An Old Undergraduate Experiment Revisited Scott D. Murray and Peter J. an sen' Northwestern College, Orange City, IA 51041
For a t least three decades a common undergraduate or-
chemistm.exneriment has consisted of the extraction -ofeanic . caffeine from tea leaves. The caffeine is extracted from
the tea leaves using hot or boiling water, extracted from this aqueous solution using a n immiscible organic solvent, isolated in crude form bv e v a ~ o r a t i n eoff the organic solvent, and purlfied by rccl~staliwatlon'oriuhllma;mn During the 60's and 70's the oraanlc solvent of choice was chloroform (131, while more recent organic laboratory manuals have specified the use of diebloromethane (4-7). Both chloroform and dichloromethane generally are recognized a s having significant toxicity. Chloroform is a "confirmed carcinogen" and dichloromethane is a "suspected buman carcinogen" (8).A recent article reported increasing concerns about "chlorinated organics a s a class and human health" (9).These potential hazards prompted the authors to seek a more suitable organic solvent for use in this experiment. The initial set of criteria used in selecting a n alternative organic solvent included: 1. immiscibility with water; 2. low toxicity; 3. low cost;
4. high stability (especially with respect to peroxide formation); and 5. appreciable caffeine solubility.
The reasons for the first four are apparent, while the last is necessary for a favorable caffeine partition coefficient between the organic solvent and water. Unfortunately, we were unable to identify a common organic solvent that met all five of these criteria. Leggett, e t al., however, reported that the salting-out effect can be employed to reduce dramatically the solubility in water of some normally water-
' Author to whom correspondence should be addressed,
miscible organic solvents (10). This suggested to us that the first criterion above could be relaxed. Employing the salting-out effect, a number of wmmon organic solvents were tested and of these l-propanol appeared to be the most suitable. In comparison to either chloroform or dichloromethane, l-propanol is less toxic (8) and less expensive (by a factor of two). In addition, it is stable under normal storage wnditions and readily dissolves caffeine. Experimental Pour 200 mL of boiling water into a beaker containing 10-12 tea bags and allow the beaker to stand for 10 min. Decant the "tea" from the tea bags and compress the tea bags to maximize solvent recovery. Return the tea bags to the original beaker, and repeat this extraction twice using 50-mL portions of boiling water. Combine the aqueous extracts, cool, add sodium chloride (26 g1100 mL of "tea") and calcium hydroxide (-1 g), the latter to precipitate tannins. Filter this solution using Celite, fast filter paper, and vacuum filtration. (This filtration may be omitted without seriously affecting one's results, but the unfiltered extract is prone to emulsion formation during the next step.) Transfer the filtrate to a separatory funnel and extract with three successive portions of l-propanol(45,35, and 35 mL). Combine the l-propanol extracts in a flask and evaporate off the solvent (-80% l-propanol and -20% water) using rotary evaporr~tionor distillation. Rmsc the reeldue in tht: flask (mostly sodlurn chloride, with two successive 10-mL portions of acetone to extract the crude caffeine. Filter this acetone extract and carefully boil off the acetone. The crude caffeine obtained can be purified by re nystallization (dissolvein 23 mL toluene and add hexane to effect crystallization) or by sublimation (5-7).
Volume 72 Number 9 September 1995
851
Discussion The ~urifiedcaffeine obtained in this ex~erimentwas identified using IR spectroscopy. The sodium chloride concentration specified above is significantly less than that required to saturate the aqueous solution. If the aaueous laver is saturated with sodium chloride, crystallizkion of iportion of the sodium chloride will occur upon addition of the l-propanol. Analysis of the two layers of a waterll-propanollsodium chloride ternary system with a composition similar to that used in this experiment yielded the results given in the table. Composition of the Waterll-PropanollSodium Chloride Ternary System Aqueous Layer (%) 1-PropanolLayer (%) water 1-propano1 sodium chloride
74
19
6
80
20
1
The partition coefficient of caffeine between the l-propanol and aqueous phases was determined to be approximately 3.7-probably lower than the corresponding values for the chlorofodwater and dichloromethanelwater systems. (Using the reported solubilities of caffeine in chloroform and in water (11),a rough estimate of 8.3 was calculated for the partition wefficient of caffeine between these two solvents. For the dichloromethane/water system a value was not established, but one might expect a value similar to that for the chlorofondwater system.) Nonetheless, given the wnditions of this
852
Journal of Chemical Education
experiment it is estimated that approximately 80% of the caffeine in the tea leaves is recovered as crude caffeine. Using a larger quantity of l-propanol or a smaller quantity of water would obviously increase the percentage of caffeine extracted from the aqueous layer, but the latter would reduce the percentage of caffeine extracted from the tea leaves by the water since the tea leaves retain a significant quantity of water. The procedures employed in this experiment do not require dry l-propanol. The l-propanollwater solvent mixture that is recovered by rotary evaporation or distillation can be saved and used repeatedly. This recycling reduces both waste generation and cost. If recycled l-propanol is used in the procedure described above, the amounts should be increased by about 25% to account for the dissolved water present in this solvent. Literature Cited 1. Helmkamp, G.K; Johnson. H. W. Sdeead Experiments in O ~ e n i Chemistry, c 2nd ed.; Freeman: Ssn Francisco, 1968; pp 157-158. 2. O'Connor, R. In The Fmman Library ofLahrofory Separates in Chpmuiv; B i d whistell, R. K.; O'Connor. R. Eds.; Freeman: San Francisco, 1971; Vol.2. 3. Pavia, D. L.:Lmpman, G. M.; Kriz. G.S. lntmduction to O~ganieLabornfo~ rich. niquas; Saunders:Philadelphia, 1976: pp 5&62. 4. Williamson, K L. Macrnscokand Micrnscok OlgonicErpprimnls; Heath: Tomno, PP 130-133. 1989 . 5. Mayo, D. W; Pike, R. M.: Butcher, S. S. Microsmle 01gonic Lobornmy, 2nd ed.: Wile": New York. 1989: oo 162-164. 6. Nimitz, J. S. Erpen'ments in Organic Chmzisfv: PrenticrHall: Englewood Cliffs, NJ. 1991: pp 61-62. 7. Landgrebe, J. A. Theory and Proelice in iha Organic Laboratory. 4th ed.: BmokYCole: Pacific Grove, CA. 1993; pp 381383. 8. Lewis, R. J.Hazardous ChemicalzDesk Refeenee, 2nd ed.:Ven Noatrand Reinhold: NewYork. 1991. ~
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