Retrosynthesis of phenylacetic acid

Disconnecting the bond between the CH₂ and the carboxyl group gives a benzyl anion and a ⁺COOH synthon. The equivalent of the latter is CO₂, which adds to Grignard reagents; that of the benzyl anion is benzylmagnesium chloride, prepared from benzyl chloride.

A second route reverses the polarity of the disconnection: a benzyl cation, whose equivalent is benzyl chloride, and a ⁻COOH synthon, whose equivalent is the cyanide ion. Cyanide replaces the chlorine of benzyl chloride and hydrolysis of phenylacetonitrile gives the acid. Both routes have two steps; the nitrile route avoids organometallics, and the Grignard route avoids the highly toxic cyanide.