12AL Experiment 1: Separating the Components of a Medicinal Mixture Safety: Proper lab goggles/glasses must be worn (even over prescription glasses). It is advised to wear gloves as well. Always ask where organic waste containers are located in the lab. Isolations and Extractions should be performed very slowly and with utmost care in working with chemicals and keeping containers labeled. Never throw anything away until you are sure you do not need it; and never put any organic chemicals in the sink. Often, students must repeat this experiment due to spilling samples, loss of samples, etc… you have two full days to complete the lab and have plenty of time to repeat it if necessary. Organic experiments are tough and you need to realize early on that you will be repeating long experiments two, three, even four times. Background: Separation of organic mixtures is a very important forensic process as many drug mixtures are illegally prepared and sold on the “black market.” One or more components of a mixture may have damaging or fatal effects on the body. Thus, being able to efficiently isolate each component to determine its purity and identity is an extremely important skill that you will use throughout your entire year of organic chemistry and beyond. It is important to consider both the physical and chemical properties of all chemicals involved in the experiment – unique physical (ie solubility) and/or chemical properties (ie acid-­‐base reactivity) will allow you to determine a suitable method or isolating each component and learning about each chemical’s structure. It is important to take the following information into account while performing your extraction and isolations involving the following: -­‐ Sucrose is insoluble in the organic solvent dichloromethane CH2Cl2 (aka methylene chloride) -­‐ Aspirin, Acetanilide, and Phenacetin are soluble in CH2Cl2 -­‐ Acetanilide and Phenacetin do not react with Sodium Bicarbonate. -­‐ Aspirin reacts with Sodium Bicarbonate to form a salt, sodium acetylsalicylate. This salt (a salt is an ionic compound) is insoluble in CH2Cl2, yet soluble in water. Please note: Your objective is to isolate aspirin; the salt is not aspirin; you will have to isolate the salt and convert it back to aspirin. -­‐ In addition, you should be able to write balanced equations and simple
mechanisms for chemical reactions occurring in this experiment. Please note: Structures for organic compounds can be drawn in an equation rather than trying to reduce the large structure to molecular compound form. Full line-­‐bond structures should always be drawn for mechanisms in order to see the flow of electrons in detail! See generic example below. Example of an Organic Equation vs. Mechanism: Objective: To separate the 3 components of a medicinal mixture: sucrose, aspirin, and an unknown chemical (acetanilide or phenacetin). You are required to SAVE your Aspirin and Unknown Chemical for Experiment 2 in which you will determine the purity of each saved component and the identity of the unknown. Procedure: Isolation of Sucrose 1. Obtain a clean & dry 8in test tube. Its mass is not necessary as we are looking for purity of individual components, not mass. 2. Accurately weigh approximately 0.400 grams of the medicinal mixture (may be labeled as Panacetin, and 8.0mL of CH2CL2. Be sure to break up any chunks. 3. Put both in the test tube, stopper with a properly fitted cork (not a rubber stopper, why?), and gently, yet thoroughly, shake the test tube to dissolve as much solid as possible. Not all of the solid mixture will dissolve – why is this? What component(s) of the mixture are dissolving? 4. Use a plastic pipette to separate the CH2Cl2 liquid layer from the solid, leaving the solid component in the bottom of the test tube (transfer the CH2Cl2 liquid layer to a clean & dry test tube) – what is this solid that you have just separated? Discard the wet solid into an appropriate organic waste jar. Isolation of Aspirin 5. Extract (add to) the CH2Cl2 liquid with ~4mL of 5-­‐10% sodium bicarbonate. What do you notice? What are the bubbles? Is a chemical reaction occuring? When the bubbles cease, you will have two liquid layers; separate the two liquid layers – one layer is aqueous and contains a component, while the other layer contains CH2Cl2 and a different component – which layer is which? How do you obtain this information in the future if needed? The density of CH2Cl2 is 1.33 g/mL, while water is about 1 g/mL. Transfer the aqueous layer to a clean & dry 50mL beaker. Label the beaker! 