In this experiment, you will extract caffeine, a naturally-occurring alkaloid from tea leaves in tea bags. The caffeine

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In This Experiment You Will Extract Caffeine A Naturally Occurring Alkaloid From Tea Leaves In Tea Bags The Caffeine 1 (162.15 KiB) Viewed 70 times

In this experiment, you will extract caffeine, a naturally-occurring alkaloid from tea leaves in tea bags. The caffeine in tea leaves can be extracted simply by boiling the leaves with water, 'brewing' the tea. However, there are other compounds in tea leaves which are extracted by hot water. You may have seen root beer-colored rivers and lakes. These waters are colored with tannins extracted into the water from tree bark and leaves, especially from oak leaves OH HO OH HO OHO -OH OH OH that have died and fallen into the river. Tannins are used to "tan" leather; especially hemlock bark, rich in tannins, was used in the tannery process to preserve and dye leather. In tea, natural caffeine is bound to the same types of tannins: dark, complex, water-soluble compounds. Black tea leaves are similar; they are dried and fermented, and are full of the tannins which give tea its dark color. Tannin mixtures contain the phenolic tannic acid; see its structure at the left. As you can see, tannic acid is not a carboxylic acid, but -OH instead is a weak, phenol-type acid. -OH a OH HO In order to get caffeine by itself, separated from the dark tannins, we will first extract the tea leaves with hot water, then 'back-extract' the caffeine dissolved in the hot water with dichloromethane (methylene chloride', CH₂Cl2) HO CH Prelab Question 1: Draw the structure of caffeine. Prelab Question 2: In the same reference you used in Prelab Question 1, you'll find the melting point (m.p.) of pure caffeine. What is it? Prelab Question 3: Why are compounds like caffeine called 'alkaloids'? What do all alkaloids have in common? Prelab Question 4: Is caffeine a primary, secondary or tertiary amine? Prelab Question 5: In the extraction below, the base Na₂CO3 is used to convert the neutral tannins, present as neutral tannic acid molecules, into their conjugate bases or tannates. The tannates are more soluble in water than the tannins, and more insoluble than the tannins in the non-polar dichloromethane, and so get left behind in the back- extraction. Why are the tannates so insoluble in dichloromethane? Procedure A: Hot Water Extraction of Tea and Back-Extraction with Dichloromethane Obtain two tea bags. Remove bags from any wrappers and remove any tags from the strings. Do not open the tea bags. Measure the bags' gross weight. Obtain the tare weight of an empty tea bag from your instructor. Then, calculate the net weight of the tea inside the two bags. Gross weight - Tare weight = Net weight Observation A1: Gross weight of two tea bags: 4.54 g Observation A2: Tare weight of one empty bag: 0.20 g Question A1: Net weight of tea leaves in two bags: 0.20 g HO HO- HO HO HO O HO OH HO -OH OH HO

Into a 250-ml beaker place the two tea bags. Add about 120 mL of reagent water and 10 g of anhydrous sodium carbonate (Na₂CO3). Bring the solution to a gentle boil on the hotplate, then reduce the hotplate's heat control to between 1-3. Keep boiling for 20 min. Keep the tea bags immersed; add a little water if they look dry. If they float, push them down gently with the glass rod. Be careful not to tear the fragile paper. If you do, the loose solids may clog the separatory funnel in the next step. Set up a ring stand and separatory funnel holder. Place a 300-mL separatory funnel into the holder. Make sure the stopcock is closed (horizontal). Decant (pour off) the hot solution into the separatory funnel. Wash the tea bags in the beaker with two or three 20-mL portions of reagent water, pressing the tea bags gently with the glass rod to squeeze out the internal solution. Add the washings to the aqueous extract solution in the funnel. Place a thermometer into the extract and allow it to cool to about 30 °C. You can cool the extract more rapidly by bathing the outside of the funnel with cold tap water while swirling. Discard the tea bags. Add 20 mL of dichloromethane (methylene chloride) to the separatory funnel. Take the funnel out of the holder. Cap the funnel and screw it on as tightly as you can. The cap tends to leak, acting as a pressure vent, so do not invert the funnel. Hold the funnel upright, at the top. Then, rapidly swirl the tip of the funnel in a small circle so that the dichloromethane layer comes into contact with as much of the aqueous extract layer as possible. Swirl for about a minute. Do not shake the funnel up and down because a hard-to-separate emulsion will form between the layers. This is because tannates are anionic surfactants; they act as emulsifiers or detergents. Question A2: How do the tannates form an emulsion? Replace the funnel in the holder and allow the two layers to separate. Question A3: Which is the top layer, the aqueous layer or the dichloromethane layer? Why? pen the stopcock and drain the organic layer into a clean, dry 50-mL Erlenmeyer flask. Try to get a minimum of the emulsion or aqueous layer into the flask, even if this means leaving a little organic layer behind. Cover the flask with Parafilm. Repeat the above back-extraction of the aqueous layer with another 15 mL of dichloromethane. Afterwards, you will have two 20-mL dichloromethane extracts combined in your 50-mL Erlenmeyer flask. Add 1.0 g anhydrous sodium sulfate (Na₂SO4) to the extract and swirl. The sodium sulfate is a 'drying agent' and will adsorb small amounts of water that may be present. EX mL 0 +5% 50 100 150 200 150 100

