EXPERIMENT
3
Extraction of Caffeine from Tea1
Purpose
To extract a molecule from an aqueous solution into an organic solvent and to explore the
role of pH in the process.
Background
CH3
N
N
C
O
C
HC
N
N
CH3
H3C
O
Caffeine
Caffeine is the principle stimulant found in tea and coffee,
and it can account for up to approximately 5% of the mass
of the tea leaves. Caffeine is soluble in water and a variety
of organic solvents, and both can be used to extract
caffeine from tea leaves or coffee beans. In fact, a wide
variety of solvents including water, water in combination
with organic solvents, methylene chloride, ethyl acetate,
and supercritical carbon dioxide are used commercially to
extract caffeine from coffee. Since we are not interested in
maintaining the flavor of our tea when we extract the
caffeine, we are going to use a two step, water followed by
organic solvent, extraction technique.
Taking advantage of caffeine’s water solubility, we will first brew a very strong cup of tea.
One we have extracted the caffeine from the tea leaves we will extract the caffeine from the
water using an organic solvent. However, we will also explore the effect that the pH of the
tea has on the extraction process; thus, Group A will add sodium carbonate to their tea to
raise the pH, and Group B will add hydrochloric acid to their tea to lower the pH.
Procedure
Place a teabag in a small beaker and add 20 mL of water. Boil the tea for 20 minutes. Add
water as needed to maintain the volume of the solution. After testing your centrifuge tubes
for leaks, transfer the tea in equal portions to two 15-mL screw-cap centrifuge tubes.
Adapted from Introduction to Organic Laboratory Techniques: A Microscale
Approach, 3rd edition. Pavia, Donald L., Gary M. Lampman, George S. Kriz, and
Randall G. Engel (1999).
1
Members of Group A, add 0.5 g of sodium carbonate to each centrifuge tube. Members of
Group B, add 1 mL of 6 M hydrochloric acid to each centrifuge tube.
Cap the centrifuge tubes and carefully shake them. Remember to vent your centrifuge
tubes. Once the sodium carbonate has dissolved and the tea has cooled to room
temperature, add 3 mL of methylene chloride (also called dichloromethane) to each test
tube. Cap the test tubes and shake the tubes. Remember to vent your centrifuge tubes.
Spin your tea–methylene chloride suspensions in a centrifuge for five minutes to break the
emulsion. Remember to balance your centrifuge tubes in the centrifuge. Carefully transfer
the organic layer from each centrifuge tube to a third 15-mL centrifuge tube. Repeat the
methylene chloride extraction and transfer with additional 3 mL aliquots of methylene
chloride, one for each centrifuge tube.
With a clean dry Pasteur pipet, transfer your methylene chloride solution to a fourth 15-mL
centrifuge tube. It is very important that no water is transferred during this step. Dry the
organic solvent with anhydrous sodium sulfate.
To remove the solvent, transfer a 3 mL aliquot of your dry methylene chloride–caffeine
solution to a tared 5-mL conical vial. Warm the vial and the solution therein in a hot water
bath (50 °C max) and gently blow compressed air onto the solution. As the methylene
chloride evaporates, transfer more of your methylene chloride–caffeine solution to the vial.
Continue gently heating the solution until all of the solvent has evaporated.
Determine the mass of your crude caffeine, and recrystallize your caffeine using acetone
and hexanes. Dissolve your caffeine in a minimum of warm acetone (start with
approximately 0.5 mL acetone) and transfer the solution to a Craig tube. Slowly layer
hexanes on top of the solution. Allow crystals to form and then cool the suspension in an
ice-water bath. Spin your Craig tube in a centrifuge to isolate the caffeine.
Report
Working with one partner, collect data for the yield of crude caffeine, making certain to
note whether the individual added hydrochloric acid or sodium carbonate, from each
student in your laboratory class, and tabulate the data. Report the average mass of caffeine
isolated for each technique and the percent error associated with the reported average
mass.
Respond to the following: the average mass of the tea in a tea bag is 2.3610 g and the
percent error associated with that measurement is 0.93 %. Is the difference in the average
mass of the caffeine collected by Group A and Group B significant? If it is significant, which
group collected more caffeine?
Consider the structure of caffeine and provide an explanation for the difference in the in the
average mass of caffeine collected. When explaining the difference consider, how HCl might
interact with caffeine. Consider how charged molecules (ions) interact with water as
compared to neutral molecules; in other words, which would be more soluble in water, a
neutral caffeine molecule or a charged caffeine molecule. Which would be more soluble in
an organic solvent, a charged molecule or a neutral molecule?