Fischer esterification is an acid-catalyzed condensation of a carboxylic acid with an alcohol to form esters. This is a reversible reaction, and it is carried out in the presence of concentrated acids such as sulphuric acid (H2SO4). This reaction was discovered by Emil Fisher and Arthur Speier in 1895. This is an important laboratory method for the synthesis of esters, which are widely used as flavorings, fragrances, rubber, and plastics.
In the above reaction, carboxylic acid and alcohol are refluxed with a small amount of concentrated sulfuric acid at equilibrium. Here, the sulphuric acid acts as a catalyst which provides the proton to protonate the carboxylic acid as well as a dehydrating agent. However, the reverse reaction is also possible with dilute sulphuric acid to hydrolyze the esters.
At equilibrium, the reaction mixture has appreciable quantities of products and reactants. By using Le Chatelier’s principle, we can shift the equilibrium toward reactants or products. For this purpose, one should use super-dried alcohol in excess. That is the reason, dried primary alcohols are preferably used in Fischer esterification. Further, water must be removed as it is produced to shift the equilibrium towards ester formation, to increase the yield. Water can be removed by simple distillation or azeotropic distillation with different solvents.
The hydrogen ion (H+) of alcohol is removed from the alcohol whereas hydroxide (OH–) is removed from the acid to form water molecules. This was proved by the isotope labeling experiment where methanol has an oxygen isotope (O18) treated with benzoic acid. This labeled oxygen atom was found in the ester. Hence, it was proved that the O-H bond of alcohol is broken in Fischer’s esterification.
For Fischer esterification, reactants must not be bulky or highly substituted. As steric crowding slows down the rate of reaction, primary alcohols are mostly used for this reaction. Tertiary alcohols due to the presence of substitution prefer elimination reaction. Therefore, special methods are being used for the synthesis of esters by using tertiary alcohols.
Acid-catalyzed Fischer esterification
The most common example of acid catalyst fisher esterification is as follows:
Mechanism of acid-catalyzed Fischer esterification
The mechanism of Fischer esterification is similar to acid-catalyzed reactions. It consists of five steps as elucidated below:
- Nucleophilic addition
In the first step, the carbonyl carbon of carboxylic acid gets a 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 (nucleophile) attacks the electrophilic carbon of carboxylic acid. 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 resulting in the formation neutral ester.
Limitations of Fischer esterification
Limitations of Fischer esterification are:
- 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 excess 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.
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 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.
Why is alcohol used in excess in this reaction?
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 they may lead to carbocation formation and give an elimination reaction. They may also 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.
- The ester synthesis (academic.macewan.ca)