the historically established name in organic chemistry for a large group of reactions that form complex compounds from two or more (several) simple compounds. condensation reactions, differentiated according to the nature of the reagents and the nature of the chemical trans-formations, include many intramolecular and intermolecular processes of formation of new carbon-carbon (C—C) bonds. Most such reactions are accompanied by the separation of some simple inorganic or organic X—Y molecule (for example, water, hydrogen, alcohol, hydrogen halide, halogen):

Examples of this type of condensation reaction are crotonic condensation, the Wurtz reaction, the Claisen condensation, the Knoevenagel reaction, the Perkin reaction, and the Friedel-Crafts reaction.

Unlike the aforementioned reactions, condensation reactions such as benzoin condensation, aldol condensation, and diene synthesis take place without separation of any simple molecule. Furthermore, all reactions that produce heterocyclic compounds are referred to in organic chemistry as condensation reactions; new bonds may emerge during these processes, for example, carbon-carbon, carbon-heteroatom, and heteroatom-heteroatom. Esterification, ester interchange, alkylation, and alkylation over oxygen or nitrogen are generally not included among the condensation reactions. The reactions of the formation of polymers according to these schemes, however, are known as polycondensation reactions.