ThinLayer Chromatography

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Thin-Layer Chromatography

• Experiment 11 (4th ed.)
• Experiment 10 (3rd ed.)

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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

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Overview of TLC

• TLCone form of solid-liquid chromatography
• Adsorbentsolid phase it wont dissolve in the
  associated liquid phase
• Ex Silica Gel (Si02) Alumina (Al2O3)
• Elutentliquid phase
• Solvated substances are spotted onto a TLC plate
  coated with the adsorbent and allowed to elute up
  the plate by capillary action

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TLC Development Setup
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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 gt coordination gt H-bonding gt
  dipole-dipole gt van der Waals
• (More Polar)
  (Less Polar)

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Overview of TLCThe 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

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TLC Visualization Methods

• Ultraviolet Lightsome organic compounds
  illuminate or fluoresce under short-wave UV light
• Iodine Vaporforms brown/ yellow complexes with
  organic compounds
• Fluorescent Indicatorscompounds fluoresce when
  placed under UV light
• Silver Nitrate Spray (for Alkyl Halides)dark
  spots form upon exposure to light
• Sulfuric Acid Spray Heatpermanent charred
  spots are produced

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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

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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 doesnt 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

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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

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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
  shouldnt 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

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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

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Analyses

• Calculate Rf values
• Compare the values of known compounds to the
  unknowns interpret the data
• Compare the visualization methods used
• Dont 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.