[Cl:1][C:2]1[C:7]([CH3:8])=[C:6]([F:9])[CH:5]=[CH:4][CH:3]=1.C([Li])CCC.[C:15](=[O:17])=[O:16].[Cl-].[NH4+]>C1COCC1.CCCCCC>[Cl:1][C:2]1[CH:3]=[CH:4][C:5]([C:15]([OH:17])=[O:16])=[C:6]([F:9])[C:7]=1[CH3:8]
29.0 g (0.2 mol) of 6-chloro-2-fluorotoluene are dissolved with stirring in 200 ml of dry THF. After cooling to -70° C., 151 ml (0.24 mol) of n-butyllithium in solution in hexane are added dropwise. After two hours, the reaction mixture, maintained at -70° C., is poured on solid carbon dioxide. After returning to room temperature, an aqueous ammonium chloride solution is added. The aqueous phase is extracted with ether, acidified with 6N hydrochloric acid and extracted with ether. The organic phases are dried over magnesium sulphate and concentrated to dryness. The residue is washed with heptane to give 9.0 g (0.048 mol) of 4-chloro-2-fluoro-3-methylbenzoic acid in the form of a white powder (yield: 24%; melting point: 198°)
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