Question

How does sulphuric acid catalyse the preparation of esters?

Answer 1

The reaction in which ester is prepared is known as esterification. Esters are prepared when carboxylic acids are heated with alcohol in presence of acid catalysts. The acid catalyst used is generally sulphuric acid. To solve this we must know the role of acid catalyst.

Complete step by step answer:
We know that the reaction in which ester is prepared is known as esterification. Esterification is a chemical reaction in which an organic acid and an alcohol combine to form ester and water.
Esters are prepared when carboxylic acids are heated with alcohol in presence of acid catalysts. The acid catalyst used is generally sulphuric acid. Water is produced as a by-product in the reaction.

The reaction of carboxylic acid and alcohol to produce ester is as follows:
${\text{RCOOH}} + {{\text{R}}^*}{\text{OH}}\xrightarrow{{{\text{Conc}}{\text{. }}{{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}}}{\text{RCOO}}{{\text{R}}^*} + {{\text{H}}_{\text{2}}}{\text{O}}$
The concentrated sulphuric acid i.e. ${{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}$ which is used as a catalyst in the esterification process has a dual roles as follows:
- We know that catalysts alter the rate of the reaction. Concentrated sulphuric acid speeds up the esterification reaction.
- Concentrated sulphuric acts as a dehydrating agent i.e. helps in removal of water. Thus, by acting as a dehydrating agent sulphuric acid shifts the equilibrium right and results in greater yield of ester.
Thus, sulphuric acid catalyses the preparation of esters by increasing the rate of the reaction and increasing the yield of ester.

Note: The mechanism of esterification occurs in five steps as follows:
1. Formation of cation
2. Delocalisation of carbocation
3. Proton transfer
4. Formation of pi-bond
5. Formation of ester