Experiment #6: Isolation of Caffeine from Tea
Chemistry 102
Background:
Tea has been a popular beverage for many centuries. The tea plant is native to
Indochina and northern India, and wasn’t widely cultivated for popular (nonmedical)
use in China until around 700 A.D. Although the first confirmed medical reference is
from 350 A.D., some experts say there were reports of tea being used medicinally as
early as 2737 B.C. The active ingredient in tea is caffeine, one of a class of compounds
called alkaloids. More specifically, caffeine is in the family called xanthines, which are
among the oldest known stimulants. Caffeine stimulates the central nervous system,
skeletal muscles, respiration and the heart. It is also a diuretic (it promotes urination).
Caffeine is highly addictive, and is possibly the most widely abused drug in this
country. Symptoms of caffeine withdrawal may include headache, lethargy, insomnia
or nausea. Caffeine is also found in other plants, such as coffee, cola and cocoa (see
table on next page).
Tea leafs contain from 2 to 5% caffeine, along with cellulose, tannins, flavonoid
pigments and chlorophylls (see structures of caffeine and tannins on next page). The
caffeine can be extracted from tea using hot water. However, the tannins, pigments and
chlorophylls will also be extracted into the water. The caffeine can then be extracted
from the water using methylene chloride (dichloromethane). Since the tannins are
acidic, base is added to the aqueous extract in order to ionize the tannins and insure
that they remain in the aqueous layer. Along with the caffeine, the chlorophylls (which
are present only in small quantities) will also go into the organic layer. Thus, the
caffeine that you obtain after evaporating the methylene chloride will not be totally
pure. Either sublimation or crystallization can be used to further purify the caffeine.
However, we will not do that in this experiment (the caffeine is already close to pure).
Decaffeinated tea (and coffee) has become more popular along with an increased
public awareness of the potential health risks associated with caffeine. Traditionally,
coffee and tea have been decaffeinated using methylene chloride. More recently,
caffeine is often removed using supercritical carbon dioxide. However, the term
“decaffeinated” can be misleading, since decaffeinated tea (or coffee) can still contain
up to 1.2% caffeine.
In this experiment you will extract caffeine from two teas, one regular and one
decaffeinated. Half of the pairs will do black tea and half will do green tea. There will
be a variety of teas available in the lab, however, you may also bring your own tea to
analyze. You will calculate the % by mass of caffeine in each tea and we will compare
the class results. You will also use TLC to look for caffeine in the organic layer when
you extract your decaffeinated tea. TLC can show trace amounts that may not be
detected by weighing. Because caffeine is a white solid, you will not be able to see it on
the TLC plate by eye. However, since caffeine is a UV-active substance (absorbs light in
the ultraviolet wavelengths), we can see it by using a UV lamp.
O
CH3
H 3C
H OR
N
N
H O
RO
O
N
N
RO
CH3
H
H
OR
H
OR
Glucose (R = H)
Caffeine
Tannin (some R's = digalloyl)
OH
OH
OH
OH
O
HO
O
O
OH
O
OH
OH
Catechin (a Tannin)
OH
OH
Digalloyl group
Caffeine Content of Some Foods and Drugs
Coffee
Decaffeinated Coffee
Tea
Cocoa
Milk Chocolate
80-125 mg per cup
2-4 mg per cup
30-75 mg per cup
5-40 mg per cup
6 mg per oz
Baking Chocolate
Coca-Cola
Anacin, Midol
Excedrin, extra strength
Dexatrim, Vivarin
Dristan
No-Doz
35 mg per oz
46 mg per 12 oz can
32 mg per tablet
65 mg per tablet
200 mg per tablet
16 mg per tablet
100 mg per tablet
Materials:
Two tea bags (one decaffeinated and one regular), methylene chloride, sodium
carbonate (Na2CO3), NaCl, 1% glacial acetic acid in acetone, distilled water, 4 beakers,
watchglass cover, hotplate, 125 mL separatory funnel, test tube, TLC plate, capillary
tubes, TLC chamber and UV lamp.
Safety Precautions:
Dichloromethane is not flammable, nor is it considered carcinogenic, but it can
cause liver damage if ingested, or with repeated inhalation or skin exposures. It should
be used in the fume hoods (inhalation of vapors can cause nausea or drowsiness).
Avoid spills, it can also pass through both latex and nitrile gloves. Do not look directly
at the UV lamp, the UV light can damage your eyes.
Procedures:
1. Carefully remove the tea from the first teabag (regular), weigh it and record the mass
of the tea. Put the tea back into the back and re-staple it.
