What is Transesterification? - Definition & Mechanism

Instructor: Sarah Pierce

Sarah has a doctorate in chemistry, and 12 years of experience teaching high school chemistry & biology, as well as college level chemistry.

This lesson describes transesterification reactions and provides examples. The mechanism of transesterification in both acidic and basic conditions is also covered.

Aspirin

Have you ever wondered how aspirin works to prevent inflammation? Aspirin makes the enzyme that causes pain and fever to stop working. It does this by reacting with the enzyme in a transesterification reaction. Once the enzyme is inactivated by the aspirin, it can't cause pain anymore. Let's find out more about what transesterification reactions are.

Aspirin uses a transesterification reaction to inhibit the enzyme that causes inflammation.
aspirin

What Is A Transesterification Reaction?

Before we get started, let's review for a minute. An ester is a molecule with a carbon that has a double bond to the oxygen, called a carbonyl carbon. The oxygen in the double bond is a carbonyl oxygen. Next to the carbonyl group, there is another oxygen called the carboxyl oxygen (note the suffix -oxyl, rather than -onyl).


An ester has a carbonyl oxygen and a carboxyl oxygen.
carbonyl


In a transesterification reaction an alcohol molecule and an ester molecule react in either the presence of an acid or base to form a new ester. Basically, one ester is transformed into another ester.


In a transesterification reaction, an alcohol (above the arrow) reacts with an ester to form a new ester. Notice that the R2 group on the ester is replaced with the R3 group from the alcohol. The new alcohol (below the arrow) now has the R2 group.
transesterification reaction


Remember, R just stands for the rest of the hydrocarbon molecule when used in organic compounds. Notice that in a transesterification reaction, the R group on the alcohol changes places with the R group bound to the carboxyl oxygen on the ester.

For example, methyl ethanoate reacts with ethanol in the presence of hydrochloric acid to form ethyl ethanoate and methanol. Notice when predicting the new name that the parent name (ethanoate) will not change. Only the substituent part of the name (methyl to ethyl changes).


Methyl ethanolate reacts with ethanol to produce ethyl ethanoate and methanol.
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Transesterification Mechanisms

There are two mechanisms for transesterification depending on whether the reaction occurs in acidic conditions or basic conditions.

Acidic Conditions

Let's take a look at the steps for acidic conditions.

Step 1: The electrons on the oxygen of the carbonyl carbon (the carbon with a double bond to the oxygen) remove the proton from the acid.


In the first step of the transesterification reaction, the proton is removed from the acid.
trans1


Step 2: The electrons rearrange to form a carbocation.


A carbocation forms.
trans2


Step 3: The electrons of the alcohol attack the carbocation, forming a new bond. This is called a tetrahedral intermediate because the carbon has four different groups attached to it.


The alcohol reacts with the carbocation
trans 3


Step 4: A base removes a proton from the oxygen of the original alcohol. The carboxyl oxygen of the original ester is protonated.


The carboxyl oxygen of the original ester is protonated.
trans 4


Step 5: The tetrahedral intermediate collapses, and the protonated carboxyl oxygen of the original ester is eliminated.


The tetrahedral intermediate collapses.
trans5


Step 6: A base removes the proton from the carbonyl oxygen and the new ester is formed. The acid is regenerated.


The new ester is formed once a base removes the proton from the carbonyl
trans 6


Basic Conditions

In basic conditions, the alcohol has lost a proton and has a negative charge.

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