Isolation of Caffeine from Tea Leaves
Introduction Caffeine is a member of the class of compounds organic chemists call alkaloids. Alkaloids are nitrogencontaining basic compounds that are found in plants. They usually taste bitter and often are physiologically active in humans. The names of some of these compounds are familiar to you even if the structures aren’t: nicotine, morphine, strychnine, and cocaine. The role or roles these compounds play in the life of the plants in which they are found is not well understood. In some cases they may act as pesticides; nicotine is found in tobacco and has been sprayed onto other plants, in which it is not found, to function as an insecticide. The structure of caffeine is shown to the right. It can be considered to be constructed from the purine ring system, which is important biologically, being found in nucleic acids and elsewhere. Caffeine is found in a number of things ingested by people. The table to the right lists some of them. Caffeine acts as a stimulant. It stimulates the heart, respiration, the central nervous system, and is a diuretic. Its use can cause nervousness, insomnia and headaches. It is physically addictive. A person who drinks as few as 4 cups of coffee a day and who attempts to stop “cold turkey” may experience headache, insomnia, and possibly nausea as the result of withdrawal. Coffee Coffee, decaf Tea Cocoa Milk chocolate Baking chocolate Coca-Cola Excedrin, extra strength
purine N N caffeine O H3C N O N CH3
H N N CH3 N N
80-125 mg per cup 2-4 mg per cup 30-75 mg per cup 5-40 mg per cup 6 mg per ounce 35 mg per ounce 46 mg per 12 ounces 65 mg per tablet
No-Doz 100 mg per tablet Tea has been consumed as a beverage for almost 2,000 years, starting in China. It is a beverage produced by steeping in freshly boiled water the young leaves and leaf buds of the tea plant, Camellia sinensis. Today, two principal varieties are used, the small-leaved China plant (C. sinensis sinensis) and the large-leaved Assam plant (C. sinensis assamica). Hybrids of these two varieties are also grown. The leaves may be fermented or left unfermented. Fermented teas are referred to as black tea, unfermented teas as green tea, and partially fermented teas as oolong. As trade routes opened to Asia in the 17th century, tea was imported to Europe. Today, you are going to make a small but strong cup of tea and extract the caffeine from it.
Isolation of Caffeine from Tea Leaves
Background to the Procedure Tea leaves consist mostly of cellulose, a water-insoluble polymer of glucose, which is a simple sugar (a monosaccharide). Cellulose performs a function in plants similar to that of fibrous proteins in animals: it is structure building material. Along with the cellulose are found a number of other things including caffeine, tannins (phenolic compounds, compounds that have an -OH directly bonded to an aromatic ring) and a small amount of chlorophyll. The idea in this experiment is to extract the water soluble materials in the tea o leaves into hot water. [The solubility of caffeine in water is 22 mg/ml at 25 C, 180 mg/ml o o at 80 C, and 670 mg/ml at 100 C.] The hot solution is allowed to cool and the caffeine is then extracted from the water with dichloromethane (methylene chloride), which is an organic solvent that is insoluble in water. Since caffeine is more soluble in dichloromethane (140 mg/ml) than it is in water (22 mg/ml), it readily dissolves in the dichloromethane. However, the tannins are slightly soluble in the dichloromethane. But we want to separate the caffeine from the tannins by having the caffeine dissolve in the dichloromethane and the tannins remain in the water. We can do this by taking advantage of the fact that phenols are acidic enough to be converted to their salts (deprotonation of the -OH group) by reaction with sodium carbonate. So, we will add sodium carbonate to the water and the tannins will be converted to...
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