• To carry out an extraction of an aqueous solution of coffee
• 100 mL coffee extract solution• 2 g solid sodium carbonate• 25 mL methylene chloride
• spoonful anhydrous sodium sulfate• 250 mL beaker
these alkaloids caffeine,1,3,7 – trimethylxanthine,occurs naturally in coffeebeans and tea leaves, but is added to soft drinks and stimulantssuch as Vivarin and No-Doz, dieting aids such as Dexatrim andDiatac and painkillers such as Anacin and Excedrin.
Alkaloids are a broad category of nitrogen containing organicmetabolites produced by plants. Since they contain nitrogen,they behave like bases (alkalis) and hence, they are termedalkaloids. Other common alkaloids are morphine, quinine, cocaineand codeine. These substances are extremely bitter and/or toxic. By producing them, plants make their leaves unattractive toeating by insects and higher animals.
In this experiment, you will extract caffeine from coffee usingmethylene chloride (CH Cl ). Methylene chloride is an organic
solvent that is somewhat nonpolar. In this experiment, we takeadvantage of the same principles we used in thin layerchromatography, especially “like dissolves like”. Methylenechloride is a liquid but it isn’t like water so they form two
layers, just like vinegar and oil. The caffeine molecule iscloser in structure to methylene chloride, so it will dissolvein that layer. The two layers can then be separated, and themethylene chloride, which has a very low boiling point, can beevaporated in the hood, leaving a residue of caffeine.
In order to verify that you have obtained caffeine, you willtake a melting point of this residue. Melting points are aunique property of pure substances. For example, pure watermelts at 0 degrees C. Pure caffeine melts at 238 ° C. If thematerial you collect melts at a temperature close to 238degrees, you can be more confident that it is caffeine. Thematerial that you have extracted will probably have a slightlylower melting point.
To a clean 500 mL Erlenmeyer flask, add 100 mL of coffeeextract. This extract will have been prepared for you inadvance.
Add approximately two grams of sodium carbonate (NaCO ) to
the coffee solution. This will react with some of thesubstances in the coffee extract and make them extremelywater soluble. Swirl the mixture until all the sodiumcarbonate dissolves.
Add 25 mL of methylene chloride (CH Cl ), and vigorously
swirl the mixture for 10 minutes. Do not shake the mixtureor an emulsion will form.
Allow the mixture to stand and separate into two layers; adark aqueous top layer and a clear methylene chloridebottom layer.
Carefully pour off into a beaker, as much of the top layeras you can, without removing the bottom layer. This processis called decanting.
Place a 12.5 cm fluted filter paper in a long stem glassfunnel. Put the funnel in a small iron ring and suspend itover a 250 mL Erlenmeyer flask.
Using a squeeze bottle of water, thoroughly wet the filterpaper.
Slowly and carefully pour the methylene chloride/watermixture into the fluted filter paper. The excess water willdrain through and the methylene chloride solution ofcaffeine will remain on the filter paper.
Using a pipet, transfer the methylene chloride solution toa 50 mL Erlenmeyer flask. To this solution, add a scoop ofanhydrous sodium sulfate (Na SO ), in order to remove the
While the solution is drying, weigh (tare) a 50 mL beakerto the nearest 0.001 g on a balance. Record this tareweight in Table 1 on the DATA SHEET.
Using a pipet, transfer the dried solution to the tared 50mL beaker.
Evaporate most of the methylene chloride in the hood on awarm hot plate. When only a fraction of a milliliter ofliquid is left, remove the beaker from the hot plate. Allowthe beaker to stand in the hood for a minute or two. Theheat remaining in the glass will cause the last amount ofmethylene chloride to evaporate and produce a solid residueof crude caffeine.
In order to determine your recovery of caffeine, reweighthe cool beaker and record this mass in the Table.
By difference, determine the mass of the caffeine in thebeaker and record this value in the Table.
Pure caffeine is a white solid. Describe the appearance ofyour product in the Table.
To a melting point tube, add a small amount of yourcaffeine. This can most easily be done by pressing the openend of the tube down on the caffeine, turning the tuberight-side-up and tapping the tube on the bench until thesolid falls to the bottom of the tube. Tap the tube on thebench a few more times so that the caffeine is compacted.
Place the tube in the slot of the melting point apparatus. Turn on the switch of the instrument. This will turn on alight that illuminates the sample. By looking through themagnifying glass, you should have a clear view of yoursample.
Turn the heating dial to a setting of about 40. This willcause a rapid heating of your sample initially, but shouldnot cause it to melt. As the temperature increases, therewill be a decrease in the rate of heating. Since yoursample will melt above 200 ° C, you may need to periodicallyincrease the setting of the heating dial. Ideally, thetemperature increase at the melting point should be only 1-2 ° C per minute.
Record the range of the melting range of your sample asfollows. In the Table record the temperature at which youfirst see liquid beginning to form. The sample willcontinue to melt. When the last of the sample has melted,record the second temperature. These two values areseparated by a dash, i.e., 63-65 ° C. For a relatively purecompound, this temperature range will generally be only afew degrees.
DATA SHEET - THE EXTRACTION OF CAFFEINE FROM COFFEE
a. Is methylene chloride more or less dense than water?
b. What evidence do you have to support your answer?
Was your melting point the same as the reported meltingpoint? If not, why not?
Why was sodium carbonate added to the coffee solution?
Explain why the caffeine that you obtained in thisexperiment may not have been white in color.
List some substances that contain caffeine other thancoffee.
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