In a condensation reaction, two molecules join to form a larger molecule and a small, stable molecule such as H2O or HCl. For example, ethyl acetate, CH3COOCH2CH3, is formed when acetic acid reacts with ethanol in the presence of an acid catalyst:
In general, condensation of a carboxylic acid and an alcohol produces an ester.
Condensation reactions are reversible: not all reactant molecules are converted to product molecules and, if product molecules are mixed, some reactant molecules form. The reverse reaction of a condensation that produces water is called a hydrolysis reaction. In hydrolysis, water breaks apart into H and OH with H attaching to one part the larger molecule and OH to another: hydrolysis comes from hydro, “water” and lysis, “breaking apart”.
Ethers can be obtained from condensation reactions involving two alcohol molecules. If the two alcohol molecules are the same, a symmetric ether forms. For example, when ethanol is treated with a limited quantity of sulfuric acid and heated to 140 °C, diethyl ether and water are formed:
If the condensation reaction involves two different alcohols, an asymmetric ethers can form. For example:
Alternatively, addition reactions between alkenes and alcohols can form ethers. For example:
Amides can be produced from carboxylic acids and primary or secondary amines (or ammonia) via condensation reactions:
It is through this reaction that amino acids (molecules containing both an amine and a carboxylic acid functional group) link together in a polymer to form peptides and proteins.