Growth and characterization of 6-chloro-2,4-dinitroaniline crystals in anti-solvent precipitation and reprecipitation methods

Abstract

The presence of superstructure and impact of solvent on the formation of different forms in the simple trimorphic system of 6-chloro-2,4-dinitroaniline (CDA) are investigated in this report. The experiment was carried out as part of sorting three forms of CDA. Forms I and III were obtained by an anti-solvent precipitation method. The filtrate, i.e. the mother liquor obtained from anti-solvent precipitation, was further used in a slow evaporative crystallization experiment. This evaporation method yielded superstructures on form I crystals in acetone (primary solvent) and water (secondary solvent). The obtained results showed that superstructures on form I resulted from combining anti-solvent crystallization and slow evaporative crystallization. Reprecipitation was carried out to verify the observed superstructure on form I using different volume fractions of water. Furthermore, the superstructure was observed in slow evaporation of CDA solution in an acetone–water mixture. The solubility data were measured for CDA in pure acetone, ethyl acetate (EtOAc), dichloromethane (CH₂Cl₂) and methyl tert-butyl ether (MTBE), and binary acetone–water, CH₂Cl₂–MTBE, EtOAc–MTBE solvent mixtures at 293 K and 298 K. X-ray powder diffraction study confirmed the formation of different forms and recorded SEM images laid a foundation for understanding the origin of the superstructure. The functional group vibrations and packing difference between form I and form III were clearly identified from the FTIR spectra. The UV-Vis-NIR results confirmed the interaction between water and solute in solution. Based on the solvent parameters and SEM images, the formation of the superstructure was hypothesized.