Why acid hydrolysis of ester is slow at the beginning when dilute hydrochloric acid is used as catalyst?

Answer 1

Step 1:

An acid/base reaction. Since we only have a weak nucleophile and a poor electrophile we need to activate the ester. Protonation of the ester carbonyl makes it more electrophilic.

Step 2:

The water O functions as the nucleophile attacking the electrophilic C in the C=O, with the electrons moving towards the oxonium ion, creating the tetrahedral intermediate.

Step 3:

An acid/base reaction. Deprotonate the oxygen that came from the water molecule to neutralise the charge.

Step 4:

An acid/base reaction. Need to make the -OCH3 leave, but need to convert it into a good leaving group first by protonation.

Step 5:

Use the electrons of an adjacent oxygen to help "push out" the leaving group, a neutral methanol molecule.

Step 6:

An acid/base reaction. Deprotonation of the oxonium ion reveals the carbonyl C=O in the carboxylic acid product and regenerates the acid catalyst.

Acid produces acts as a catalyst enhances the rate of reaction

Answer 2

The slow rate of acid hydrolysis of ester at the beginning when dilute hydrochloric acid is used as a catalyst is due to the activation of the ester bond. Initially, the acid needs to protonate the carbonyl oxygen of the ester to make it more susceptible to nucleophilic attack by water. Once this activation step occurs, the hydrolysis reaction proceeds more quickly.