Solid-state dehydrochlorination and decarboxylation reactions. Part 2. Reactions of three crystal habits of p-aminosalicylic acid hydrochloride and the crystal structure of p-aminosalicylic acid hydrochloride

Abstract

Three crystal habits of p-aminosalicylic acid hydrochloride were obtained and the crystal structure determined. Crystals are monoclinic, space group P2₁/c, with a= 5.316(2), b= 8.464(3), and c= 17.888(9)Å, β= 93.30(3)°, and Z= 4. The structure was solved by direct methods and refined to R 0.099 for 923 observed reflections. The crystal packing of p-aminosalicylic acid is dominated by H ⋯ Cl^– hydrogen bonds and there are no carboxydimers. The carboxy-proton is instead hydrogen-bonded to a Cl^– ion.

Upon heating at 95 °C all three crystal habits lost HCl from one crystal face and then decarboxylated. The anisotropic behaviour of two of the three crystal habits is consistent with crystal packing while the third is not.

Crystals of all the three habits also lost HCl when placed in contact with Na₂CO₃ at room temperature and 60°C. This reaction always started at the point of contact of the base and proceeded through the crystal anisotropicall, resulting in dehydrochlorination and formation of the p-aminosalicylate anion. The anisotropic behaviour of two of the crystal habits is consistent with crystal packing. Different reactivity of these crystal habits with gaseous amines was also observed.