Two Concomitant Polymorphs of 1,2-Naphthoquinone-2-semicarbazone

1,2-Naphthoquinone-2-semicarbazone (i.e., naftazone) has been characterized by structural analysis (single-crystal X-ray diffraction, powder X-ray diffraction) and by thermal analysis (differential scanning calorimetry, DSC). Although this molecule has no chiral center, the two forms crystallize in chiral space groups: the stable form I (P212121, Z = 4, a = 5.1187(7) Å, b = 5.6092(7) Å, c = 34.847(5) Å; V = 1000.5(2) Å3) and the metastable form II (P21, Z = 2, a = 6.3349(1) Å, b = 3.9263(1) Å, c = 19.790(5) Å, β = 92.424(4)°; V = 491.8(2) Å3). At high concentration (c = 30 g·L−1), the pure stable form I crystallizes regardless of the antisolvent (water) addition rate. At low concentration (c = 15 g·L−1), the pure metastable form II crystallizes at low water addition rate (ψ = 100 mL·h−1); however, the structurally pure form I is also obtained with a high water addition rate (ψ = 72 × 103 mL·h−1). Concomitant polymorphs crystallize at intermediate antisolvent addition rate and concentration. Therefore in the majority of the operative conditions, these results appear to be in contradiction with Ostwald’s rule of stages.