Concomitant colour polymorphs of (Z)-N-(4-fluorophenyl)-2-oxopropanehydrazonoyl chloride.

The title compound, (Z)-N-(fluorophenyl)-2-oxopropanehydrazonoyl chloride, C₉H₈ClFN₂O, was found to form concomitant colour polymorphs upon recrystallization from acetone. Block-shaped pale-orange crystals of form I (space group P2₁/n, No. 14) and prism-shaped yellow crystals of form II (space group P2₁/c, No. 14) both belong to the monoclinic crystal system. N-H...O hydrogen bonds resulting in zigzag chains [graph-set descriptor C(6)] are the dominating intermolecular interactions in both crystal forms. The hydrogen-bonded zigzag chains so formed extend by 2₁ screw symmetry in form I and by c-glide symmetry in form II, and are arranged in layers, which are more corrugated in form II than in form I. The polymorphs are virtually indistinguishable by their calculated densities, packing indices and melting points. Form I sublimes to yield crystals of form II, whereas form II sublimes to afford crystals of the same polymorph with characteristic crystal morphology.