In retrosynthetic analysis, a synthon is a hypothetical unit within a target molecule that represents a potential starting reagent in the retroactive synthesis of that target molecule. The term was coined in 1967 by E. J. Corey. He noted in 1988 that the "word synthon has now come to be used to mean synthetic building block rather than retrosynthetic fragmentation structures".
It was noted in 1998 that the phrase did not feature very prominently in Coreys 1981 book The Logic of Chemical Synthesis', as it was not included in the index. Because synthons are charged, when placed into a synthesis an uncharged form is found commercially instead of forming and using the potentially very unstable charged synthons.