Esters (Higher Tier Overview)

Definition and Formation of Esters

Esters are a class of organic compounds formed by a condensation reaction between a carboxylic acid and an alcohol. This reaction involves the elimination of a water molecule (H₂O) as the acid and alcohol combine.

The general reaction can be summarised as:

$$\text{Carboxylic acid} + \text{Alcohol} \xrightarrow[\text{heat}]{\text{acid catalyst}} \text{Ester} + \text{Water}$$

An acid catalyst, usually concentrated sulfuric acid (H₂SO₄), is required to speed up the reaction and help remove water, shifting the equilibrium towards ester formation.

This type of reaction is called esterification. Esterification is a reversible equilibrium reaction, meaning the reaction can proceed in both directions depending on conditions such as concentration and temperature.

For example, when ethanoic acid reacts with ethanol in the presence of sulfuric acid, the ester ethyl ethanoate and water are formed:

$$\text{CH}_3\text{COOH} + \text{CH}_3\text{CH}_2\text{OH} \xrightarrow[\text{heat}]{\text{H}_2\text{SO}_4} \text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{H}_2\text{O}$$

Esters have a characteristic structure with the functional group -COO-, where the carbon atom is double bonded to one oxygen atom and single bonded to another oxygen atom which is connected to an alkyl group from the alcohol.

For example, the structure of an ester functional group can be represented as:

Naming Esters

The name of an ester is derived from the names of the alcohol and the carboxylic acid it is made from:

• The alcohol part forms the first part of the name and ends with -yl.
• The acid part forms the second part of the name and ends with -oate.

For example, the ester formed from ethanol and ethanoic acid is called ethyl ethanoate.

If the acid is methanoic acid and the alcohol is methanol, the ester is methyl methanoate.

This naming system helps identify the source alcohol and acid easily.

For instance, the ester formed from propanol and propanoic acid is propyl propanoate.

Properties of Esters

Esters have several distinctive properties:

• Fruity smells: Many esters have pleasant, fruity odours and are responsible for the aromas of fruits like pineapples, apples, and bananas.
• Volatile liquids: Esters tend to be volatile, meaning they evaporate easily at room temperature, which helps their smell spread.
• Uses: Because of their smells, esters are widely used in perfumes, food flavourings, and cosmetics.

For example, the ester isoamyl acetate smells like bananas and is used in flavourings.

Hydrolysis of Esters

Esters can be broken down by hydrolysis, which is the reverse of esterification. Hydrolysis means reacting the ester with water to break it back into the original acid and alcohol.

There are two main types of hydrolysis:

• Acid hydrolysis: The ester is heated under reflux with dilute acid (usually dilute sulfuric acid). This reaction is reversible and produces the original carboxylic acid and alcohol.
• Base hydrolysis (saponification): The ester is heated with a dilute alkali (e.g. sodium hydroxide). This reaction is irreversible and produces an alcohol and a salt of the carboxylic acid (carboxylate salt).

The general equations are:

Acid hydrolysis:

$$\text{Ester} + \text{Water} \xrightarrow[\text{heat}]{\text{dilute acid}} \text{Carboxylic acid} + \text{Alcohol}$$

Base hydrolysis:

$$\text{Ester} + \text{Sodium hydroxide} \xrightarrow[\text{heat}]{} \text{Alcohol} + \text{Sodium salt of carboxylic acid}$$

For example, hydrolysis of ethyl ethanoate with sodium hydroxide produces ethanol and sodium ethanoate:

$$\text{CH}_3\text{COOCH}_2\text{CH}_3 + \text{NaOH} \xrightarrow[\text{heat}]{} \text{CH}_3\text{CH}_2\text{OH} + \text{CH}_3\text{COONa}$$

This base hydrolysis is important in soap making (saponification), where fats (esters of glycerol and fatty acids) are hydrolysed to produce glycerol and soap (the salt).

Below is a diagram illustrating acid and base hydrolysis of esters:

Learning example: Acid hydrolysis of methyl ethanoate

Methyl ethanoate is heated under reflux with dilute sulfuric acid and water. The products are ethanoic acid and methanol.

Equation:

$$\text{CH}_3\text{COOCH}_3 + \text{H}_2\text{O} \xrightarrow[\text{heat}]{\text{dilute acid}} \text{CH}_3\text{COOH} + \text{CH}_3\text{OH}$$

Since the reaction is reversible, heating with excess water shifts equilibrium to hydrolysis.