3-TLCCaffeineAspirinF07 - Thin Layer Chromatography...

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Thin Layer Chromatography: Analysis of Isolated Caffeine and Aspirin Reading assignment: Fessenden, R.J., Fessenden, J.S., Feist, P. Organic Laboratory Techniques , 3 rd ed.; Brooks/Cole: Pacific Grove, 2001, pp 133-138. Introduction In last week’s experiment you attempted to isolate aspirin and caffeine (figure 1) from the over-the-counter drug, Anacin®. This week your goal is to gather evidence to determine the purity of the compounds isolated and to prove whether or not you successfully isolated caffeine and aspirin. aspirin C C N C N C N HC N O O CH 3 H 3 C caffeine C O OH O C O CH 3 CH 3 Figure 1. The structures of caffeine and aspirin. Chromatography Chromatography is the general method used to separate mixtures of two or more compounds. Some examples include column and gas chromatography, which you will learn later this semester, and thin layer chromatography (TLC), the focus of today’s experiment. TLC is an extremely simple, fast, and inexpensive technique involving separation of compounds based on differences in polarity, adsportivities and solubilities . TLC has many uses including determination of the number of components in a mixture, identification of the components in a mixture, to monitor the progress of a reaction, and to measure the effectiveness of a purification. In today’s lab you will use TLC to determine the identity of the compounds isolated in last week’s experiment and also to make a determination of the purity of these compounds. As you have learned from reading Fessenden and Fessenden, chromatography typically involves the use of two phases: a mobile phase, or eluent and a highly polar stationary phase , or adsorbent . The mixture of compounds to be separated/analyzed is dissolved in an easily evaporated solvent and placed on the adsorbent. The eluent travels via capillary action through the stationary phase and as it travels past the mixture of compounds an equilibrium is established for each component of the mixture between the molecules of that component which are adsorbed on the solid and the molecules which are in solution. In principle, the components will differ in solubility and in the strength of their adsorption to the stationary phase and some components will be carried farther up the plate than others. Generally, polar molecules adsorb more tightly to the polar stationary phase than non-polar molecules and consequently travel a shorter distance up the plate.
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Many solvents can be used as the mobile phase and some examples are listed below in the order of decreasing polarity. A non-polar solvent will carry a non-polar component up the stationary phase and leave behind the polar compounds. A polar solvent will carry non-polar and polar compounds up. Combinations of solvents are useful because the polarity of the mobile phase can be fine-tuned by varying the ratio of the more polar solvent to non-polar solvent. Popular combinations include 0-30% ethyl acetate/hexanes and 0-40% ether/pentane. Methanol (CH
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3-TLCCaffeineAspirinF07 - Thin Layer Chromatography...

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