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Thin-Layer Chromatography

Thin-Layer Chromatography. Experiment 11 (4 th ed.) Experiment 10 (3 rd ed.). Required Reading Before Class. 4 th ed.—Exp. 11 Sections 1-4 of Techniques 19 Sections 1, 2, 4, 5, 6, 7, 9, & 10 of Technique 20 3 rd ed.—Exp. 10 Sections 1-4 of Technique 12

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Thin-Layer Chromatography

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  1. Thin-Layer Chromatography Experiment 11 (4th ed.) Experiment 10 (3rd ed.)

  2. Required Reading Before Class • 4th ed.—Exp. 11 • Sections 1-4 of Techniques 19 • Sections 1, 2, 4, 5, 6, 7, 9, & 10 of Technique 20 • 3rd ed.—Exp. 10 • Sections 1-4 of Technique 12 • Sections 1, 2, 4, 5, 6, 7, 9, & 10 of Technique 14

  3. Overview of TLC • TLC—one form of solid-liquid chromatography • Adsorbent—solid phase; it won’t dissolve in the associated liquid phase • Ex: Silica Gel (Si02) & Alumina (Al2O3) • Elutent—liquid phase • Solvated substances are spotted onto a TLC plate coated with the adsorbent and allowed to elute up the plate by capillary action

  4. TLC Development Setup

  5. Overview of TLC • This technique manipulates POLARITY • More polar substances bind strongly to the adsorbent and elute SLOWER • Less polar substances bind weakly to the adsorbent and elute FASTER • The strength of interactions between the adsorbent and eluting components vary approximately in this order: Salt formation > coordination > H-bonding > dipole-dipole > van der Waals (More Polar) (Less Polar)

  6. Overview of TLC—The Rf Value • A given compound will always travel a fixed distance relative to the distance the solvent travels • This ratio is called the Rf value and is calculated in the following manner: .distance traveled by substance.distance traveled by solvent front

  7. TLC Visualization Methods • Ultraviolet Light—some organic compounds illuminate or fluoresce under short-wave UV light • Iodine Vapor—forms brown/ yellow complexes with organic compounds • Fluorescent Indicators—compounds fluoresce when placed under UV light • Silver Nitrate Spray (for Alkyl Halides)—dark spots form upon exposure to light • Sulfuric Acid Spray + Heat—permanent charred spots are produced

  8. Applications of TLC • TLC has several important uses in organic chemistry. Some examples are: • To establish that two compounds are identical • To determine the number of components in a mixture • To determine the appropriate solvent for a column-chromatographic separation • To monitor the progress of a reaction

  9. Procedural Notes • TLC Chamber Preparation • Cut the filter paper so that it fits in the jar, touching the bottom and reaching a height of about 1cm from the top of the jar • To ensure that the filter paper will work, put it in the jar, and then place an unused TLC plate in the jar. If the above criteria are met and the plate doesn’t make any contact with the filter paper, the setup should work • Remove the TLC plate, and then completely saturate the filter paper with the development solvent using a pasteur pipet. • Fill the jar with development solvent to a depth no greater than 0.5cm • Put the lid on the jar to preserve the saturated conditions

  10. Procedural Notes • TLC Plate Preparation • Do not touch the coated surface of the TLC plate! It is best to wear gloves when handling, but you can also manage by just grabbing the plate on the edges • Using a PENCIL, draw a line across the plate about 1cm from one end • Label your spotting lanes. You should be able to get at least three lanes per plate

  11. Procedural Notes • Spotting the TLC Plate • Dip the open end of a capillary tube into the solvent containing the compound to be eluted • Touch the end of the capillary tube lightly and very briefly to the coated surface of the TLC plate • Your spots should be made on the line drawn across the plate in the correct lanes and shouldn’t have a diameter much larger than the capillary tube • After spotting the plate, place it in the saturated chamber and close the lid • Substances should be eluted until the solvent front reaches a height of about 0.5cm from the top of the TLC plate

  12. Procedural Notes • Marking the TLC Plates • Before allowing the plate to dry, mark the solvent front w/a pencil • After drying, observe the developed plate under short-wave UV light. Circle all visible spots w/a pencil • Perform the I2 vapor visualization method after doing the UV method

  13. Analyses • Calculate Rf values • Compare the values of known compounds to the unknowns & interpret the data • Compare the visualization methods used • Don’t forget to sketch your TLC plates for your lab report. You do not need to turn in your TLC plates, although you may attach them to your lab report if you wish.

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