Experiment 4

Experiment 4 - Experiment 4: Determination of the Partition...

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Experiment 4: Determination of the Partition Coefficient of Caffeine and Sublimation ( Mohrig Chapter 12) Liquid-liquid extraction techniques will be used to determine the partition coefficient (K) of caffeine. A purification technique, sublimation , will be conducted on the isolated caf- feine. Caffeine is a member of the class of natural products called alkaloids, which contain a ni- trogen atom. Other members of this class include morphine, mescaline, and strychnine. The biological activity of these compounds has led to their thorough investigation by many different disciplines. Caffeine possesses the oxidized skeleton of purine (see above), a related compound which is a constituent of DNA. Caffeine is found in many plants, with coffee and tea being the most familiar. A cup of coffee or tea provides a 25 to 100- mg “dose” of caffeine which stimulates the central nervous system. Tea can be made by dipping a tea bag in hot water, thereby extracting out the flavors from the leaves, and also the caffeine. This means that caffeine is soluble in water, but just how soluble is it? Chemists use a value called a partition coefficient to determine how soluble a compound is in water relative to another immiscible layer, usually an organic liquid. Knowing this partition coefficient (K value) can help determine the number of ex- tractions necessary to partition all of the material into one layer. This is crucial to process chemistry because no material that costs money to acquire should be wasted. Procedure 1 : Weigh out approximately 100 mg of caffeine and place this in a clean and dry 5 mL conical vial. Add 1.5 mL of water and 1.5 mL of dichloromethane to the conical vial containing the caffeine. Cap the vial and shake until fully dissolved. Do not shake too vigorously because dichloromethane and water can form an emulsion (If an emulsion happens ask your TA to show you how to centrifuge it using a centrifuge tube) Let the conical vial sit on the desktop until the layers equilibrate and two clear layers form. Using a Pasteur pipet, remove the bottom organic layer and transfer to a small Erlenmeyer flask. Add drying agent, Na 2 SO 4 , to the flask to absorb any water contaminating the organic liquid. After drying for 10 minutes, make a filter pipet by stuffing a small amount of cotton down the top of a Pasteur pipet with your small spatula. Add about 1 cm of Na
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Experiment 4 - Experiment 4: Determination of the Partition...

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