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A condensation reaction is a chemical reaction in which two molecules or moieties, often functional groups, combine to form a larger molecule, together with the loss of a small molecule. Possible small molecules that are lost include water, acetic acid, hydrogen chloride, or methanol, but most commonly in biological reactions it is water. Condensations producing water as a byproduct are the opposite reaction of transformations involving hydrolysis, which split a reactant into two new species through addition of a water molecule.

Condensation can be intermolecular (between two different molecules) or intramolecular (involving different groups within the same molecule). A simple example of an intermolecular condensation is the joining of two amino acids in the peptide bond, as is characteristic of all proteins. Examples of intermolecular condensations often lead to ring formation, and include the synthesis of cyclic peptides via the same bond forming process as just described, as well as Dieckmann condensations, in which the two ester groups within a diester molecule react with release of an alcohol molecule to form a β-ketoester product.

The condensation reaction-speed can be catalyzed, by simply adding a concentrated acid to the reaction. It effects it by acidifying the environment whereas the reaction takes place the acid thereby binds with the water molecules and speed up the process.

Mechanisms

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Condensation reactions can follow a variety of different reaction mechanisms, depending on the groups reacting and the conditions employed to perform the reaction (solvent, temperature, reaction additives, etc.).

Applications

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Many artificial, man-made chemical reactions, and many biological transformations are condensation reactions. In the latter case (reactions in nature), phosphorylation and glycosylation reactions are generally all condensations, as are the key bond-forming reactions in all polypeptide and polynucleotide syntheses, and much of polyketide and terpene biosynthesis as well. Examples of the large number of condensation reactions are used in synthetic organic chemistry include:

• Acyloin condensation
• Aldol condensation
• Claisen condensation
• Claisen–Schmidt condensation
• Darzens reaction (glycidic ester condensation)
• Dieckmann condensation
• Guareschi–Thorpe condensation
• Knoevenagel condensation
• Pechmann condensation
• Rap–Stoermer condensation
• Self-condensation or symmetrical aldol condensation
• Thorpe–Ziegler reaction

The reactions that form acid anhydrides from their constituent acids are also typically condensation reactions.

Condensation polymerization

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Condensation polymerization produces 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. In condensation polymerization or "step-growth polymerization", multiple condensation reactions take place, joining monomers and monomer chains into long chains called polymers. It occurs for example in the synthesis of polyesters or nylons. It can be homopolymerization of a single monomer A-B with two different end groups that condense, or copolymerization of two co-monomers A-A and B-B.

Condensation polymerization releases multiple small molecules, in contrast to polyaddition reactions, which do not. 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. A high conversion rate is required to achieve high molecular weights, per Carothers' equation.

Bifunctional monomers lead to linear chains, and therefore thermoplastic polymers, but, when the monomer functionality exceeds two, the product is a branched chain that may be a thermosetting polymer.

See also

• Anabolism
• Hydrolysis, the opposite of a condensation reaction
• Condensed tannins

References

1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (1994) "Condensation Reaction". doi:10.1351/goldbook.C01238

• Condensation reactions