Introduction of recrystallization of naphthalene

EXP 3-1 RECRYSTALLIZATION (MICROSCALE) UNLV CHEM 241L SUMMER 2020 RECRYSTALLIZATION: Introduction • Read INTRODUCTION and THEORY OF CRYSTALLIZATION sections in the provided lab manual. • Understand the basic concept of recrystallization • Review the polarity concept from the previous lab (solubility and solution) if necessary RECRYSTALLIZATION : Introduction • Recrystallization → a purification technique for solid compounds → the solubility of a compound in a specific solvent is the most important property • Good recrystallization solvent? → sparingly soluble at room temperature → highly soluble at elevated temperature (boiling temperature) RECRYSTALLIZATION : Experimental • Purification of impure naphthalene by two recrystallization solvents → ethanol → hexane • Determine which solvent is a good recrystallization solvent for naphthalene → based on experimental melting point provided in the video → based on the % recovery of naphthalene from each recrystallization Experiment # 3-1: Recrystallization (Micro-scale) INTRODUCTION In most organic chemistry experiments, the desired product is first isolated in an impure form. If this product is a solid, the most common method of purification is crystallization. The general technique involves dissolving the material to be crystallized in a hot solvent (or solvent mixture) and cooling the solution slowly. The dissolved material has a decreased solubility at lower temperatures and will separate from the solution as it is cooled. This phenomenon is called crystallization, if the crystal growth is relatively slow and selective, or precipitation, if the process is rapid and nonselective. Crystallization is an equilibrium process and produces very pure material. A small seed crystal is formed initially, and it then grows layer by layer in a reversible manner. In a sense, the crystal “selects” the correct molecules from the solution. In precipitation, the crystal lattice is formed so rapidly that impurities are trapped within the lattice. Therefore, any attempt at purification with too rapid a process should be avoided. Because the impurities are usually present in much smaller amounts than the compound being crystallized, most of the impurities will remain in the solvent even when it is cooled. The purified substance can then be separated from the solvent and from the impurities by filtration. THOERY OF CRYSTALLIZATION The first problem in performing a crystallization is selecting a solvent in which the material to be crystallized shows the desired solubility behavior. In an ideal case, the material should be sparingly soluble at room temperature and yet quite soluble at the boiling point of the solvent selected. The solubility curve should be steep, as can be seen in line A of Figure 1. A Figure 1. Graph of solubility vs. temperature. 1 curve with a low slope (line B) would not cause significant crystallization when the temperature of the solution was lowered. A solvent in which the material is very soluble at all temperatures (line C) also not be suitable crystallization solvent. The basic problem in performing a crystallization is to select a solvent (or mixed solvent) that provides a steep solubility-vs.-temperature curve for the material to be crystallized. A solvent that allows the behavior shown in line A is an ideal crystallization solvent. A successful crystallization depends on a large difference between the solubility of a material in a hot solvent and its solubility in the same solvent when it is cold. When the impurities in a substance are equally soluble in both the hot and the cold solvent, en effective purification is not easily achieved through crystallization. A material can be purified by crystallization when both the desired substance and the impurity have similar solubilities, but only when the impurity represents a small fraction of the total solid. The desired substance will crystallize on cooling, but the impurities will not. In this experiment, you will perform a micro-scale crystallization. Impure naphthalene will be recrystallized from hexane and ethanol separately, and you will determine a good recrystallization solvent for naphthalene based on melting point measurement and % recovery.