Extraction of Caffeine from Tea
The purpose of this activity is to learn about liquid-liquid extractions, and how to extract caffeine from tea.
No formal lab report is required.
Review the PROCEDURE (below) and answer each question. Submit the answers in Word .doc or .rtf format. Please number your answers to correspond with the questions. Copy each question to your answer sheet.
Include the following at the top of your report:
Title of Exp.
Frequently, organic chemists must separate an
organic compound froma mixture of compounds, often derived from natural sources
or as products of synthetic reactions. One technique used to separate the
mixture compounds is called extraction. Extraction is a process that
selectively dissolves one or more of the mixture compounds into an appropriate
solvent. The solution of these dissolved compounds is often referred to as the extract.
(See Figure 1 below).
Extraction processes include removal of
soluble compounds from a solid matrix, such as occurs in brewing coffee or tea
or in decaffeinating coffee with liquid carbon dioxide. In the organic chemistry
laboratory, however, extraction almost always refers to the transfer of
compounds from one liquid solvent to another liquid solvent.
Washing is the reverse process, in which the
impurities are removed to the second solvent, leaving the desired compound in
the original solvent, as shown in Figure 2.
Extraction is a particularly effective means of separating organic compounds if one compound in the mixture can be chemically converted to an ionic form. The ionic form is soluble in an aqueous layer and can be extracted into it. Other non-ionized organic compounds in the mixture will remain dissolved in the nonpolar solvent layer. Separation of the two layers results in the separation of the dissolved compounds.
A separatory funnel is used to perform liquid-liquid extractions.
Ionic forms of some organic compounds can be produced by reacting them with aqueous acids or bases. Reacting organic acids with bases such as sodium hydroxide (NaOH) converts these acids to water-soluble anions (see structure below). Reacting basic amines with dilute aqueous acid solutions such as hydrochloric acid (HCI) converts the amines to water-soluble cations.
Acid reacting with base to form a salt. Salts are ionic and soluble in water.
Once an organic acid (or base) has been converted to an ionic form, like the carboxylate salt of a carboxylic acid, its solubility in in water is greatly increased and its solubility in a hydrocarbon solvent is decreased. For this reason, acids can often be separated from hydrocarbon contaminates by dissolving the mixture in a hydrocarbon solvent such as cyclohexane and extracting the acid component into an aqueous layer of dilute sodium hydroxide. The hydroxide converts the acid to its carboxylate salt which is more soluble in the aqueous layer than the cyclohexane layer. The dissolved hydrocarbon is unaffected by the hydroxide and remains in the cyclohexane layer.
Tea and coffee have been popular beverages for centuries, primarily because they contain the stimulant caffeine. It stimulates respiration, the heart, and the central nervous system and is a diuretic (promotes urination). It can cause nervousness and insomnia and, like many drugs, can be addicting, making it difficult to reduce the daily dose. A regular coffee drinker who consumes as little as four cups per day can experience headache, insomnia, and even nausea upon withdrawal of the drug.
Caffeine may be the most widely abused drug in the United States. During the course of a day, an average person may unwittingly consume up to a gram of this substance. The caffeine content of some common foods and drugs is given in the table below.
Caffeine belongs to a large class of compounds known as alkaloids. These are of plant origin, contain basic nitrogen, often have a bitter taste and complex structure, and usually have physiological activity. Their names usually end in -ine, many are quite familiar by name if not chemical structure-nicotine, cocaine, morphine, strychnine. In general, their function within the plant is obscure. Tea leaves contain tannins, which are acidic, as well as a number of colored compounds and a small amount of un-decomposed chlorophyll (soluble in dichloromethane).
The solubility of caffeine in water is 2.2 mg/mL at 25°C, 180 mg/mL at 80°C, and 670 mg/mL at 100°C. It is quite soluble in dichloromethane, the solvent used in this experiment to extract the caffeine from water.
Caffeine can be extracted easily from tea bags. The procedure one would use to make a cup of tea, simply "steeping" the tea with very hot water for about 7 min, extracts most of the caffeine. Since caffeine is a white, slightly bitter, odorless, crystalline solid, it is obvious that water extracts more than just caffeine. When the brown aqueous solution is subsequently extracted with dichloromethane, primarily caffeine dissolves in the organic solvent, leaving the other substrates in the aqueous layer. Evaporation of the solvent leaves crude caffeine, which on sublimation yields a relatively pure product.
Here is the overall procedure used to extract caffeine from tea leaves:
Tea leaves steeped in hot water.
Sublimation of Crude Caffeine Extract
The crude caffeine extract contains green chlorophyll and other impurities. Sublimation is a method of purifying the extract. In this drawing, the crude caffeine extract is placed in the bottom of the 25-mL Filter flask and heated. The 15-mL centrifuge tube is filled with ice to provide a cool surface. The sublimated caffeine will deposit (sublimate) on the outer surface of the cool centrifuge tube.
