General procedure for the synthesis of 3,4-dichloropyridazine from 4-chloro-3-pyridazinone: 4-chloro-3-pyridazinone (25 g, 0.19 mol) was dissolved in acetonitrile (200 ml). Subsequently, phosphoryl chloride (73 g, 0.48 mol) was added to the solution and the reaction mixture was refluxed at 80 °C for 3 hours. After the completion of the reaction was monitored by TLC, the reaction solution was cooled to room temperature and slowly poured into ice water (200 ml). The pH of the mixture was adjusted to 7-8 with sodium bicarbonate and then extracted twice with ethyl acetate (300 ml). The organic phases were combined, washed with saturated brine (400 ml) and dried over anhydrous sodium sulfate. The organic phases were concentrated under reduced pressure and the resulting crude product was purified by column chromatography to give 3,4-dichloropyridazine (18.0 g, 0.12 mol) in about 64% yield.
