Organic Lab 1 Make-up Experiment. Extraction of Caffeine from Beverages. Introduction

Transcription

1 Organic Lab 1 Make-up Experiment Extraction of Caffeine from Beverages Introduction Few compounds consumed by Americans are surrounded by as much controversy as caffeine. One article tells us that caffeine will cause anxiety, whereas another article tells us that the consumption of a cup of coffee before an exam will improve our grade. Approximately 50% of Americans drink coffee daily at a rate of over three cups per day. Caffeine is also found in soda pop, tea, chocolate, and pills that keep you awake. In addition to caffeine, tea and chocolate contain a related compound, theobromine. While the effects of theobromine are mild when compared to caffeine, it is believed that the effects of theobromine are enhanced by the presence of the caffeine in chocolate. The structures of caffeine and theobromine, the main component in chocolate, are shown below. Caffeine is one of the oldest known stimulants. Caffeine stimulates both the central nervous system and skeletal muscles. This stimulation results in increased alertness, which is believed to be the reason for increased ability to concentrate, in the short term. The increased alertness also aids in the ability to stay awake when tired. The downside to this effect is that excessive intake of caffeine may result in restlessness, insomnia, and irritability. Heavy caffeine use can lead to both a tolerance to and a dependence on the consumption of caffeine. Anyone who has tried to kick the caffeine habit can tell you about the headache from caffeine withdrawal. The symptoms are real and in extreme cases can be so severe that the symptoms can include nausea and lethargy. Check your caffeine consumption by examining the list of common caffeine-containing products below. How much caffeine do you consume in a day? Product Content Caffeine Coffee (7 oz) mg espresso (2 oz) 100 mg stay-awake pill 100 mg instant coffee mg tea mg cola mg chocolate bar 30 mg cold relief tablet 30 mg chocolate milk (8oz) 8 mg 1-7

2 In this experiment, you will extract the caffeine from some typical caffeine-containing products. The extraction will be a crude one, but it will give you an indication of the varying amounts of caffeine found in different products. The caffeine is first extracted in an aqueous solution made basic with sodium carbonate. The sodium carbonate converts acids present such as gallic acid and tannic acid into their sodium salt that are highly soluble in water. Although caffeine is soluble in water, it is much soluble in the organic solvent, methylene chloride, but the sodium salts of the gallic acid and tannins remain in the aqueous layer. Any aqueous layer that may have not have been completely separated is "dried" with anhydrous sodium sulfate, a desiccant. The methylene chloride is evaporated leaving you with nearly pure caffeine, which you will also analyze for purity using thin-layer chromatography (TLC). Thin-Layer Chromatography In this experiment, you will use a separation technique called thin-layer chromatography (TLC). TLC is often used in organic chemistry to monitor the progress of a reaction or to identify the components of a mixture. In TLC, a small drop of the mixture to be separated is applied near one end of a flexible plate coated with silica gel. The end of the plate is then dipped into a developing solvent, called the mobile phase, which flows up the plate by capillary action. As the developing solvent flows up the plate, it can carry along the components of the mixture. The rate at which a particular component moves depends on whether it tends to dissolve in the developing solvent or to remain adsorbed (or stuck) on the surface of the silica gel, called the stationary phase. A component that moves rapidly is spending more time dissolved in the developing solvent, whereas a component that moves slowly is spending more time adsorbed to the silica. The results of a chromatographic separation are expressed in terms of Rf values. The Rf is the relative distance that a sample component has moved, that is, relative to the distance moved by the developing solvent. As an example of R f calculation, let's use the diagram of a typical chromatogram shown below. Distance is measured from the point where the sample is applied (the origin) to the middle of each component spot. It is also necessary to measure the distance from the origin to the location of the solvent front. In the above example, Component A traveled 4 cm and the developing solvent traveled 10 cm, the Rf of component A is calculated as follows. The Rf value of each spot is calculated and is a mathematical presentation of the separation of the components of a mixture. The closer the Rf value of a component is to one Rf = distance A traveled = 4 cm distance solvent traveled 10 cm In cases where a solution or liquid contain more than one components R f values for each of the components is calculated. 2-7

