Fischer Esterification Mechanism

What is Fischer Esterification Reaction?

The Fischer Esterification Mechanism must include the continuous removal of water from the system or the usage of a large excess of alcohol since the overall reaction is reversible. Fischer Esterification is an organic reaction which is employed to convert carboxylic acids in the presence of excess alcohol and a strong acid catalyst to give an ester as the final product. This ester is formed along with water. A few examples of Fischer Esterification reactions are given below.

Fischer Esterification

Fischer Esterification Reaction

The techniques used to remove water from the system during this esterification include the removal of water by azeotropic distillation or adsorption by molecular sieves. The reaction is an example of a nucleophilic acyl substitution reaction. The substitution is based on the nucleophilicity of the alcohol and electrophilicity of the carbonyl carbon.

Fischer Esterification Detailed Mechanism

Step 1

The carbonyl oxygen is protonated by the acid catalyst, activating it toward a nucleophilic attack from the ethanol as illustrated below:

Fischer Esterification Mechanism Step 1

Step 2

The alcohol executes a nucleophilic attack on the carbonyl. A lone pair of electrons from the oxygen atom of the alcohol forms a bond with the carbonyl carbon, breaking its pi bond with the other oxygen. The pi bond electrons move up to the oxygen and neutralize its positive charge. This results in an oxonium ion.

Fischer Esterification Mechanism Step 2

Step 3

Now, a proton transfer occurs from the oxonium ion to the OH group, giving rise to an activated complex. This can be divided into two further steps where the alcohol first deprotonates the oxonium ion giving a tetrahedral intermediate after which the OH group accepts the proton from the alcohol. This step can be illustrated as:

Fischer Esterification Mechanism Step 3

Step 4

Now, the 1,2 elimination of water occurs, giving the protonated ester. A lone pair of oxygen forms a pi bond with the carbon, thereby expelling the water as shown below.

Fischer Esterification Mechanism Step 4

Step 5

The remaining positively charged oxygen is deprotonated, giving the required ester as a product (as shown below).

Fischer Esterification Mechanism Step 5

Thus, the required ester is produced. The Fischer esterification Mechanism can simply be described as – Protonation of the carbonyl, followed by the nucleophilic attack on the carbonyl, the proton transfer to the OH group, the removal of water and finally the deprotonation step. The reverse reaction of Fischer esterification can employ both acids and bases as catalysts and is referred to as hydrolysis.

FAQs

1. How does Fischer esterification work?
Ans: If alcohol and an acid catalyst are used to treat a carboxylic acid, an ester (along with water) is formed. This reaction is called the esterification of Fischer. In large excesses, the alcohol is generally used as a solvent.

2. Why is Fischer esterification important?
Ans: The esterification of Fischer is one of the most common carboxylic acid reactions. Treatment with alcohol of carboxylic acids in the presence of acid catalyst contributes to the formation with esters along with the removal of a water molecule.

3. What is a disadvantage of Fischer esterification?
Ans: The equilibrium existence of the reaction is a major disadvantage in Fischer esterification. When the reaction is conducted in a closed vessel, it is even more difficult because it is necessary to remove either the product or the water generated to drive the reaction to completion.

4. What is the limiting reagent in Fischer esterification?
Ans: The Fischer esterification uses sulfuric acid as a catalyst. It protonates the carboxylic acid carbonyl group and not the feature of hydroxyl. The resulting cation is a stable resonance. Because the reaction is a 1:1 reaction (carboxylic acid: alcohol), the limiting reagents are the carboxylic acid.

5. What is direct esterification?
Ans: Esterification is the reaction of an acid (acid carboxyl group condensation) in the presence of a catalyst with alcohol (alcohol hydroxyl group).

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