Caffeine is a white crystalline xanthine alkaloid that acts as a stimulant drug. Caffeine is found in varying quantities in the seeds, leaves, and fruit of some plants. It is most commonly consumed by humans in infusions extracted from the seed of the coffee plant and the leaves of the tea bush, as well as from various foods and drinks containing products derived from the kola nut. The water soluble materials in the tea leaves are extracted into hot water. The hot solution is allowed to cool and the caffeine is then extracted from the water with dichloromethane (methylene chloride), which is an organic solvent that is insoluble in water. Since caffeine is more soluble in dichloromethane than it is in water, it readily dissolves in the dichloromethane.
This experiment is aims to isolate crude caffeine from tea leaves. The purity of caffeine is determined through melting point determination, the comparison of melting points of an unknown substance and a pure substance, and the comparison of physical characteristics.
Extraction is a chemical method of separating a specific component of a solution from the rest of the solution. This is done by using a solvent in which the substance to be isolated is very soluble, while the rest of the solution is not as soluble.
Tea leaves consist mostly of cellulose, a waterinsoluble polymer of glucose, which is a monosaccharide. Cellulose performs a function in plants similar to that of fibrous proteins in animals: it is structure building material. Along with the cellulose are found a number of other things including caffeine, tannins (phenolic compounds, compounds that have an OH directly bonded to an aromatic ring) and a small amount of chlorophyll.
The group was tasked to take 5 bags of commercially sold tea (caffeinated) and to boil for 5 minutes in water to extract the water soluble materials in the tea leaves into hot water which in this case would be caffeine. Using a separatory funnel, the organic layer was isolated and then evaporated to get crude caffeine. Crude caffeine was purified by recuptallization and then tested for purity with the melting point as a criterion.
Through the experiment, the group should be able to accomplish the following: (1) Isolate and purify caffeine from tea leaves, (2) characterize the purity of the isolated caffeine through a comparison of melting points with standard caffeine and (3) determine the percentage yield of caffeine in the tea leaves.
5 bags of tea are opened and weighed using the analytical balance. The leaves are then returned to the bags and boiled for 5 minutes in 100 mL of distilled water in a 250 mL beaker with a cover (a watch glass may do). After boiling it is then cooled in running tap water and an ice cube is added to the boiled solution to cool it down. The solution is then transferred to a separatory funnel then 20 mL of dichloromethane is added. The funnel is gently shaken several times and is opened once in a while to release pressure. The organic layer of CH2Cl2 being more dense than water is the lower layer. It is collected in a clean and dry Erlenmeyer flask. 20 mL of CH2Cl2 is added again and the process is repeated until 60 mL of CH2Cl2 has been added and the 3rd organic layer is taken and combined in the Erlenmeyer flask.
The combined organic layer in the Erlenmeyer flask is again transferred to in a separatory funnel and washed with 20 mL of 6M NaOH. The funnel is shaken vigorously until the 2 layers are clearly separated. The upper layer (NaOH) is discarded and the lower layer (organic layer) is collected in a 100 mL beaker. The solution is dried with 1-2 spatulas of anhydrous Na2SO4 to remove excess water. It is then filtered through a funnel with cotton and collected in an evaporating dish covered with aluminum foil with holes. The solution is left to evaporate to dryness. The dried solution is then termed as crude caffeine.
The crude caffeine is collected and purified through recuptallization. The collected crude caffeine is washed with ethanol and heated in a water bath until the caffeine has completely dissolved in the ethanol. The beaker containing the dissolved caffeine is then put in an ice bath to cool and crystallize. It is then filtered through a funnel with a fluted filter paper. The filter paper is dried and then weighed on the analytical balance for the weight of the pure caffeine.
2 capillary tubes are prepared. One end of both tubes are heated to close. Capillary tube 1 (C1) is filled with standard caffeine 0.5 cm in length and capillary tube 2 (C2) is filled with a 2:1 ratio mixture of standard and pure caffeine. The caffeine in both tubes should be tightly packed to avoid air within the powder that may cause an error when melting point determination is being executed.
Both tubes labeled C1 and C2 are tied to a thermometer with a rubber band. The thermometer is then clamped by the upper end onto an iron stand and is submerged in a 100 mL beaker with cooking oil. The beaker is suspended via a tripod over a Bunsen burner. The oil is heated until the caffeine within both tubes is completely melted.
The initial temperature when caffeine in the tubes start to melt is recorded as T1 and the final temperature wherein the caffeine is completely melted is recorded as T2. T1-T2 is the range of the melting point of the substances.
The melting points of the substances in C1 and C2 are compared. If they are the same or are almost the same then the unknown compound or the compound being tested for purity is confirmed to be purely of the same species.
Percentage yield is computed through the following formula for computation:
% Yield of caffeine in tea leaves = Weight of pure caffeine in gramsWeight of tea leaves in grams x 100
III. Results and Discussion
Sample: 5 bags of Lipton Yellow Label Tea
Table 1. Data Collected from Extraction, Purification, and Melting Point Determination of Caffeine from Tea Leaves
Table 1 shows the data recorded during the experiment. It includes the weight of the tea leaves in grams, over-all volume of used CH2Cl2, physical characteristics of crude caffeine and pure caffeine, weight of the pure caffeine in grams, and the melting point of pure and standard caffeine.
It can be observed in the table that crude and pure caffeine share similar physical characteristics. When both are also compared to standard caffeine provided in the laboratory, similarities can also be perceived. This can add to the evidence of purity of the pure caffeine.
To further aid in the testing of purity of the caffeine melting points of the pure caffeine and standard caffeine were compared. The results show that the temperature range of the melting point of both substances were almost alike, only that standard caffeine is fully melted at a temperature 1 degree Celsius higher than that of when the mixture of pure and standard caffeine was fully dissolved.
Through the experiment, caffeine was isolated from commercially sold tea in bags through boiling it in water. The caffeine in tea leaves is highly soluble in water thus it is boiled to be isolated.
Crude caffeines characteristics were highly similar to that of standard caffeine. After purification it was then dried and was termed as pure caffeine. The pure caffeine also showed the same physical characteristics. To further confirm the purity of the caffeine, melting points of both the pure caffeine and standard caffeine were compared. Results have shown that the melting points of both substances were almost the same; only varying by 1 degree Celsius in the T2 of the standard caffeine.
It is therefore concluded that the caffeine extracted from the tea leaves and purified through recuptallization is pure due to the similarity in physical characteristics and melting point range and a percentage yield of 0.61899% of caffeine was computed.
Bayquen, A.V., Cruz, C.T., et al (2009).Laboratory Manual in Organic Chemistry. Quezon City: C&E Publishing Inc. Experiment 6: Isolation of caffeine from tea leaves. (n.d.). Retrieved from http://employees.oneonta.edu/knauerbr/chem226/226expts/226_expt06_pro.pdf Mann, FG & Saunders, BC (1960) Practical Organic Chemistry (4th ed), Longman, London, p.387 Selinger, B (1978) Chemistry in the Market Place (2nd ed), John Murray (Publishers) Ltd, London p415-7