Transesterification is a reaction in which one ester is transformed into another ester by treating it with a large excess of alcohol. The strong acids and strong bases catalyze this reaction. It can also be catalyzed by enzymes such as lipases. It has many applications on the laboratory as well as industrial scale.

What is transesterification?

When we treat ester with excess second alcohol in the presence of strong acids such as H2SO4 and HCl and strong alkali. The alkoxide group of an ester is displaced by the other alkoxide group. The reaction is known as transesterification or alcoholysis.

Mechanism of transesterification

Alcoholysis is catalyzed by both acid and base.

Base catalyzed transesterification

Under basic conditions, an addition elimination reaction occurs.

Step 1

In the first step, alkoxide attacks the carbonyl carbon. It causes the shifting of pi electrons towards the carbonyl oxygen. In this way, an intermediate is formed.

Step 2

This step is the elimination step. Which involves the replacement of an alkoxide that is initially present on the ester.

Acid-catalyzed transesterification

Step 1

In the first step, the protonation of carbonyl oxygen occurs. Which increases the electrophilicity of carbonyl carbon.

Step 2

In this step, alcohol (nucleophile) attacks the carbonyl carbon. This led to the formation of an intermediate.

Step 3

The proton of alcohol is transferred to the alkoxide group of an ester.

Step 4

The positively charged oxygen of the alkoxide group is highly electron-withdrawing. In this way, the electron pair is shifted towards oxygen. Which results in the breaking of the carbon-oxygen bond.

Step 5

In the last step, the carbonyl oxygen is deprotonated to produce another form of ester.

Intramolecular transesterification

If an ester and alcohol are present in the same molecule then intramolecular transesterification can occur. It results in the formation of lactones and macrocycles.

Applications of transesterification

The application of transesterification is not only restricted to the laboratory scale. However, it is also used in many industrial processes to synthesize different compounds.

Synthesis of PET

Alcoholysis is used in the synthesis of PET(polyethylene terephthalate). This is involved in the transesterification of the dimethyl terephthalate with ethylene glycol. This reaction is catalyzed in the presence of zinc acetate.

Synthesis of acrylic acid derivative

The transesterification of methyl acrylate with different alcohols yields acrylic acid derivatives. The acid catalyst can also catalyze this reaction.

Transesterification of vegetable oil

In the transesterification of the vegetable oil, triglyceride reacts with the alcohol to produce a mixture of fatty acids. This is catalyzed by the strong acid or a base.

Production of biodiesel

It is used in the production of biodiesel. It yields biodiesel that has the best properties. Such as viscosity, calorific value, and cetane number.

Concepts Berg

What is the transesterification process?

It is a process in which one ester is transformed into another ester by reaction with an alcohol.

What is transesterification and esterification

Esterification is a reaction of alcohol with carboxylic acid to form an ester. Whereas transesterification is a reaction of an ester with alcohol to replace the alkoxide group with another alkoxide.

What catalyst is used in transesterification?

Strong acids such as sulphuric acid and strong bases are used as catalysts in transesterification.

Reference Books

  • A textbook of Organic Chemistry by Ghulam Rasool Chaudhary

Reference links