Chemistry Olympiad Training Program Sec 4/2010
PRACTICAL: EXTRACTION OF CAFFEINE FROM TEA LEAVES
Name:
AIM
To isolate caffeine from tea leaves.
TECHNIQUES TO BE USED
§ Liquid-liquid extraction with a separating funnel
§ Using a solid drying agent to remove water
§ Gravity filtration
§ Evaporation of a solvent using a rotary evaporator
BACKGROUND
Caffeine is a naturally-occurring organic compound containing nitrogen and having basic properties. Its structure is displayed below:
SAFETY CONSIDERATIONS
Dichloromethane is a volatile chlorinated organic solvent. It must be used in the fumehood at all times so as to prevent excessive inhalation of its fumes. It also should be disposed of in the appropriate waste containers.
Appropriate care should be ensured whenever operating a heating implement such as the hot plate. Handle all hot glassware carefully by using the oven mitts provided.
PROCEDURE
Extracting the soluble components present in tea leaves (In groups of four)
1. To a large beaker containing 5 tea bags, add 100 cm3 of water and boil for 10 minutes on a hot plate. Stir carefully to avoid breaking the tea bags.
2. Decant [pour off the liquid] the hot tea solution into a 250 cm3 conical flask. Ensure that you have collected as much of the hot tea as possible.
3. Extract the tea from the tea bags a second time by using 50 cm3 of water and heating for a few minutes.
4. Add the second extract to the same conical flask.
5. Cool the aqueous solution to room temperature using an ice bath.
Liquid-Liquid Separation
(The following steps should be conducted in the fumehood)
6. Pour the cooled tea extract into a 250 cm3 separating funnel.
7. Gently extract [ask your tutor how to do this!] the tea solutions two times, first with 30 cm3 of dichloromethane, and again with 25 cm3. Combine both extracts in a 125 cm3 conical flask. [NOTE: Do not shake too vigorously so as to reduce the formation of an emulsion.]
8. Dry the combined extracts with anhydrous sodium sulfate.
9. Filter the dry extract into a dry pre-weighed 100 cm3 round-bottom flask. Wash with a little dichloromethane (approximately 2 cm3).
Removal of the dichloromethane solvent and collection of crude caffeine
10. Using a rotary evaporator, remove the dichloromethane solvent until about 5 cm3 of liquid remains in the round-bottom flask. Do not allow the flask to go dry or the caffeine may decompose.
11. Label your round-bottom flask, and let it evaporate to dryness overnight.
12. Record the crude mass of the caffeine you have extracted.
Results and Observations
Record all your observations throughout the progress of your experiment.
Analysis Questions
1. Identify the functional groups present in caffeine.
2. Explain briefly why water can be used to extract caffeine from the tea leaves.
3. The extraction with water also causes other components of the leaf to be extracted, such as catechin (shown below). Suggest how the use of dichloromethane can help to separate caffeine from catechin.
catechin
4. Explain the principle behind using a rotary evaporator to separate caffeine the solvent.
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