Difference Between Esterification and Transesterification

Main Difference – Esterification vs Transesterification

Esterification and Transesterification are two terms related to esters. Esters are chemical compounds that have been derived from an acid by replacing its –OH group with an alkoxy group. Esters are typically formed from carboxylic acids. They are polar molecules and can form hydrogen bonds with other molecules due to the presence of oxygen atoms. Esterification is the process used to produce an ester. Transesterification is the modification of esters through chemical reactions. The main difference between esterification and transesterification is that esterification includes an ester as the end product whereas transesterification includes an ester as a reactant.

Key Areas Covered

1. What is Esterification
      – Definition, Mechanism, Examples
2. What is Transesterification
      – Definition, Mechanism, Examples
3. What is the Difference Between Esterification and Transesterification
      – Comparison of Key Differences

Key Terms: Alcohol, Alkoxy Group, Carboxylic Acids, Esterification, Esters, Transesterification

What is Esterification

Esterification is the process where an ester is formed. Most often, this is done with carboxylic acids. Esterification occurs when a carboxylic acid reacts with an alcohol. This reaction is possible only when an acid catalyst and heat are provided to the reaction mixture. Otherwise, there will be no reaction even if the carboxylic acid and the alcohol are mixed together. This is because this process needs a high energy to remove the –OH group from the carboxylic acid. Therefore, a catalyst is needed to reduce the activation energy of the reaction and heat is required as an energy source.

The byproduct formed in this reaction is water. Therefore, in the presence of a dehydrating agent, we can obtain the pure ester product. By altering the carboxylic acid or the alcohol, the ester with desired carbon atoms can be obtained. The esterification reaction is an equilibrium reaction. Therefore, to obtain a high yield of ester, we can use either an excess amount of alcohol or a dehydrating agent to remove water from the system. If not, water can be removed by some other method such as distillation.  

Figure 01: Synthesis of Methyl Acetate

The above image shows the production of methyl acetate using ethanoic acid and methanol as reactants. The byproduct is a water molecule. The water molecule is formed from the H⁺ comes from alcohol and the ^–OH of the carboxylic acid.  The catalyst used here is sulfuric acid.

What is Transesterification

Transesterification is the exchanging of the alkyl group attached to the oxygen atom of the ester with the alkyl group of an alcohol. This reaction requires acidic or basic catalysts in order to reduce the activation energy of the reaction.

Mechanism

Transesterification of an ester is initiated with the nucleophilic attack by the alcohol. The alcohol, removing the proton attached to the oxygen atom, becomes a nucleophile due to the presence of lone electron pairs. This nucleophile can attack the carbon atom which is attached to the two oxygen atoms. The carbon atom gets a partial positive charge because the two oxygen atoms attract the bond electrons towards themselves since the electron affinity of the oxygen atoms is higher than that of the carbon atom. Thus, this carbon with the partial positive charge is a good location for the nucleophile to attack.

Figure 02: Transesterification Reaction

This forms an intermediate molecule which is composed of ester and alcohol bonded to the carbon atom (with partial positive charge) through its oxygen. Since this intermediate is unstable, a rearrangement occurs. There, the –OR group of carboxylic acid is removed. But the alcohol group remains attached to the carbon. Now, a new ester is formed.

This mechanism is used in polyester production.

Difference Between Esterification and Transesterification

Definition

Esterification: Esterification is the process in which an ester is formed.

Transesterification: Transesterification is the exchanging of the alkyl group attached to the oxygen atom of the ester with the alkyl group of an alcohol.

Ester

Esterification: The ester is the end product of esterification.

Transesterification: The ester is a reactant of the transesterification.

Byproduct

Esterification: The byproduct of esterification is a water molecule.

Transesterification: The byproduct of transesterification is a nucleophile/ an alcohol molecule.

Catalyst

Esterification: Esterification requires an acid catalyst.

Transesterification: Transesterification requires acid or base catalysts or enzymes.

Energy Requirement

Esterification: Esterification requires heat energy to produce the ester.

Transesterification: Transesterification does not require heat. 

Conclusion

Esterification is the process in which an ester is formed. Transesterification is the process in which an ester is modified. The ester produced from the esterification can be altered from transesterification in order to obtain the desired ester. The main difference between esterification and transesterification is that esterification includes an ester as the end product whereas transesterification includes an ester as a reactant.

References:

1.”Esterification: Definition, Process & Reactions.” Study.com. Study.com, n.d. Web.  Available here. 04 Aug. 2017. 
2.”Transesterification.” Wikipedia. Wikimedia Foundation, 01 Aug. 2017. Web. Available here. 04 Aug. 2017.

Image Courtesy:

1.”Esterification (acetic acid and methanol)” By Doxepine – Own work, Public Domain) via Commons Wikimedia
2. “General transesterification mechanism” By Rifleman 82 – Own work, Public Domain) via Commons Wikimedia