Isolation of Caffeine from Tea or Coffee

In Experiment 13A, caffeine is isolated from tea leaves. The chief problem with the isolation is that caffeine does not exist alone in tea leaves but is accompanied by other natural substances from which it must be separated. The main component of fea leaves is cellulose, which is the principal structural material of all planf cells. Cellulose is a polymer of glucose. Because cellulose is virtually insoluble in water, it presents no problems in the isolation procedure. Caffeine, on fhe ofher hand, is wafer soluble and is one of the main substances extracted into the solution called tea. Caffeine constitutes as much as 5% by weight of the leaf maferial in fea plants. 

When tannins are extracted into hot water, some of these compounds are partially hydrolyzed to form free gallic acid. The taimins, because of their phenolic groups, and gallic acid, because of its carboxyl groups, are both acidic. If sodium carbonate, a base, is added to tea water, these acids are converted to their sodium salts that are highly soluble in water. 

Although caffeine is soluble in water, it is much more soluble in the organic solvent methylene chloride. Caffeine can be extracted from the basic tea solution with methylene chloride, but the sodium salts of gallic acid and the tannins remain in the aqueous layer. 

The brown color of a tea solution is due to flavonoid pigments and chlorophylls and to their respective oxidation products. Although chlorophylls are soluble in methylene chloride, most other substances in tea are not. Thus, the methylene chloride extraction of the basic tea solution removes nearly pure caffeine. The methylene chloride is easily removed by evaporation (bp 40°C) to leave the crude caffeine. The caffeine is then purified by sublimination at reduced pressure to prevent decomposition. 

Solid phase extraction (SPE), described in Technique 12, Section 12.14, avoids the use of mefhylene chloride, and fhe technique will be employed in Experiment 13B for exfracfing caffeine from fea or coffee. The material used in this SPE procedure is prepared by alkylating silica with 18-carbon chains. This process changes the silica from a polar subsfance to a relatively nonpolar one. The resulting material, called C-18 silica, is placed in a tube that resembles a syringe body. Because the silica is now nonpolar, water and other polar substances such as tannins and gallic acid will pass through the column, whereas the C-18 silica retains caffeine. To remove fhe caffeine from the column, ethyl acetate is added. 

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