6A_Lecture_5

6A_Lecture_5 - Lecture 5 TLC Experiment 5 NMR Spectroscopy...

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Unformatted text preview: Lecture 5 TLC Experiment 5 NMR Spectroscopy Chromatography ◦ Separation of complex mixtures of compounds ◦ Name insinuates separation of colored compounds Three types we will discuss: Thin-Layer Chromatography ◦ Liquid Chromatography (LC) and High Pressure LC (HPLC) ◦ Gas Chromatography Concept is based on: ◦ Mobile Phase: Liquid or gas that carries compounds through the solid phase ◦ Solid Phase: A stationary composite through which the compound and the mobile phase interact TLC tells us: ◦ Progress of a reaction ◦ Rough number of compounds in a mixture ◦ Rough indication of purity ◦ Relative polarity differences between molecules Overview of TLC procedure ◦ Making the TLC Chamber Let the solvent (mobile phase) equilibrate Prepare the TLC plate Spot your compound on the TLC Develop the TLC plate in the chamber Visualize the spots Destructive Staining (KMNO 4 ) Non-Destructive UV-light Iodine (I 2 ) Calculate R f values (ratio to the front) R f = spot dist divided by solvent dist Must report solvent with R f Why do the compound spots move differently?? Answer = Polarity ◦ Solid phase (Silica, Alumina, Cellulose) is polar ◦ Polar compounds can bind to solid phase by: Hydrogen bonding, van der Walls forces, dipole-dipole “like interacts with like” so polar compounds move slower ◦ Non-Polar compounds can bind only by weak vdW These compounds move faster ◦ Solvent effects the distance traveled too Compounds move slower in non-polar solvents Common Solvent Systems (mobile phase) *You don’t have to use a pure solvent, you can use a mixture to cater to the polarity you want Hexanes (non-polar) Dichloromethane (non-polar) Ether (moderately polar) Ethyl acetate (moderately polar) Acetone (polar) Water not advised because it dissolves silica gel Most common system is a ratio of hexanes/ ethyl acetate So, TLC tell us what again??...
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This note was uploaded on 09/09/2011 for the course CHEM 6a taught by Professor Pettus during the Spring '07 term at UCSB.

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6A_Lecture_5 - Lecture 5 TLC Experiment 5 NMR Spectroscopy...

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