What is Fischer Esterification?

Final Fischer Esterification Step

Fischer Esterification

Fischer Esterification is an organic chemical reaction that reacts a carboxylic acid group with an alcohol group to form an ester. Esterification is defined as any reaction that produces an ester.

Fischer Esterification is one of the most common reactions that forms esters.

Ester Structure
Ester Structure

Catalyzed with a strong acid, Fischer Esterification also forms water as an additional product. Some strong acids often used in this reaction include sulfuric acid (H2SO4), tosic acid (TsOH), and hydrochloric acid (HCl).

Esterification Reaction

Fischer Esterification Mechanism and Reaction

Fischer esterification is an example of a nucleophilic substitution reaction. This reaction often conducts with an excess of alcohol, and has a slower rate of reaction. Fischer Esterification is therefore an easily reversible reaction.

The nucleophile is the electronegative oxygen in O-H and the electrophile is the carbon in the carboxylic acid group. During the reaction, the alcohol O-H group substitutes into an O-R group in the ester product.

Below are the 5 reaction steps in this mechanism with a longer description of each step below that:

Esterification Mechanism
Esterification Mechanism

1. Protonation: Step 1 of Fischer Esterification

The carbon in the carboxylic acid is protonated by the acid catalyst. By adding a hydrogen atom, this creates a positive charge on the oxygen atom. Protonation begins the esterification process and allows for a nucleophilic attack.

Protonation Step of the fischer esterification
Protonation Step

2. Nucleophilic Addition

The nucleophilic O-H group attacks the carboxylic acid’s carbon atom. This bonds the O-R’ and carboxylic acid group, and neutralizes the positive oxygen atom’s charge. The O-R’ group now also has a positively-charged oxonium ion. The R groups on this molecule can be many different structures. That is part of what makes this reaction so useful.

Nucleophilic Addition Step of the fischer esterification
Nucleophilic Addition

3. Proton Transfer

Usually completed in a two-step process, the H+ proton transfers from the oxonium ion to the carboxylic acid’s O-H group. This transfer neautralizes the oxonium ion’s positive charge and forms a water molecule.

Proton Transfer Step of the fischer esterification
Proton Transfer Step

4. Elimination of H2O

Next, the lone pair on the oxygen atom forms a pi-bond with the carbon atom. This eliminates the water molecule as a reaction byproduct. The water is the leaving group in this reaction.

Water Elimination Step of the fischer esterification
Water Elimination Step

5. Deprotonation

Next, the acid catalyst deprotonates the positively-charged oxygen atom, and then gives an ester as the final product. The R groups attached to the final product can be many different things, making the synthesis extremely versatile.

As shown in this reaction process, this esterification process transforms carboxylic acid into an ester!

Step five of the fischer esterification
Deprotonation/Termination Step

Examples of Fischer Esterification Products

1. Aliphatic Esters

In chemistry, the term aliphatic means constructed of single carbon-carbon bonds. An aliphatic ester contains the ester functional group and a long carbon chain. Fischer Esterification most commonly forms these esters.

An example of an aliphatic ester is ethyl acetate.

Ethyl acetate is formed through Fischer Esterification by reacting acetic acid and ethanol. The byproduct is water and the reaction is catalyzed by a strong acid. The reaction is drawn out below.

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Ethyl Acetate Synthesis

2. Aromatic Esters (Lactones)

Aromatic esters, also known as lactones, are made up of the ester functional group and an aromatic ring. Condensing a carboxylic acid group and an alcohol forms these aromatic esters. Fischer Esterification most commonly forms these esters.

An example of an aromatic ester is methyl benzoate, an organic compound often found in perfumes.

Reacting methanol and benzoic acid in a Fischer Esterification forms mathyl benzoate. Sulfuric acid catalyzes the reaction and water is the byproduct. The reaction is seen below.

Methyl Benzoate Synthesis

Examples of Esters in Everyday Life

All esters end in the suffix -oate or -ate. When occurring in the natural world, esters are responsible for fruity and flowery scents. Artificial flavoring replicates these scents in many products. Esters are often used in artificial flavouring, perfumes, and cosmetics. Found in many manufactured products, esters are crucial to everyday life.

For example, isopentyl acetate is responsible for artificial banana flavouring. Ethyl butyrate is responsible for artificial pineapple flavouring.

Artificial Flavouring: Many artificial flavors have esters in them that may be synthesized using the Fischer Esterification

Further Reading