Shown below is a retrosynthetic analysis of phenylacetic acid:
In planning the synthesis, two synthons are identified. A nucleophilic "-COOH" group, and an electrophilic "PhCH" group. Both synthons do not exist as written; 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:
: PhCHBr + NaCN → PhCHCN + NaBr
: PhCHCN + 2 HO → PhCHCOOH + NH
In fact, phenylacetic acid has been synthesized from benzyl cyanide, itself prepared by the analogous reaction of benzyl bromide with sodium cyanide.