups (-OH) are needed. For example, in the formation of polyethylene terephthalate (PET), reactions involve terephthalic acid (C6H4(COOH)2), a dicarboxylic acid, and ethylene glycol (C2H4(OH)2), a diol: \[ n \, C6H4(COOH)2 + n \, C2H4(OH)2 \rightarrow [C6H4(COOC2H4O)]_n + 2n \, H2O \]

Step-by-step solution

Understand the formation of an ester

In the condensation reaction between a carboxylic acid and an alcohol, a water molecule is eliminated, and an ester is formed. The general structure of an ester can be represented as R-COO-R', where R and R' represent different hydrocarbon chains.

Provide an example of an ester-forming reaction

Let's take acetic acid (CH3COOH) and ethanol (CH3CH2OH) as an example. The reaction between these two compounds can be represented as follows: \[ CH3COOH + CH3CH2OH \rightarrow CH3COOCH2CH3 + H2O \] In this reaction, acetic acid reacts with ethanol to form ethyl acetate (CH3COOCH2CH3) and a water molecule.

Explain how the reaction can be extended to form a polyester

To form a polyester, we need to have a carboxylic acid with two or more carboxyl groups (-COOH) and an alcohol with two or more hydroxyl groups (-OH). These compounds are called dicarboxylic acids and diols, respectively. When a dicarboxylic acid reacts with a diol, a polyester is formed through the continuous condensation reaction between the carboxyl and hydroxyl groups, eliminating water molecules in the process. For example, let's consider the formation of a polyester called polyethylene terephthalate (PET), which is widely used in making plastics and fibers. The reaction will involve terephthalic acid (C6H4(COOH)2), a dicarboxylic acid, and ethylene glycol (C2H4(OH)2), a diol. The formation of polyester can be represented as follows: \[ n \, C6H4(COOH)2 + n \, C2H4(OH)2 \rightarrow [C6H4(COOC2H4O)]_n + 2n \, H2O \] In this reaction, a continuous chain of ester-linked terephthalic acid and ethylene glycol monomers is formed, creating the polyester PET.

Key concepts

Condensation Reaction

A condensation reaction is a chemical process where two molecules join together, losing small molecules like water in the process. This reaction is quite important in organic chemistry and is commonly used to synthesize esters.

In the formation of an ester, a carboxylic acid and an alcohol react, and through a condensation reaction, they produce an ester and a water molecule. The process involves the hydroxyl group (-OH) from the carboxylic acid and the hydrogen atom (H) from the alcohol's hydroxyl group, combining to form water.

You can think of condensation reactions as nature's way of constructing more complex molecules. These reactions are vital for producing many types of organic compounds, such as esters, amides, and peptides. The process allows the creation of large and intricate molecular structures from simpler units.

• Two molecules join together.
• A small molecule, like water, is released.
• Forms complex compounds efficiently.

Carboxylic Acid

Carboxylic acids are organic acids characterized by having one or more carboxyl groups (-COOH). They are essential in forming esters through condensation reactions, as one of the reactants.

The carboxyl group is highly reactive, making these acids a perfect choice for reactions that form esters. During the reaction, the hydroxyl part (-OH) of the carboxylic acid combines with a hydrogen atom from an alcohol to release water. This is crucial in the ester formation process.

Common examples of carboxylic acids include acetic acid, which gives vinegar its acidic taste, and citric acid, found in citrus fruits. Due to the acidic nature of carboxylic acids, they are also reactants in neutralization reactions that produce salts and water.

• Contains carboxyl group (-COOH).
• Highly reactive and versatile.
• Key player in ester formation.

Alcohol

Alcohols are organic compounds with one or more hydroxyl groups (-OH). In ester formation, they are the second essential ingredient, reacting with carboxylic acids.

The hydroxyl group in alcohols is crucial for the condensation reaction, where it combines with the carboxyl group from the carboxylic acid to form water. This reaction results in the creation of an ester linkage.

Alcohols can vary from simple, like methanol (CH₃OH) and ethanol (C₂H₅OH), to more complex structures. They play a significant role in both chemistry and everyday life, being involved in reactions that produce flavors, fragrances, and biofuels.

• Contains hydroxyl (+OH) group.
• Essential for ester reactions.
• Versatile in chemical synthesis.

Polyester

Polyesters are types of polymers formed from the repeated condensation reactions of carboxylic acids and alcohols, where each subunit is connected by an ester linkage. This leads to long chains of these repeating units, providing materials with different properties depending on the specific monomers used.

The polyester formation process typically involves a dicarboxylic acid and a diol. The continuous reaction processes cycle through many stages, each time releasing water molecules, and effectively building the polymer chain.

One of the most well-known polyesters is polyethylene terephthalate (PET), renowned for its use in making plastic bottles and fibers. Polyesters, known for their durability and versatility, are used in various industries ranging from textiles to engineering.

• Formed by polymerization of esters.
• Involves repeating ester linkages.
• Common and versatile material.