Abstract: To synthesize 4-chloro-5-methoxy-2-(β-trifluoroethyl)-3 (2 H)-pyridazinone (4), the reactions of 4-chloro-5-methoxy-3 (2 H)-pyridazinone (5) with RCH 2 CF 3 (R= I, TsO, MsO, TfO) in different solvents were studied. It was found that methyl group migration took place during this reaction. An oxonium salt 9 was suggested as the active intermediate for the formation of the byproduct 4-chloro-5-methoxy-2-methyl-3 (2 H)-pyridazinone (7) and 4- ...