E x p e r i m e n t 18
Isolation of Caffeine from Tea
Objectives
To extract caffeine from tea
To purify the caffeine via recrystallization
To monitor the extraction and purification steps via Thin Layer Chromatography
(TLC)
In the Lab
Students work in pairs
After Lab
Complete the lab report on Chem21Labs.com
Waste
Place the used potassium carbonate in the waste container in the hood.
Place aqueous solutions that remain at the end of lab in the waste container labeled
Aqueous Waste located in the Instructor’s hood.
Place organic solutions that remain at the end of lab in the waste container labeled
Organic Waste.
Safety
Students must wear goggles for this experiment.
Gloves are available.
Caffeine belongs to a class of organic compounds called alkaloids. Alkaloids are naturally
occurring compounds that contain nitrogen – a structural feature that gives these compounds
basic / alkaline properties. Common alkaloids include caffeine (stimulant), nicotine, and
morphine (analgesic).
Other common alkaloids include cocaine (stimulant), atropine (spasmolysis agent),
vincamine (vasodilator), quinine (anti-malarial), ephedrine (anti-asthma), and codeine
(analgesic). Note the “ine” ending for the alkaloids.
Caffeine is commonly found in coffee, tea, soft drinks, chocolate, and "stay-awake" pills
such as NoDoz (see below). Tea leaves contain cellulose, chlorophyll, tannins, and caffeine
(~4%). Caffeine stimulates the heart, central nervous system, and the respiratory system. It
is a diuretic and has the effect of delaying fatigue. One can develop both a tolerance and a
dependence on caffeine. The dependence is real, and a heavy user (> 5 cups of coffee per
day) will experience lethargy, headache, and perhaps nausea after about 18 hours of non-use.
Caffeine Source
Caffeine Amount
Coffee
80 to 125 mg per cup
Coffee, decaffeinated
2 to 4 mg per cup
Tea
30 to 75 mg per cup
Cocoa
5 to 40 mg per cup
Milk chocolate
6 mg per oz
Baking chocolate
35 mg per oz
Coca-Cola
46 mg per 12 oz
Anacin
32 mg per tablet
Excedrin, extra strength
65 mg per tablet
Dexatrim
200 mg per tablet
No-Doz
100 mg per tablet
Pure Caffeine has a melting point of 235
o
C - 238
o
C, and a molecular weight of 194.19
g/mol. It is very soluble in Methylene Chloride and partially soluble in water. The isolation
of Caffeine from tea appears rather simple, while in fact it utilizes a number of rather
N
N
N
N
O
O
Caffeine
N
N
Nicotine
N
H
O
O
Morphine
advanced chemical processes. To isolate the Caffeine in a sample of tea, it is necessary to
chemically separate the Caffeine from the rest of the tea (mainly cellulose). The procedure
used around the world to make a cup of tea will be used in the lab today. The tea is
“steeped" in very hot water for about 10 minutes – this extracts most of the caffeine. There
is no advantage to boiling the tea leaves with water for 10 minutes. Not only is caffeine
more soluble in hot water, but hot water is a critical component in that it causes the tea leaves
to swell releasing the caffeine trapped in its cells.
The resulting brown tea solution is then extracted with dichloromethane, dissolving primarily
caffeine and leaving the other water-soluble substances in the aqueous layer. In fact, the
extraction is performed three times with fresh Methylene Chloride since multiple extractions
remove more Caffeine than a single extraction with the same total amount of solvent.
Evaporation of the Methylene Chloride leaves behind crude caffeine, which on
recrystallization yields a relatively pure product.
The solubility of caffeine in water is 2.2 mg/mL at 25°C, 180 mg/mL at 80°C, and 670
mg/mL at 100°C. It is quite soluble in dichloromethane, the solvent used in this experiment
to extract the caffeine from water.
