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PART 2 BIOCHEMICAL PHARMACEUTICALS
Unit 6 Isolation of Caffeine from Tea
In this experiment，Caffeine will be isolated from tea leaves. The major problem of the
isolation is that caffeine does not occur alone in tea leaves，but is accompanied by other natural substances from which it must be parated. The major component of tea leaves is cellulo，which is the major structural material of all plant cells. Cellulo is a polymer of gluco. Since cellulo is virtually insoluble in water，it prents no problems in the isolation procedure. Caffeine，on the other hand，is water soluble and is one of the major substances extracted into the solution called "tea.”Caffeine compris as much as 5 percent by weight of the leaf material in tea plants. Tannins also dissolve in the hot water ud to extract tea leaves. The term tannin does not refer to a single homogeneous compound，or even to substances which have similar chemical structure. It refers to a class of compounds which have certain properties in common. Tannins are phenolic compounds having molecular weights between 500 and 3000. They are widely ud to "tan
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”leather. They precipitate alkaloids'z and proteins from aqueous solutions. Tannins are usually divided into two class: tho which can be hydrolyzed and tho which cannot. Tannins of the first type which are found in tea generally yield gluco and gallic acid when they are hydrolyzed. The tannins are esters of gallic acid and gluco. They reprent structures in which some of the hydroxyl groups in gluco have been esterified by digalloyl groups. The non-hydrolyzable tannins found in tea are condensation polymers of catechin. The polymers are not uniform in structure，but catechin molecules are usually linked together at ring positions 4 and 8.
When tannins are extracted into hot water，the hydrolyzable ones are partially hydrolyzed，meaning that free gallic acid is also found in tea. The tannins，by virtue of their phenolic groups，and gallic acid by virtue of its carboxyl groups，are both acidic. If calcium carbonate，a ba，is added to tea water，the calcium salts of the acids are formed. Caffeine can be extracted from the basic tea solution with chloroform，but the calcium salts of gallic acid and the tannins are not chloroform soluble and remain behind in the aqueous solution.
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国家安全监管总局官网The brown color of a tea solution is due to flavonoid pigments and chlorophylls，as well as their respective oxidation products. Although chlorophylls are somewhat chloroform soluble，most of the other substances in tea are not. Thus，the chloroform extraction of the basic tea solution removes nearly pure caffeine. The chloroform is easily removed by distillation(by 61'C)to leave the crude caffeine. The caffeine may be purified by recrystallization or by sublimation.
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Catechin Gallic Acid
In a cond part of this experiment，Caffeine will be converted to a derivative. A derivative of a compound is a cond compound，of known melting point，formed from the original compound by a simple chemical reaction. In trying to make a positive identification of an organic compound，it is often customary to convert it into a derivative. If the first compound，Caffeine in this ca，and its derivative both have melting points which match tho reported in the chemical literature (e. g.，a handbook)，it is assumed
that there is no coincidence and that the identity of the first compound，Caffeine，has been definitely established.
Caffeine is a ba and will react with an acid to give a salt. Using salicylic acid，a derivative salt of Caffeine，Caffeine salicylate，will be made in order to establish the identity of the Caffeine isolated from tea leaves.
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Special Instructions Be careful when handling chloroform. It is a toxic solvent，and you should not breathe it excessively or spill it on yourlf. When discarding spent tea leaves，do not put them in the sink becau they will clog the drain. Dispo of them in a waste container.龙骨怎么做好吃
Procedure Place 25g of dry tea leaves，25g of calcium carbonate powder，and 250ml of water in a 500ml three neck round bottom flask equipped with a condenr for reflux. Stopper the unud openings in the flask and heat the mixture under reflux for about 20 minutes. U a Bunn burner to heat. While the solution is still hot，filter it by gravity through a fluted filter using a fast filter paper such as E&D No. 617 or S&S No. 595. You
may need to change the filter paper if it clogs.
Cool the filtrate (filtered liquid)to room temperature and，using a paratory funnel， extract it twice with 25ml portions of chloroform. Combine the two portions of chloroform in a 100ml round bottom flask，Asmble an apparatus for simple distillation and remove the chloroform by distillation. U a steam bath to heat. The residue in the distillation flask contains the caffeine and is purified as described below (crystallization). Save the chloroform that was distilled. You will u some of it in the next step. The remainder should be placed in a collection container.三垟湿地公园
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Crystallization(Purification)Dissolve the residue obtained from the chloroform extraction of the tea solution in about 10m1 of the chloroform that you saved from the distillation. It may be necessary to heat the mixture on a steam bath. Transfer the solution to a 50ml beaker. Rin the flask with an additional 5m1 of chloroform and combine this in the beaker. Evaporate the now light-green solution to dryness by heating it on a steam
bath in the hood.
The residue obtained on evaporation of the chloroform is next crystallized by the mixed solvent method. Dissolve it in a small quantity (about 2 to 4ml) of hot benzene and add just enough highboiling (60'C to 90'C)petroleum ether (or ligroin) to turn the solution faintly cloudy. Alternatively，acetone may be ud for simple crystallization without a cond solvent. Cool the solution and collect the crystalline product by vacuum filtration using a Hirsch funnel. Crystallize the product the same way a cond time if necessary，and allow the product to dry by allowing it to stand in the suction funnel for a while. Weigh the product. Calculate the weighty percentage yield bad on tea and determine the melting point. If desired，the product may be further purified by sublimation as described in the next experiment.
