In a condensation reaction, two molecules or parts thereof combine, releasing a small molecule. When this small molecule is water, it is known as a dehydration reaction. Other possible small molecules lost include hydrogen chloride, methanol, or acetic acid.
When two separate molecules react, their condensation is termed intermolecular. A simple example is the condensation of two amino acids to form a peptide, as seen in Figure 1. This reaction example is the reverse of hydrolysis, which splits a chemical entity into two parts through the action of the polar water molecule, which itself splits into hydroxide and hydrogen ions.
When a condensation is performed between different parts of the same molecule, the reaction is termed intramolecular condensation. In many cases this leads to ring formation. An example is the Dieckmann condensation, in which the two ester groups of a single diester molecule react with each other to lose a small alcohol molecule and form a β-ketoester product.
Many condensation reactions follow a nucleophilic acyl substitution or an aldol condensation reaction mechanism. Other condensations, such as the acyloin condensation, are triggered by radical conditions.
Condensation Polymerization Reactions
In one type of polymerization reaction, a series of condensation steps take place whereby monomers or monomer chains add to each other to form longer chains. This is termed "condensation polymerization" or "step-growth polymerization," and occurs in processes such as the synthesis of polyesters or nylons Figure 3. This reaction may be either a homopolymerization of a single monomer A-B with two different end groups that condense, or it may be a copolymerization of two co-monomers A-A and B-B. Small molecules are usually liberated in these condensation steps, as opposed to polyaddition reactions with no liberation of small molecules.
In general, condensation polymers form more slowly than addition polymers, often requiring heat. Condensation polymers are generally lower in molecular weight. This type of reaction is used as a basis for the making of many important polymers such as nylon, polyester, and various epoxies. This reaction is also the basis for the laboratory formation of silicates and polyphosphates. Many biological transformations, such as polypeptide synthesis, polyketide synthesis, terpene syntheses, phosphorylation, and glycosylations are condensations.