Esters: formation, properties and uses

In a nutshell

Esters have the functional group $$(-COO)$$. They are used in many products, such as in perfumes and solvents. Esters can undergo hydrolysis as well as other multiple types of reactions.

Equations

These are general chemical equations. You will need to be able to use these to write equations with specific substances.

$$alcohol + carboxylic \space acid \rightarrow ester + water$$

​$$ester + water \xrightleftharpoons[reflux]{H^+} carboxylic \space acid + alcohol$$

$$ester + hydroxide \xrightarrow{reflux} carboxylate + alcohol$$

$$triester + 3 \, methanol \xrightarrow{KOH}\, glycerol + methyl \,ester$$​​​​

Formation of esters

Esters have the functional group $$-COO$$. They are formed by reacting an alcohol and carboxylic acid together in the presence of an acid catalyst, such as concentrated sulfuric acid.

​$$alcohol + carboxylic \space acid \rightarrow ester + water$$​​

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Example  

​$$methanol + ethanoic \space acid \rightarrow methyl \space ethanoate + water \\ CH_3OH + CH_3COOH \rightarrow CH_3COOCH_3 + H_2O$$

Uses of esters

Esters have many uses. Here are a few uses and properties that you need to know.

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​​Hydrolysis

Hydrolysis is the breaking of a molecule using water. Esters can be hydrolysed in the presence of a catalyst to give a carboxylic acid (or carboxylate) and alcohol. 

Acid hydrolysis

Ester is split in the presence of water and an acid catalyst, such as dilute sulfuric acid.

​$$ester \,+\, water \xrightleftharpoons[reflux]{H^+} carboxylic \space acid \,+\,alcohol$$

Example

Base hydrolysis

Ester can be split in the presence of water and a base catalyst, such as sodium hydroxide.

​$$ester \,+\, hydroxide\, \,\xrightarrow{reflux}\,\, carboxylate \, +\,alcohol$$​​

​​Example

Fats and oils

Fatty acids are long chains of carboxylic acids. They can be saturated or unsaturated. 

Animal fat tends to have saturated hydrocarbon chains, allowing the molecules to pack closely together and increasing the number of Van der Waals forces. This leads to a higher melting point and hence why fats are solid at room temperature. 

Vegetable oils have unsaturated hydrocarbon chains so the molecules are not able to pack closely together due to the presence of double bonds, creating kinks. This reduces the number of Van der Waals forces, hence a lower melting point. This is why they are liquids at room temperature.

Glycerol

Fatty acids join with glycerol to make esters. 

Oils and fats can be hydrolysed to form glycerol and soap. This is done by heating fats and oils in the presence of sodium hydroxide.

Biodiesel

Vegetable oils are converted to biodiesel. This is done by reacting the vegetable oil with methanol in the presence of potassium hydroxide (catalyst). The result is glycerol and a mixture of methyl esters of fatty acids, which is biodiesel.

$$triester\,+\,3\,methanol \, \xrightarrow{KOH}\,\,glycerol\,+\,methyl \space ester$$​