Table of Physical Constants
Chemical Name
Chemical
Formula
Molecular
Weight
Melting
Point
Boiling
Point
Density
20
D
n
Caffeine
C
8
H
10
O
2
N
4
194.19
Methylene Chloride
(Dichloromethane)
CH
2
Cl
2
84.93
40°C
1.325
1.4240
Calcium carbonate
CaCO
3
100.09
Acetone
C
3
H
6
O
58.08
56°C
.791
1.3590
Petroleum Ether
(low-boiling)
30 - 60°C
Petroleum Ether
(high-boiling)
[Ligroin]
60 - 80°C
.656
1.3760
Table 1
18
E x p e r i m e n t 18 • Isolation of Caffeine from Tea
4
Procedure:
1. If the Instructor has prepared the tea,
go to Step 9. Enter 8.0 g for the mass of
tea and obtain 50.0 mL of the prepared tea.
2. Place 50 mL hot water and 2.5 grams
calcium carbonate in a 250 mL beaker.
3. Heat the beaker on a hot plate (setting
10) until the water is boiling.
4. Turn the heat off and place 3 tea bags
of black tea in the hot water.
5. Let the tea “steep” in the hot solution
for 10 minutes – leave the beaker on the
hot plate during this time.
6. Remove the tea bags from the solution
and squeeze all the liquid from them.
7. Vacuum filter the tea solution through
a pad of filter aid (Celite) to remove the
Calcium carbonate.
8. Place the filtrate in a beaker in an ice
bath for 10 minutes.
9. Transfer the filtrate (or 50 mL if
already prepared) to a separatory funnel
and add 25 mL of methylene chloride.
10. Shake vigorously for 3 seconds and
allow the layers to separate for 10 minutes.
11. Remove the lower organic layer(s)
(the two layers below the dark brown
aqueous layer) and place it in a 250 mL
Erlenmeyer flask.
12. Add 25 mL of fresh methylene
chloride to the aqueous layer remaining in
the separatory funnel and shake vigorously
for 10 seconds and allow the layers to
separate for 5 minutes.
13. Remove the lower organic layer and
place it in the Erlenmeyer flask containing
the first 25 mL of methylene chloride.
14. Repeat Steps 12 and 13 – this will
make a total of ~75 mL of methylene
chloride.
15. Rinse the separatory funnel with water
(don’t dry it) and place the ~75 mL of
methylene chloride back into the funnel.
Allow the lower organic layer to slowly
pass through potassium carbonate granules
(the instructor will give you the K
2
CO
3
when you are ready) that have been
placed in the fluted filter paper in the
apparatus shown. The glass funnel and
Erlenmeyer flask must be dry. If water
droplets are observed in the Erlenmeyer
flask after filtering, add ~ 1 g K
2
CO
3
to
the liquid, swirl vigorously and pass the
solution through a fluted filter paper into a
dry Erlenmeyer flask.
16. Save 0.5 mL of this liquid in a clean
test tube (label Step 15).
17. Place the rest of the organic layer into
a 250 mL RBF (24/40 ground glass joint)
and remove the CH
2
Cl
2
via rotary
evaporation (see apparatus below).
18. Stop the evaporation when ~ 5 mL
remain (if the solvent completely
Stopcock
Closed
Separating
Draining
Stopcock
Open
Remove Stopper
18
E x p e r i m e n t 18 • Isolation of Caffeine from Tea
5
evaporates or you have < 5 mL left, add
~ 5 mL CH
2
Cl
2
to dissolve the caffeine).
19. Use a pipette to transfer the caffeine /
CH
2
Cl
2
solution to a 50mL beaker.
20. Rinse the RBF several times with
distilled methylene chloride (don’t use
more than 10 mL total) and add these
rinses to the 50mL beaker.
21. On a hot plate in the hood evaporate
the CH
2
Cl
2
solution to almost dryness
(leave ~ 1 mL). Swirl the solution
continually during evaporation, otherwise
spattering will occur and your product will
be lost.