6. To the remaining CH2Cl2 liquid in the test tube, add another 4mL portion of sodium bicarbonate. Please note: if you have lots of bubbling, feel free to add more sodium bicarbonate in order to obtain all the component the sodium bicarbonate is reacting with. Again, you will have two liquid layers form; transfer the aqueous layer to the aqueous layer already in the 50mL beaker, leaving all the CH2Cl2 liquid in the test tube. What component of your original mixture is the sodium bicarbonate reacting with? *At this stage in the experiment, you have a test tube with CH2Cl2 and one component of your original medicinal mixture AND you have a beaker with an aqueous mixture containing the salt form of another component of your original medicinal mixture. 7. In order to isolate your aspirin, you will need to convert the salt in the aqueous mixture back to aspirin by adding hydrochloric acid. To your aqueous mixture in your beaker, slowly add 1.0mL of 6.0M hydrochloric acid while gently swirling the beaker. You may notice the formation of a precipitate immediately or in step 8. Test the pH of the solution with pH paper – the pH should be 2 or lower. If necessary, add a little more acid. Don’t panic if more acid is needed… afterall, the acid serves to not only convert the salt back to aspirin, but to neutralize any remaining base – what is this base? 8. Cool the mixture in an ice water bath for at least 5 minutes – you should have an abundance of white precipitate in your beaker – what is the identity of the precipitate? You may have to induce crystallization by scratching the inside of the beaker with a glass stirring rod. You may have to ice for quite awhile. And often, you may have to let it precipitate overnight in your lab drawer. At a last resort, you may have to start over due to incorrect separation or mislabeling of layers, etc… 9. Using your microkit filter funnel, filter adapter, and micro filtering flask, filter the mixture in order to collect the solid – what is this solid? Rinse the solid in the funnel with ice cold water to clean it and safely store the solid somewhere in your lab drawer to dry. SAVE this solid for Experiment 2. Make sure to label your chemical! Isolation of the Unknown Component 10. Under the hood, evaporate the CH2Cl2 from the liquid mixture in the test tube by heating very gently in a warm-­‐water bath. CH2Cl2 has a very low boiling point. Stop heating when a solid residue remains – what is this solid? SAVE this solid for Experiment 2. Make sure to label your chemical! Aspirin Component Isolated Instructor Signature______________________ Unknown Component Isolated Instructor Signature______________________ (Purity will be checked in Experiment 2) 12AL Prelab 1: Separating the Components of a Medicinal Mixture 1. What is an analgesic drug? Antipyretic drug? 2. What is a Bronsted-­‐Lowry Acid? Bronsted-­‐Lowry Base? 3. What is a Lewis Acid? Lewis Base? 4. Draw Lewis Structures for aspirin, acetanilide, acetaminophen, and phenacetin (all bonds and lone pairs must be shown neatly). 5. Look up the pKa values of each of the molecules in #4. What does this mean in terms of the molecules in #4. 12AL Postlab 1: Separating the Components of a Medicinal Mixture 1. a. Describe the lab evidence that a chemical reaction occurred when you added HCl to the aqueous solution containing sodium acetylsalicylate (the aspirin salt). b. Why did this occur? (meaning what type of chemical reaction occurred?) 2. Clearly & specifically explain how the % sucrose, % aspirin, and %unknown would be affected (increase, decrease, stays same) by the following changes: a. After adding dichloromethane to the Panacetin mixture, you didn’t shake the mixture long enough. b. During the sodium bicarbonate extraction, you failed to thoroughly mix up the layers. c. You added HCl to your aqueous layer to a pH of 7, instead of a pH of 2. 3. Acetaminophen (like aspirin) is a fairly strong acid. What problem would you encounter if the unknown component was acetaminophen and you extracted the aspirin/unknown filtrate with NaOH (instead of NaHCO3)? In addition, give the TWO reaction MECHANISMS that are occurring (draw full structures out, clearly show arrows attacking etc, make sure to show all bonds, lone pairs, and formal charges – be neat!). Mechanism 1: Mechanism 2: 4. Show reaction MECHANISMS for the reactions involved when… a. aspirin dissolves in aqueous NaHCO3 b. aspirin is precipitated from the sodium acetylsalicylate solution by HCl (draw full structures out, clearly show arrows attacking, make sure to show all bonds, lone pairs, and formal charges – be neat!). 5. HCl was used to precipitate aspirin from the aspirin salt AS WELL AS to….. (show the balanced reaction).