What is Transesterification?
Transesterification is an organic reaction in which the R group of an alcohol is exchanged with an R’ group of an ester. This is generally done via the introduction of an acid or base catalyst to the reaction mixture. However, it can also be done using certain enzyme catalysts (such as lipases). An illustration detailing the exchange of an R’ group belonging to the alcohol with the R’’ group of an ester in a transesterification reaction is provided below.
Also Read: Esterification
When catalyzed by an acid catalyst, this reaction proceeds via the conversion of the carbonyl group through the donation of a proton to it. On the other hand, base catalysts take a proton away from the alcohol group, resulting in the formation of a highly nucleophilic alkoxide ion.
It can be noted that methyl & ethyl esters can be used to form esters with relatively large alkoxy groups via the process of transesterification. This is usually done by heating the ester (methyl or ethyl) with the acid/base catalyst and the alcohol having a large alkoxy group, and subsequently evaporating off the smaller alcohol in order to drive the equilibrium reaction in the required direction.
Mechanism under Basic Conditions
The alcohol is deprotonated by the basic medium, resulting in the formation of an alkoxide ion. This alkoxide executes a nucleophilic attack on the carbonyl carbon of the ester, resulting in the formation of an intermediate. The double bond between the carbonyl carbon and the oxygen is broken and the negative charge is transferred to the carbonyl oxygen, as illustrated below.
The R’ group of the initial ester reactant acts as a leaving group and is removed from the intermediate. The oxygen retains the bond pair of electrons, resulting in the formation of a new alkoxide. Finally, the double bond between the carbonyl carbon and the negatively charged oxygen is reformed, as illustrated below.
Further protonation of the alkoxide yields the required transesterification products.
Mechanism Under Acidic Conditions
First, the carbonyl oxygen is protonated by the acidic medium. The resulting positive charge on the oxygen makes it more electron-withdrawing, activating the carbonyl carbon towards a nucleophilic attack.
The presence of 2 lone pairs on the oxygen of the alcohol gives it a nucleophilic nature. This oxygen executes a nucleophilic attack on the carbonyl carbon and binds to it. This results in the formation of an intermediate.
An intramolecular proton transfer occurs in this intermediate and the positive charge is relayed from the oxygen of the alcohol to the oxygen of the ester, as illustrated below.
The protonated oxygen acts as a leaving group and the carbon-oxygen bond is broken. The bond pair is retained by the oxygen atom and the positive charge is relayed through the carbonyl carbon to the carbonyl oxygen (the carbon-oxygen double bond is reformed, as illustrated below).
Note that the required alcohol product is formed in this step.
Finally, the positively charged carbonyl oxygen on the ester is deprotonated, yielding the required ester product and the regenerated acid.
Applications of Transesterification
- Transesterification reactions play a crucial role in the synthesis of polyester. Here, di-esters and diols are subjected to transesterification in order to obtain macromolecules.
- This process is also used in plastic recycling to reduce polyester into its monomers.
- Biodiesel can be prepared from triglycerides via transesterification.
- This reaction is also used in the synthesis of certain derivatives of enols. For example, vinyl ethers can be prepared from vinyl acetate via transesterification.
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