Esters are organic compounds that are prepared by the condensation of a carboxylic acid and an alcohol. When esters are added to water containing an acid or alkali catalyst, they break down back to the carboxylic acids and alcohols. The phenomenon of the hydrolysis or breakdown of esters in the presence of a catalyst is known as esters hydrolysis.
It is the reverse reaction of Fisher esterification and is also known as the de-esterification reaction. Ester hydrolysis is a nucleophilic addition elimination reaction. The catalyst used is a strong mineral acid such as concentrated sulphuric acid or a strong base(alkali). Note that, acid ester hydrolysis is a reversible reaction whereas basic ester hydrolysis is an irreversible reaction.
The overall equation for the hydrolysis of an ester is as follows:
Ester + H2O → Carboxylic acid + Alcohol
In acid-catalyzed hydrolysis, the hydronium ion (H3O+) attacks the carbonyl carbon of the ester to form a tetrahedral intermediate that breaks apart into carboxylic acid and alcohol. Whereas, in the presence of a base catalyst, hydroxide ion (OH-) attacks the carbonyl carbon of the ester to form an unstable intermediate which subsequently breaks into the same compounds.
It is to be noted that the rate of hydrolysis of esters depends upon the nature of the ester and the acid or base being used as a catalyst. It is also influenced by the electron-withdrawing groups attached to the carbonyl carbon, such that a strong electron-withdrawing group tend to increase the rate. Moreover, this reaction is preferably performed in basic conditions for maximum yield.
General equation of ester hydrolysis
Acid-catalyzed Esters Hydrolysis
This type of hydrolysis takes place in an acidic medium. All the steps of acidic ester hydrolysis are reversible. The position of equilibrium depends upon the concentration of water and solvent. If water is present in excess hydrolysis occurs whereas when anhydrous alcohol (solvent) is present in excess then it favors esterification.
Acidic hydrolysis results in the formation of carboxylic acid and alcohol shown in the example below:
Reaction Mechanism of Acid-catalyzed Esters hydrolysis
Acidic ester hydrolysis takes place in the following steps:
Protonation of Oxygen of carbonyl compound
Attack of oxygen of water molecule to the electron-deficient carbonyl carbon
Loss of proton from the positively charged oxygen atom
Removal of Alcohol (leaving group)
Loss of proton (acidic hydrogen)
The steric factor affects acid-catalyzed ester hydrolysis. If large groups are present near the reaction site, they slow down the reversible reaction.
Base-catalyzed Esters Hydrolysis
This type of ester hydrolysis takes place in a basic medium. It results in the formation of alcohol and salt of carboxylic acid. This is an irreversible reaction.
Basic hydrolysis is also called saponification. This is an important reaction used for the synthesis of soap. The reaction takes place by refluxing ester aqueous sodium hydroxide. It results in the formation of alcohols and sodium salt of carboxylic acid.
Mechanism of Base-catalyzed Ester hydrolysis
Attack of a nucleophile to acyl carbon
Removal of leaving group
Loss of proton (acidic H+)
Formation of carboxylate ion(salt of carboxylic acid)
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What is meant by Fischer Esterification?
The Fischer esterification is a condensation reaction of carboxylic acid and alcohol to form an ester and water molecules as by-products.
What is an ester?
Esters are the derivatives of carboxylic acid. These are naturally occurring low molecular weight neutral compounds having pleasant smells.
What is meant by Hydrolysis?
Hydrolysis is the breakdown or splitting of larger molecules into fragments when reacted with a water molecule. Hydrolysis is the opposite of condensation.
What are the end products of acidic ester hydrolysis?
The acidic ester hydrolysis is a reversible reaction and it never goes to completion. But the end products in the forward direction are carboxylic acid and alcohol.
What are the end products of basic ester hydrolysis?
The end products of basic ester hydrolysis are salt carboxylic acid and alcohol.
Write down the steps of acid-catalyzed ester hydrolysis.
Steps of acid-catalyzed ester hydrolysis:
- Protonation of Carbonyl oxygen.
- Attack of nucleophile (H2O) to acyl carbon.
- Deprotonation of oxonium ion.
- Removal of leaving group.
- Deprotonation of Carboxylic acid.
What is meant by saponification?
The term saponification is derived from the Latin word Saponis means Soap. it is the process of soap formation through basic hydrolysis ester of long-chain fatty acid.
Why is base-promoted ester hydrolysis an irreversible reaction?
It is an irreversible reaction because carboxylate ions are the conjugate base of carboxylic acid and it is less reactive toward nucleophilic substitution reaction.
Why acid-catalyzed ester hydrolysis is a reversible reaction?
It is a reversible reaction because both the ester and carboxylic acid have the same or comparable reactivities toward nucleophilic substitution reaction.
Write down the steps of saponification.
Steps of saponification
- Attack of nucleophile
- Elimination of leaving group.
- Proton transfer(highly exothermic)
- Formation of carboxylate ion(salt of carboxylic acid).
- 2nd edition of Organic Chemistry by Joseph M. Hornback
- 12th edition of Organic Chemistry by T.W Graham Solomons, Craig B. Fryhle, and Scott A. Synder.
- 9th edition of Organic Chemistry by Leroy G. Wade, JR.
- 4th edition of Organic Chemistry by Francis A. Carey.
- Chapter 20 (chem.ucalgary.edu)