In planning the synthesis of phenylacetic acid, two synthons are identified: a nucleophilic "COOH" group, and an electrophilic "PhCH" group. Of course, both synthons do not exist by themselves; 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:
where Ph stands for phenyl.
* C2 synthons - acetylene, acetaldehyde
* -CHOH synthon - ethylene oxide
* carbocation synthons - alkyl halides
* carbanion synthons - Grignard reagents, organolithiums, substituted acetylides