Organic Chemistry: Condensation Reactions

Condensation reactions are common reactions in the organic chemistry laboratory. Learn what a condensation reaction is, some named condensation reactions, and see examples of the various condensation reactions. Vocabulary terms listed at the end for reference.

What is a condensation reaction?

A condensation reaction combines two molecules into a single molecule and a small molecule (such as water).

Below is an example of a condensation reaction where the small molecule released is water, and the new molecule formed is AB.

AOH + BH –> AB + H2O

Example of  condensation reactions
Example of a Condensation Reaction

Some common small molecules formed in the reaction include water, acetic acid, ammonia, hydrogen sulfide, etc. If water is the small molecule formed, the reaction is also a dehydration reaction.

There are many sub-types of condensation reactions. A self-condensation is the combining of two of the same molecules. Esterfication, Aldol condensations, and Claisen condensations are other condensation reactions. These topics discussed further below.

Condensation Reactions with Amino Acids

Amino acids are the building blocks of proteins. Amino acids can undergo a condensation reaction with each other to form a peptide bond. The new molecule is called a dipeptide. When multiple amino acids form a chain of many amino acids it results in a peptide. A long peptide chain forms a protein.  

The reaction occurs between the amine on one amino acid and the carboxylic acid on the other amino acid. The reaction releases water.

Here is an example of two amino acids reacting in a condensation reaction. A new bond is formed between the nitrogen of one amino acid and the carbon in the carboxylic acid of the other.

condensation Reaction of two amino acids to form a dipeptide
Reaction between two amino acids to form a dipeptide

Above is a reaction between two amino acids. The R1 and R2 will depend on the amino acids. The red bond formed in the condensation reaction forms between the nitrogen in one peptide and the carbon from the other peptide.

Named Condensation Reactions

The esterification reaction, Claisen condensation, and Aldol condensation are shown below. There are other condensation reactions, and these are just a few of the many possible examples.

Esterfication Reaction

An esterification reaction is a condensation reaction that forms an ester and releases a small molecule. A covalent bond forms from a carboxylic acid and an alcohol.

An example of this type of reaction is below:

Esterfication Reaction.
An esterfication reaction is a type of condensation reaction.

Claisen Condensation

The Claisen condensation is a reaction between two esters. Each ester must contain an α-hydrogen. And generally, a base catalyzes the reaction. The esters can be the same or different. When the esters are different, a mix of four different products will form.

An example of the Claisen Condensation reaction is shown below. In this example, two of the same esters react together to form the product.

Claisen Condensation Reaction
Claisen Condensation Reaction

Aldol Condensation

An aldol condensation is another reaction that forms a carbon to carbon bond. An aldehyde or ketone reacts with the carbonyl of another molecule to form. First, an enolate forms, followed by attack by a nucleophile. Then finally, the result is protonated.

An example Aldol condensation below.

Aldol condensation Reactions
Aldol Condensation Reaction

Condensation Reaction Examples

There are many examples of condensation reactions in nature and biology. The reaction between peptides is one such example. Below are some examples of other condensation reactions.

The first example is between an alcohol and an aldehyde.

Example of a Condensation Reaction

Here is another example reaction where nitrogen is involved in the reaction, and the resulting small molecule is water.

Example condensation reaction


Condensation Reaction– Combining two molecules into a single molecule and releasing a small molecule such as water.

Dehydration Reaction– A reaction in which one of the products is water. Often, a condensation reaction will be a dehydration reaction because it produces the small molecule water.

Esterification Reaction– A condensation reaction that forms an ester from a carboxylic acid and an alcohol. This is a subcategory of a condensation reaction.

Hydrolysis-The opposite of a condensation reaction. One molecule breaks into two new molecules due to a reaction with water.

Saponification-A hydration reaction breaks an ester bond. The result is a carboxylic acid and an alcohol.

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