3 Materials Equipment 125-mL Erlenmeyer flask 50-mL Erlenmeyer flask (2) mortar and pestle for No Doz tablets 100-mL graduated cylinder spatula funnel test tube 250-mL beaker watch glass TLC plates UV lamp microcapillary tubes Chemicals tea bags Surge, Dr. Pepper, or other caffeinated beverage Instant coffee No-Doz tablets methylene chloride Na 2 CO 3 anhydrous Na 2 SO 4 ethyl acetate caffeine standard 0.2 g/5ml of methylene chloride PROCEDURE Safety: Dichloromethane (methylene chloride) is a chlorinated hydrocarbon. It should be used in the hood and kept in the hood. Separatory funnels are very expensive. Use it with care! Part A: Isolation of caffeine 1. You will need 100 ml of beverage for the experiment. Your assigned sample will be on the table. For the coffee and tea, make up the equivalent of a strong cup of tea or coffee. For the soda pop, measure out 100 ml. For the group assigned tthe No-Doz tablet, crush it using the mortar and pestle, and dissolve the powder in 100 ml of water. Use a 125 ml erlenmeyer flask to contain your beverage or NO-Doz solution. 2. Add 2 g of sodium carbonate to your sample and swirl to dissolve. 3. Transfer the solution into a separatory funnel. 4. In a hood, add about 8 ml of methylene chloride, CH 2 Cl 2, (IUPAC - dichloromethane) to the solution in the separatory funnel. Place a small dot of silicone grease on the penny head stopper. Put the lid on and rotate to completely grease the ground glass. 5. Mix gently as demonstrated and vent frequently. Continue to mix until no more gas is evolved. 6. Decant off the organic layer (bottom layer) into a 50 ml erlenmeyer flask. 7. To the aqueous layer remaining in the separatory funnel, add another 8 ml of methylene chloride. Use a funnel to prevent grease contamination. Repeat the mixing and venting. Add the organic layer to the first extract (50 ml erlenmeyer flask). Repeat the extraction with another 8 ml of methylene chloride. Add the organic layer to the previous extracts. 8. Add 1 gram of anhydrous Na 2 SO 4 to the methylene chloride. Parafilm the flask and allow the solution to 3-7

4 stand for 10 minutes. The anhydrous sodium sulfate will absorb any residual water in the solution. If the Na 2 SO 4 is solidly clumped up, add another 0.5 grams of anhydrous Na 2 SO 4, swirl and let stand for another 5 minutes. Have your instructor check your flask before proceeding to the next step. 9. Weigh a clean, dry 50 ml erlenmeyer flask or a 50 ml boiling flask. Record its mass. 10. Using a pasteur pipet, transfer the extract (do not transfer the Na 2 SO 4 ) into the preweighed boiling flask. Cover the flask with parafilm and give the flask to your instructor for evaporation. 11. Cool the flask. Reweigh the flask and determine the amount of caffeine. Part B: Analysis of the purity of the isolated caffeine Safety: Use the ethyl acetate in the hood. 1. For a development chamber, you will use a 250-mL beaker lined with a piece of filter paper covered with a watch glass. The purpose of the filter paper is to provide an atmosphere in the developing chamber that is saturated with solvent's vapor. Prepare the developing chamber by obtaining a small amount of the development solvent (ethyl acetate). Fill the chamber with the development solvent to a depth of about 1/4 inch. Be sure that the liner is saturated with the solvent. 2. Obtain a TLC plate, handle the plate carefully by the edges and do not bend. Using a lead pencil (not a pen), lightly (don t score) draw a line about 1.5 cm from the edge on the matted side. Mark off two intervals equally spaced on the line. These are the points at which the samples, your isolated caffeine and reference caffeine, will be spotted. 3. In the hood, add 5 ml of methylene chloride to the flask containing the caffeine and swirl gently to dissolve the crystals. 4. Spot the solution of your caffeine on the plate. To spot the plate, dip a capillary tube into the solution. Lightly touch the capillary tube to the plate. Allow just a small spot of liquid onto the plate. 5. Next, spot the caffeine reference solution on the plate. 6. When the plate has been spotted, it is ready to be placed in a development chamber. The solvent level must be blow the spots on the plate or the samples will dissolve off the plate into the reservoir instead of developing. Place the spotted plate in the chamber and allow the plate to develop. 7. When the solvent has risen to a level about 1 cm from the top of the plate, remove the plate from the chamber and, using a lead pencil, mark the position of the solvent front. Let the plate dry. 8. When the plate is dry, observe it under a short-wavelength UV lamp. Lightly outline all the observed spots with a pencil. Also observe under ordinary light and circle any spots. Wear uv goggles when using the uv lamp. 4-7

5 Name Class DATA AND RESULTS Part B: Analysis of the purity of the isolated caffeine 1. Draw a diagram of the chromatogram. 2. Record any observations about the appearance of the spots under ordinary light and UV illumination. 5-7

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7 Name Class Post lab Questions 1. How did your results compare with the expected caffeine amounts listed in the table? Can you account for any difference? 2. From the appearance of your isolated caffeine, do you believe that your sample was pure caffeine? If not, what do you think are the possible contaminants? 3. Did the TLC provide you with any additional information about the purity of your isolated caffeine? Explain. 7-7