Recrystallization Lab Report- Organic Chemistry

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Experiment 1 Post-lab Recrystallization W19 CHEM 43A Name: Page Limit 2 pages Unknown ID# 364 Objective (1-2 sentences) State the objective of the experiment. (what?) The goal of this experiment is to use microscale and macroscale techniques and recrystallize a given unknown. The unknown will be purified using melting point determination. Motivation (1-2 sentences) State why the experimental techniques or results are important. (why?) Recrystallization is an important method used to purify solids that are nonvolatile Scientific Reasoning Questions (Answer in 1-2 sentences each) Recall - What evidence did you use to identify your compounds? Understand - Was your recrystallized compound pure? How do you know? Apply - What should you do if you don't recover any crystals after the recrystallization? (Where'd the compound go?) Critical Analysis - Why is it important to slowly cool the hot solution of dissolved compound? What might happen if you cool the solution too quickly? Evaluate - What are the characteristics of a good recrystallization solvent? Experimental Summary Briefly (in ~100 words) summarize the objective of the experiment, the method/s you used to accomplish the objective, and your results. Include only the key experimental data that directly supports the outcome of your experiment. ©2018 Jeremy K. Klosterman. Do not copy or distribute without permission.
W19 CHEM 43A Klosterman Experiment 1 Recrystallization Tasks 1. Successfully recrystallize an impure unknown using microscale and macroscale techniques 2. Identify the purified unknown using melting point determination. Learning Objectives 1. Explain how purification via crystallization works in terms of solubility differences. 2. Contrast gravity and vacuum filtrations and explain how the techniques are best applied to separate compounds of differing solubility Reading Assignment - Heating & Cooling Methods, Recrystallization Techniques in Organic Chemistry1 4th edition: pages 73-86; 221-235 Video Assignment - Recrystallization Techniques MIT Digital Lab Techniques Manual Weizman CHEM143A Recrystallization Video Clips Introduction Crystallization is a technique where a solid compound is dissolved into a solvent and then slowly recrystallized. Typically, hot solvent is used so that the compound readily dissolves. Upon cooling the solubility of the compound in the solvent should decrease. The solution becomes supersaturated and crystals begin to form. Recrystallization is commonly used as a purification technique that takes advantage of differences in solubility and crystal lattice formation between the desired compound and any impurities that may be present. After dissolution, any insoluble impurities can be filtered from the solution. After recrystallization, any soluble impurities remain in the solution and the crystals are recovered by filtration. The key steps to a successful purification by recrystallization are dissolution, purification, Figure 1. Solubility Curve hot filtration, crystallization; and isolation/Recovery. 1. Dissolution Dissolve the compound to be purified in the minimum amount of solvent just below the boiling point. The concentration should be as high as possible. If too much solvent is used, the concentration will be too low, cooling will not yield a supersaturated solution (See Fig 1.), and the compound will not crystallize from solution. Start with a small amount of solvent, ~5mL (this will depend on how much compound you are using), and heat the solution. If the sample did not dissolve, add 1– 2 mL and heat again. Continue until the solid completely dissolves. Be Careful - Insoluble impurities will not dissolve and you may add too much solvent. A good rule of thumb is find the point where crystallization begins as soon as you stop heating. Then add a little more solvent. We don't want crystals to form before or during filtration. SAFETY NOTE – Using Hot Plates • Plan ahead - Never return a hot plate to the shelf while it is still hot. Let it cool to room temp first. • Watch the cord - Damaged cords can spark and cause fires. Restrain loose cords so they don't melt on the hot plate or knock over chemicals. 