4-Bromoacetophenone (20.0 g, 0.10 mol) was mixed with N,N-dimethylformamide dimethyl acetal (28.5 mL, 0.20 mol) in N,N-dimethylformamide (12 mL) and heated to 110 °C for 4 hours. During the reaction, the resulting methanol and water (6.2 mL total) were removed by distillation. After completion of the reaction, the mixture was cooled to 25 °C. Subsequently, methyl tert-butyl ether (100 mL) and methylhydrazine (21.2 mL, 0.40 mol) were added to the mixture and stirred overnight at room temperature. The reaction mixture was washed sequentially with 1M aqueous ammonium chloride (3 x 40 mL) and water (40 mL). The organic phase was dried by azeotropic distillation through a Dean-Stark apparatus or, as an alternative, through anhydrous magnesium sulfate column. The dried solution was filtered through a silica gel column (60 g) and the product was eluted from the column with methyl tert-butyl ether. The eluates containing the product were combined and concentrated to about 70 mL by distillation. Heptane (120 mL) was added to the concentrate and distillation was continued until the tank temperature reached 98.4°C. Approximately 100 mL of the distillate was collected. The remaining mixture was cooled to 40°C, inoculated with crystalline seed and maintained at this temperature for 30 minutes to induce crystallization. Subsequently, the mixture was slowly cooled to 0°C over 90 minutes and held at 0°C for 30 minutes. The solid was collected by filtration, washed (3 times) with pre-cooled (0 °C) heptane and dried over a filter to give a cream-colored crystalline solid (16.3 g, 68% yield).The nuclear magnetic resonance (NMR) data of 5-(4-bromophenyl)-1-methyl-1H-pyrazole were in agreement with Alternative 1 .
