Condensation Reaction

What is the general equation of a condensation reaction?

The general equation of condensation goes as follows:

$\mathrm{AH}+\mathrm{BOH}\to \mathrm{AB}+{\mathrm{H}}_2\mathrm{O}$

What is an example of a condensation reaction?

Let's use the condensation of galactose and glucose as an example.

Glucose and galactose are both simple sugars - monosaccharides. The result of their condensation reaction is lactose. Lactose is also a sugar, but it is a disaccharide, meaning that it consists of two monosaccharides: glucose and galactose. The two are linked together with a chemical bond called a glycosidic bond (a type of covalent bond).

The formula for lactose is ${\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}$, and galactose and glucose is ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$.

The formula is the same, but the difference is in their molecular structures. Pay attention to the placement of the -OH on the 4th carbon atom in Figure 1.

Fig. 1 - The difference in molecular structures of galactose and glucose is in the position of the -OH group on the 4th carbon atom

If we remember the general equation of condensation, it goes as follows:

$\mathrm{AH}+\mathrm{BOH}\to \mathrm{AB}+{\mathrm{H}}_2\mathrm{O}$

Now, let us swap A and B (groups of atoms) and AB (a compound) with galactose, glucose, and lactose formulas, respectively:

data-custom-editor="chemistry" ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+{\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\to {\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}+{\mathrm{H}}_2\mathrm{O}$

As a new covalent bond forms, one of the sugars loses a hydrogen atom (H), and the other loses a hydroxyl group (OH). From these, a molecule of water is formed (H + OH = H2O).

Since a water molecule is one of the products, the resulting lactose has 22 hydrogen atoms (H22) instead of 24 and 11 oxygen atoms (O11) instead of 12.

The diagram of condensation of galactose and glucose would look like this:

Fig. 2 - The condensation reaction of galactose and glucose

The same thing happens during other condensation reactions: monomers join to form polymers, and covalent bonds form.

Therefore, we can conclude that:

• A condensation reaction of monomers monosaccharides forms covalent glycosidic bonds between these monomers. In our example above, disaccharide forms, meaning two monosaccharides join together. If multiple monosaccharides join together, a polymer polysaccharide (or complex carbohydrate) forms.

• The condensation reaction of monomers that are amino acids results in polymers called polypeptides (or proteins). The covalent bond formed between amino acids is a peptide bond.

• The condensation reaction of monomers nucleotides forms a covalent bond called a phosphodiester bond between these monomers. The products are polymers called polynucleotides (or nucleic acids).

Although lipids are not polymers (fatty acids and glycerol are not their monomers), they form during condensation.

• Lipids form in a condensation reaction of fatty acids and glycerol. The covalent bond here is called an ester bond.

What is the purpose of a condensation reaction?

The purpose of a condensation reaction is the creation of polymers (large molecules or macromolecules), such as carbohydrates, proteins, lipids, and nucleic acids, all of which are essential in living organisms.

They are all equally important:

• Condensation of glucose molecules allows for creating complex carbohydrates, for instance, glycogen, which is used for energy storage. Another example is the formation of cellulose, a carbohydrate that is the main structural component of cell walls.

• The condensation of nucleotides forms nucleic acids: DNA and RNA. They are crucial for all living matter as they carry genetic material.

• Lipids are essential energy storage molecules, building blocks of cell membranes and providers of insulation and protection, and they form in the condensation reaction between fatty acids and glycerol.

Without condensation, none of these essential functions would be possible.