Isolation of Caffeine from Tea

Transcription

1 Isolation of Caffeine from Tea Introduction A number of interesting, biologically active compounds have been isolated from plants. Isolating some of these natural products, as they are called, can require interesting extraction strategies. Caffeine, shown below along with some of its close relatives, is a nitrogen-containing compound. Since nitrogen containing organic compounds behave as bases, these natural products are often called alkaloids. The alkaloids, which include a large number of medically important compounds and some poisons, are bitter to the taste and require special ph conditions for proper extraction. Although the extraction of caffeine is not considered particularly difficult, its isolation from tea will give you an appreciation of some of the considerations and techniques commonly used by natural products chemists. Amines and other nitrogen-containing organic compounds react with water as is shown below. The "R" groups in the structure represent various possible organic structures that may be attached to nitrogen. This reaction is reversible, i.e. a state of equilibrium exists. Since only the uncharged free base can be extracted into a nonpolar solvent, the equilibrium should be shifted as far to the left as possible. This can be done by adding hydroxide ion (OH ) to the solution. According to Le Chatelier's principle, stressing the system with added product will cause the equilibrium to shift to the left to the reactant side to relieve the stress. In this experiment, the hydroxide concentration will be raised by dissolving sodium carbonate, a basic salt, in the extraction mixture. Under these conditions, when dichloromethane is added as a nonpolar extracting solvent, most of the caffeine will be in the neutral, free base form and will readily extract into the dichloromethane layer. Note that a number of different substances, including tannins, pigments and other watersoluble components will also extract from the tea into the aqueous or water phase. However, at basic ph they will not extract to any significant extent into the dichloromethane layer. Dichloromethane has a very low boiling point and can be easily distilled away from the crude caffeine. The final purification step in this laboratory exercise will be to sublime the crude caffeine. Sublimation, a direct phase change from solid to gas, will be performed under vacuum at high temperature. Although not all solid organic substances are candidates for purification using sublimation, caffeine sublimes quite well. The caffeine crystals collected in the sublimation apparatus should be very pure. Your instructor will show you how to collect a reflectance infrared spectrum of your purified caffeine sample. A good infrared spectrum will provide confirmation of the identity of the caffeine Isolation of Caffeine from Tea, Page 1

2 Caffeine Found in coffee, tea, cola nuts, guarana paste. Procedure Theobromine Found in cacao beans, tea, cola nuts. Theophylline Synthetic drug used as cardiac stimulant, diuretic, smooth muscle relaxant, bronchodilator. Weigh two large or three small tea bags and record the mass. Take an additional tea bag, find its mass, then empty the contents into a small beaker. Weigh the empty tea bag and record the mass. The mass of an empty tea bag will allow you to estimate the mass of tea you extracted in your tea bags. Dissolve approximately 11 grams of sodium carbonate in about 100 ml of water in a 250 ml beaker and add the tea bags. Boil this mixture for about 20 minutes using a hot plate. Place a watch glass on top of the beaker to minimize water loss. If the water boils away, you may add an additional 25 to 50 ml of water. Remove the tea bags, squeezing them carefully to remove as much of the extract as possible. Allow the tea extract to cool to room temperature before extracting it with dichloromethane. This cooling step is essential, since dichloromethane has a boiling point of 37 C, and it will immediately boil if it is brought into contact with warm tea extract. Transfer the tea extract to a separatory funnel (on cart). Extract the tea three times with 20 ml portions of dichloromethane. The dichloromethane is more dense than water and will form the lower layer in the funnel. Gently swirl the two layers together to allow contact between the two liquids. Do not shake the funnel, since an emulsion can form and you will have to wait for an extended period of time for the two layers to separate. Occasionally invert the funnel and open the stopcock to release any excess pressure that may form. Remember that you cannot drain the lower dichloromethane layer until the ground-glass stopper in the top of the funnel has been removed. After collecting the three dichloromethane extracts, combine them and dry the combined extract by passing over a layer of magnesium sulfate contained in filter paper cone in a funnel. Pre folded filter paper is on the cart. A layer of magnesium sulfate about 1 cm deep should be sufficient the dry the extract. Wash the magnesium sulfate layer with an additional 10 ml of dichloromethane. If water, which is darkly colored, makes it through this drying system, dry the extract a second time with magnesium sulfate. Transfer the clear, dry dichloromethane extract to a small, pre-weighed, round-bottom flask. Use the rotary evaporator to remove the solvent. Your instructor will show you how to operate it; it can be tricky to use until you get used to it. The white residue left in the round-bottom flask is the crude caffeine. Weigh and record the mass of the flask and the caffeine. If the time is short in the lab period, store the flask in your lab drawer for further work next lab period. Isolation of Caffeine from Tea, Page 2

3 Using a small spatula, carefully scrape as much of the crude caffeine loose from the roundbottom flask as you can. Transfer the loose caffeine to the small conical vial of the sublimation apparatus. Alternatively, you may use a small volume, approximately 5 ml, of dichloromethane to transfer your crude caffeine from the round bottom flask to the conical vial. If you use dichloromethane to make the transfer, you will need to evaporate it with the rotary evaporator. Once you have transferred your crude caffeine to the small conical vial, attach it to the cold finger. Load the cold finger with ice and water, and connect the side-arm of the apparatus to the vacuum system. Be sure to include a trap between your apparatus and the vacuum system. The tubing used in a vacuum system must be thick-walled rubber hoses that will not collapse under the influence of a vacuum. Heat the vial with a hot plate or heat gun. (The temperature setting on the hot plate should be fairly high. If you use the heat gun, run it on the hot setting.) You should also make sure that the tubing and stoppers in the system are tightly seated so that they will not leak. Once the sublimation system is in operation, it should only take a few minutes before pure caffeine crystals become visible collecting on the surface of the cold finger. Allow the system to operate long enough to collect several milligrams of pure caffeine. Remember to replenish the ice in the cold finger if necessary. Once enough caffeine has been collected to test, stop the system by turning off the heat, vent the vacuum system to room air pressure and turn off the vacuum. Carefully remove the cold finger and scrape the pure caffeine crystals from the cold finger onto a preweighed sheet of weighing paper. Weigh and record the mass of the weighing paper and pure caffeine. Allow your instructor to inspect and sign-off on your caffeine sample. Your instructor may require you to test the melting point and/or run an infrared spectrum of your caffeine. If so, he/she will show you how to run those tests. Isolation of Caffeine from Tea, Page 3

4 Report Isolation of Caffeine from Tea Mass of empty tea bag Mass of full tea bags Mass of tea in one bag Mass of tea extracted Mass of round-bottom flask and crude caffeine Mass of empty round bottom flask Mass of crude caffeine Mass of weighing paper and sublimed caffeine Mass of weighing paper Mass of pure sublimed caffeine Name Calculate the percent by mass of crude caffeine in tea. Show setup. ` Calculate the percent by mass of sublimed caffeine recovered from crude. Show setup. Melting point of sublimed caffeine Sample approval by instructor. Isolation of Caffeine from Tea, Page 4

5 1. Draw the structure of caffeine. 2. Describe how caffeine differs in its physiological action from its relatives theobromine and theophylline. 3. Why was the water extraction system made basic? 4. What is meant by the term sublimation? 5. What is the literature melting point of caffeine? 6. What are two properties of dichloromethane which make it a good solvent for extractions? Isolation of Caffeine from Tea, Page 5