Which solvent system is recommended for recrystallizing the crude product?

In recrystallization, selecting the right solvent system is crucial for effectively purifying a compound. The goal is to dissolve the crude product when the solvent is hot and allow it to crystallize as the solution cools down.

50% aqueous methanol is often recommended as it strikes a balance between solubility and the ability to form crystals upon cooling. Methanol is a polar solvent that can dissolve a wide range of organic compounds while providing a suitable environment for recrystallization to occur. The 50% aqueous mixture allows for some degree of water solubility, facilitating the separation of impurities that are more soluble in water compared to the desired product. During cooling, the reduction in solubility leads to the crystallization of the desired compound while keeping impurities dissolved in the solvent.

In contrast, other solvent choices may either not dissolve the product adequately when heated or may not promote crystal formation effectively upon cooling. For instance, pure methanol might lead to higher solubility and insufficient crystallization, while dimethyl sulfoxide is generally too polar and can result in a less efficient crystallization process. Aqueous ethanol may not have the ideal properties for recrystallization, such as the necessary balance between solubility and crystallization.