Abstract

The crystallization of 5-nitrouracil (5NU) from
pure aqueous solution yields two anhydrous polymorphs and a
monohydrate depending on the temperature at which the
process is carried out. 5NU mimics true polymorphism in that,
when retained in aqueous solution, both metastable (anhydrous)
forms undergo solvent mediated phase transformations
(SMPTs) into the more thermodynamically stable hydrated
form as would be predicted by Ostwald’s rule of stages. The
phase transformations can occur either in the classical manner
of dissolution and recrystallization or, in one case, may in some
circumstances be nucleated and morphologically templated by
the original crystalline form. There is no appearance from
aqueous solution of a known third acentric anhydrous form,
previously prepared from acetonitrile solutions. Preliminary experiments suggest this nonappearance may result from the
relatively high solubility of this form in aqueous solution.