Sulfapyridine (polymorph III), sulfapyridine dioxane solvate, sulfapyridine tetrahydrofuran solvate and sulfapyridine piperidine solvate, all at 173 K.

The X-ray crystal structures of solvates of sulfapyridine have been determined to be conformational polymorphs. 4-Amino-N-(1,2-dihydropyridin-2-ylidene)benzenesulfonamide (polymorph III), C(11)H(11)N(3)O(2)S, (1), 4-amino-N-(1,2-dihydropyridin-2-ylidene)benzenesulfonamide 1,3-dioxane monosolvate, C(11)H(11)N(3)O(2)S·C(4)H(8)O(2), (2), and 4-amino-N-(1,2-dihydropyridin-2-ylidene)benzenesulfonamide tetrahydrofuran monosolvate, C(11)H(11)N(3)O(2)S·C(4)H(8)O, (3), crystallized as the imide form, while piperidin-1-ium 4-amino-N-(pyridin-2-yl)benzenesulfonamidate, C(5)H(12)N(+)·C(11)H(10)N(3)O(2)S(-), (4), crystallized as the piperidinium salt. The tetrahydrofuran and dioxane solvent molecules in their respective structures were disordered and were refined using a disorder model. Three-dimensional hydrogen-bonding networks exist in all structures between at least one sulfone O atom and the aniline N atom.