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Column chromatography works on the same principle as TLC

Column chromatography works on the same principle as TLC. The adsorbent (alumina or silica gel) should be packed with a stream of air or nitrogen to drive out air pockets in the column. This leads to better separation. Selection of the eluting solvent is an important factor in a good separation.

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Column chromatography works on the same principle as TLC

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  1. Column chromatography works on the same principle as TLC • The adsorbent (alumina or silica gel) should be packed with a stream of air or nitrogen to drive out air pockets in the column. This leads to better separation.

  2. Selection of the eluting solvent is an important factor in a good separation • The more polar the eluant, the faster compounds will move through the column. If a solvent is too “fast”, everything will come out with the solvent front.

  3. More polar compounds travel more slowly through the column. • Compounds with more polar groups will adhere to the adsorbent (alumina or silica gel) more strongly than less polar molecules.

  4. The column should have a level surface so that the bands stay even as they travel through the column. • The sample should be applied to the column in a minimum amount of solvent. Wide band widths lead to poor separation.

  5. Narrow bands traveling through the column prevent overlap.

  6. Isolating the separated compounds • Run TLC’s of the fractions after the column to decide which ones to combine.

  7. The C=O bond has some single bond character.

  8. If Z is an electron-withdrawing group, then resonance structure 2 becomes less important

  9. The wavenumber is the inverse of the wavelength. It is directly proportional to energy.

  10. Expt. 16 – Separation of an Alkane Clathrate • Preparation: You will add urea dissolved in methanol to a mixture of 2,2,4-trimethylpentane and hexadecane. After a white solid forms, cool with an ice/water bath until crystallization of the clathrate is complete.

  11. Confirm identity of the hydrocarbon by comparison of its IR spectrum to that of hexadecane and 2,2,4-trimethylpentane

  12. Clathrates in the News • Methane hydrate deposits on the ocean floors are twice the size of the known coal and gas reserves on earth. Could they be tapped as an energy source? Methane is a potent greenhouse gas and could contribute to the global warming phenomenon.

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