Reminder: These notes are meant to supplement, not replace, the laboratory manual.
Extraction: Isolation of Caffeine from Tea Leaves Notes
Application and History
Tea has been enjoyed in china for the past four thousand or so years i and within
the west for about the past 400 years. Tea originated in China and this was the
sole producer until the British Empire colonized India and began large tea
plantations. This tea colonization spread to many other countries and had huge
worldwide socio-political ramifications. Tea drinking is still a very important ritual
in Japan and England from the choreographed Japanese tea ceremony to the
daily tea time held in homes and offices all across the United Kingdom. Currently
the United States is the 13th highest consumer of tea in the worldii.
Many people want to enjoy the flavor of tea without the caffeine.
Decaffeinated teas and decaffeinated coffees are in high demand. Today
caffeine is removed from whole coffee beans using either a methylene chloride
process (KVW or European Process) akin to the method carried out in this
experiment or a Swiss Water Method. There are health concerns with using the
methylene chlorine method but the water method removes some of the flavor iii.
1. Terminology.
Partitioning. This is a distribution of a material between two or more phases.
Drying. As applied to organic chemistry, this means the removal of water from a
liquid. Organic liquids can be wet (containing water) or dry (containing little or no
water).
Decanting. This is the process of pouring off a liquid and leaving a solid or
sediment behind.
Emulsion- One liquid held in a suspension within another liquid. Mayonnaise is
an emulsion of oil in water and lemon juice. Emulsions are usually an undesirable
occurrence when attempting a separation or extraction.
Distribution coefficientiv is a numerical assessment of the ratio of a compounds
solubility in an organic solvent over that in water.
The larger the
the more efficient the extraction of a compound from water into the
other solvent.
2. Composition and role of each material in extraction
Tea-Leaves of Camellia Senesisv which are used to brew a popular beverage.
The leaves contain cellulose, chlorophyll, flavonoids, tannins and caffeine
Tea bag- Contains the solid tea.
Water- This is the initial solvent used to extract the caffeine from the solid tea in
the initial solid-liquid extraction step.
Sodium carbonate (Na2CO3)- This is a base which deprotonates the phenolic OH
of tannins, rendering these compounds anionic and hence highly soluble
in water but with a low solubility in the methylene chloride.vi If Na2CO3 was
not present the isolated caffeine would also contain extracted tannins.
Methylene Chloride- CH2Cl2, This is also called dichloromethane. This is a
halogenated organic solvent. This is used in the liquid-liquid extraction
portion of the experiment. The caffeine is extracted from the water solvent
to the CH2Cl2 due to a favorable distribution coefficient.
Sodium Sulfate- Na2SO4- This is a material which binds with water and removes
it from a liquid. It is a drying agent.
Ethyl Acetate: Acetic Acid, This 95% ethyl acetate 5% acetic acid mixture is the
mobile phase in the TLC analysis.
3. Solvents for extraction experiments should have the following desirable properties.
For both liquid-liquid and liquid-solid extractions, the solvent should have a
relatively low boiling point for easy removal by evaporation; and it should
not react with any of the substances present (unless you are performing
an acid-base extraction).
In liquid-liquid extraction, the compound being extracted should have a favorable
distribution coefficient in the extracting solvent; in liquid-solid extraction,
the solvent must dissolve the compound being extracted.
4.
The caffeine extraction procedure has several steps which are required but the
reason for doing them may not be obvious. Here are some explanations.
Sodium carbonate is a base. Caffeine is an alkaloid, an organic base. Tannins
contains phenolic –OH groups on aromatic rings which are acidic. Sodium
carbonate is added in the first extraction to make sure that 1) the caffeine
remains in the free base form (that is, to prevent it from reacting with any
acids that may be present and form a cation) and 2) to reacts with the
acidic phenolic groups of the tannins rendering them ionic and keeping
them in the aqueous phase when the liquid-liquid extraction is carried out.
An emulsion is a suspension of one liquid as droplets in another (the two liquids
must be insoluble in one another). Emulsions are almost always
undesirable. To avoid them, you can shake mixtures of insoluble liquids
gently, and add salt to aqueous layers. You will use a centrifuge in this
experiment break them up once they form.
5.
The equation for calculating the percentage of caffeine in tea is as follows:
percentage of caffeine =
6. The purity and identity of the isolated material will be determined via TLC. Review
methods and calculations related to TLC before coming to lab. The TLC
plate is white and caffeine is white. Caffeine cannot be seen on a TLC
plate without the use of an ultraviolet (UV) light or other imaging
technique. The caffeine absorbs the UV light and gives off visible light.
