Ester Reactions: Formation of Carboxylic Acid from Ester using Acids

Carboxylic acids (and alcohols) can be formed from esters using a strong acids (H3O+, H2SO4/H2O, etc…):

This reaction is a known as a hydrolysis reaction. The ester molecule is split at the “ether” oxygen, resulting in the formation of a carboxylic acid (major/desired product) and an alcohol (minor product). If the starting ester is part of a ring, the ring will be hydrolyzed and the resulting molecule will be have both a carboxylic acid group and alcohol group :

Intramolecular Saponification

The reaction mechanism is depicted below:

In the first step, the free electrons from the double bonded oxygen atom remove a proton from a nearby H3O+ molecule.

In the second step, a nearby H2O molecule attacks the ester-carbon, breaking the double bond with the oxygen sending those electrons to the oxygen atom.

In the third step, the lone pair electrons from the ester-oxygen atom attack a nearby proton on an H3O molecule while simultaneously, an H2O molecule removes a free proton from the newly bonded H2O molecule.

In the fourth step, one of the 2 OH groups attack the ester-carbon, forcing the recently protonated ester-oxygen off the original molecule.

In the fifth step, a nearby water molecule strips the proton from the double bonded oxygen atom, completing the reaction.

This reaction is conducted using an acid such as H3O+ or H2SO4/H2O. This reaction is similar to treating an ester with a strong base (NaOH, KOH, etc…) under hot conditions as it results in the same product but proceeds through a different mechanism.