22. The heat remaining in the beaker
should evaporate off the remaining CH
2
Cl
2
leaving only solid caffeine in the beaker.
If not, heat the beaker to remove any
remaining CH
2
Cl
2
.
23. Add just enough acetone (0.5 – 2.0
mL) to the impure caffeine to completely
dissolve it when the acetone is boiling.
24. Immediately remove the heat and add
10 drops low-boiling petroleum ether
(primarily pentane) to precipitate caffeine
from the solution. If caffeine does not
precipitate, heat the solution on a hot plate
to remove ~ 0.25 mL acetone and add 10
more drops of low-boiling petroleum
ether. Keep removing 0.25 mL of acetone
and adding the petroleum ether until
crystals form.
25. Cool the crystals in an ice / water bath
for 5 minutes. Also cool 5 mL of low-
boiling petroleum ether.
26. Remove the “mother liquor” (the
liquid that remains) from the crystals by
suction using a disposable pipette – press
the tip of the pipette on the bottom of the
beaker and withdraw the liquid into the
pipette taking care not to remove any of
the caffeine crystals. If no liquid is
present, go to Step 28.
27. Save this liquid in a clean, dry, labeled
test tube (label Step 26).
28. Wash the caffeine with ~ 5 mL cold
petroleum ether and remove this solvent
with a transfer pipette as described in Step
26 – add this solvent to the test tube in
Step 27.
29. Place a tiny amount (a speck) of your
caffeine crystals in a clean test tube (label
Step 29).
30. Place the rest of your crystals in the
drying oven until you have completed the
TLC analysis.
Thin Layer Chromatography
31. You should have three labeled test
tubes containing caffeine at various purity
levels. At the front of the class is another
test tube containing pure caffeine from the
stockroom. Spot these four solutions on a
Thin Layer Chromatography (TLC) plate
and develop the plate with 50% CH
3
OH /
H
2
O. Place a line 1 cm from the top of the
plate and remove the plate from the
Distillate
Bath
Your
Compound
Water Bath
Condenser
Distillate
18
E x p e r i m e n t 18 • Isolation of Caffeine from Tea
6
solvent when the solvent just touches this
line. Use UV light to visualize your spots.
Determine the R
f
value of your caffeine.
32. Weigh the product and obtain its
melting point.
33. Place your product in a labeled vial
and turn it in to your instructor.
34. Calculate the percent recovery from
the weight of tea used.
Lab Report
Once you have turned in your Instructor
Data Sheet, lab attendance will be entered
and you will have access to enter your lab
data online and begin the lab submission
process. Enter you lab data before exiting
the lab - enter your data accurately to
avoid penalty. The lab program will take
you in order to each calculation. Mouse
over the orange “TOL” link to see the
points and tolerances for each calculation.
Lab 18
Isolation of Caffeine from Tea
Name:
Mass: g
f
Distance from start to center of substance spotted
R=
Distance from start to solvent front
Interpreting the Data
The R
f
value for caffeine must be calculated. R
f
stands for “ratio of fronts” and is characteristic
for any given compound on the same stationary
phase using the same mobile phase. In other
words, you and any other person in the lab today
should calculate the same R
f
value for caffeine
since you are both using the same type of
chromatography medium, the same developing
solvent and essentially the same experimental
conditions.
7
Isolation of Caffeine from Tea
Student Data Sheet
Weight of Tea - g
Weight of recrystallized Caffeine g
Melting Point Range of Recrystallized Caffeine ºC
Results from Thin Layer Chromatography: “Reproduce”
your TLC plate on the rectangle provided.
Isolation of Caffeine from Tea
Instructor Data Sheet
Weight of Tea - g
Weight of recrystallized Caffeine g
Melting Point Range of Recrystallized Caffeine ºC
Results from Thin Layer Chromatography: “Reproduce”
your TLC plate on the rectangle provided.
Name:
Partner:
Spots
Spots