2. Place the teabag and about 40 mL of distilled water into a 250 mL beaker. Cover it
with a watchglass and heat to a gentle boil. Boil for 5 minutes.
3. Decant the liquid into a 150 mL beaker. Add about 20 mL more water to the teabag
and boil again for 5 minutes.
4. Decant the liquid into the the 150 mL beaker (with the first liquid). Use a glass rod to
gently squeeze out as much liquid as possible (without breaking the bag). Add about 1
g of Na2CO3, swirl to dissolve, and allow the liquid to cool to room temperature.
5. Transfer the liquid to a 125 mL separatory funnel (supported with a ring clamp).
Add 10 mL of methylene chloride, shake gently (vent frequently) and allow mixture to
separate. Collect the organic layer in a 100 mL beaker (label and weigh the beaker first
and record the mass). Add another 10 mL of methylene chloride to the separatory
funnel, shake, separate and collect in the 100 mL beaker. *If an emulsion forms, try
adding a little NaCl (1 or 2 scoops) and shake gently.
6. Discard the aqueous layer and rinse out the separatory funnel. Allow the methylene
chloride to evaporate while you extract your second teabag.
7. Repeat the above procedures with the second teabag (decaffeinated), collecting the
methylene chloride (step 5) in a 50 mL beaker.
8. If the methylene chloride from the first extraction (regular tea) is all evaporated, then
weigh the beaker + caffeine and record the mass. If it’s not dry, run a stream of air over
it to complete drying before weighing.
9. After you record the mass, scrape out a small amount of caffeine and place it in a
small test tube. Add about 0.5 mL of methylene chloride.
10. Prepare a TLC plate by drawing a line about 1 cm from the bottom of the plate, and
then marking 3 places on the line for spotting. Obtain a TLC chamber and two capillary
tubes. Place about 8 mL of TLC solvent (1% glacial acetic acid in acetone) into the
chamber.
11. Spot the plate with your samples. First, spot the left and center marks with your
first tea extract (one spot each). Next, with a new capillary tube, spot the second tea
extract (from the 50 mL beaker) on the right-hand mark. Spot 3 or 4 times, allowing the
spot to dry between each (this solution is more dilute). Then spot the second tea extract
in the same way over the center mark, right on top of the first spot. Label the spots
under the marks.
12. Develop the TLC plate and mark the solvent line before drying it. Observe the plate
under short-wave UV light and circle any spots you see. Note the appearance of the
spots (bright, dark, colored, etc.). Make a sketch of the plate (with observations) in your
notebook. Measure the Rf of each spot.
13. If the methylene chloride from the second extraction (decaffeinated tea) is all
evaporated, then weigh the beaker + caffeine and record the mass. If it’s not dry, run a
stream of air over it to complete drying before weighing.
14. Calculate the mass of the caffeine obtained from each tea, and then calculate the %
by mass of caffeine for each tea.
Waste Disposal:
Teabags can go into the trash. Aqueous solutions should go into the aqueous
waste container (they contain base). Any organic chemicals should go into the organic
waste container. Capillary tubes go into the glass waste (not the trash).
Pre-Lab:
Prepare your notebook as described in the lab notebook handout. Include:
name, partner’s name, title of experiment, date, purpose, structure of caffeine,
procedures (bulleted list), separation scheme (flow chart) and data (make spaces to
record masses and volumes). Leave spaces for observations.
Report:
1. In your notebook, make tables for your results. One should include the brand of tea,
regular or decaffeinated, mass of tea, mass of caffeine and % by mass of caffeine in the
tea for each tea tested (two in this case). The other should include the R f values and
appearance for each spot for each of the three lanes on your TLC.
2. In your notebook, write a brief discussion of the experiment. In this section you
should summarize your results (brand, regular or decaffeinated and % by mass
caffeine) and note any interesting observations (odor, color etc.). Also summarize your
TLC results and discuss their relevance. Make any possible conclusions about the
experiment (successful vs. unsuccessful and reasons why; compare your results to
expected results).
3. Answer the post-lab questions.
4. Make a copy of your lab notebook pages (do not tear out the originals) and attach the
question sheet with your answers. Staple all pages and turn it in.
Post-Lab Questions:
1. Before extracting the caffeine with methylene chloride, you added Na2CO3 to the
aqueous portion. Explain why. Also, write the chemical reaction that would occur
between phenol and NaOH and incorporate it into your explanation.
2. Caffeine is soluble in methanol, as well as methylene chloride. Why couldn’t you use
methanol to extract the caffeine from the aqueous portion?