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Example 3 Synthesis of 2,5-difluorobenzaldehyde Procedure: to a stirred solution of 194.5 g (1.70 mol) of 1,4-difluorobenzene dissolved in 2 ml of anhydrous tetrahydrofuran was added dropwise 1.70 mol (2.2 M hexane solution) of n-butyllithium at -60°C, at a rate maintaining the temperature below -55°C. The reaction mixture was stirred below -50°C for 45 minutes, followed by continued stirring in the range of -50°C to -45°C for 1.5 hours. The solution was cooled to -30°C and 230 g of N-methylformanilide dissolved in 300 ml of tetrahydrofuran was added dropwise over 30 minutes. The mixture was stirred at -50°C for 1 hour, followed by a slow warming to -30°C over 15 minutes. The reaction mixture was poured into ice water and neutralized to pH 6-7 with 10% sulfuric acid solution, followed by three extractions with hexane. The combined organic phases were sequentially washed once each with 1N sulfuric acid solution and saturated sodium chloride solution and concentrated to give an oil. Purification by reduced pressure distillation (64°-65°C, 20 mm Hg) gave 187.5 g (77.5%) of 2,5-difluorobenzaldehyde.