1 2nd edition: pages 47-56; 100-116; 3rd edition: pages 49-58; 183-197 ©2019 Jeremy K. Klosterman. Do not copy or distribute without permission. W19 CHEM 43A Klosterman 2 • Clean up spills immediately - Unplug and cool the hot plate immediately. Do not use again until clean and dry. Heating spilled chemicals can release toxic vapors and/or start fires. • Keep solvents closed and far away - Close solvent containers and keep them far from ignition sources. Hot plates are not explosion proof. The internal electronics can spark and ignite organic vapors. Many solvent vapors can also auto-ignite upon contact with the hot plate surface. Diethyl ether is particularly dangerous; the vapors are heavier than air and auto-ignite at 160 °C.. • Don't use excess heat – Use the lowest setting possible. Too much heat increases the likeliness of uncontrolled boiling, burns and can damage the hot plate. Hot plates heat slowly and cool slower. Keep below settings 3 or 4 (330 °C or 390 °C) • Always use a boiling chip - Hot liquids can form large bubbles of superheated vapor that can suddenly boil over in a violent eruption (called bumping), spattering hot solvent that can cause burns. Boiling chips (small chemically inert, black pieces of porous carborundum) help small bubbles quickly form and prevent bumping. Add boiling stones before heating liquids; they can cause bumping when added to hot liquids. 2. Purification with Activated Carbon Most organic molecules are colorless and appear as white solids. During crystallization, colored soluble impurities can be removed by the addition of activated carbon (charcoal). Colored molecules have very large numbers of conjugated double bonds and will adsorb (stick) to the surface of charcoal. Only add a pinch of carbon (20-30 mg per gram of compound), as the compound being purified may also stick to the carbon. Do not add activated carbon to hot solvent as it can cause bumping. 3. Hot Filtration Once the desired compound has dissolved, solid impurities (including boiling chips and charcoal) can be removed by hot filtration using folded/fluted filter paper. The filtration is performed hot to prevent cooling and premature recrystallization. The filter paper, filter funnel and receiving flask are pre-warmed with hot solvent (discard before filtering to avoid dilution). Gravity filtration is used to avoid evaporation and cooling that occurs in vacuum filtration. If crystallization occurs, small amounts of hot solvent can be added to redissolve the crystals from the filter paper. If charcoal passes through the paper, collect and filter again. 4. Crystallization Growing big, beautiful crystals is an art. Crystal formation is dependent on the concentration of the solute, the temperature of the solvent and the cooling rate. If the solution cools too quickly, the crystals become unstable and rapidly precipitate, often with entrapped impurities (Fig 2). Small crystals also are harder to collect via filtration. Solvent choice is important. If the solute (compound) is very soluble in the selected solvent, it may never crystallize, even at low temperatures. Occasionally, crystals refuse to spontaneously form. Crystal nucleation can be induced by the addition of previously obtained crystals (called a seed crystal) or by scratching the side of the flask. It may also be necessary to increase the concentration by reheating the solution to remove excess solvent by evaporation. (Always add a boiling chip!) Once crystals have formed, the Figure 2. The Crystallization Process solution is often placed in an ice bath to maximize recovery. 5. Isolation and Recovery Part A Macroscale Crystals of the purified compound are collected on pre-wetted filter paper in a Büchner funnel by vacuum filtration (Fig. 9.6 Mohrig 4ed, p138). A vacuum trap must always be used to prevent liquids from entering the vacuum system. Small amounts of cold solvent can be used to rinse any remaining crystals from the Erlenmeyer flask. Collected crystals on the filter paper should also be rinsed with small amounts of cold solvent to wash any remaining soluble impurities on the surface. The crystals are then dried to remove residual solvent, often by evaporation (with or without vacuum). Desiccators are typically used to speed the removal water. Part B Microscale For small quantities (10-100 mg), a Craig tube is used with a centrifuge to isolate the crystal from the solution (mother liquor); The crystals cannot pass through the narrow space between the plunger and Craig tube (Fig 15.6 Mohrig 4ed, p 232). The tube is inverted and spun in a centrifuge which ©2019 Jeremy K. Klosterman. Do not copy or distribute without permission. W19 CHEM 43A Klosterman 3 drives the mother liquor out, leaving the crystals behind. The crystals are collected, washed with cold solvent and dried. Lab Notebook Prelab write-up Write water and activated carbon and all compounds from Table 1 into the reagent table. Include IUPAC and common names, molecular weights and melting points. Draw a gravity and vacuum filtration set ups under the equipment section. Look up and fill in the CAS numbers and IUPAC names to the chemical hazard sheet (see appendix) from the SDS