To the reactor was added 3,5-dichlorobenzoic acid (190 g, 1.0 mmol), which was dissolved with anhydrous THF (1 L) and cooled to -78 °C. Under the condition of maintaining the reaction temperature, n-butyllithium (2.5 M, 0.5 L) was slowly added dropwise. Subsequently, N,N-dimethylformamide (95 g, 1.3 mmol) was added slowly and the titration process lasted for 2 hours. The reaction mixture was slowly warmed up to room temperature until the feedstock completely disappeared. After completion of the reaction, the reaction was quenched with a half-saturated ammonium chloride solution and the organic phase was extracted with ethyl acetate. The organic phase was concentrated to give a white solid. Finally, purification by pulping using mixed petroleum ether/ethyl acetate solvent gave 3,5-dichloro-4-formylbenzoic acid (179 g, 82% yield).
