Fischer esterification is an acid-catalyzed condensation reaction. In this reaction, carboxylic acid reacts with alcohol to form esters and a water molecule as a by-product. This is a reversible reaction. This reaction is carried out in the presence of strong acid. This reaction was discovered by Emil Fisher and Arthur Speier in 1895. That’s why it is also known as Fischer-Speier esterification.
In this reaction, carboxylic acid and alcohol are refluxed with a small amount of concentrated sulfuric acid at equilibrium. This reaction must be carried out in the presence of strong acids. Otherwise, it is a very slow reaction. This reaction is reversible when a water molecule is present. The reverse reaction is called hydrolysis of the ester. This will decrease the yield of the product.
At equilibrium reaction mixture has appreciable quantities of products and reactants. By using le Chatelier’s principle we can shift the equilibrium toward reactants or products. In order to shift the equilibrium towards products, we should use super died alcohol in excess. Primary alcohols are preferably used for Fischer esterification.
At the same time remove the water molecule from the reaction mixture as it is formed. It also shifts the reaction in the forward direction. This will increase the yield of the product. A water molecule can be removed by simple distillation or azeotropic distillation with different solvents. Similarly, dehydrating agents can also be used for the removal of a water molecule.
Similarly, we have to use concentrated acids rather than dilute acids. This will increase the yield of the reaction. Strong acids like hydrochloric acid, sulfuric acid, p-toluene sulfonic acids, etc can be used.
At equilibrium reaction mixture has appreciable quantities of products and reactants. By using le Chatelier’s principle we can shift the equilibrium toward reactants or products. We can either increase esterification or hydrolysis.
In this reaction, hydrogen is removed from the alcohol whereas OH is removed from acid to form water molecules. This was proved by an experiment. Benzoic acid was treated with methanol having an oxygen isotope (O18). This labeled oxygen atom was found in the ester. This reveals the bond breaking in Fischer esterification.
For Fischer, esterification reactants must not be bulky or highly substituted. Because steric crowding slows down the rate of reaction. Primary alcohols are mostly used for this reaction. Tertiary alcohols instead of substitution go for elimination reaction. Special methods are used for the synthesis of esters by using tertiary alcohols.
Acid-catalyzed Fischer esterification
Mechanism of Fischer esterification
The mechanism of Fischer esterification is similar to acid-catalyzed reactions. It consists of five steps.
- Nucleophilic addition
In the first step, the carbonyl carbon of carboxylic acid gets proton from the acid catalyst.
The carbonyl carbon of carboxylic acid can also get proton from the protonated alcohol.
2. Nucleophilic addition
In the second step, alcohol attack the electrophilic carbon of carboxylic acid. This is a nucleophilic attack because alcohols act as nucleophiles. This results in the formation of oxonium ions.
This step involves the intramolecular migration of hydrogen atoms. This is called tautomerism.
In this step, a water molecule is removed which will result in protonated ester.
In the last step, the base removes the proton and results in a neutral ester.
What are the major limitations of Fischer esterification?
Limitaions of Fischer esterification:
- It is a very slow reaction without a catalyst
- In the presence of a small amount of water this reaction shifts in the backward direction.
- Phenol esters can not be prepared by the Fischer esterification method.
Why is a large excess of acetic acid used in Fischer esterification?
A large amount of acetic acid is used to increase the yield of the product by shifting equilibrium in a forward direction.
Why primary alcohols are used in Fischer esterification?
Because there is no steric hindrance in primary alcohols. Similarly, they can not form a stable carbocation and do not undergo an elimination reaction like tertiary carbocations.
How can we increase the yield of the product?
We can increase the yield of the product by
- Using super dried alcohol
- Using concentrated acid
- Using an excess alcohol
- Removing water molecule as it is formed
Why highly substituted acids can not be used for esterification reactions?
We can not use highly substituted acids for the esterification reaction because of steric hindrance reaction will slow down and equilibrium will shift toward reactants.
Why do we use a dry tube in the setup for Fischer esterification?
Because the presence of water molecules or moisture tends to shift the reaction in a backward direction and decrease the yield of the reaction.
Why is alcohol used in large excess?
Alcohol is used in large excess to remove water molecules by azeotropic distillation.
How can we monitor the progress of a chemical reaction?
We can monitor the progress of the reaction by using thin layer chromatography.
What is meant by azeotropic distillation?
Azeotropic distillation is the process of separation of liquids mixture on the basis of volatilities.
Why we can not use tertiary alcohols for Fischer esterification?
Tertiary alcohols can not be used for Fischer esterification because these may lead to carbocation formation and gives an elimination reaction. Similarly, these may decrease the rates of reaction due to steric hindrance.
- 2nd edition of Organic Chemistry by Joseph M. Hornback
- 10th edition of Organic Chemistry by Francis A. Carey and Robert M. Giuliano.
- 9th edition of Organic Chemistry by Leroy G. Wade and Jan William Simek.