This phenomenon is called fluorescence. The caffeine is clearly visible on
the TLC plate under UV light but invisible under normal lighting.
7. There are some systematic errors in this experiment as it is performed. For example,
the tea is not weighed by itself but instead it is weighed along with the string, tea
bag and a staple. When the liquid tea is separated from the tea bag, some of the
liquid (containing caffeine) remains behind in the wet bag. The distribution
coefficient of caffeine between water and dichloromethane is not perfect; some
caffeine will remain dissolved in the water. Dichloromethane evaporates rapidly,
cooling the watch glass as it does; water can condense on the cool watch glass,
influencing the weight of your final product.
8.
A variety of chemicals are necessary to successfully carry out this extraction.
Caffeine is a naturally occurring alkaloid; an organic nitrogenous base vii. The
organic solvent used is methylene chloride (common name) or dichloromethane.
An ethyl acetate/acetic acid mixture is used as the TLC solvent. The other
chemicals are inorganic salts and bases.
9. Methylene chloride and water are not miscible and have low solubilities in each other.
This means when they are added together they do not mix and form two distinct
layers. Water has a density at 25°C of 1.00 g/mL. Most organic liquids have a
density less than 1.0 g/mL. That is why gasoline and oil float on top of water.
Methylene chloride contains 2 very heavy chlorine atoms (35.5 g/mol) which
results in a high density of 1.33 g/mLviii. Density determines which liquid will be
on top and which on the bottom. When methylene chloride and water are added
together, which is predicted to float?
10. The solubility of caffeine varies substantially based on the temperature of the
solvent and the solvent identity. ix
Solvent, Temperature
Caffeine Solubility (g caffeine / 100g solvent)
o x
Water, 25 C
2.26
Water, 80 o C
20
o xi
Water, 100 C
67
o viii
Methylene Chloride, 25 C
8.45
Caffeine has a much higher solubility in hot water than room temperature (25°C) water.
Caffeine is almost four times more soluble at room temperature in methylene chloride
as it is in water. Therefore the caffeine is extracted from the solid tea leaves into boiling
water. The extract is allowed to cool. The caffeine is then extracted from the room
temperature water into the liquid methylene chloride due to a favorable distribution
coefficient.
11.
Safety considerations for this experiment.
All of the compounds in the experiment are at least slightly toxic, even the tea
(which is more basic than usual because of the sodium carbonate). Avoid
direct contact with these compounds. Evaporate the dichloromethane in
the fume hood, not in the open lab, to keep exposure to a minimum.
Halogenated organic materials have an increased toxicity compared to nonhalogenated materials. Do not breathe the vapors, avoid contact with skin.
References:
i
Tea Revives the World, G.S. Haley Company http://www.gshaly.com/resources/teashistory.htm (March 24, 2012)
NationMaster, Food Consumption: Tea http://www.nationmaster.com/graph/foo_tea_con-food-tea-consumption
(March 24, 2011)
iii
Roasting Coffee, Moore Coffee and Tea Company, http://www.moorecoffee.com/french/roasting.html (March 24,
2012)
iv
Ramette, Chemical Equilibrium and Analysis, Addison-Wesley Publishing, London 1981 p574-575
v
Encyclopedia Britanica, Camellia, http://www.britannica.com/EBchecked/topic/90798/Camellia (November 2,
2012)
vi
D. Pavia, G. Lampman, G.Kriz, R. Englel, A Microscale Approach to Organic Laboratory Techniques, 5 th Edition,
Brooks/Cole, Belmont CA, 2013, p100-101.
vii
Hackh’s Chemical Dictionary, 5th Edition, McGraw-Hill, New York,1969 p26.
viii
CRC Handbook of Chemistry and Physics, 65th edition, CRC Press, 1985, C-372
ii
ix
K. L. Williamson, Macroscale and Microscale Organic Experiments, Houghton Mifflin Company, 3rd Edition,
1999.. pp146-150
x
Shalmashi, A., Golmohammad, F., Latin American Applied Research, vol. 40., no 3, July 2010
http://www.scielo.org.ar/scielo.php?pid=S0327-07932010000300012&script=sci_arttext (accessed December 11,
2013)
xi
Sigma-Aldrich, Product Information Caffeine https://www.sigmaaldrich.com/etc/medialib/docs/SigmaAldrich/Product_Information_Sheet/c0750pis.Par.0001.File.tmp/c0750pis.pdf
(Accessed December 11, 2013)
Revised: October 26, 2014 S. L. Weaver