Background: Caffeine is a natural product belonging to a group of compounds called alkaloids. It is found in tea (30 to 75 mg per cup), Coca-Cola (46 mg per 12 oz) and coffee (80-125 mg per cup). No-Doz contains 100 mg per tablet. It stimulates respiration, the heart and the central nervous system, is a smooth muscle relaxant and a diuretic. It is also addicting.
Hot water swells the tea leaves to release caffeine and other compounds such as tannins. Dichloromethane, CH2Cl2, is used to selectively extract the caffeine from the water which retains most of the other organic compounds. After the CH2Cl2 solution is separated, it is passed through some solid Na2SO4 to remove traces of water. The CH2Cl2 is removed by evaporation to give crude caffeine. Finally the caffeine is purified by recrystallization with ethyl acetate.
Preparation: Read Mayo, et al., p.113-118 and 122-124. Look up and record the structure of caffeine which is in the Handbook in the list of organic compounds; the structure will be on a separate page, with the number referenced to the initial entry.
To create as concentrated sample of tea as possible, boil a solution of 0.3g Na2CO3 in 8 mL of deionized water in a 50 mL beaker. Remove the beaker form the hot plate and add a teabag (which contains about 2 g of tea); virtually all the water will be soaked up by the tea. Use a 30 mL beaker as a plunger to release the tea into the aqueous solution. Allow it to extract the tea for 3 min. then press the teabag while pouring the brown liquid into a 16x125 mm test tube, releasing as much liquid from the bag as convenient. Return the beaker to the hot plate and add 4 mL of water, warm and repeat the release of tea and add this to the centrifuge tube. Repeat the extraction once again with 4 more mL of water.
Cool the centrifuge tube containing the concentrated tea solution for 2-3 min. in ice water, and then add 1 mL of CH2Cl2 (note 1). Stopper the tube with a cork and shake the mixture vigorously for 15 sec.; remove the stopper. Make a counterbalance by filling another centrifuge tube with water to the within 0.2 g as the first (or counterbalance with another student's tube, adding water dropwise to the lighter tube), place the tubes in opposite positions in the centrifuge which is in the hood, behind a safety shield. Lower the hood window and start the centrifuge by plugging it in (2 min at 70% setting). Prepare a pipet with a cotton plug as in fig 3.30b and pour about 1 cm of Na2SO4 above the plug (this is a drying agent - see Mayo, et al. p. 123). Clamp the pipet vertically above the 25 mL (clean and preweighed to the nearest milligram) Erlenmeyer. Hold the centrifuge tube close to the top of the pipet and transfer the CH2Cl2 layer (note 2) from the centrifuge tube (is it the upper or lower?) into the pipet containing Na2SO4 so that it drains directly into the beaker. Cool the tea in the centrifuge tube again, add another 1 mL of CH2Cl2, re-extract, centrifuge, and transfer the CH2Cl2 layer to the beaker via the pipet filter. Repeat with a third mL of CH2Cl2.
Insert a thin wooden boiling stick (splint) into the beaker, and warm it on the steam bath to evaporate the CH2Cl2. Remove the stick and weigh the pale yellow solid, crude caffeine.
The caffeine is now purified by sublimation (note 3). A 10 x 75 mm test tube is fitted with a 2-3 mm thick section of a thin walled rubber hose. The rubber band is slid up the tube so that when the it is placed inside the 25 mL Erlenmeyer it is suspended 1-2 mm above the bottom of the Erlenmeyer. Remove the tube momentarily and place Ice water inside it; make sure the outside of the tube is dry and set it back into the Erlenmeyer. The assembly is now buried into a sand-bath heated to 120 oC (no stirring is necessary). Periodically replace the tube's water with ice water - but do not spill any water into the flask. If sublimation - the formation of white crystals on the outside of the tube - does not begin within 5 minutes, increase the temperature of the sand. When all the pure caffeine has sublimed, remove the assembly from the sand bath.
Submit the purified in a plastic bag. To transfer the caffeine, set a preweighed (to the nearest mg) bag into a 400 or 600 mL beaker so that it remains open and able to receive the scraped caffeine from the test tube. Remove the test tube and scrape it thoroughly. Use a melting point capillary to scrape just enough - either from the tube or the bag - for a melting point determination (next step). Then reweigh the bag before it is sealed and submitted.
Measure a sealed melting point (note 4) of the crystals of the purified caffeine. To do this, place a few crystals of the solid at the bottom of a capillary tube (first laboratory handout); attach the open end of the tube to a hose with a septum cap (stockroom), attach the other end of the hose to the aspirator (running) in the hood. Light the torch and obtain a small, narrow flame. Lay the torch on the bench of the hood with the flame pointing up and to the back. Touch the closed end of the capillary to a piece of ice with the right hand, lower the hose and capillary to the flame with the left hand to collapse the walls of the capillary approximately 1 cm from the closed end. Obtain the melting point of your product. If time permits, extract the brown aqueous thrice more with CH2Cl2 to determine if the yield can be improved.
Report the masses of crude and purified caffeine, melting point. Compare the melting point to the Handbook's value. A 13C NMR of a sample previously isolated by a student will be supplied. On it, use symbols to make clear correlations between each signal and each carbon atom in the molecule.
1. CH2Cl2, dichloromethane or methylene chloride, has a boiling point of 40 oC. Although this solvent is not flammable, contact with the skin should be avoided. The water solution clearly contains lots of brown compounds so extraction of caffeine into CH2Cl2 is necessary. If equal volumes of water and CH2Cl2 were used as a solvent, caffeine would favor dissolving in the later solvent by a factor of 8.2 to 1. Organic solids are almost always obtained from an organic solvent, not water; water is difficult to remove from organic crystals once they are formed.
2. This transfer is not easy, and you should expect to have some of the CH2Cl2 solution run out of your transfer pipet at the wrong time. The problem is that CH2Cl2 vapors are warmed by your hand even while it is inside the pipet and expand, forcing the liquid to dribble out of the pipet. With some practice you can counteract this by constantly sucking air into the pipet. In any case, nothing is lost if the CH2Cl2 dribbles back into the centrifuge tube.
3. Caffeine could be purified by recrystallization instead of sublimation: add 1 drop of ethyl acetate for every 2 milligrams of crude caffeine obtained. Warm the mixture in the steam bath until the solid has dissolved. (If not all solid is dissolved while hot, add more drops and rewarm) Then cool the solution with ice water; fluffy crystals will form. Prepare a Hirsch funnel with a properly fit paper disk for suction filtration. When the crystallization is complete, carefully scrape the solid portion onto the filter first, then filter the rest of the solution. Weigh the purified caffeine.
4. Caffeine normally sublimes before it melts. Thus the melting point capillary must be sealed.
"Mayo et al." Mayo, D.W., Pike, R.M., Butcher, S.S. and Trumper, P.K. Microscale Techniques for the Organic Laboratory; Wiley: New York, 1991.
The "Handbook" recent editions of: Weast, R.D. Handbook of Chemistry and Physics; The Chemical Rubber Co.: Cleveland, OH, 1960-present.
The "Aldrich IR Library" refers to any edition of: Pouchert, C.J. The Aldrich Library of Infrared Spectra; Aldrich Chemical Co. Milwaukee WI 1970-present.
Rev. January, 2001