The purpose of this experiment was to perform a liquid-liquid extraction method to extract the caffeine from the tea bags that were provided, and then recrystallize the caffeine. The solvents used in the experiment were an aqueous sodium carbonate and dichloromethane (DCM). Anhydrous calcium chloride pellets were used to dry the solution and emulsion layer and the DCM was then decanted. After washing the anhydrous calcium chloride pellets with more DCM, the solvent was evaporated, leaving greenish-white crystalline caffeine residue weighing about .25 mg.
In order to recrystallize the caffeine, we used a mixed-solvent method, consisting of hot acetone and hexanes. The solution was cooled and a vacuum filtration was done to remove the caffeine crystals. The final product weighed about 3 mg. Introduction
Caffeine is an organic compound that is found in tea leaves and coffee beans. It is a basic substance (due to the nitrogen atoms in its structure) and it appears as a white crystalline solid at room temperature. In this experiment, we aimed to extract caffeine from the tea leaves in the tea bags provided beginning with a solid-liquid extraction method and then a liquid-liquid extraction. Extraction techniques are used to isolate and remove particular compounds form another substance. For both solid-liquid and liquid-liquid extraction techniques, solvents should be chosen by their miscibility in water (should be immiscible), they should have relatively low boiling points for faster and easier extraction, and they should be unreactive with the other substances being used in the experiment. In this experiment, a solid-liquid extraction method was used first to extract the caffeine from the tea leaves/tea bags to by dissolving sodium carbonate in hot water and creating an aqueous sodium carbonate solvent.
Sodium carbonate and hot water were added to the tea bags and was let to stand for about 7 minutes in order to bring the caffeine molecules out of the tea bags and into the aqueous solution. Because caffeine is water-soluble and is a base, sodium carbonate must be added to the hot water to prevent it from reacting with the acids present and to ensure it remains a base. A liquid-liquid extraction was then performed to extract the caffeine from the mixture by adding dichloromethane. In a liquid-liquid extraction, two layers are needed- an organic layer and an aqueous layer- that are both immiscible in water. Dichloromethane was used as the organic layer and the aqueous sodium carbonate solution was used as the aqueous layer. Caffeine is more soluble in organic substances so the dichloromethane was used with a separatory funnel to extract the caffeine from the aqueous sodium carbonate (the aqueous layer) and into the organic layer.
The remaining organic layer that included the caffeine was dried using anhydrous calcium chloride pellets since they are neutral and unreactive and would not disrupt any further reactions. It was then distilled, and then the remaining dichloromethane was evaporated, leaving crude, greenish-white crystalline caffeine. The most common method for purifying solid compounds is from recrystallization. Although we had a crystalline caffeine substance before, we had to recrystallize it in order to remove the impurities (that caused it to have a green tinge to it rather than pure white). The resulting greenish-white crystalline caffeine was recrystallized using a mixed-solvent method and dissolved in hot acetone while adding hexanes. Hexanes had to be used because of caffeine’s high solubility in acetone compared to hexane. A vacuum funnel using a Buchner funnel was used to remove the liquid and impurities and the remaining crystals were washed and transferred using a few drops of hexanes. Chemical Structures and Calculations
Percent Yield = Actual Yield / Theoretical Yield x 100
= .03 g / .25 g x 100 = 12 %
Procedure
1.) 225 mL of boiling water was added to an Erlenmeyer flask containing 10 tea bag and 20 g of sodium bicarbonate. The solution was decanted into a separate Erlenmeyer flask after it soaked for about 8 minutes. An additional 50 mL of hot water was added to the Erlenmeyer flask with the remaining tea bags and was then immediately decanted and added to the first extracted solution. Total water used = 275 mL. 2.) The aqueous sodium carbonate solution was cooled and we extracted it twice with separate 30-mL portions of dichloromethane into a separatory funnel. We rocked the separatory funnel several times and then extracted the dichloromethane from the funnel into a beaker, excluding the emulsion layer that had formed. 3.) We extracted some more dichloromethane into the same beaker, but included the emulsion layer and added anhydrous calcium chloride pellets to dry the solution and emulsion layer. We added several spoon fulls until the anhydrous calcium chloride pellets stopped clumping together.
4.) The dichloromethane solution was then filtered into a clean Erlenmeyer flask using filter paper and a Hirsch funnel. 5.) The anhydrous calcium chloride was then washed with dichloromethane and then placed on a hot plate to evaporate it. A wood stick served as a boiling stick to prevent superheating. 6.) A greenish-white residue was left over, coming out to weigh .25 g 7.) We added hexane to the left over residue and then dissolved the greenish-white caffeine residue in 5 mL of hot acetone the solution was a cloudy white. 8.) After we let the solution cool, we vacuum filtered it, using a small Buchner funnel. We added some additional hexane to help transfer and wash the crystals through the vacuum filtration. 9.) We did not repeat the process.
10.) Final weight was 30 mg
Data & Results
Once we conducted the solid-liquid extraction and liquid-liquid extraction, we weighed the resulting greenish-white caffeine crystals and recorded .25 g (this included the impurities). After we recrystallized it with the acetone and hexane to remove the impurities, the final weight was .03 g.
Discussion
We were successful in extracting caffeine from the tea bags, but based on the percent yield, we were not successful in extracting a large amount of caffeine. We weighed the first extraction that included the impurities in it to be .25 g and the final extraction without the impurities to be .03 g resulting in a low percent yield of 12%. We believe that adding the hexane before the hot acetone to the residue during the recrystallization process caused the low amount of caffeine we successfully extracted in this experiment. We did not think it would make much of a difference at the time, but in hindsight, realize that this messes up the mixed-solvent method that recrystallizes the caffeine, thus not getting a higher percent yield because not all of the caffeine precipitated out.
The purity of our extract should not have been affected because we vacuum filtered the cool solution that contained the crystals twice. We were also able to remove the impurities by adding the dichloromethane to the aqueous solution in a separatory funnel, which created two different colored layers, a clear organic layer, which included the caffeine, and a brown aqueous layer that contained the impurities. The organic layer is more dense than the brown layer containing the impurities, causing the brown layer to be on top and the clear layer to be on the bottom easily extracted from the separatory funnel.
