Extracción De Cafeína

Extraction of Caffeine from Tea Leaves

Introduction
Extraction is a common technique used in organic chemistry to separate or isolate a desired compound from a mixture. The two major types of extractions used in the organic laboratory are solid-liquid extraction and liquid-liquid extraction. Solid-liquid extraction is often used to isolate a natural product from its biological source, suchas the leaves or bark of a tree. For example, the anti-cancer drug paclitaxel (Taxol®) can be isolated from the bark of the Pacific yew tree by solid-liquid extraction with methanol. Liquid-liquid extraction is the most common extraction technique, and involves partitioning a solute between two immiscible liquid phases, usually an aqueous (water) phase and an organic solvent phase. Therelative concentration of solute in each phase depends on the solubility of the solute in each phase and is given by the partition coefficient (K):

[g/100 mL] in organic phase
Partition coefficient (K) =
[g/100 mL] in aqueous phase

Caffeine is an alkaloid (a class of naturally occurring compounds containing nitrogen and exhibiting the properties of an alkali, hence the term alkaloid)which is found in many sources, such as tea and coffee. In fact, the amount of caffeine in tea leaves can be as much as 5%.


[pic]

Caffeine


In this experiment, you will first use an aqueous medium (sodium carbonate in water) to extract caffeine (along with tannins, gallic acid, flavinoid pigments, and some other compounds) from tea leaves by a solid-liquid extraction process, inother words, you will make tea (albeit tea that is basic). Caffeine, tannins, gallic acid, and the flavinoid pigments are soluble in this alkaline medium, but the main component of tea leaves – cellulose – is not. You will then perform a liquid-liquid extraction of the tea solution with an organic solvent, methylene chloride, to separate the caffeine from the basic tea solution and away from thetannins, gallic acid and the remainder of the tea components. Since caffeine is more soluble in the organic solvent methylene chloride (CH2Cl2) than in water, it partitions primarily into the methylene chloride. The tannins and gallic acid, being acidic compounds, are converted to water soluble salts by the sodium carbonate, and remain in the aqueous layer during the extraction. The othercomponents of tea are not very soluble in methylene chloride. Therefore, the extraction of tea solution with methylene chloride removes virtually no other compound but caffeine. The solid caffeine can be then be isolated by evaporation of the low-boiling methylene chloride.
Required Reading:
Pavia, Lampman, Kriz, and Engel:
a) Technique 12 “Extractions, Separations, and Drying Agents”, p. 669(Parts A, C, D)
b) “Caffeine”, p. 87
Special Safety Notes:

Methylene chloride is a somewhat toxic solvent, so handle it as much as possible in a hood, and avoid spilling it on yourself.

Procedure:
Preparation of Tea
Place about 20 mL of water into a 100 mL beaker (use the beaker markings to estimate the volume). When the solution has reached a gentle boil, remove the watch glass andplace a tea bag (the instructor will give you the weight of the tea in the bag) into the gently boiling water so that it is covered as completely as possible by the water. Note the approximate level of the liquid in the beaker at this point. Replace the watch glass and continue gently boiling the mixture for about 15 minutes. Water may evaporate during this step, so do not be afraid to add water tomaintain the liquid level. Occasionally push down (GENTLY!) on the tea bag with the closed end of a test tube to maintain contact of the tea in the bag with the solution. After the 15 minute boiling period, remove the beaker from the hot plate and squeeze any excess liquid out of the tea bag into the beaker (use a stir rod or test tube to roll the tea bag against the side of the beaker – the...