Crystallization Lab Report, Lab Reports of Organic Chemistry

Download Crystallization Lab Report and more Lab Reports Organic Chemistry in PDF only on Docsity! Organic Chemistry: Techniques and Transformations 1 LABORATORY 3 Crystallization Concept goals: Relationship between structure, polarity and solubility, role of temperature on solubility, characteristics of a good recrystallization solvent. Operational goal: Develop the skill to perform a recrystallization effectively, including the use of minimal solvent, use of charcoal if needed, filtering a hot solution, washing and drying crystals. Introduction Organic compounds synthesized in the laboratory or isolated from natural sources are often contaminated with impurities. Recrystallization is a method for removing impurities from organic compounds that are solid at room temperature. This method relies on the observation that the solubility of a compound in a solvent generally increases with temperature. Conversely, the solubility of a compound will decrease as a solution cools until the solution becomes saturated, and crystals form. Recrystallization can produce very pure compounds. When a warm solution of a compound cools and reaches the saturation point, small seed crystals of the compound form in the solution. Slowly, additional molecules of the compound attach to the seed crystal and the crystals grow. Since molecules in the crystals have a greater affinity for other molecules of the same type than they do for any impurities, the process of crystal formation gives rise to relatively pure crystals. The impurities originally present in the compound are left in solution. In this laboratory you will have the opportunity to carry out the recrystallization of two organic compounds: benzoic acid and naphthalene. The first experiment will give you practice in the technique as you recrystallize benzoic acid from water. In the second experiment, your goal will be to purify an impure sample of naphthalene by recrystallization. You will then assess its purity by taking a melting point. Reading and Pre-Lab Assignments Read the following sections in The Organic Chem Lab Survival Manual: A Student’s Guide to Techniques by James W. Zubrick, John Wiley & Sons, Inc. • Interpreting a Handbook. You will need to use these resources to look up data for benzoic acid, naphthalene, and methanol. • Recrystallization. See especially “General guidelines for Recrystallization”, “Activated Charcoal,” and “Working with a Mixed-Solvent System,”. C O OH Benzoic Acid Naphthalene Organic Chemistry: Techniques and Transformations 2 Before you come to the laboratory, do the Pre-Lab assignments for this laboratory as assigned by your instructor. Prepare your laboratory notebook as required by your instructor. Procedure Experiment 1 – Recrystallization of Benzoic Acid Weigh 1.0 g of benzoic acid, recording the exact amount, and place it into a 50 mL Erlenmeyer flask. Place 20 mL of distilled water into a second 50 mL Erlenmeyer flask. Add a boiling stick and, using a hot plate, heat the water to boiling. CAUTION! Handle the hot flasks with a towel or tongs to prevent burning your fingers! Using a Pasteur pipette, add 0.5-1 mL of the boiling solvent to the flask containing the benzoic acid. Swirl the flask with each addition and place it on the hot plate to keep it a boiling. Continue to add water in 0.5 mL portions until the benzoic acid just dissolves. Remove the flasks from the hot plate and allow the benzoic solution to cool to room temperature. When the solution has reached room temperature, place the flask in an ice-water bath for 5 minutes to further cool the solution and complete crystallization. Collect the crystals of benzoic acid by vacuum filtration using a small Hirsh funnel. Let the crystals air dry. Weigh it and calculate your percent recovery. If you pre-weigh your filter paper, calculation of percent recovery will be easier. (% yield is a term reserved for synthesis experiments, % recovery is a more accurate description of what you are doing in this experiment). Use melting point to assess the purity of your crystals. Experiment 2 – Recrystallization of an Impure Sample of Naphthalene Obtain a small sample of impure naphthalene and take its melting point. Observe its physical appearance and record this melting point range as an assessment of its purity. CAUTION! Methanol boils at a much lower temperature than water. You will need to turn the temperature of your hot plate down to prevent boil-over. This step should be done in the hood to minimize exposure to fumes. Prepare your small funnel for the filtering step by placing a small piece of glass wool loosely into the bottom of the funnel. Weigh 1.0 g of impure naphthalene and place it in a 50 mL Erlenmeyer flask along with 12 mL of methanol and a boiling stick. Heat the mixture to boiling on a hot plate. Remove the flask from the hot plate and allow for it to cool slightly. Filter the solution through the glass wool into a clean 50 mL Erlenmeyer flask. Add 2 mL of methanol to the used flask, warm it on a hot plate. With a pipette, use this liquid to wash the glass wool. When the filtration is complete, the total volume should be approximately 8-10 mL. If not, evaporate the excess carefully on the hot plate.