and staple onto the cover of your notebook. Write out the steps you will perform under the procedure section. Observations Write down what you did and what you observed directly into the notebook. Data that was not written into the notebook cannot be used for the lab report. Calculations All calculations must be written directly into the lab notebook. Procedure Safety • • • • Waste • • • Hot Plates are HOT and can cause severe burns Boiling water and steam are HOT can cause severe burns. Do not add boiling chips or activated carbon to a hot solution. Wear gloves when transferring liquids. Used filter papers and gloves go into the solid waste container by hood #6. Solvent waste goes into liquid organic waste in hood #6. Unknown vials are returned to the TA. Part A - Macroscale Recrystallization Table 1. Possible Unknowns Compound CAS Number mp (°C)* Acetanilide 103-84-4 113 – 115 Benzoic acid 65-85-0 121 – 125 Benzamide 55-21-0 125 – 128 2-Chlorobenzoic acid 118-91-2 138 – 140 Salicylamide 65-45-2 140 – 144 Benzilic acid 76-93-7 149 – 151 Salicylic acid 69-72-7 158 – 161 4-Acetamidophenol 103-90-2 168 – 172 *Sigma Aldrich Catalog: Your TA will show you the proper gravity filtration setup and vacuum filtration set up with vacuum trap. You will be provided a small, impure sample of one of the compounds in Table 1. Record the ID number of your sample and the associated hazards. This is your unknown; the identity of the compound will not be disclosed to you. Add boiling chips and ~ 150 mL of de-ionized water to an Erlenmeyer flask. Heat to a light boil. You will use the hot water throughout this experiment so keep it on the hot plate until you are done. Add 0.5g of the impure unknown, 5mL of hot water and a boiling chip to a 50 mL Erlenmeyer flask. Place the Erlenmeyer flask on the hot plate and reheat to a light boil. Use tongs to gently swirl and heat the contents of the flask until the solid has dissolved. Do not touch the hot flask directly with your hands. If needed, add hot water in small amounts (1-2 mL at a time) until the solid dissolves. If the solution is colored (hint: all unknown compounds are colorless), cool the solution to stop boiling and add about 4 – 6 charcoal pellets. Heat again to boiling and allow it to stand for a few minutes. Prepare a pre-warmed filter funnel by pouring hot water through the funnel (with fluted filter paper) into an Erlenmeyer. Dispose of the cooled water after warming. Clamp everything to avoid spills. Filter the still warm solution through the pre-warmed filter. If crystals form on the filter paper, wash with a small amount of hot water to redissolve the crystals. Collect the resultant filtrate solution. The solution should be clear and colorless. If not, filter again. ©2019 Jeremy K. Klosterman. Do not copy or distribute without permission. W19 CHEM 43A Klosterman 4 Allow the filtrate solution to slowly cool to room temperature undisturbed. Then cool the mixture in an ice bath for about 10 minutes to maximize the recovery. Collect the crystals by vacuum filtration using a Büchner funnel fitted with the appropriate-size filter paper. Do not forget the vacuum trap between the filter flask and the vacuum line. Wash the crystals with a few portions of cold water and remove excess water by pressing the pure crystals down by using a spatula while applying vacuum. Transfer the crystalline sample to a dry piece of filter paper and place in the desiccator to dry until the next lab period. Weigh the dry crystals and use a small portion in Part C to determine the identity via melting point determination. Place the remaining sample in a small plastic bag, label properly, and turn in to the TA. Part B - Microscale Recrystallization Add 75 mg of the impure unknown from Part A and a boiling chip to a 10 x 75 mm test tube. Add 0.5 – 1.5 mL of hot water dropwise until the unknown has dissolved. Keep the test tube hot during this process by placing it in a warm water bath. Use a microspatula to stir the solution as you add the hot water to improve dissolution of your unknown. If the solution is colored (and the desired compound is colorless), remove from heat and cool the solution to below the boiling point. Add about two charcoal pellets. Place back in the hot water bath to return to boiling. Allow the solution to stand for a few minutes. Pre-warm plastic Hirsch funnel with hot water. Perform a hot gravity filtration using the Hirsch funnel and collect the filtrate in a 2 mL Craig tube. The filtrate should be clear and colorless. If not, filter it a second time. Allow the Craig tube to cool to room temperature. Then cool the mixture in an ice bath for about 10 minutes. Insert the plunger into the Craig tube, attach a fine wire