Making and breaking esters

An ester is made from a carboxylic acid and an alcohol. The carboxylic acid and alcohol are heated in the presence of a catalyst, usually concentrated sulphuric acid.

carboxylic acid + alcohol → ester + water

This is a condensation reaction, where two molecules join together to form one larger molecule (the ester) and a small molecule, usually water.

Example: formation of ethyl propanoate.

Propanoic acid and ethanol react to produce ethyl propanoate and water. The ester link is formed by the reaction of a hydroxyl group with a carboxyl group.

The reaction of an alcohol with a carboxylic acid is a reversible reaction. In the reaction, some of the ester molecules formed are split up again into the alcohol and carboxylic acid they are made from. This happens when water reacts with the ester link. This is a hydrolysis reaction.

A hydrolysis reaction is one where a large molecule is split into two smaller molecules by reaction with water.

When an alcohol reacts with a carboxylic acid, an equilibrium is produced. The equation for the reaction contains a double arrow. The forward reaction is a condensation reaction, and the reverse reaction is a hydrolysis reaction.

Example: formation of methyl ethanoate.

Ethanoic acid reacts with methanol in the forward condensation reaction to produce methyl ethanoate and water. The reverse hydrolysis reaction occurs when methyl ethanoate and water react, producing ethanoic acid and methanol.

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