The preparation of magnesium benzyl fluoromalonate and other equivalent materials, the synthetic equivalents of the --CH.sub.2 F moiety, is described. Reaction between these reagents and the in situ-formed imidazolides of various carboxylic acids gives beta-keto-alpha-fluoroesters, which upon hydrogenation and spontaneous decarboxylation yields fluoromethyl ketones in excellent yields. The overall transformation from RCOOH to RCOCH.sub.2 F is thus illustrated.