to the plunger and place a centrifuge tube over the plunger. Now, invert the assembly and centrifuge the sample. Scrape the crystals from the Craig tube and dry in a desiccator. Record the total mass of pure compound obtained. Place your product in a small plastic bag and label the bag properly; turn over the contents to your TA. Part C - Melting Point Determination of the Recrystallized Unknown Table 2. Melting Point Standards Compound CAS Number mp (°C)* Benzhydrol 90-01-0 65 – 67 Vanillin 121-33-5 81 – 83 Acetanilide 103-84-4 113 – 115 Benzamide 55-21-0 125 – 128 Salicylic acid 69-72-7 158 – 161 Succinic acid 110-15-6 184 – 186 Dimethylglyoxime 95-45-4 240 – 241 *Sigma Aldrich Catalog: Using the recrystallized, pure samples from Part A and Part B, determine “crude” and careful melting point ranges for both. Next, calibrate the thermometer by selecting a compound from Table 2 that has a melting point near your observed melting point ranges. Confirm the identity of your unknown by determining a mixed melting point range. Use purified compound from either Part A or B. Record the observed and corrected melting point ranges. You only need one mixed melting point to support your identification. Repeat if your first attempt is ambiguous. Calculations • Identify the unknown compound from Part A and B using your observed melting point range. Use the mixed melting point data to support your conclusion. ©2019 Jeremy K. Klosterman. Do not copy or distribute without permission. W19 CHEM 43A Klosterman 5 Experiment 1 Chemical Hazards - Fill in the missing data CAS Number IUPAC Name Chemical Structure mp / bp (°C) 113 – 115 H N O O 121 – 125 OH O GHS Hazard Codes WARNING H302, H315, H317, H319, H335 DANGER H315, H318, H372 125 – 128 WARNING H302, H341 138 – 140 WARNING H319 140 – 144 WARNING H302, H315, H319, H335 149 – 151 Not Hazardous 158 – 161 DANGER H302, H318 168 – 172 WARNING H302, H315, H319, H335, H411 NH2 O Cl OH OH O NH2 OH HO2C OH O OH H N O HO C H O 100 H OH H3CO O WARNING May form combustible dust concentrations in air Not Hazardous 65 – 67 WARNING 303, H315, H319, H335 81 – 83 WARNING H319, H412 158 – 161 DANGER H302, H318 184 – 186 DANGER H318 240 – 241 DANGER H228, H301 HO OH O OH O OH HO HO N O CH3 N OH GHS Pictogram CH3 H228 ___________________________________ H302 Harmful if swallowed. H303 May be harmful if swallowed. H315 Causes skin irritation. H317 May cause an allergic skin reaction H318 ___________________________________ H319 Causes serious eye irritation. H335 May cause respiratory irritation. Hazard Statements H341 ___________________________________ H372 ___________________________________ H401 Toxic to aquatic life H402 ___________________________________ H410 Very toxic to aquatic life with long lasting effects. H411 Toxic to aquatic life with long lasting effects. H412 Harmful to aquatic life with long lasting effects ©2019 Jeremy K. Klosterman. Do not copy or distribute without permission.
Experiment 1 Data Report Recrystallization Last Name: First Name: W19 CHEM 43A Student ID: Unknown ID# 364 Compound Name 4-Acetamidophenol Transfer data from your notebook to the tables below. Remember to write out the units and use the correct significant figures associated with the accuracy of each measurement. Mathematical calculations should be written in your notebook unless Excel is required. Part A - Macroscale Recrystallization Table 1. Starting and Final Mass Starting Mass (impure) Recovered Mass (pure & dry) % Recovery1 0.5124 g 0.2257 g 44.1 % Part B - Microscale Recrystallization Table 2. Starting and Final Mass Starting Mass (impure) Recovered Mass (pure & dry) % Recovery1 0.0778 g 0.0511 g 65.5% Part C - Melting Point Determination of a Recrystallized Unknown Table 3. Melting Point Determination of recrystallized Unknown # Low High Crude mp range A 163.8 ℃ 170.1 ℃ Careful mp range A Careful mp range B 167.3 ℃ 169.9 ℃ Average mp range Potential unknown identity - Write out the names of 2-3 possible matches for your unknown. 4-Acetamidophenol NA NA Table 4. Thermometer Calibration MP Standard Low Reported mp Observed mp range Correction Factor Corrected mp range of unknown 169.0 ℃ 165.3 ℃ 3.0 ℃ 168.3 ℃ High 172.0 ℃ 169.9 ℃ 172.9 ℃ Table 5. Melting Point Determination of Unknown #+ Known Known compound Mixed mp range 1 Organic Chem Lab Survival Manual 10th ed. page 22 ©2019 Jeremy K. Klosterman. Do not copy or distribute without permission. Experiment 1 Data Report Recrystallization Unknown # 364-part A+ Unknown # 364-part B + W19 CHEM 43A 4-Acetamidophenol 4-Acetamidophenol 166.2 – 169.4 ℃ 168.2 – 170.1 ℃ ©2019 Jeremy K. Klosterman. Do not copy or distribute without permission.

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