Extraction of caffeine from tea

Extraction of Caffeine from Tea
Purpose is to learn some of the basic techniques of organic chemistry: extraction, filtration, evaporation of a solvent and drying methods-in the context of working with a chemical known to all, caffeine. Pure caffeine is a white, tasteless substance that makes up as much as 5% of the weight of tea leaves. By structure, caffeine is closely related to the purine bases, guanine and adenine, found in deoxyribonucleic acids (DNA). A number of plants contain caffeine and its use as a stimulant predates written history. The origins of tea and coffee are lost in legend. In addition to being in tea leaves and coffee beans, caffeine is a natural constituent of kola nuts and cocoa beans. Cola soft drinks contain 14-25 mg of caffeine per 100 mL (3.6 oz), and a sweet chocolate bar weighing 20 g (0.7 oz) contains about 15 mg of caffeine. "Stay awake" preparations such as No Doz have caffeine as a main active ingredient. The caffeine content of tea leaves depends on the variety and where they were grown; most tea has 3-5% by weight. Coffee beans contain only about 2% caffeine by weight, yet a cup of coffee has about 3.5 times as much caffeine as does a cup of tea. How can this be? Coffee is usually boiled in its brewing or else ground extremely fine: tea leaves are simply steeped in hot water for a few minutes Furthermore more ground coffee than tea is used to brew one cup of beverage. A cup of tea contains about 25 mg of caffeine. The biological action of caffeine includes cardiac and respiratory stimulation, and it has a diuretic effect as well. Tea also contains a trace of the alkaloid theophylline, which is similar in structure to caffeine; it stimulates muscle action and relaxes the coronary artery. Theophylline also has veterinary applications as a diuretic and a cardiac stimulant. Obtaining pure caffeine from tea requires a method for separating caffeine from the other substances found in tea leaves. Cellulose, the primary leaf component, poses no problem, because it is virtually insoluble in water. However, a large class of weakly acidic molecules called tannins also dissolve in the hot water used to dissolve the caffeine from tea leaves. Tannins are colored compounds having molecular weights between 500 and 3000 and phenolic groups that make them acidic. If calcium carbonate, a base, is added to tea water, calcium salts of these acids form in the tea solution. The caffeine can then be separated from the alkaline tea solution by a process of extraction using dichloromethane, an organic solvent in which caffeine readily dissolves. The calcium salts of the tannins remain dissolved in the aqueous solution. Flavinoid pigments and chlorophylls also contribute to the color of a tea solution. Although chlorophylls have some solubility in dichloromethane, the other pigments do not. Thus, the dichloromethane extraction of a basic tea solution removes nearly pure caffeine, which has a slight green color from the chlorophyll impurity. After the extraction procedure, the organic solution of dichloromethane and caffeine is dried with an anhydrous inorganic salt. Crude caffeine is recovered as a solid residue by evaporation of the dichloromethane. The solubility of caffeine in water at 20°C is 2.2 g per 100 mL, so there is no problem in keeping it in water solution while you filter off the spent tea leaves and calcium salts. Caffeine is far more soluble in dichloromethane: 10.2 g per 100 mL at 20% So this extraction takes advantage of distribution coefficient (k) of 4.6. To Conserve dichloromethane and time, we will settle for two 15 mL extractions of the aqueous tea solution. This method does not extract all the caffeine but yields more than enough for the purification step. The 10 g of tea that you boil with water should contain at least 300 mg of caffeine. You will be able to recover 10-30% of this amount. A comment about filtering the boiled tea solution should be made before you begin. If it is filtered when it is too hot, messy bubbling occurs in the filtrate and some solution may be lost. Yet if it is filtered when it is too cool, the gelatinous material that separates on cooling will clog the pores of the filter paper. Fast, non retentive filter papers such as Schleicher and Schuell (S&S) No. 410 and Whatman No. 54 work well. Safety:
Dichloromethane is toxic, an irritant, absorbed through the skin, and harmful if
