Crystallization in Final Stages of Purification

Methods are described for the laboratory-scale crystallization of “small” organic compounds. The process of crystallization from solution can be used as a purification step in its own right, or to produce crystals for molecular structure determination by single-crystal or powder X-ray diffraction. Both aspects are discussed, with particular emphasis on growing crystals for structure determination in natural product chemistry. The processes detailed for the slow growth of diffraction-quality crystals include solvent selection and solution supersaturation by evaporation, cooling, liquid/vapor diffusion, and thermal gradient methods. Common problems and solutions, including solid-state polymorphism and solvate formation, are highlighted and modern approaches to parallel crystallization and crystal structure determination from X-ray powder diffraction data are also introduced.