A condensation reaction is a class of organic addition reaction that typically proceeds in a step-wise fashion to produce the addition product, usually in equilibrium, and a water molecule (hence named condensation). The reaction may otherwise involve the functional groups of the molecule, and formation of a small molecule such as ammonia, ethanol, or acetic acid instead of water. It is a versatile class of reactions that can occur in acidic or basic conditions or in the presence of a catalyst. This class of reactions is a vital part of life as it is essential to the formation of peptide bonds between amino acids and the biosynthesis of fatty acids.
Many variations of condensation reactions exist, common examples include the aldol condensation, Claisen condensation, Knoevenagel condensation, and the Dieckman condensation (intramolecular Claisen condensation).