Polymorph-specific chlorination of maleic-hydrazide

The reaction of chlorination of maleic-hydrazide (1) with POCl3 is specific to the substrate polymorph. Chlorination of either the triclinic polymorph of maleic-hydrazide (1), denoted MH1, or the monoclinic polymorph MH2 leads to a single product of 3,6-dichloropyridazine (2), while chlorination of the monoclinic polymorph MH3 under the same conditions results in two products: 3,6-dichloropyridazine (2) and 6-chloro-3-pyridazinone (3). The structural differences between polymorphs and their topochemical chlorination in phosphorus oxychloride are discussed. The crystal structure of 6-chloro-3-pyridazinone (3) monohydrate, determined in this study by X-ray diffraction, is built of ribbons of hydrogen-bonded 3 and water molecules. It has been established that the chlorination of ground crystals of polymorph MH3 yields exclusively the dichloropyridazine, which suggests that the crystal habit of MH3, the size of crystallites and the lower solubility of MH3 than MH1 and MH2 are the main reasons for the different course of the reaction. [Copyright &y& Elsevier]