A condensation reaction, also commonly referred to as dehydration synthesis, is a chemical
reaction in which two molecules or moieties combine to form a larger molecule, together
with the loss of a small molecule.
Possible small molecules lost are water, hydrogen chloride, methanol, or acetic acid but most
commonly in a biological reaction it is water.
The word "condensation" suggests a process in which two or more things are brought "together"
to form something "dense", like in condensation from gaseous to liquid state of matter; this
does not imply, however, that condensation reaction products have greater density than
reactants.
When two separate molecules react, the condensation is termed intermolecular.
A simple example is the condensation of two amino acids to form the peptide bond characteristic
of proteins.
This reaction example is the opposite 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.
Hence energy is released.
If the union is between atoms or groups of the same molecule, the reaction is termed
intramolecular condensation, and in many cases 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.
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.
Condensation reactions in polymer chemistry 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
for example in the synthesis of polyesters or nylons.
It may be either a homopolymerization of a single monomer A-B with two different end
groups that condense or a copolymerization of two co-monomers A-A and B-B.
Small molecules are usually liberated in these condensation steps, in contrast to polyaddition
reactions with no liberation of small molecules.
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 as per Carothers' equation.
Bifunctional monomers lead to linear chains, but, when the monomer functionality exceeds
two, the product is a branched chain that may yield 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.
Many biological transformations are condensation reactions.
Polypeptide synthesis, polyketide synthesis, terpene syntheses, phosphorylation, and glycosylations
are a few examples of this reaction.
A large number of such reactions are used in synthetic organic chemistry.
Other examples include: Acyloin condensation
Aldol condensation Benzoin condensation
Claisen condensation Claisen–Schmidt condensation
Darzens condensation Dieckmann condensation
Guareschi–Thorpe condensation Knoevenagel condensation
Michael condensation Pechmann condensation
Rap–Stoermer condensation Self-condensation or symmetrical aldol condensation
Ziegler condensation See named reactions
See also Anabolism
Hydrolysis Condensed tannins
References