Column Chromatography Isolation of Caffeine D o u g l a s F. Taber and R. Scott H o e m e r University of Delaware, Newark, DE 19716 Column chromatography is the technique commonly used for routine purification of synthetic intermediates in organic chemistry research laboratories. We report a n undergraduate laboratory procedure using this technique.' In the first semester of the sophomore organic laboratory course, it is common to do an experiment with qualitative thin-layer chromatography. We have for many years used the separation of t h e common analgesics acetominophen, aspirin, phenacetin, and ~ a f f e i n eWe . ~ have now found t h a t chromatographic purification of caffeine, extracted from tea.Vits well with this e x ~ e r i m e n t . T L C of analgesics, column ~ o t procedures h chromatography) can be carried o u t within a single 3-h Iaboratory period, if the T L C determinations are done a t slow periods of the extraction (e.g., while the hot aqueous extract is cooling). We currently take two laboratory periods and include quantitatiue determination of the same "unknown" analgesiE mixture, using high performance liquid chromatograph~.~ Experimental Extraction Distilled water (40 mL) in a 125-mL Erlenmeyer flask containing two tea bags is brought gently to a boil (foams!). After 2 min the heat is removed, and the solution is allowed to cool (may be packed in ice). The cooled aqueous extract is tben extracted with C H ~ C(4 ~Z X 10 mL). The CH4X extracts are combined, dried over NanSO4, and ennce&ted a n t h i rotarv. evaoorator (note: the distillate, and all . other orpmie msldue~.should he retained for proper disposal) st reduced pressure until they just hecumecloudy (5-10mL remains). ~
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concurrent tapping on the side of the column to free air bubbles. After the silicagel is settled, it is covered with another 0.5-cm layer of sand. The residue from the extraction (above) is added to the top of the column. The solvent is allowed to drop to the level of the upper layer of sand. The evaooratine flask is rinsed with an additional 5 mL of CH2CI?,andrhisalwiieddcd tothe ropoithecolumn. Eluant irom thecolumnian~llertedin rest tubes, heldina rack. I t is important w keep these tubes in order. The column is eluted with 5 mL each of 5% 10% 20% and 40% ethyl acetateICHzCln,followed by 5 mL of pure ethyl acetate. After each addition, the level of the solvent in the column is allowed to drop to the level of the upper layer of sand, and the column is switched to a new receivine tube. If elution is too slow.. .. eentle air pressure can be applied to the top of the cdumn. Be sure to remove the air pressure beiore the solvent dmps helow the level uf the sand. The ronrentsofeaeh teat tubeare checked by TLC', with comparison ro an suthentrc sample of caffeine. Several tubes can be sported on the same plate, if care is taken. I t helps to mark the spurting painmlightly with a pencil. If the column has worked pn,perly, early yellow fractions (evaporate a sample, and amell rt!, wrll be folhucd by colorless fractio& tbat contain caffeine. Combine in a tared round-bottom flask the fractions tbat contain caffeine, evaporate the solvent on the rotary evaporator, and weigh the fluffy white residue. About 65 mg of caffeine should be obtained. ~
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Chromatography Commercially available open glass tubes, 19 mm i.d. X 10 em, drawn down at one end to a drip tip, were used (figure).A small wad of glass wool is tamped down in the bottom of the column, tben covered with a 0.5 cm layer of sand. Silica gel (0.7 gm, 60-200 mesh, "slhea . . gel for chromatography") is added in a thin stream: with
VL eluting solvent
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sand
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A very nice procedure for the purification of carvone by simple column chromatography was repotled several years ago: Mitchell. R. H.: West, P. R. J. Chem. Educ. 1974,51, 274. Despite this, isolation of a natural product by preparatlve column chromatography has apparently been considered to be too tedious to include in the first semester of the beginning organic laboratory wurse. Moore, J. A,; Dalrymple, D. L.; Rodig, 0. R. Experlrnentalhfethcds in Organic Chemistry, 3rd ed.; Saunders: Philadelphia. 1982: pp 7289. We have found that 2:9:9 Rbutanollmethyl isobutyl ketonelethyl acetate works well for TLC separation of the analgesics. We use TLC plates having fluorescent indicator, with UV detection of the analgesics. Wilcox. C. F...~ Jr. EXDerImental hoanlc Chernlstrv: A Small-Scale ,~~ ~ ~ p r o a c h ; ~ & n i i i i aNew n : York. t 9 k p 96. We h&e founo that the CH2C19extract irom coffee is too thick to apply to the chromatography column. We have not yet investigated other sources of caffeine (e.g.. soft drinks). 4Kagel, R. A.; Farwell, S. 0. J. Chern. Educ. 1983, 60, 163. (b) Haddad, P.: Hutchins, S.; Tufty, M. J. Chem. Educ. 1983, 60, 166. "he procedure outlined here, suitable for smalldiameter columns. is a modification of the usual aooroach: Taraeit. N. M.: Kilcoyn;. J. P.; Green. 6. J. Org. Chern. 1979,44,4962ind references cited therein.
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silica gel
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Silica gal chromatography. Note that wnh the 1.9-cmi.d. column, it is not necessary to make provisions to stopper Me column at the bonom. Column flow stops when the eluting solvent reaches the upper layer of sand. Volume 68
Number 1 January 1991
73