The caffeine isolated from tea leaves can be purified by sublimation.Caffeine melts and sublimes at 2380C. In addition to finding the melting point of pure caffeine, we can react caffeine with salicylic acid to form a salt. The salt is an ionic compound and has a sharp distinct melting point. We call the salicylate a "derivative" of the original compound, caffeine. Preparing derivatives of organic compounds is a common method of identifying compounds.
Caffeine is an organic base and can therefore accept a proton from an acid to form a salt. Reaction 1 (below) shows the formation of the caffeine salicylate derivative.
A. Read and review concepts in this handout.
B. Access Extraction of Tea experiment Simulation. Click here. Or type following URL address into your browser.
C. Read THEORY section of Simulation.
D. Read PROCEDURE section of Simulation.
E. Run Simulation under the SIMULATOR tab. Record observations and data.
F. Answer the following questions in your own words (no copy and pasting) for your report. Submit the answers in Word .doc or .rtf format. Please number your answers to correspond with the questions. Copy each question to your answer sheet.
Note - questions asking for explanation should be answered in detailed complete sentences.
1. The extraction process selectively dissolves one or more of the mixture compounds into an appropriate solvent. The solution of these dissolved compounds is referred as the ______________.
2. What organic solvent is used to extract the caffeine in this simulated experiment?
3. What type of funnel is used to perform liquid-liquid extractions in this experiment?
4. Why is extraction a particularly effective means of separating organic compounds if one compound in the mixture can be chemically converted to an ionic form? Explain.
5. What is the solubility of caffeine in dichloromethane?
6. How many milligrams of caffeine will dissolve in 20 mL of water at 80oC? Show Calculations.
7. What is the purpose of adding Na2CO3 to the tea solution? Explain.
8. What is produced when an organic acid reacts with a base?
9. What is produced when an organic base (amine) reacts with an acid?
10. What are the major chemical components of tea?
11. How would reacting acetic acid with NaOH affect its solubility in water?
12. In the simulated extraction procedure, why is dichloromethane the bottom layer after separating from the water? Explain.
13. List 4 physical properties of caffeine.
14. What is an alkaloid? Explain.
15. What is sublimation, and how is it used in this experiment? Explain.
16. What is the purpose of filling the centrifuge tube with ice in the sublimation procedure?
17. What are the good and bad physiological properties of caffeine? Explain.
18. What is the IUPAC (chemical name) for caffeine?
19. What is an organic derivative? Explain.
20. Why does the crude caffeine residue appear green or yellow?
21. What is the minimum and maximum amounts of caffeine (in mg) in an average 5-ounce cup of tea?
22. Which has a greater density, dichloromethane or water?
23. What is the role of sodium carbonate in extracting caffeine?
24. What is the sublimation temperature of caffeine?
25. Identify (list) all the equipment, glassware, and reagents used in the Simulated experiment.
26. How many tea bags were used in the experiment?
27. What is the color of the water solution after the tea has been squeezed out of the tea bags?
28. What volume of tea solution (after heating the tea bags in water) was transferred to the empty separatory funnel?
29. How many layers is observed after adding the dichloromethane to the separatory funnel containing the tea solution? Identify each layer.
30. What is the purpose of shaking the separatory funnel containing the dichloromethane and tea solution?
31. What is the purpose of venting the separatory funnel periodically during the shaking process?
32. Additional volumes of dichloromethane are added to the separatory funnel. What is the final total volume of dichloromethane extract?
33. What is the purpose of adding sodium sulfate to the dichloromethane layer after extraction in the separatory funnel?
34. What is the purpose of boiling the flask containing the dichloromethane after addition of sodium sulfate and decantation?
35. What is the mass of the clean empty watch glass?
36. What is the mass of the watch glass and caffeine?
37. What is the mass of caffeine recovered in the experiment?
38. What is the molecular formula for caffeine?
39. What is the molecular formula for salicylic acid?
40. What is the molecular formula for caffeine salicylate?
41. What is the formula mass for caffeine?
42. What is the formula mass for salicylic acid?
43. What is the formula mass for caffeine salicylate?
44. Write the balanced reaction (using molecular formulas) for the reaction between caffeine and salicylic acid to form caffeine salicylate.
45. Suppose the mass of caffeine recorded in step 37 is used to prepare the caffeine salicylate derivative. What is the theoretical yield of caffeine salicylate that can be made from the mass of purified caffeine. Show ALL steps in this calculation.
46. Suppose 8.2 mg of caffeine salicylate is recovered after reacting caffeine with salicylic acid. What is the percent recovery of the caffeine salicylate? Show ALL steps in this calculation.
47. What is the melting point of caffeine salicylate? How does it compare with the melting point of caffeine?
48. What would be the reason for preparing the caffeine salicyate derivative of caffeine?
49. Which step in the procedure for isolating caffeine could lead to the most error in recovering pure caffeine? Why?
50. What did you like best about this experiment?
Copyright © 2007 M.W. McClure, All Rights Reserved
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