SC CHEM 333 - Thin Layer Chromatography of Analgesics (TLC) Pre-Lab (7 pages)

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Thin Layer Chromatography of Analgesics (TLC) Pre-Lab



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Organic Chemistry 333L section 03M Laboratory Pre Lab Thin Layer Chromatography of Analgesics TLC By Cameron Kahn 14 January 2019 TA James Collie 1 Page Objective In this experiment one will learn how to set up the apparatus and perform a thin layer chromatography analysis on various standard analgesics The experiment will also teach one to calculate the Rf value and determine the identity of an unknown analgesic by comparing its Rf value to those of the standard analgesics CITATION Geo121 l 1033 By doing the experiment one will learn to identify the content of over the counter analgesics Theory TLC is a simple quick and inexpensive procedure that gives the chemist a quick answer as to how many components are in a mixture TLC is also used to support the identity of a compound in a mixture when the Rf of a compound is compared with the Rf of a known compound preferably both run on the same TLC plate A TLC plate is a sheet of glass metal or plastic which is coated with a thin layer of a solid adsorbent usually silica or alumina A small amount of the mixture to be analyzed is spotted near the bottom of this plate The TLC plate is then placed in a shallow pool of a solvent in a developing chamber so that only the very bottom of the plate is in the liquid This liquid or the eluent is the mobile phase and it slowly rises up the TLC plate by capillary action As the solvent moves past the spot that was applied 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 adsorbent and some components will be carried farther up the plate than others When the solvent has reached the top of the plate the plate is removed from the developing chamber dried and the separated components of the mixture are visualized If the compounds are colored visualization is straightforward Usually the compounds are not colored so a UV lamp is used to visualize the plates The retention factor or Rf is defined as the distance traveled by the compound divided by the distance traveled by the solvent For example if a compound travels 2 1 cm and the solvent front travels 2 8 cm the Rf is 0 75 2 Page The Rf for a compound is a constant from one experiment to the next only if the chromatography conditions below are also constant solvent system adsorbent thickness of the adsorbent amount of material spotted temperature Since these factors are difficult to keep constant from experiment to experiment relative Rf values are generally considered Relative Rf means that the values are reported relative to a standard or it means that you compare the Rf values of compounds run on the same plate at the same time The larger an Rf of a compound the larger the distance it travels on the TLC plate When comparing two different compounds run under identical chromatography conditions the compound with the larger Rf is less polar because it interacts less strongly with the polar adsorbent on the TLC plate Conversely if you know the structures of the compounds in a mixture you can predict that a compound of low polarity will have a larger Rf value than a polar compound run on the same plate The Rf can provide corroborative evidence as to the identity of a compound If the identity of a compound is suspected but not yet proven an authentic sample of the compound or standard is spotted and run on a TLC plate side by side or on top of each other with the compound in question If two substances have the same Rf value they are likely but not necessarily the same compound If they have different Rf values they are definitely different compounds Note that this identity check must be performed on a single plate because it is difficult to duplicate all the factors which influence Rf exactly from experiment to experiment CITATION Dep13 l 1033 Procedure REMINDER Close and cap all reagent and waste containers Start by obtaining a TLC plate and jar Next pick an end of the TLC plate and make a light pencil line about 1 cm from one side to the other this will now be the bottom of your TLC plate Now add the TLC solvent system made of 95 ethyl acetate 5 acetic acid to the TLC jar Make sure the solvent does not go above 1 cm the TLC plate Check this by placing the TLC plate beside the jar and eyeing the line you placed on the TLC plate Now spot one small circle of each of the three standards standards aspirin acetaminophen caffeine ibuprofen and one unknown onto the pencil line on the TLC place Take CAUTION to make sure that the spots are applied along the drawn pencil line Label these spots lightly with a pencil and make sure not to dig deep into the TLC place with the pencil CITATION Geo121 l 1033 Now gently place the TLC plate into the jar and be sure not to splash the solvent on the paper nor should the solvent go higher than 1 cm Wait for the solvent to reach approximately 1 cm to the top of the plate remove the plate and draw a line where the solvent has traveled This is known as the solvent front Now take the UV light and observe your TLC plant Place circles around the highlighted spots Now measure the distance from the center of each spot to the pencil line and record the values Next measure and record the distance travelled by the solvent starting from the pencil 3 Page line Use this information to calculate the Rf value of each of the spots and compare this information with the Rf value of the unknown With this information determine the identity of the unknown Apparatus Figure 1 UV Light Figure 2 TLC Apparatus 4 Page Reagent Table Name Structure Molecular weight g mol Melting point oC Boiling point oC Density g mL 88 10 83 6 77 1 0 8945 60 0524 16 6 117 9 1 0492 32 042 98 64 6 0 791 180 1598 135 140 1 35 151 1646 169 1 293 194 1926 238 1 23 206 28 76 C4H8O2 Ethyl Acetate CH3COOH Acetic Acid CH3OH Methanol C6H8O4 Aspirin C8H9NO2 Acetaminophe n C8H10N4O2 Caffeine C13H18O2 Ibuprofen MechanismNo mechanisms for this lab Disposal Please dispose of the TLC plate in the proper container and this does not mean throwing it in the trash can The excess standard solution can be washed down the drain with excess water The 5 Page organic solvents go in the predestinated organic liquid waste containers Soiled gloves and paper towels go in there proper containers If any broken glass be sure to place it in the broken glass container If at any time


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