What is the 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.
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Fischer esterification reactions
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
The carbonyl oxygen is protonated by the acid catalyst, activating it toward a nucleophilic attack from the ethanol as illustrated below:
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.
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:
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.
The remaining positively charged oxygen is deprotonated, giving the required ester as a product (as shown below).
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.