Condensation reaction

Overview
A condensation reaction is a chemical reaction in which two molecules or moieties combine to form one single molecule, together with the loss of a small molecule. When this small molecule is water, it is known as a dehydration reaction; other possible small molecules lost are hydrogen chloride, methanol, or acetic acid.

A condensation reaction may be considered as the opposite of a hydrolysis reaction (the cleavage of a chemical entity into two parts by the action of water).

Mechanism
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 or single electron transfer conditions.

The synthesis of polymers, where monomers react and become covalently bonded with one another through the loss of a water molecule.

Condensation reactions in polymer chemistry
In polymer chemistry, a series of condensation reactions take place whereby monomers or monomer chains add to each other to form longer chains. This may also be termed as 'condensation polymerization' or 'step-growth polymerization'. It occurs either as a homopolymerization of an A-B monomer or a polymerization of two co-monomers A-A and B-B. Small molecule condensates are usually liberated, unlike in polyaddition where there is no liberation of small molecules.

A high conversion rate is required to achieve high molecular weights as per Carothers' equation.

In general, condensation polymers form more slowly than addition polymers, often requiring heat. They are generally lower in molecular weight. Monomers are consumed early in the reaction; the terminal functional groups remain active throughout and short chains combine to form longer chains. Bifunctional monomers lead to linear chains (and therefore thermoplastic polymers), but when the monomer functionality exceeds two, the product is a thermoset polymer.

Applications
This type of reaction is used as a basis for the making of many important polymers for example: nylon, polyester and other condensation polymers and various epoxies. It is also the basis for the laboratory formation of silicates and polyphosphates. The reactions that form acid anhydrides from their constituent acids are typically condensation reactions. Other organic condensation reactions are Aldol condensations, self-condensation, the acyloin condensation and the benzoin condensation.

Many biological transformations are condensation reactions. Polypeptide synthesis, polyketide synthesis, terpene syntheses, phosphorylation, and glycosylations are a few examples.