What is "condensation reaction"

n. (context chemistry English) any reaction in which two molecules react with the resulting loss of a molecule of water (or other small molecule); the formal reverse of hydrolysis

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, hydrogen chloride, methanol, or acetic acid, but most commonly in a biological reaction it is water.

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 required to form chemical bonds via condensation.

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.