It is a beverage
produced by steeping in freshly boiled water the young leaves and leaf buds of the tea plant,
Camellia sinensis. They have been reported as a good sources of Caffeine. Tea leaves consist
mostly of cellulose, a water-insoluble polymer of glucose. 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 leaves into hot
water. [The solubility of caffeine in water is 22 mg/ml at 25 oC, 180 mg/ml at 80 oC, and 670
mg/ml at 100 oC insoluble material.] The hot solution is allowed to cool and the caffeine is
then extracted from the water with dichloromethane (methylene chloride), which is anorganic
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 phenolic anions, which are not soluble in the dichloromethane but are soluble
in highly polar water. There is one practical disadvantage in converting the tannins to their
salts – they become anionic surfactants. Detergents and soap are surfactants. It is the
purpose of surfactants to cause materials that do not dissolve in water (like oil, grease and
dichloromethane) to form an emulsion with water. We want to be able to separate the
aqueous phase from the dichloromethane phase, so the last thing we want is an emulsion of
the two. Consequently, as you extract the caffeine from the water into the dichloromethane
do not shake the separatory funnel vigorously. The flow diagram below summarizes the