Extraction of Caffeine from Tea Leaves

 

 

Introduction

The active ingredient that makes tea and coffee valuable to humans is caffeine. Caffeine is an alkaloid.  Alkaloids are a class of naturally occurring compounds containing nitrogen and having the properties of an organic amine base.

Caffeine is a naturally occurring alkaloid found in over 60 plant species.  Caffeine belongs to a family of naturally occurring compounds known as xanthines.  The xanthines, which come from plants, are possibly the oldest known stimulants.  Caffeine is the most powerful xanthine in its ability to increase alertness, put off sleep and to increase ones capacity for thinking. Caffeine is a vasodilator (relaxes the blood vessels) as well as a diuretic (increases urination).

Caffeine does not exist alone in tealeaves; the leaves are mainly cellulose, pigments and chlorophylls, and tannins.  Tannins are phenolic compounds of high molecular weight that have certain properties in common. 

Some of the better-known plant sources are coffee and cocoa beans, tealeaves, and kola nuts.  While coffee and tea are both popular products containing caffeine, the amounts vary widely in a single serving.  To further confuse the matter, coffee beans contain less caffeine than tea leaves when measured dry.  However, a serving of coffee contains roughly twice the caffeine of tea.  Much of the flavor of coffee and tea comes from tannins and other flavoring agents.  Caffeine has a slightly bitter flavor.  As a result, decaffeinating coffee beans and tea leaves will leave the flavor slightly changed even if no other compounds are lost.

 

Beverage

Caffeine (mg)/cup

Coffee

80 – 125

Coffee, decaffeinated

2 - 4

Tea

30 – 75

Chocolate milk/cocoa

3 – 30

Soft drink

20 – 50

 

Several health concerns have been raised over the consumption of caffeine.  The Food and Drug Administration (FDA) has extensively studied the consumption of caffeine and its health effects.  In 1987 the FDA concluded that normal caffeine consumption does not increase risk to health.  These studies included cancer risk, coronary heart disease, osteoporosis, reproductive function, birth defects, and behavior of children.

Many consumers prefer to avoid caffeine partially or altogether due to its stimulant effects and others still have health concerns.  This makes decaffeinating coffee and tea an important industrial process.  Decaffeination is also significant for the world’s economy; approximately eight billion pounds of coffee are grown a year making it second only to oil as an international commodity.  It should be noted that decaffeinated coffee and tea are not caffeine free.  These products can be labeled decaffeinated as long as 97% of the caffeine has been removed.

 

 

Theory

Caffeine (C8H10N4O2) is an alkaloid with the structure given below.
                                               
Alkaloids are bitter tasting, natural nitrogen-containing compounds found in plants.  The basic property of alkaloids comes from the lone pair of electrons found on at least one nitrogen.  Alkaloids are often found to have potent physiological activity.  Some better-known examples are morphine, heroin, lysergic acid (LSD), cocaine, quinine, strychnine, and nicotine.

The basic N in caffeine can be used to increase or decrease its water solubility.   

Acidic conditions will form the conjugate acid salt giving caffeine increased water solubility as a cation.  On the other hand if caffeine is in a basic environment it takes the neutral form and it is only somewhat polar. 

In order to successfully extract any substance from one solvent into another, we must maximize differences in solubility.  The key step in this entire procedure is the extraction. Extractions can have many forms, and the most common is the liquid-liquid extraction.  Today’s lab uses a liquid-liquid extraction.  For this process to work, the liquids cannot be soluble in one another. Our two liquids are water (aqueous layer) and dichloromethane (organic layer).  When these two liquids are vigorously shaken together, compounds that are dissolved in each layer will have the opportunity to pass into the other liquid.  If a compound is more soluble in water than dichloromethane, it will be primarily dissolved in the water layer at the end of the extraction. After the liquids have been mixed, they will begin to separate.

 

 

Procedure

http://www.okbu.edu/academics/natsci/chemistry/courses/cheml310/caffeine.htm

Carefully open two tea bags, record the mass of the leaves and the brand of the tea. 

Dissolve approximately 2.0 g sodium carbonate in 10. mL water in a beaker and heat the solution to near boiling on the hot plate.  Once the solution is hot, add two tea bags and gently boil for ~15 minutes.  Place a watch glass on top of the beaker.  Remove the bags, and force out as much water as possible by pressing the bags against the side of the beaker using a stirring rod.  Be careful not to break the bags. 

Remove the beaker from the hot plate and let the dark solution cool to room temperature.  (It is very important for the solution to be completely or nearly completely at room temperature.) Transfer the entire solution into a vial.  Add 2 mL CH2Cl2 to the aqueous solution, seal the vial tightly, shake and invert to mix the layers.  Carefully open the vial since pressure may have developed during the shaking.  Remove the CH2Cl2 layer (make sure that you have the correct layer!!) from the tube with a pipet.  Try to remove as much CH2Cl2 possible without removing any water (it may not be possible to remove every bit of the CH2Cl2.)  Note that small amounts of water (dark liquid) that are accidentally removed will be taken care of later in the procedure.

Place the CH2Cl2 into a clean flask and set aside. 

Continue the extraction procedure with two more fresh portions of 2 mL CH2Cl2.  Each time be careful with pressure building up in the vial, and try to remove only CH2Cl2 from the centrifuge tube.  Combine all CH2Cl2 washes. 

Add a small amount of MgSO4 to the organic layer and swirl the mixture.  Some of the MgSO4 will clump up, and the rest should remain as a fine powder.  If there is no fine powder, add a little more MgSO4 and swirl again.  Keep adding more until the drying agent does not completely clump up.

 

Transfer the dried organic solution using a pipet with a small cotton plug into the beaker into a clean dry reaction tube.  Rinse the MgSO4 residue with a little fresh CH2Cl2 and filter this into the reaction containing the organic layer.  Add a boiling chip or stick.  In the hood, drive off the methylene chloride by gently warming the flask on a sand bath.  Remove the reaction tube from the heat once you begin to a white solid.  Let the reaction tube cool for a few minutes, and record the weight of the extracted caffeine.  Determine the amount of caffeine that has been extracted, and the percent recovery from the two starting tea bags.

 

Cleaning up:  Glass vials should be disposed of in the glass waste. The brown aqueous layer goes into the aqueous waste

 

 

 

 

 

Purify your caffeine by sublimation, carefully monitoring the heat so as to minimize the decomposition of your caffeine.  Determine the mass of the pure product and the melting point.  Use the FT-IR to identify your product as caffeine.

 

 

Alternate purification:  Purify by recrystallization from warm acetone. 

 

 

 

Questions

1.  A student neglected to add sodium carbonate when extracting the tea leaves with hot water.  Yet a very high yield of caffeine was obtained.  The crystals melted at 202 – 214 °C.  Explain. 

2.  What two things does the addition of Ca(OH)2 do to aid the extraction of caffeine?

3.  Is it better to do two extractions with each 2mL of one extraction with 4mL?  Please explain. 

 


References

Murray, D.S.; Hansen, P.J., J. Chem. Educ., 1995 (72) 851.

Hampp, A., J. Chem. Educ., 1996 (73) 1172.