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Procedure B: Measuring a Melting Point of Pure Caffeine Obtain a melting point capillary tube. Pour a little of the pure caffeine into a weighing boat. Press the open end of the capillary into the pile of caffeine to get a plug of caffeine about 2 mm into the tube. Hold the tamping tube so that its open end is on the floor or lab bench. Place the capillary, closed end down, into the tamping tube and drop it so that it bounces on the floor or bench. This will tamp the caffeine in tightly so that there are no air pockets. Repeat. Turn on the melting point apparatus. Set the plateau temperature by holding down the 'set' key and pressing the up- arrow key until you reach a temperature about 10 °C lower than the expected melting point of pure caffeine (see Prelab Question 2). Now press the 'start' button. The 'HEATING' light will come on. When the 'PLATEAU' light comes on, you've reached the plateau temperature. You can read the temperature of the block on the display. The heater will hold this temperature until further notice. Place the capillary into one of the holes on its side so that you can clearly see the plug of caffeine through the magnifying viewer. This end of the capillary is now seated in an aluminum block heater. There are two holes so two capillaries can be viewed at the same time. Watch the plug carefully at all times; make sure it hasn't already melted at the plateau temperature. It should look like a white, crystalline solid. Now, press the 'start' button again. The 'RAMPING' light will come on and a very slow increase in temperature will begin. Watch the plug of caffeine continuously. When it melts, you will see a big change in its appearance. Press the 'stop' button to stop the ramp, then press the 'start' button again to reset the plateau temperature for the next trial. Observation B1: Record the actual melting point (m.p.) of pure caffeine: 263°C Procedure C: Filtration, Evaporation and Collection of Crude Caffeine Get your flask with the dichloromethane extract from Procedure A. If all the dichloromethane has evaporated, add 10-20 mL of dichloromethane and re-dissolve the crude product. Weigh an empty 50-mL Erlenmeyer receiving flask. Observation C1: Record the tare weight of this receiving flask: 41.16 g

Place a long-stem funnel into the empty receiving flask. Flute-fold an 11-cm diameter piece of filter paper; your instructor will demonstrate the art of flute-folding. Place the fluted filter paper into the funnel. Pour your dichloromethane extract from Procedure B through the filter paper to filter out the wet sodium sulfate. The filtrate is what drips through the filter paper, into the receiving flask. Once all the liquid has been filtered, rinse the extract flask with about 5 mL more of dichloromethane, then use this rinse to rinse the filter paper into the receiving flask. Remove the funnel and place the flask with the filtrate on a hotplate set to '4'. Evaporate off the dichloromethane. The solid that remains is your crude caffeine product! Determine the gross weight of the flask and caffeine: Observation C2: Gross weight of flask + caffeine: 41.35 g Observation C3: What does the crude caffeine look like? Question C1: Why doesn't the crude caffeine look like the pure caffeine you saw in Procedure B? Now, subtract the tare weight of the flask and record the net weight of the crude caffeine: Question C2: Net weight of caffeine: Question C3: Calculate the % by weight (w/w) of caffeine in the dry tea: % caffeine (w/w) = (mass caffeine / mass tea) %:

Procedure D: Measuring a Melting Point of Crude Caffeine As in Procedure B, measure the m.p. of your crude caffeine. However, this time, set the plateau temperature to 20 °C below the expected m.p. of caffeine. Observation D1: Record the m.p. of your crude caffeine: 231-232 °C Question D1: Compare this mlp to the m.p. you measured in Procedure B. Why is there a difference? How can the map, tell you if a product is pure or not? Question D2: Although it was the extraction solvent of choice at one time, dichloromethane is no longer used to decaffeinate tea and coffee. Instead 'supercritical' or liquefied carbon dioxide is used to extract the caffeine. The CO₂ is pressurized to liquefy it at room temperature, then used to extract the caffeine. The pressure is then removed and the CO₂ is simply allowed to evaporate. What advantages do you think supercritical CO₂ extraction would have over the water/dichloromethane method you used? What could the extracted caffeine be used for?