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 Corey's 1981 book The Logic of Chemical Synthesis, as it was not included in the index. Because synthons are charged, when placed into a synthesis a neutral form is found commercially instead of forming and using the potentially very unstable charged synthons. Example In planning the synthesis of phenylacetic acid, two synthons are identified: a nucleophilic "COOH−" group, and an electrophilic "PhCH2+" group. Of course, both synthons do not exist per se; synthetic equivalents corresponding to the synthons are reacted to produce the desired reactant. In this case, the cyanide anion is the synthetic equivalent for the COOH− synthon, while benzyl bromide is the synthetic equivalent for the benzyl synthon. The synthesis of phenylacetic acid determined by retrosynthetic analysis is thus: PhCH2Br + NaCN → PhCH2CN + NaBr PhCH2CN + 2 H2O → PhCH2COOH + NH3 C2 synthons - acetylene, acetaldehyde -C2H4OH synthon - ethylene oxide carbocation synthons - alkyl halides carbanion synthons - Grignard reagents, organolithiums, substituted acetylides Alternative use in synthetic oligonucleotides This term is also used in the field of gene synthesis—for example "40-base synthetic oligonucleotides are built into 500- to 800-bp synthons". Carbanionic synthons In 1967, E. J. Corey introduced the concept of a synthon in retrosynthetic analysis. Planning the steps of a complex molecule synthesis requires recognizing key synthons and identifying how they can be assembled into a desired product. In many retrosynthetic bond disconnections, the bond is broken heterolytically instead of homolytically, generating a carbocationtic and a carbanionic synthon. Most polar-mechanism reactions, whether they are nucleophilic displacements, 1,2-carbonyl additions, Michael reactions, or other processes, involve the fusion of a nucleophilic fragment and an electrophilic fragment. There are a wide variety of carbanionic synthons available to the organic chemist for the construction of complex molecules. These include enolates, organometallics, acetylides, malonates, and carbanions generated in situ after addition to olefins. An example of a carbanionic synthon for an ester enolate is shown in fig. 1. A carbon-carbon bond alpha to