A synthon is a concept in retrosynthetic analysis. It is defined as a structural unit within a molecule which is related to a possible synthetic operation. The term was coined by E.J. Corey. It is noted  that the phrase does not feature very prominently in Corey's book, The Logic of Chemical Synthesis, as it is not included in the index.
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 product. 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
- C1 synthons - carbon dioxide, carbon monoxide, cyanide
- C2 synthons - acetylene, acetaldehyde
- -C2H4OH synthon - ethylene oxide
- carbocation synthons - alkyl halides
- carbanion synthons - Grignard reagents, organolithiums, substituted acetylides
- Organic Synthesis, the science behind the art. W.A. Smit, A.F. Buchkov, R.Cople, Royal Society of Chemistry ISBN 0-85404-544-9