swallowed or inhaled. Use it in a well ventilated hood. Wear gloves and wash
your hands thoroughly after handling it.
Solid caffeine is toxic and an irritant. Avoid contact with skin, eyes, and clothing
Place approximately 10 g of tea leaves in a 400-mL weighed (tared) beaker; record the mass of the tea leaves. If you use teabags, four bags should contain about 10 g of tea; remove the tea leaves from the bags and place the tea in the beaker. Add 4.8 g of. Calcium carbonate and pour 100 mL of water over the tea. Boil the mixture gently on a hot plate for 15 min, stirring every minute or two with a stirring rod. Let the tea mixture cool to about 55oC, then filter it, using vacuum filtration through S&S No. 410 or Whatman Filter paper. Pour the tea mixture in the Buchner funnel in two portions. If the filter paper clogs while the first portion is filtering, replace it with a fresh piece before filtering the remainder of the tea mixture. Cool the filtered solution to 15-20°C by adding a few ice chips. Set up a 125-mL separatory funnel and pour the cooled tea solution into the separatory funnel (be sure the stopcock is closed). Add 15 mL of dichloromethane to the funnel. Stopper the separatory funnel, hold the stopper firmly in place with your index finger, and invert the funnel. Open the stopcock to vent the vapors. Rotate the inverted funnel for 2-3 min, so that the two layers swirl together many times, opening the stopcock frequently to vent the funnel. Allow the layers to separate and then drain the dichloromethane layer into a 50- mL Erlenmeyer flask. If a small emulsion layer is present at the interface between the organic and aqueous phases, add it to the Erlenmeyer flask. Cork the Erlenmeyer flask to prevent evaporation of the dichloromethane. Add 15 mL of fresh dichloromethane to the separatory funnel (still containing the tea solution) and repeat the extraction process. Again, allow the layers to separate and drain the dichloromethane layer, including any emulsion layer, into the Erlenmeyer flask containing the dichloromethane solution from the first extraction. Pour the tea solution out of the top of the separatory funnel into a beaker. Rinse the separatory funnel with water before pouring the combined dichloromethane solutions into the funnel; add about 20mL of water. Stopper the funnel, invert and rock it gently to mix the two layers. Some emulsion layer may be present at this point. If only a thin layer of emulsion exists at the interface between the aqueous phase and the dichloromethane solution, push a small piece of glass wool to the bottom of the dichloromethane layer with a large stirring rod. The glass wool will break the membranes of the emulsion. Drain the lower dichloromethane layer slowly into a clean, dry 50-mL Erlenmeyer flask. Add anhydrous magnesium sulfate to the dichloromethane solution. Cork the flask and allow the mixture to stand for at least 10 min, swirling the flask occasionally. Weigh (tare) a dry 50-mL Erlenmeyer flask on a balance that measures to 0.001 g. Place a fluted filter paper in a dry conical funnel and filter the drying agent from the dichloromethane solution collecting the filtrate in a tared 50 mL Erlenmeyer flask. Rinse the magnesium sulfate remaining in the flask with approximately 2 mL of dichloromethane and also pour this rinse through the funnel. Add a boiling stick or boiling chip to the flask containing the dichloromethane solution so that it boils without bumping Evaporate the dichloromethane on a steam bath or water bath heated on a hot plate in a hood. Alternatively, the dichloromethane may be removed by
evaporation, using a stream of nitrogen, in a hood, or with a rotary evaporator.
Continue the evaporation until a dry greenish residue of crude caffeine forms on the
bottom of the flask. Weigh the flask and determine the mass of crude caffeine.
Calculate the percent recovery. Cork the flask and store it in your laboratory drawer
for purification and analysis.
Place the tea leaves in the non-hazardous solid waste container. Wash the tea solution remaining from the initial extractions and the water remaining in the 125-mL separatory funnel down the sink. Allow the flask containing the magnesium sulfate drying agent to dry in a hood before putting the spent drying agent in the inorganic waste container.

Source: http://www.polaris.nova.edu/~shanbhag/chemistry/oc1labs/caffeine.pdf

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