Chem 333L Experiment 1 Pre-Lab Objectives 1. Use recrystallization to purify a sample of phthalic acid. 2. Learn to use mass and melting point of recrystallized product to determine purity 3. Calculated the percent recovery Intro/Theory Most organic solids are naturally impure. For precise work in the laboratory, it is necessary to refine the solids quickly and efficiently. The most important method for doing this is recrystallization. 1. Solid Properties A. Solids are held together by a variety of forces that range greatly in strength and determine many of the properties of the solid. Solids are a regular pattern of molecules due to these forces. When impurities exist, some of the Intermolecular forces are disrupted and the energy required to melt the solid is lowered. 1. Ionic Attraction is the strongest of these forces. It is the attraction between an anion and cation when an electron is taken by the anion. This force is occasionally found in organic compounds although usually found in inorganic. 2. Hydrogen bonding occurs only between molecules with an N,O, or F atom covalently bonded to an atom of H. The force acts as an intermolecular force, binding molecules of a substance together rather than just its atoms. 3. Dipole-Dipole Interaction occurs between polar molecules. The negative end of one molecule attracts the positive end of the other and creates moderately strong intermolecular bond between the two molecules. 4. London Dispersion Forces (van der Waals) are the weakest of the intermolecular forces. These occur between every molecule as electron imbalances causes even nonpolar molecules to become polar for an instant, or a instantaneous dipole. B. The energy required to break the regular pattern of a solid and convert the substance to a liquid is the melting point. The melting point for a solid a unique physical property that can be used for identification. Impurities can also be detected by any varying of the melting point. When recrystallization is used to purify a substance, the melting point will rise to a more accurate reading for the pure solid. 2. Solvent Properties A. Solubility of a substance is dependent on its temperature. It increases as the temperature does and vice versa. Recrystallization can make use of this principle by saturating solvent at one temperature before cooling the solvent, causing its solubility to decrease in turn. As a result, the excess solute will precipitate out as crystals. B. An ideal solvent solvent is specific to the desired solute for the experiment. There are many factors, some of which depending greatly on what substance is being dissolved. 1. The intermolecular forces and polarity of the solute and solvent should be as similar as possible. 2. A high dependence on temperature is desirable (high solubility only when hot). 3. The solvent must either dissolve the impurities completely or not at all. This is imperative to segregate the solute from the impurities during the recrystallization process. 4. Solvent ideally should not be expensive, flammable, or toxic. It is important, however, that it doesnot react with the solute and will evaporate off of the crystals easily. 3. Impurities A. Soluble impurities are left behind when the solute precipitates out of the solution. B. Insoluble impurities can be separated from the solute by hot gravity filtration. 4. Recrystallization Method A. Decide on an ideal solvent for the experiment B. Dissolve the solute in the minimal amount of solvent necessary (try for full saturation) C. Remove impurities (see section 3 on methods depending on solubility of the impurity) D. Crystallize the solute by allowing the solvent to cool, then cooling further with an ice bath. E. Gather the crystals of the pure substance, wash the residual solvent off, and dry. 5. Melting Point to Determine Purity A. Melting Point Depression is the amount that the melting point is lowered by impurities B. Possible impurities can be caused by incomplete reaction, inorganic impurities, byproducts, and water. Mechanism Procedure 1. Fill a test tube with 100mg of phthalic acid and cover solid with water (1-1.5ml). 2. Add a boiling stone to the test tube and heat solution on a heating mantle with a Variac setting of 50. 3. When water reaches boiling, slowly add drops of water until the solid dissolves. By doing this, no more than the minimal amount of water necessary to dissolve the solute is added. 4. If the impurities are insoluble, use hot gravity filtration to remove. 5. Allow 5-10 minutes to allow solution to cool. 6. Cool further by adding test tube to an ice bath. This will facilitate the process of recrystallization. 7. Use suction filtration to obtain the pure crystals. 8. Wash the product with COLD solvent. Must be cold to prevent any the crystals dissolving during washing. 9. Use Buchner funnel to allow wash to drain off. 10. Move pure sample to filter paper, spread the crystals out and cover with a beaker. Allow at least a day to dry. 11. Record mass of the dried sample and calculate its percent recovery. 12. Determine melting point and calculate percent difference to gauge the purity of the substance. ApparatusReactants Name, Structure, MW Melting Temp C Boiling Temp C Density g/mL Properties Safety Phthalic Acid C8H6O4 230 dec 1.59 White crystals Irritant, Forms an explosive dust if heated Disposal • Dispose of acid and filter paper in the solid organic waste container. • Wash filtrate down drain with excess water. • Dispose of gloves and paper towels in designated trash containers. Dispose of all broken glass in the “broken glass container.” • Always consult TA with question regarding safety and disposal. References Chemistry 333L&334L: Microscale Experiments in Organic Chemistry Organic Chemistry 10th